I don't understand quantum physics (2018)

Posted by rramadass 5 days ago

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Comment by fasterik 4 days ago

So many entry-level explanations of quantum mechanics start with classical physics, waves, particles, the double-slit experiment, etc. and then jump straight to infinite-dimensional Hilbert spaces. In my opinion, there's way too much conceptual baggage that comes with these concepts [0].

As a non-physicist, QM only clicked for me when I learned the linear algebra of one- and two-qubit systems and observables on those systems [1]. The math is accessible at a pre-calculus level, and the most "counter-intuitive" aspects of QM (interference, the uncertainty principle, entanglement) make perfect sense in this framework. I think it's the best starting point for understanding what QM really says.

[0] That's not to say they aren't important historically or for a complete understanding of QM. I'm talking about conceptual understanding for someone, especially an amateur, encountering QM for the first time.

[1] Chapter 2 of Nielsen and Chuang is what did it for me. It's heavygoing if you've never seen linear algebra before, but I don't know of a clearer explanation of the postulates of quantum mechanics.

Comment by sampo 3 days ago

> As a non-physicist, QM only clicked for me when I learned the linear algebra of one- and two-qubit systems and observables on those systems

Before quantum computers, the only reason to learn quantum mechanics was so that you could learn to explain the electron orbitals of the hydrogen atom. And after that proceed to understand how other atoms and other matter works.

And for this you need vector calculus, partial differential equations, Schrödinger equation, orthogonal functions (spherical harmonics, Laguerre polynomials) (with hint of the theory of Hilbert spaces).

The idea that someone would like to learn the basic principles of QM without applying it to solve the 3-dimensional probability distribution of electrons around the atomic nucleus, is rather new.

Comment by ninkendo 3 days ago

> the most "counter-intuitive" aspects of QM (interference, the uncertainty principle, entanglement) make perfect sense in this framework

Scott Aaronson’s “Quantum Computing Since Democritus” makes a similar assertion about learning QM from a linear algebra perspective and completely sidestepping all the thorny interpretation questions. It’s mostly a book about complexity theory and what quantum computers actually buy you, but it gives a lot of insight into the mathematics of it.

Comment by thenfcm 3 days ago

Tbf, on my physics course (not necessarily all physics courses mind) you start of with waves and such, but soon move on from that kind of understanding and just focus on the maths.

That applied for almost every module, not just quantum.

Comment by kurthr 3 days ago

Yeah, I don't understand where this is coming from. If you're not teaching the math, what are you even doing? Maybe it's from pop-sci books or something where they're really selling woo rather than comprehension?

My books went from 2 slit experiments, to particle in a box (1d and then 2d harmonic oscillators) with tunneling, and then the Hydrogen atom potential with electron orbitals as an extension, to perturbations for He, to Hartree-Fock and Born-Oppenheimer for large atoms and molecular models. Learning the <bra/ket> formalism can be useful for thinking about it, but it has to be motivated by the math.

I guess black body and scattering were pretty early too as motivation for the developments (as they historically were). Whereas EPR/Bell, Decoherence, and "Interpretations" were mostly discussed as interesting flavor and philosophical arguments.

Comment by BobbyTables2 4 days ago

The math is one thing, but I find the idea that the fundamental pieces of reality aren’t particles a little unsettling.

The classic double-slit experiment is kinda bizarre when one considers the “single electron” case.

Comment by fpoling 3 days ago

Particles do exist for practical purposes. If one can distinguish individual photons, then the picture is nothing but a bunch of particles with only averages producing wave-like pictures. But the puzzle is that they do not travel in straight lines and we cannot predict individual point where they will hit.

Comment by OkayPhysicist 1 day ago

The whole point of the double slit experiment is that you get the interference patterns you'd expect from double slits, even when there is no more than one photon in the chamber at a time. One photon enters, one photon hits the detector, it's probability distribution suggests it's being interfered with.

The one-two punch of the photoelectric effect and the double slit experiment (the classic first two labs of QM) force you to accept that photons behave both like waves, and like particles.

Comment by fpoling 1 day ago

There is no interference pattern from a single photon as it hits the registration device at a particular place. But collect a big enough sequence of single photon measurements and it emerges.

This is the reason for statistical interpretation of QM that states that a wave function is just a way to calculate probability with no inherent meaning.

So wave-particle duality is just an artifact of Copenhagen interpretation of QM, not something that one can observe. What one clearly observes that photons travel through a region where they should not classically be and never appear in the region where they should be classically.

Note that even classically waves does not exist as they are just an artifact of motion of collection of molecules that on average can be described by wave-like equations.

Comment by OkayPhysicist 20 hours ago

> There is no interference pattern from a single photon as it hits the registration device at a particular place. But collect a big enough sequence of single photon measurements and it emerges.

The same could be said of the entirely unsurprising many-photon case. The individual photons aren't demonstrating the interference pattern, just the "big enough sequence of single photon measurements". Again, that's not the interesting part of the double slit experiment. We're perfectly used to the concept of a collection of many interacting particles having wave-like properties.

The double slit experiment has non-interacting photons demonstrating wave-like properties.

Comment by fpoling 17 hours ago

The experiment does not demonstrate wave-like properties. It can be interpreted that way, but purely from experimental properties one can also interpret the result that photon traveled by curved path which brings path-integral formulation of quantum mechanics when there is nothing wave-like but there are just probabilities of a particular path.

Comment by OkayPhysicist 16 hours ago

The photon travels a path, with a probability distribution that results in behavior that is identical to a wave, complete with interference. That's wave like.

Comment by diedyesterday 3 days ago

The fundamental claim of QM is essentially a claim about the shape and size of the state space: That states were not limited to the edges (axes) of that high (or infinite-dimensional) space (as classical mechanics thought) and that the whole interior points were available. This is essentially the central claim of QM (imagine it as not being limited to the edges of a very high dimensional cube and that you can choose points in its interior as your states).

Comment by throwaway894345 3 days ago

It’s crazy to me that reality is just waves in quantum fields that interact to create the illusion of particles, which interact to create atoms and chemistry and biology and neurology and society.

Comment by yatopifo 3 days ago

This is the best mathematical model in existence but I’d be cautious equating it with reality. There are lots of gaps that may eventually necessitate that we switch to a more convenient formalism. It has happened many times already.

Comment by namero999 4 days ago

"Particles" don't exist in the naive sense. What a particle is, is an excitation of a field. A "particle" is a dynamic pattern.

Comment by themgt 3 days ago

But that just kicks the can.

The modern record for a matter-wave superposition stands at a nanoparticle composed of roughly 7,000 sodium atoms (about 8 nanometers wide), achieving a massive "Schrödinger's cat" state at the scale of a small virus.

So then particles don't exist, and viruses also don't exist? Is "reality" just a dynamic pattern that doesn't exist in the naive sense? As astute stoics we should not worry ourselves with such matters, "shut up and calculate"?

There is something about quantum theory that is different in character from any physical theory posed before. To put a finger on it, the issue is this: The basic statement of the theory—the one we have all learned from our textbooks—seems to rely on terms our intuitions balk at as having any place in a fundamental description of reality. The notions of “observer” and “measurement” are taken as primitive, the very starting point of the theory. This is an unsettling situation! Shouldn’t physics be talking about what is before it starts talking about what will be seen and who will see it? Perhaps no one has put the point more forcefully than John Stewart Bell:

> What exactly qualifies some physical systems to play the role of ‘measurer’? Was the wavefunction of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for some better qualified system . . . with a PhD?

https://arxiv.org/pdf/1003.5209

Comment by soon_to_be_sb 3 days ago

> But that just kicks the can.

"reality" is just a picture painted on consciousness. A particle exists because consciousness, for some reason, perceive a particle. Physicists should stop looking for smaller particles, and look into space and time as something that emerge out of consciousness...

Comment by throwaway894345 3 days ago

I think you’re asking physicists to pursue a fundamentally metaphysical question, no?

Comment by soon_to_be_sb 3 days ago

Look, imagine you are a scientists in a 3D shooting game, like Quake for example. And you want to figure out the fundamental reality. So in the game, you take game objects and look really close. You find that everything is made up of very small triangles. Then you very carefully slice out a single triangle, and shoots right at its center with some particle like thing from game. You find that something inside the triangle reflect the particle.

You are enthralled! "There is something there still!" you say to yourself.

But there is nothing there! The game engine just figured that the particle would collide with the triangle, and altered its velocity accordingly. But you are too caught up in your assumption that you do more experiments (I think this is what our physicists are doing in our world). But what ever you do, you can only observe the behavior implemented by the game engine, and not the game engine itself, or the computer that is running it....

But you can imagine it. You can imagine an explicitly implemented program and a computer with limited memory that is running it. And if you are allowed to consider it, they can do other experiments. Instead of looking into smaller and smaller things, you can look for other observations that might confirm this. Maybe you can go really far and see if they are not allowed to go beyond some threshold (Ref: The movie "The thirteenth floor").

And would that be pursuing something metaphysical?

Comment by throwaway894345 2 days ago

> But what ever you do, you can only observe the behavior implemented by the game engine, and not the game engine itself, or the computer that is running it....

Yes, physics is about learning about the rules that the game engine implements. It’s not about learning about some reality outside our own, that’s the domain of metaphysics. Physics can’t answer questions about what is happening outside our own reality, and indeed neither can anything else inside reality because (as you pointed out) we can only test and observe what the game engine allows.

Comment by soon_to_be_sb 2 days ago

>Physics can’t answer questions about what is happening outside our own reality..

But the observation that I describe does happen in our reality. It is just that if you look for a reason for such observations here, you are not going to find it.

It is just like when say, biology have to reach out to chemistry and chemistry to physics and physics to math for deeper explanations for processes and observations that are native to their domain. So as unprecedented as it may be, physics should may be try to reach out to metaphysics as a way to explain observations that happen in its domain..

Comment by throwaway894345 1 day ago

> But the observation that I describe does happen in our reality

I don't know what observation you're talking about--you were giving an analogy, right? What am I misunderstanding about your claim?

> It is just like when say, biology have to reach out to chemistry and chemistry to physics and physics to math for deeper explanations for processes and observations that are native to their domain.

Yes, this is emergent complexity and physicists already look into this (e.g., holographic principle). In your game/engine analogy, we're still talking about examining the game, not the game engine.

Comment by soon_to_be_sb 1 day ago

>I don't know what observation you're talking about..

In my example, the observation would be something alters the path of a particle that is shot at the center of the triangle (where it is empty space). This happens in the game reality, but cannot be explained by looking within the same (for example, if you expect to find a smaller triangle at the center of the bigger triangle)

Comment by JCattheATM 3 days ago

particles exist independent of consciousness.

Comment by soon_to_be_sb 3 days ago

Can you describe what you mean by "exist"?

Without a common understanding of that word, any further discussion is useless..

Comment by JCattheATM 1 day ago

The Merriam-Webster definition is sufficient.

Comment by namero999 1 day ago

Nobody denies the existence of an objective, external world. However "particle" is but a description of what appears on your screen of perception. Making the move that reality is made of "particles" is exactly identical of me concluding that you are made of pixels because I look at a picture of you on my screen.

Comment by throwaway894345 3 days ago

This is a bit like saying the universe is made of tiny pink elephants, and arguing that this is only untrue in some “naive sense” because we redefined “tiny pink elephant” to mean “excitation of a field”. It doesn’t seem more naive to me to admit that the world is not in fact made of tiny pink elephants than it is to argue “yes it still is made of tiny pink elephants [but only because we redefined the term to match the evidence]”.

Comment by My_Name 4 days ago

Not if you imagine the single electron going out from the emission point along every path the apparatus allows and back again, forwards and backwards in time, with the whole sequence taking no time at all. Each trip goes through one slit, so the electron goes through both slits on different trips, and the trips combine where they meet at the screen, which gives the fringes (bands). It never turns into a wave and back. It ends up as one dot when one trip doesn’t make it back, because it was absorbed at the screen. The equations of physics are time-symmetric to a very good approximation, and Feynman read antiparticles as particles running backwards in time, so nothing in the maths forbids it. To me it’s the simplest explanation of a lot of “quantum woo”.

Comment by podocarp 4 days ago

It makes sense in the framework but the framework makes no sense. The model contradicts regular intuition.

Comment by fasterik 4 days ago

If we've learned anything from modern physics, it's that our regular intuitions are often wrong. The framework makes perfect sense mathematically, and it makes the right predictions experimentally. It's still controversial how exactly the framework relates to reality, which is where you get into interpretations of quantum mechanics.

Comment by hansvm 4 days ago

One of my favourite examples of this pedagogically is how kinetic energy is quadratic with velocity. It's easy to tie into things people think they understand, like braking distance. Our intuitions are wrong even in shockingly simple problems.

Comment by oasisaimlessly 4 days ago

Agree, but kinetic energy being mv^2/2 rather than mv can be pretty intuitive in the right context:

> Would you father get hit with:

> * a 1 kg object moving at 1 m/s or

> * a 1 g object moving at 1 km/s?

Comment by hansvm 3 days ago

I think that context obscures the kinetic energy thing though. The 1g object is more dangerous because it's able to actually pierce you. I wouldn't particularly want to be hit with a 1 mg object at 1 km/s either, especially near any major arteries, despite it having the same kinetic energy as a 1 kg object at 1 m/s, which I wouldn't care about at all.

And to illustrate the point a bit further, that same argument doesn't work in other speed/mass regimes. Would you rather get hit with:

- A 1000 kg object moving at 1 mm/s or

- A 1 kg object moving at 1 m/s

It's suddenly no longer intuitively obvious because both are totally fine. Like...roll a die, and my squishy human body will be equally happy with either result. Or, shifting the argument the other direction, would you rather get hit with:

- A 1000 kg object moving at 1 km/s or

- A 1 kg object moving at 1000 km/s

Pretty obviously the 1 kg object, despite its much higher kinetic energy. If I'm lucky it just takes off an arm or something, whereas the 1000 kg object is definitely splattering me.

And that sort of thing is why kinetic energy can be hard to reason about. Our intuition is calibrated around our squishiness, which only loosely relates to energy. There are other factors at play.

Comment by pwdisswordfishq 3 days ago

> The 1g object is more dangerous because it's able to actually pierce you.

Why?

Comment by hansvm 3 days ago

Not sure if this is a rhetorical device or if you're legitimately curious, so here's a longer answer than you probably want :)

- Why more dangerous: A 1g steel ball is around 1/4 inch in diameter. A 1/4 inch hole in any major artery is an immediate life-threatening event, same if it hits the lungs, heart, or brain (gut-reaction: I think it'd be quite hard to penetrate the skull with this projectile, so to hit the brain you'd have to go through the eyes or pterion or something, but ignoring that sort of potential quibbling). It'll have pretty significant negative effects on any other system if it can actually pierce that system. If we pick a denser material like lead, the hole is smaller but still life-threatening. If we pick a less-dense material like wood, the hole is larger, and you're still pretty majorly screwed. Contrast that with the 1kg object, which is roughly equivalent to dropping two cans of beans from a height of 2 inches; it just doesn't matter.

- Why pierce: That's a big can of worms with a lot of answers. A halfway okay way to think about it for common hard materials and common shapes in the speed regime being discussed is that you have a higher energy to surface-area ratio.

Critically (in case you're using some sort of Socratic method to guide me to the increased energy being the rationale), the important number for piercing injuries is closer to being that ratio than the total energy, hence my example of 1mg at 1km/s in some recent comment. That has the same energy as our 1kg at 1m/s object, but it's still extremely dangerous because it physically punches out a 1mm hole through your body. If that strikes a location like your eye or a superficially exposed major artery (like the one on the inside of your elbow), you're still going to have a very bad day.

Comment by skeptic_ai 3 days ago

I guess first. But 2nd seems like speed of bullet. But of first object is metal I’m fucked. If second is a feather of 1g I think won’t hurt me. So depends what it is too

Comment by hansvm 3 days ago

500J is a lot of energy. Ballpark estimate, the feather would hurt quite a bit too. It'd have a similar impact to a 1kg steel ball at 31.6m/s (assuming it's 1000x less dense than steel) in terms of energy dissipated per unit area of skin, which is something like the mass of 7 baseballs at the speed of a typical baseball in the major leagues. I'd expect significant blunt-force trauma from a broadside feather at that speed. I'd expect much more severe injuries if it struck with a different cross-section.

Comment by adwn 3 days ago

> But of first object is metal I’m fucked.

1 m/s is just 3.6 km/h, or slow walking speed. You'll be fine.

Comment by adwn 3 days ago

[dead]

Comment by whatever1 4 days ago

This. A tesla model Y going 60mph has almost 2 times the kinetic energy (75MJ) of the same car going 45mph (40MJ)

Your stupid brain thinks, oh I am just 10ish over the limit.

Comment by hansvm 3 days ago

Yep yep. I think the most dangerous version of this failed intuition happens in places where the speed limit is 25mph. 5-10 over is the difference between a few minor injuries and a lot of major injuries or fatalities.

The exact numbers are more nuanced (both in simulation and in terms of available data on the topic), but to illustrate the effect, suppose typical reaction time allows you to slow to 0mph from 25mph just before striking a child with your car, assuming somebody with perfect training and a typical car applying proper emergency braking and decelerating at around 0.8 gs. I'll consider both the 0.5s reaction time estimate (median results in places where people are primed to have to react) or 1.5s (median for an alert driver expecting hazards occasionally). Let's assume we're going 10 over.

The fact that you were going faster means you cover more distance before you even hit the brakes. The 25mph traversed either 44.43ft in 1.92s (at 0.5s reaction time) or 81.1ft in 2.92s (at 1.5s reaction time). By the time the 35mph even applies the brakes, it only has either 18.77ft left or 4.1ft left (0.5s vs 1.5s reaction time).

Then how much can you slow down in that time? In the 0.5s reaction time case the impact speed is 27.85mph. In the 1.5s case it's 33.57mph.

In either case, you're actually striking the child above the posted speed limit in the same emergency-braking scenario, at a speed likely to cause significant injury.

Details obviously vary depending on how you set up the parameters, but 25mph is an approximate cutoff below which we empirically tend to have pretty safe traffic outcomes in residential areas and above which we tend to have pretty dangerous traffic outcomes. I don't know if people choose to speed in those situations because they don't know or because they don't care, but either way, it makes our roads much more dangerous than they need to be.

Comment by whatever1 3 days ago

Maybe we should replace the car speed measurement with something exponential?

Instead of mph/kph the speed limit and speedometers could be on QuadraticSpeedUnits.

Comment by eigenspace 4 days ago

Intuition is much more adjustable than youre giving it credit for.

One can build new intuitions.

Comment by SJC_Hacker 4 days ago

Nature gives jack crap about our intuition

Comment by mattm 4 days ago

Do you have any recommended resources that you used to help learn the linear algebra?

Comment by fasterik 4 days ago

Start with 3Blue1Brown's "Essence of Linear Algebra" series https://www.youtube.com/playlist?list=PLZHQObOWTQDPD3MizzM2x...

Axler's "Linear Algebra Done Right" https://linear.axler.net/LADR4e.pdf

Nielsen and Chuang's "Quantum Computation and Information" Chapter 2. (Available online, but legality depends on where you live)

If you're interested primarily in QM rather than linear algebra, I'd recommend not working through all of Axler, but instead using Nielsen and Chuang to learn about qubits and observables, and using Axler to fill in the gaps.

Comment by d_tr 4 days ago

"A Course in Modern Mathematical Physics: Groups, Hilbert Space and Differential Geometry" by Peter Szekeres has several chapters on linear algebra early on, starting with sets and groups and including tensors (which you absolutely need for QM).

I loved it.

Comment by brooke2k 4 days ago

not sure about its specific applications to quantum mechanics, but I worked through MIT OpenCourseWare's Linear Algebra class (taught by Gilbert Strang) and I thought it was amazing. The lectures are very very good.

Comment by GlibMonkeyDeath 4 days ago

https://linear.axler.net/

Comment by anotherpaulg 4 days ago

Agreed.

Quantum Processes, Systems, & Information by Benjamin Schumacher and Michael Westmoreland is another good treatment along those lines.

Comment by wnoise 4 days ago

Mike and Ike is almost always my standard recommendation.

Comment by the-mitr 4 days ago

Of possible interest

Basic Concepts Of Quantum Mechanics by L. V. Tarasov

https://archive.org/details/tarasov-basic-concepts-of-quantu...

The dialogues of the author engages with various personas and the reader that are interspersed as Prelude and Interludes are really enlightening.

For example:

Classical Physicist: Can one seriously speak about an object without having an idea of what it looks like? Isn’t it strange that we study, for example, the behaviour of an electron in a crystal while we don’ t even know what an electron actually is?

Author: I don’t agree that we don’t even know what an electron is. I have just indicated a number of precisely determined characteristics and properties of an electron. More detailed properties of microparticles in general and electrons in particular were considered in the preceding sections of the book (and will be considered in the following sections). In fact, we know quite a lot about the electron and know, in particular, about its behaviour in a crystal. This is evidenced by the large number of semiconducting devices fabricated and used by us in practice. As you will see, the absence of a “physically intuitive” model of the electron has in no way turned out to be a serious obstacle. We may even go a step further and state that an understanding of natural phenomena in which Planck’s constant plays an important role is possible only through a significant rejection of a graphic description. By the way, this idea was given by Heisenberg, in whose works much attention was given to questions of the use of physically intuitive methods.

Comment by rini17 4 days ago

Classical physicists had conceptual difficulty when something stopped behaving as "body" even macroscopically. For example when they tried to determine coefficient of friction of pure coper, throughly cleaned, in vacuum to prevent any oxidation, the surfaces cold-welded together. The copper did not get the memo that it is supposed to be two physical bodies.

Comment by rramadass 4 days ago

Nice; a classic "Mir Titles" book!

Also checkout Tarasov's other books - https://mirtitles.org/?s=Tarasov Relevant here are his books on Probability and Lasers.

The Physics of the "very small" is essentially probabilities applied to Atomic, Molecular and Optical (AMO) domains where light and matter interact.

Comment by myth_drannon 3 days ago

Archiv.org has a collection of many Mir books

https://archive.org/details/mir-titles

Comment by the-mitr 2 days ago

I am the maintainer/curator of the collection and the blog at https://mirtitles.org

Comment by the-mitr 4 days ago

His book on Symmetry also discusses symmetry aspects of the microworld.

Comment by bobvanluijt 5 days ago

Somewhat related: a book that really helped me is Picturing Quantum Processes by Bob Coecke. It's a visual representation of quantum processes, which really helped me understand the concept

Comment by yboris 5 days ago

Full PDF https://www.cs.ox.ac.uk/people/aleks.kissinger/PQP.pdf

Comment by iorekz 4 days ago

Thanks for sharing! Just read the first pages and it looks promising

Comment by newsy-combi 4 days ago

Funny, both books scream default LaTeX

Comment by internetter 4 days ago

Both books are default LaTeX. What’s wrong with that?

Comment by newsy-combi 4 days ago

It stands out as not too common a sight for books, so having two like this, more or less in the same style, on the same topic, both posted in this thread, is amusing. Maybe it's not that rare and I'm just not reading enough books from academics for whom a book is like a weird long paper with hand-holding for amateurs.

Comment by 2b3a51 4 days ago

Physicists use LaTeX and less often plain TeX a lot as you say because of writing papers. I think that the basic LaTeX book document style provides a workable set of features, and that lets the author just focus on the content. I'd imagine that commercial publishers e.g. OUP would redefine the basic macros to produce output in their house style.

Comment by wasabi991011 4 days ago

Finished reading this book not too long ago, one of my top three textbooks. An absolute joy to read full on great insight.

Not sure how it feels to a beginner, but to someone with a good deal of familiarity in the field it put everything in such a nice tight conceptual package.

Comment by rramadass 4 days ago

This looks quite interesting; thanks for sharing.

Comment by 5 days ago

Comment by dbvdh 4 days ago

"This rather fanciful interpretation of Quantum Physics has now become accepted by many theoretical physicists as being more satisfactory than the Copenhagen interpretation, despite the fact that the mechanism which causes the splitting of the Universe is not explained in any more detail than wavefunction collapse. As you can probably tell, I am in the minority of physicists who finds the Many Worlds interpretation dificult to accept, and not particularly illuminating."

I think these lectures are a good intro to QM. This statement feels very emotional though. Many-worlds just states there is no such thing as wave function collapse. The many worlds are a logical conclusion following. Copenhagen simply refuses to elaborate to when wave-function collapse happens, which to me is not a complete theory.

You could have some critique of many-worlds in relation to statistics, although I think many people have given quite satisfactory explanations.

Many worlds is just bare bones QM.. If any other theory is proposed, it should be falsifiable (like pilot wave theory for example). Doesn't mean it won't be difficult to measure.

Comment by linuxhansl 4 days ago

FWIW (and it's not worth a lot, since I'm a layperson), I think the interpretation that makes the most sense to me is Quantum Decoherence (https://en.wikipedia.org/wiki/Quantum_decoherence).

My interpretation of the interpretation is that the universe is a giant constraint resolution system. Particles are free to be anywhere randomly as long as they do not randomly interact with other particles. As soon as they interact they have fewer "degrees of freedom", since now they interacted. When they interact with anything macroscopic they have no freedom left. Makes sense to me anyway.

Comment by monktastic1 4 days ago

Decoherence isn't an interpretation, but an experimentally established fact. What makes interpretations interpretations is that they cannot be experimentally distinguished (or in some cases, can only be with great difficulty). That said, the study of decoherence has certainly made aspects of the measurement problem easier to come to terms with, which in turn has bearing on the intuitive draw of some interpretations over others. In particular, it helps answer when universes should be said to "split" in MWI (though, as many will rightly tell you, that's not a complete solution to the measurement problem, as decoherence is still unitary--i.e., there's no in-principle irreversibility there).

Comment by rcxdude 4 days ago

Decoherence is how you get from "Maybe the wave function never collapses" to "Maybe there are many parallel 'universes' in QM.".

Comment by kgwgk 4 days ago

Decoherence gets you a diagonal density matrix description of the world. It doesn't get you a single-state description of the world. Going from "There are many parallel universes" to "and we are in one of them" still takes a lot of handwaving.

Comment by mitthrowaway2 4 days ago

But we're not in one of them. We're in all of them. Yours is a misunderstanding of how it feels to be in a quantum superposition. There's no component of the wave function in which Schrodinger's cat experiences anything other than being alive or being dead. There's no component in which it feels like it's in some ghostly combination of both outcomes. Both components of the wave function exist in superposition, and to the extent that they are decoherent, they do not interact and do not even notice that they have company. I don't mean this only as an academic statement; it's also true of you and me, at this very moment.

Comment by tsimionescu 2 days ago

> There's no component of the wave function in which Schrodinger's cat experiences anything other than being alive or being dead.

The problem is that this is not what Schrodinger predicts, though. If |alive> and |dead> are valid solutions to the Schrodinger equation of some system, then so is any linear combination of them whose norm is one, such as 1/sqrt(2) |alive> + 1/sqrt(2) |dead>. So the math of QM predicts that there should be infinitely many cats that should feel partly alive and partly dead, many times more than the one live cat and the one dead cat. And yet you don't find this in any classical experiment.

Comment by mitthrowaway2 2 days ago

Maybe I have to say it again.

1/sqrt(2) |alive> + 1/sqrt(2) |dead> is a valid solution to the Schrodinger equation. That cat is half alive and half dead.

That is different than |A cat that feels half alive and half dead>, which is its own completely different state. For a cat to feel like it is half alive and dead, or to notice that it is half alive and half dead, there are distinct things that happening in its brain that don't happen in either the |alive> or |dead> basis vectors, let alone any linear combination of the two.

We could even put all those states in a superposition!

1/sqrt(3) |alive> + 1/sqrt(3) |dead> + 1/sqrt(3) |half alive and half dead>.

Comment by tsimionescu 2 days ago

But you can pick those other values as basis vectors, and they are just as good a choice as alive/dead. The only reason that you can say "the cat's brain doesn't experience those other states" is that you've presupposed a classical cat, that has some preferred basis. But this is just begging the question. We only get classical behavior when the wavefunction gets entangled with a classical object, but the math predicts you should also have non-classical "half-here" objects that would experience non-classical "half states" - and those are actually infinitely many.

So how did the classical world arise? Or how come we happen to be in the classical world, when there are infinitely many other non-classical observers? And note that the anthropic principle doesn't help here - all of the real laws of physics are the same for those other observers, you get the same chemistry etc, it's just the classical approximations that fail.

Comment by monktastic1 4 days ago

It's a bit like:

"Doc, I feel pain in my toe."

"No, sorry, the math says you don't."

"Okay, but like... I do."

"Sorry, you're just not good at accepting math!"

Not a perfect analogy, of course, but people have competing intuitions of what to do when the map fails to match the territory. MWI abandons the latter, most prefer the former.

Comment by diedyesterday 3 days ago

Every state is a superposition. Even eigenstates (of some operator, e.g. X) are superpositions (of eigenstates of some other operator, e.g. P). So everything is a superposition and being superposition or not is relative.

Comment by podocarp 4 days ago

If that isn't absurd, then I don't know what is.

Comment by mitthrowaway2 4 days ago

I think the alternatives would be more absurd:

- That humans can't be put into quantum superpositions, or

- That a human being in a quantum superposition of two states A and B would feel at all to that human like being in a quantum superposition of two states A and B, when neither of those states is "a human feeling like they're in a superposition of two states A and B".

Follow this to its conclusion, and you end up with a superposition of A and B meaning: "the universe is in a superposition, containing a component where a human believes that they're in state A, and a component where a human believes that they're in state B. Within both branches, the human thinks their wavefunction collapsed in that state". This is exactly the way that all interpretations of QM already understand things to work with particles in superpositions.

Comment by kgwgk 4 days ago

There's the handwaving.

Comment by goatlover 4 days ago

Decoherence only hides entanglement by having the entire world go into a complex superposition. It's entirely compatible with MWI. Decoherence does not resolve the measurement problem on its own. It's not a separate interpretation. It only provides us a mathematical reason for why the world appears classical.

Comment by tsimionescu 2 days ago

Specifically, decoherence provides a mathematical reason why we don't see self-interference and similar quantum phenomena on classical objects. It doesn't on its own explain why the single classical world arises from the infinitely many solution pairs to any quantum equation.

Comment by EvgeniyZh 4 days ago

Many-worlds is an interpretation and not a theory. It makes no additional predictions beyond what QM does. Believing in any interpretation (or shutting up and calculating) is purely emotional; it has no scientific content.

Comment by edanm 4 days ago

(Disclaimer: I'm nowhere near truly knowledgeable about any of this)

Quantum computing was explicitly proposed as a way to prove the truth of many worlds. As far as we can tell, quantum computing appears to work.

I don't think most people believe QC truly proves many-worlds, but I think it is considered some evidence in its favor, and historically it was I think one of the reasons for the development of QC originally.

Comment by antognini 4 days ago

I've heard this claim, but I don't think it holds up. The idea is that supposedly the calculation is done in parallel across the different universes which allows you to see an exponential speedup in some calculations. But the whole point of many-worlds is that after a branch event the various branched portions of the wavefunction no longer interact. Even if it were true that quantum computing was really a bunch of parallel classical computations (which it is not), there would be no way for any of the other universes to communicate the result back to the universe you observe it in.

Comment by rcxdude 4 days ago

Many worlds doesn't really feature 'hard branches'. In fact the interpretation is pretty specific that quantum interactions becoming de-correlated and stopping to react is the mechanism by which you get the 'many worlds', and this is a soft and messy process not some global hard split.

OTOH, I don't think any of the other interpretations really struggle to explain quantum computing so I agree it's not much of a distinguishing factor.

Comment by thayne 4 days ago

I don't think there is anything in quantum computing that is specific yo the many worlds interpretation.

Comment by card_zero 4 days ago

But if what you want is understanding, you want a good explanation. (Or to quote TheOtherHobbes from elsewhere in these comments, a reasonable metaphor: https://news.ycombinator.com/item?id=50220700)

If you only like predictions and disparage explanations, I might start calling you names, specifically instrumentalist.

Comment by spolitry 4 days ago

What do call someone who wants good "Just So" story explanations that don't explain anything, but posit the existence of meaningless things?

Comment by card_zero 4 days ago

Here you are taking "good explanation" and combining it with the term "just so story" which indicates a bad (arbitrary) explanation. So you cast doubt on the idea that explanations can be good. What's this, sabotage?

Comment by goatlover 4 days ago

It has mathematical and philosophical content. People can be convinced for reasons other than empirical without it being emotional. Sean Carroll makes logical arguments for why he's convinced MWI is correct.

Comment by spolitry 4 days ago

I'm not Sean Carroll. I'm interested in logical arguments for why I should be convinced that something completely impossible to ever test, not even in theory, is correct.

Comment by mitthrowaway2 4 days ago

How do you test whether the dinosaurs once existed as living animals, vs as fossils that were embedded in the Earth when the universe began 6000 years ago?

Because I only believe one of these propositions, even though I can't imagine any test that would discriminate between the two.

Comment by qlte 3 days ago

...uh, by using the abundant evidence from radiometric dating, geology and archeology that the world is much older than 6000 years old?

Plus astronomical observations used in cosmology to model the state of the universe milliseconds after the big bang which eventually evolved into our locally observable region of space around 13.8 billion (not 6000) years later?

That doesn't prove the dinosaurs were in fact living beings but this isn't necessary to disprove the other assumptions of the alternative hypothesis. By parsimony that leaves the hypothesis that dinosaur fossils emerged by natural processes like everything else we've observed on Earth to date.

Am I missing a joke? Because this is definitely not a good example of a hypothesis impossible to disprove with available evidence.

(If the response is "how do you test whether all that evidence was faked 6000 of years ago to deceive scientists" then we're not actually doing science anymore but games of make believe)

Comment by mitthrowaway2 3 days ago

That's exactly what we're doing. Obviously I don't believe the initial conditions of the universe were set 6000 years ago, and just look billions of years older by mere coincidence. My point -- directed at its parent comment -- is that we can scientifically prefer one hypothesis over the other, even though they're indistinguishable by any experiment because they make identical predictions.

Instead, we can use other forms of reasoning, such as probability and Ockham's razor. These are perfectly valid approaches.

You seem to agree with me on this point, so I direct your attention to the grandparent comment:

> I'm interested in logical arguments for why I should be convinced that something completely impossible to ever test, not even in theory, is correct.

Comment by habinero 3 days ago

Sure you can. There is no predictability and no fundamental laws in a purpose-built universe run by an omnipotent creator because ultimately the creator can do anything they please.

Not to get all reddit atheist here, but as far as we can tell, the universe is predictable and explainable by fundamental mathematical laws. (We don't see, for example, unexplainable violations of conservation as Michael Landon pops in and out of existence.)

This could mean there is no god or they don't interfere or whatever, but in any case it's pretty good evidence that we can reason about how the universe works.

That's about as good as you can know anything. Beyond that is the "Are you a brain in a jar" kinds of philosophical arguments that are fun when you're 21 and get old after.

Comment by unparagoned 3 days ago

Pretty much all physics for since forever has been about the ontological explanation of what’s happening. If you don’t want to subscribe to an ontological interpretation then that’s something new and the opposite of how people have always done physics. It’s something that has only ever come about with qm.

In normal physics people often use occamz razor and subscribe to what the interpretation with the simplest postulates says.

So the reality is the opposite. You don’t like what the physics is saying so for emotional reasons you’ve come up with a completely new and novel framework to discount what the simplest set of equations is saying.

Comment by EvgeniyZh 3 days ago

> You don’t like what the physics is saying so for emotional reasons you’ve come up with a completely new and novel framework to discount what the simplest set of equations is saying.

Nah, physics is not saying anything. I do physics because I enjoy discovering and modeling new phenomena. If someone wants to do philosophy and ask questions like "but what is __really__ happening when we measure an atom", it's his business, but I don't believe we'd be doing the same thing, because these questions are impossible to answer with empirical evidence and don't help us model reality in any way. I don't see any difference between QM interpretations; one of them might be "true"

Comment by dbvdh 4 days ago

Does it not make any predictions? It states there is no such thing as wavefunction collapse. This means that if you entangle a state with a large system, in theory you could ‘disentangle’ it. Of course, this is no easy feat. But indeed quantum computing to me, is an example of this feat. We can entangle systems on a large scale, then use this feat to make calculations. Inspiration for quantum computing actually came from many worlds.

Many worlds is falsifiable, if you can show wave function collapse actually happens (not just decoherence). I believe that in the not so distant future we will get more and more proof that wave function collapse never happens. Quantum computing at scale to me is an indication that Everett was just right..

Comment by unparagoned 3 days ago

Nope it’s the opposite. The Copenhagen wavefunction collapse is untestable or unfalsifiable.

With many words, every postulate has been tested and very verified. What more can you ask from a theory?

Now objective collapse is a testable postulate but none expects that to pan out.

Comment by EvgeniyZh 3 days ago

> This means that if you entangle a state with a large system, in theory you could ‘disentangle’ it.

This can be done in any other interpretation? Any unitary evolution on an isolated system can be reversed. This is a mathematical statement.

> Many worlds is falsifiable, if you can show wave function collapse actually happens

So, what experiments do you think would falsify it? How can you show that collapse actually happened?

Comment by dbvdh 3 days ago

In the Copenhagen interpretation, when I look at schrodingers cat, the wavefunction will collapse. After I have looked at it, no way to go back. Information is lost. In many worlds, I just become entangled with the system - and I could theoretically disentangle after. Like quantum computing going to a deterministic outcome in a certain basis.

Showing collapse happened is really tricky because of decoherence. Similar to unmixing coffee and milk. But all systems we have observed so far do not show wave function collapse (granted these systems are generally small, but otherwise decoherence starts messing about).

These are just fundamentally different theories. Not just an interpretation, physically different outcomes.

Comment by EvgeniyZh 2 days ago

> In the Copenhagen interpretation, when I look at schrodingers cat, the wavefunction will collapse. After I have looked at it, no way to go back.

In the Copenhagen interpretation, there is a difference between measuring a system and entangling with it. There exists a unitary that entangles you with the cat, just like there exists a unitary that entangles the cat with the quantum device. By definition, unitary evolution is not a measurement in the Copenhagen interpretation.

> Showing collapse happened is really tricky because of decoherence. Similar to unmixing coffee and milk.

Unmixing would show the lack of collapse (in the specific case), though? To show collapse you'd need somehow experimentally prove that it is impossible to unmix. And to falsify collapse, you'd need to somehow experimentally prove that it is always possible to unmix. And if we go with your analogy, would you say the coffee with milk hasn't forgotten its initial state because it is technically possible to unmix them? Being able to manipulate the whole universe's wavefunction to unentangle particular subsystems is akin to having access to Maxwell's daemon, since decoherence increases entropy.

Comment by abc123abc123 4 days ago

This is the way! It is absolutely amazing that the supposedly "brilliant brains" at lesswrong and other similar outfits, have not learned about the difference between an interpretation and a theory.

Since QM goes beyond our material world intuitions, I am personally loathe to clothe nrs in words, until we have more and better experiments. I think prematurely translating it into material-level intuitions just leads to a waste of time and much confusion.

Comment by vlovich123 4 days ago

Given we haven’t bridged quantum with gravity yet, my bet is the function collapses right at the boundary when gravity starts to play a meaningful role.

That’s a more physically plausible interpretation than ever “tick” of the universe actually creates infinitely more universes because every fundamental particle is quantum and each one is constantly interacting under the wave function every plank second. And the infinite number of possible events that a single particle would experience is multiplied by the joint combination of all particles in the entire universe. It doesn’t seem plausible.

Comment by JanosProjnow 1 hour ago

You haven't bridged. I have bridged. https://doi.org/10.5281/zenodo.18445925

Comment by 1 hour ago

Comment by dbvdh 4 days ago

How would you define exactly ‘when gravity starts playing a role’. It always plays a role, from photons to planets.

And on the ‘every tick creates more universes’: we are completely fine with superposition in nanoscopic systems. It’s just uneasy to follow that conclusion to the logical end. Just my two cents. I think just because the conclusion seems hard to grasp, the reasoning to get there is completely sound..

Comment by vlovich123 4 days ago

“Starts to play a role” means when the effect of gravity becomes significant to the quantum equations which currently don’t have any placeholder for gravity.

Just like Newton’s equations are accurate to describe gravity and motion but only because the full equations Einstein worked simplify to them at small speeds.

So similarly it would make more sense that quantum equations and relativity are two sides of the same coin and there’s some missing mathematical bridge where at small particle sizes it simplifies to quantum and at large particle scales gravity matters and we get Newton and then further relativity.

Using Occam’s razor that’s at least my prior vs some undefined mechanism of action where all possible interactions of every particle actually creates an infinite superposition of universes where that did happen and you get an infinite number of such superpositions ad infinitum. Much easier to believe the huge gaping hole between two very accurate theories explains how quantum mechanics turns into classical mechanics, especially since we know gravity exists, what it is, and quantum mechanics can’t explain gravity.

Comment by analog31 4 days ago

The place where quantum and gravitational effects are thought to become significant to one another is at energy levels that cannot be obtained in our experiments, either in the lab or through astronomical observation.

Comment by vlovich123 4 days ago

And so as a result there’s multiple worlds? That’s the simpler more testable prediction?

Comment by analog31 4 days ago

I'm just an old industrial physicist, but as I understand it, quantum gravity has yielded no testable predictions so far, and neither have multiple-worlds theories.

Comment by vlovich123 4 days ago

I never said anything about quantum gravity. String theory is also trying to bridge things. But many worlds is pointing to a gap in the math and claiming god. All I’m saying is we don’t have any model or math and many worlds is the more fantastical explanation.

Comment by card_zero 4 days ago

It's simple and elegant if you like it, overcomplicated if you don't.

Comment by vlovich123 4 days ago

Many worlds is a philosophical interpretation akin to god. It doesn’t add anything to our understanding and instead invokes some kind of supernatural untestable phenomenon.

Comment by mr_mitm 3 days ago

The many worlds are a consequence, not a postulate. It is the Copenhagen interpretation that postulates that all branches but one disappear at measurement.

Comment by habinero 3 days ago

Both of them are interpretations. We have no reason to believe one over the other.

Comment by 4 days ago

Comment by antognini 4 days ago

This is more or less Roger Penrose's position: https://web.archive.org/web/20150327124054id_/http://image.s...

He argues that the wavefunction collapses on a timescale of h-bar / E_G, where E_G is the gravitational self-energy of the quantum system.

Comment by vlovich123 4 days ago

Didn’t know this but glad I’m in such distinguished company.

Comment by tempaccountabcd 4 days ago

[dead]

Comment by dualvariable 4 days ago

The quantum world involves states that are a complex linear superposition of all possible states. It isn't just that the world bifurcates into one where the cat is |alive> or |dead>, but also all c1 |alive> + c2 |dead>, where c1^2+c2^2 = 1 and c1 and c2 are complex. The MWI has no explanation for how this other infinity of possible states disappears into just two states. You still wind up with the collapse of the wave function--involving a lot of hand-waving or appealing to some nebulous property of consciousness.

Comment by virgil_disgr4ce 2 days ago

> The MWI has no explanation for how this other infinity of possible states disappears into just two states

They don't disappear. This is the entire point of MW.

Comment by trenchgun 4 days ago

>The MWI has no explanation for how this other infinity of possible states disappears into just two states. You still wind up with the collapse of the wave function--involving a lot of hand-waving or appealing to some nebulous property of consciousness.

The real issue here is the preferred basis problem, not an infinity of worlds: a single state vector can be decomposed many ways, and the bare math doesn't say which decomposition counts as "the branches." Decoherence picks it out: once the cat is entangled with its environment, the alive/dead decomposition becomes effectively unique and stable, through ordinary unitary dynamics. No collapse, no consciousness (that's von Neumann–Wigner, not Everett). MWI does have an open problem, but it's a different one: the Born rule, i.e. why |c|^2 should count as probability when every branch is real. Also, the normalization is |c1|^2 + |c2|^2 = 1.

Comment by kgwgk 4 days ago

Sure, the MWI still has an open problem which is why it looks like there is a collapse of the wave function.

Comment by rcxdude 4 days ago

No, it explains that well enough, it just struggles to explain why you would expect to see the collapse with the probabilities that you do (though, to be fair, most other interpretations don't presume to explain this at all, it's just a rule. MWI just reframes it in a way that makes it odd that there is a rule).

Comment by soVeryTired 4 days ago

AI comment

Comment by red75prime 4 days ago

When the future has multiple instances of you, it becomes problematic to define what the Born rule means.

For example, "Many Worlds, the Born Rule, and Self-Locating Uncertainty" notices that the rule doesn't mean anything, but it is rational to behave like it does. It's not a very satisfying position, I'd say.

Comment by AIorNot 4 days ago

Many worlds as real ontology are just as wild as saying their is a God. Physicists are thus making claims that just as wild as the theologians .. I find this fascinating.

Just like the AI nerds and the AGI singularity or Musk saying we are in simulation...

ie so many crazy ideas.. for the rationalist, its like being in an insane asylum and trusting one mental patient or another..

I suppose at some level its just not rational why we exist. We must accept some wild, fundamental metaphysical assumption

Comment by pamcake 4 days ago

I think a common mistake when approaching MWI is reading too much into it. These Other Worlds "exist" in similar sense to the number i. MWI does _not_ claim anything about there being infinite alternative equivalent planes of existence where these physically manifest or anything like that.

Comment by spolitry 4 days ago

You make an extraordinary claim.

Where in MWI do you believe the preferred unique macroscopic reality is defined?

Comment by y1n0 4 days ago

This has never not been true. There are always hand wavy axioms at the bottom.

Comment by bazoom42 4 days ago

Is many worlds falsifiable?

Comment by mr_mitm 4 days ago

Yes. Many worlds is just the Schrödinger equation without the wave function collapse. It makes less assumptions. Falsify the Schrödinger equation, and you have falsified many worlds. However, is the wave function collapse falsifiable? It's one postulate which many worlds doesn't make, and it cannot be confirmed or falsified by experiment.

Comment by tsimionescu 4 days ago

> Many worlds is just the Schrödinger equation without the wave function collapse.

It is not, it is the Schrödinger equation plus an extra rule that says that classical observers are only certain kinds of quantum states. There's no explanation in the MWI for why a device never reads out that a particle is 1/sqrt(2) |here> + 1/sqrt(2)|there>, even though that is exactly as probable a state as being |here>. And it turns out that this is exactly equivalent to positing wave function collapse / the Born rule.

And no, decoherence has nothing to do with this. Decoherence explains why there is no observable interference at the classical level between these states, not why you can only observe some states.

This is known as the preferred basis problem, for anyone who wants to read more about it.

Comment by spot5010 4 days ago

Doesn’t the structure of the Hamiltonian (local interactions) set what the preferred basis are?

Comment by tsimionescu 3 days ago

Well, when we set an experiment up, we can measure in any basis we like, and get the corresponding results as probabilities per the Born rule, so I don't think there are any constraints on this. Plus, local interactions can't explain why the same classical basis arises globally - after all, when we investigate a distant star, it's never detected as being in some other basis then the local area.

Comment by Gimpei 4 days ago

This is not what falsifiability means in the Popperian sense. You are confusing falsifiability of a theory’s predictions with falsifiability of an additional claim of what those theories mean. We might as well say that the simulation hypothesis is falsifiable because the laws of physics govern our simulated universe, so if you can falsify those laws you can falsify the simulation hypothesis. That being said, the concept of falsifiability is not without its weaknesses.

Comment by baxtr 4 days ago

Exactly.

Any good scientific theory must create falsifiable hypotheses.

Otherwise it’s just a nice story.

Comment by soon_to_be_sb 4 days ago

>Any good scientific theory must create falsifiable hypotheses.

For any behavior inside the world, this is good.

But for the world itself and what harbor's it, asking for falsifiability is like measuring the expansion of space using a ruler that inhabits the same space.

Even science, that works in terms of cause and affect, which implies space and time, cannot be used to describe space and time itself.

Vanilla "scientists" have been trying to do that, but that is a dead end.

Comment by aleph_minus_one 4 days ago

> But for the world itself and what harbor's it, asking for falsifiability is like measuring the expansion of space using a ruler that inhabits the same space.

Which exactly is what general relativity is centrally about and the reason why general relativity is defined in terms of intrinsic properties of the Einstein manifold describing the space of the universe. Intrinsic properties are, colloquially, indeed properties that can be measured "using a ruler that inhabits the same space".

Gauß' Theorema Egregium [1] is an early predecessor of intrinsic properties of general manifolds, and says that the curvature of a surface can indeed be "be determined entirely by measuring angles, distances and their rates of change on a surface, without reference to the particular manner in which the surface is embedded in the ambient 3-dimensional Euclidean space" ([1]).

[1] https://en.wikipedia.org/w/index.php?title=Theorema_Egregium...

Comment by zmgsabst 4 days ago

Also why LIGO works but Michelson-Morley failed: you can use the ruler distortion as a signal, with some caveats.

In LIGO, we detect the far away distortions propagating through our rulers and the differential effect of those waves.

But Michelson-Morley failed because the ruler distorted synchronously with the signal we tried to measure — since we mistakenly believed EM and matter were different fields.

Comment by senderista 4 days ago

How are general-relativistic effects relevant for the outcome of the Michelson-Morley experiment?

Comment by zmgsabst 3 days ago

What do you mean?

You seem to think I’ve said something very technical, while I’ve merely pointed out a well-known effect of Lorentz invariance and interferometry: you can’t detect an “aether” signal coincident with the detector, because the detector is distorted synchronously.

Michelson-Morley was never going to work because it relied on the assumption matter wasn’t subject to aether wind — which turned out to be wrong, as matter is subject to relativistic effects and fields unify. You needed that independence to generate the differential the interferometry amplified.

By contrast, LIGO measures an effect far removed and not co-moving with the detector. So it works.

Comment by aleph_minus_one 4 days ago

> How are general-relativistic effects relevant for the outcome of the Michelson-Morley experiment?

General-relativistic effects are relevant for the outcome of the Michelson-Morley experiment in the sense that the general theory of relativity is a generalization of the special theory of relativity. :-)

Otherwise: the additional predictions that the general theory of relativity makes about gravity, curvature of space etc. are not relevant for the outcome of the Michelson-Morley experiment.

Comment by senderista 4 days ago

I mean, if you're claiming that general relativity is relevant here, then you need to show that quantitatively.

Comment by zmgsabst 4 days ago

> measuring the expansion of space using a ruler that inhabits the same space

We do that, routinely: that’s how we do cosmology.

Comment by soon_to_be_sb 4 days ago

> We do that, routinely:

Yea, that is why we are not getting any where...

Comment by aleph_minus_one 4 days ago

> Yea, that is why we are not getting any where...

Which alternative way of doing measurements do you propose? As far as I know, measuring intrinsic properties, i.e. "using a ruler that inhabits the same space" is with our current technology the only possible way of doing measurements.

Comment by soon_to_be_sb 4 days ago

Assuming that we are trying to understand the ground truth about existence by doing these measurements, may be we can acknowledge the limitation and stop trying to do it. Then we can be open minded and stop being hung up of the idea of "falsifiability", and could come up with or give more consideration to radically different hypothesis that we might come up with alternative measurements that we could make, that can prove or disprove them.

Comment by russdill 4 days ago

So time extending into the future is not a useful hypothesis? Space continuing beyond the cosmic horizon is not a useful hypothesis? The past entropy of the past being arbitrarily low (vs what is essentially last tuesdayism) is not a useful hypothesis?

Comment by spolitry 4 days ago

Time extending into the future is validated by direct experience.

A continuum of variants of myself is bought.

What is the value of assuming that literally every possible trajectory actually happens, but somehow I am only in one of them, while also being a continuum of other ones? If everything is true, than nothing is true.

Comment by russdill 4 days ago

If time did not continue into the future, it would not be observable. Therefore hypothesis that it will continue to do so is not disprovable.

As far as multiverse goes, there isn't any inherit value in assuming that the wavefunction as given actually happens, it's just a simpler explanation than trying to come up with (arguable unprovable) ways to cull the wavefunction.

Comment by unparagoned 3 days ago

Yes, and all the postulates have been evidenced and panned out.

Comment by habinero 3 days ago

> Many worlds is just bare bones QM.. If any other theory is proposed, it should be falsifiable

Many worlds isn't falsifiable, either. There's no reason to accept one over the other.

Comment by unparagoned 3 days ago

You look at if the postulates are falsifiable. All the postulates from me have been evidenced or are falsifiable.

The Copenhagen wavefunction collapse is unfalsifiable.

Comment by zarzavat 4 days ago

The Copenhagen interpretation doesn't even deserve to sit with the other interpretations because it's not well defined. It's protoscience, like alchemy.

Comment by wnoise 4 days ago

It's "rules of thumb for a working physicist".

Comment by mohamedkoubaa 4 days ago

"Many-worlds just states there is no such thing as wave function collapse" is the emotional statement. The contempt for many-worlds is both principled and rational.

Comment by isolli 5 days ago

In a podcast, Alain Aspect (Nobel Prize winner) explained that he does use the power of visualization and his imagination to generate new ideas, but that he always needs to check them with actual calculations because no mental picture can be an accurate representation of quantum physics.

Comment by asystole 5 days ago

This piece looks great, adding to my ever-growing "to read" collection on Zotero.

If anyone is interested in self-studying quantum physics, QFT for the Gifted Amateur [1] is very well regarded.

1: https://academic.oup.com/book/36442?login=false

Comment by ktallett 5 days ago

Second this book, but you need a source before this to study QP successfully. This requires a lot of UG maths and physics knowledge that I think most would not have.

Comment by rustyhancock 4 days ago

I like QM for engineers.

It is much more shut up and calculate.

But I do believe that is atleast in part Feynmans point. The way we have explored QM for the past... 120 years (? Wow is it really that old!?) is you don't need to understand it, here are the rules and tools to carry out extraordinary calculation.

I believe the likes of Sean Caroll are trying to bring understanding back to physics and meet philosophy.

https://web1.eng.famu.fsu.edu/~dommelen/quantum/

Comment by ktallett 4 days ago

It really depends on your goal. If you wish to use it day to day then you do need to understand it but yes you can use the handy Dirac notation to let you calculate lots without ever going too deep.

Comment by spolitry 4 days ago

Sean Carroll is trying to bring storytelling fantasy back to physics. No good comes from "studying" the angels dancing on the head of a pin.

Comment by rramadass 5 days ago

See also, Atomic Physics for Everyone: An Introduction to Atomic Physics, Quantum Mechanics, and Precision Spectroscopy with No College-Level Prerequisites. - https://news.ycombinator.com/item?id=46961595

Comment by jasrys 4 days ago

Sharing in case useful for someone. Started getting into this subject in 2015, and full disclosure: I'm no physicist by any means. Without the formal background, the math and concepts were really hard to conjure and comprehend purely from the page, so I built an interactive self-study guide to "feel" the equations from Newton to holography. https://apples-to-entanglement.jasonrys.com/

Comment by 4 days ago

Comment by Mabusto 3 days ago

This is amazing, thank you!

Comment by ridiculous_fish 4 days ago

Link looks to be broken.

Comment by quietFerry94 5 days ago

That pre-determined value idea is exactly local hidden variables, and Bell's inequality rules it out, Aspect's 1982 CHSH experiment measured the violation.

Comment by caturopath 5 days ago

To be precise, Bell's inequality rules out a local hidden variable theory. Pilot wave theories/Bohmian mechanics is an attempt to describe a nonlocal hidden variable theory of quantum mechanics.

Comment by antonvs 5 days ago

To be even more precise, it rules out hidden variable theories that satisfy Bell locality, i.e. the assumptions that the theorem makes.

Superdeterminism and retrocausality both allow for local hidden variables - it’s just that they don’t satisfy Bell’s assumptions.

Comment by rcxdude 4 days ago

They're also generally regarded as weirder than the problem they are trying to solve.

Comment by antonvs 4 days ago

Intuitive notions of “weird” aren’t necessarily a good guide here. We already know time doesn’t behave the way we intuitively imagine, because of relativity: there can’t be universal agreement on what is past, present, or future. Once you’ve accepted that, retrocausality seems much less strange.

Similarly, superdeterminism is simply the application of the determinism that was believed to apply to physics before QM, to QM itself. In a way, one might have expected it to be the default interpretation.

Comment by drdeca 4 days ago

No? Superdeterminism says that there’s no way to pick your experimental choices in a way that reveals things because any possible random number sources you could use have contrived correlations that prevent that? Why would one ever consider that as “the default”?

Comment by antonvs 4 days ago

The strong belief before QM was discovered was that the universe was deterministic - that given sufficient knowledge of a physical system's present state, you could accurately predict its specific future states (not just possible future states.)

Laplace expressed this in "A Philosophical Essay on Probabilities" in 1814: "We ought then to regard the present state of the universe as the effect of its anterior state and as the cause of that which is to follow. [...] For [an intelligence that has sufficient information] nothing would be uncertain, and the future, as the past, would be present to its eyes."

Superdeterminism essentially extends this idea of deterministic physical evolution to quantum mechanics. It could be considered an appropriate default simply because it was previously considered to be, not only the default, but the strongly believed truth.

> Superdeterminism says that there’s no way to pick your experimental choices in a way that reveals things because any possible random number sources you could use have contrived correlations that prevent that

You're arguing against a consequence of superdeterminism, not its motivation. Its motivation is as I've described.

The word "contrived" is doing some heavy lifting in your statement. Nothing needs to be contrived in superdeterminism, even if it seems that way to an observer within the universe.

The explanatory problem you've alluded to only arises because we're trying to understand the universe from within the universe. The universe is under no obligation to make itself sensible to us.

Essentially, you're saying little more than that you don't like the idea of superdeterminism.

Comment by rcxdude 4 days ago

Can you explain how determinism applied to quantum mechanics results in superdeterminism? I feel like you've just skipped over an important step here. Determinism isn't a controversial idea, and indeed in QM the wavefunction is fully deterministic anyway, it's only interpretations which require a collapse where non-determinism appears. I can understand roughly why the many worlds in MWI are a consequence of assuming the wavefunction is reality and that there is never a collapse, I do not understand how you get to the consequences of superdeterminism from applying determinism to quantum mechanics.

(And yeah, preference for any of these interpretations will essentially boil down to what idea you like the most, or more likely what idea you dislike the least. With QM you pick your reasonable base assumptions and then take the uncomfortable conclusions that result)

Comment by kubanczyk 3 days ago

Superdeterminism so far seems to be prone to Occam's razor.

Actual models of superdeterminism (e.g. Donadi–Hossenfelder (2022)) seem inelegant and mathematically more complex than MWI. Which doesn't mean it's physically wrong or "weirder than MWI".

Comment by drdeca 2 days ago

The contrivance is the conspiracy that it requires.

A theory that says that unicorns exist but only go places where no one will discover them (even though they aren’t invisible), is dumb.

A scientific theory should be compatible with experimentation.

Comment by quietFerry94 1 day ago

[flagged]

Comment by HarHarVeryFunny 4 days ago

I've got a feeling that our intuitive ick around spooky action at a distance is due to out intuitive view on locality and the nature of space, maybe time too, being wrong. IOW remote is really not remote.

Perhaps we've got it backwards and quantum entanglement is what defines "locality" rather than mysteriously existing in the face of it.

Comment by spolitry 4 days ago

Ironically everyone assumed non spooky action at a distance for most of human history, until relativity.

Comment by antonvs 5 days ago

If you saw that in section 15, the very next paragraph covers Bell.

Comment by mono442 5 days ago

it doesn't if you assume superdeterminism

Comment by salsa_catsup 5 days ago

I don't understand the superdeterminism idea. Wasn’t that already the classical/Newtonian view - that everything was predetermined, a universe of billiard balls following their destiny exactly?

Comment by MattPalmer1086 5 days ago

Newtonian is just plain determinism. One thing causes another, but does not imply things are fixed in any particular way.

Superdeterminism implies that things are fixed in advance in such a way that the things we "choose" to measure always looks as if there are no local hidden variables (e.g. Bell's inequality) even though there are.

Comment by gyrovagueGeist 4 days ago

You can imagine the general idea by imagining the state space of a bunch of billiard balls: their position and velocity in a 6D space.

This is a metaphor and not the state or experiment that superdeterminism seeks to answer. I am also speaking well outside the areas of physics I was trained in. So huge grain of salt.

In classical determinism, the trajectories of all billard balls can be derived precisely from their initial conditions. Due to sensitivity to initial conditions and our ability to know them precisely, we can see chaotic mixing behavior where the trajectory of even two billiard balls that start at nearly the same place become more and more uncorrelated as time goes on as they bounce around and into each other. One expects that many accessible observables and functions loose detectable correlations as this billiard ball universe goes on and the billiard balls go on to roughly fill the full 6D state space up to conservation of momentum.

Now imagine the weirder scenario, that there exists some function and statistical comparison you can make between state variables from sets of billiard balls. Although these state variables themselves seem uncorrelated after enough time has passed, there are these properties that always hold with perfect correlation even after infinite chaotic time bouncing around. The points in state space that the balls cover is restricted to some manifold where these statistical properties are true when checked. The system cannot evolve in a way where those correlations of bulk quantities would disappear.

In this toy metaphor it’s not so horrifically odd. Superdetermism gets more flak because: (i) the state space and property in question is so much weirder and more complicated than the kinematic example, the correlations are between objects that measure and the objects being measured! (ii) if these properties are always true under any check we can make there is no way to falsify. Then it doesn’t matter if the behavior is from some unimaginable underlying universal state space geometry or local quantum effects. This same argument could also be applied more generally and inappropriately to reduce any experiment to “that’s just the way the structure of the universe is”.

Comment by triMichael 4 days ago

No, they are completely different. Determinism is the idea that if you have a perfect snapshot of the universe at one time, you could run that snapshot forwards or backwards using the laws of physics and have a complete history of the entire universe. It's a valid scientist idea, but Bell's Inequality experiments pretty much prove it false by showing that either realism or locality is false, and you kind of need both to have determinism.

Meanwhile, superdeterminism is the idea that you can still have realism and locality by claiming that Bell's Inequality experiments are invalid because the universe could somehow "know" you are performing a Bell's Inequality experiment and rig it to get the "wrong" answer. Superdeterminism is equivalent to saying that we can't prove smoking causes lung cancer, because the universe might rig any science experiments to make it look like smoking causes cancer. Anytime you ask why the universe would do that or how you can prove anything in such a universe, they shrug their shoulders. It's a completely BS idea that got traction because specific people pushed it and others don't understand quantum physics enough to know how unserious of an idea it is.

Comment by rcxdude 4 days ago

It's quite deeply weird. It's basically looking at the seemingly impossible correlations that occur in quantum mechanics and saying that they happen because the decision about what to measure is correlated with the state of the system somehow (Bell's inequality assumes that these are independent). This is very strange in a universe where we apparently could base that decision on many different seemingly unrelated processes. (including processes that have been causally seperated for near the lifetime of the universe, e.g. by choosing the measurements based on noise in the CMB).

Comment by sampo 5 days ago

I started to read from "3.1 The Ultraviolet Catastrophe" and this sentence

"The number of different wavelengths of electromagnetic radiation, N(λ)dλ, with wavelengths in a small interval between λ and λ + dλ, (dλ means a small interval in wavelength) can be shown to be proportional to 1/λ⁴."

doesn't make sense. It should not talk about a number of wavelengths, but the number of allowed standing-wave modes (number of eigenmodes that a mathematical solution to the electric field in the relevant situation has) in the small wavelength interval.

Comment by squiffy 5 days ago

How strange to see a familiar name here! I wasn't his PhD student but worked in the same department as Douglas Ross, and can only say that he was warm, extremely intelligent, with a very strong natural intuition about high energy physics, enjoyed puzzles, and wouldn't shy away from quizzing people giving talks. I remember he had a back and forth with a visiting speaker for about 20 minutes on some minutia!

I don't have much to add on the article itself except that it's almost certainly worth reading.

Comment by rramadass 4 days ago

Also see https://news.ycombinator.com/item?id=50220891

Comment by mietek 4 days ago

Unfortunately, this work perpetuates the misunderstanding of Bell's theorem and its implications with respect to hidden variable theories. See the Scholarpedia article on Bell's theorem for an in-depth discussion, including quotes from Bell himself, deeply frustrated at the misunderstanding.

https://scholarpedia.org/article/Bell's_theorem

Comment by ant6n 4 days ago

I see 502 bad gateway

Comment by mietek 4 days ago

It seems to be intermittent right now. Internet Archive can help.

https://web.archive.org/web/*/https://scholarpedia.org/artic...

Comment by torcete 3 days ago

One of the things that fascinates me is that after studied and used probability and statistics, quantum physics seems to have a different approach to the world of probability. The only common point I find is that the probability of an event must bee somewhere between 0 and 1, and that's it, that's the only one.

Comment by us-merul 3 days ago

https://www.scottaaronson.com/democritus/lec9.html

“Now, what happens if you try to come up with a theory that's like probability theory, but based on the 2-norm instead of the 1-norm? I'm going to try to convince you that quantum mechanics is what inevitably results.

Let's consider a single bit. In probability theory, we can describe a bit as having a probability p of being 0, and a probability 1-p of being 1. But if we switch from the 1-norm to the 2-norm, now we no longer want two numbers that sum to 1, we want two numbers whose squares sum to 1.”

Comment by torcete 22 hours ago

This is really interesting!

Comment by mahboi 4 days ago

Don't know if it's just me, but whenever I read or asked about quantum physics, I'd immediately get stuck on what it means that a photon is a particle and a wave, usually that exact phrasing. A wave of what? Some explanations even make it look like the particles move through space in a sine wave. And then they keep going on the math around waves, introducing the Schrödinger equation, and I still don't get what these waves physically are.

This doc is the first one I've seen that explains upfront what a wave means here: "The quantity which travels in this way is known as a 'disturbance'. Precisely what this disturbance is depends on the wave," and for EMR, "it is an electric field and a magnetic field which are perpendicular to each other and also perpendicular to the direction of the wave." So now I'm going to read the rest.

Comment by habinero 3 days ago

QM is a mathematical framework that started with "ok but what if matter does act like a wave" and then throwing the classical wave equation at the problem.

A lot of weird predictions pop out if you do that. Those predictions (where we can test them) turn out to be true.

This kicked off an argument about what exactly this means in a physical sense and a hundred years later, it's still going.

Comment by Vaslo 3 days ago

I somehow graduated from the upper class QM and Thermodynamics courses with A or A-. I think the average grade on exams was between 30-40 out of 100. I saw our TA at the bar and he gave me the most reassurance I’ve ever had - he said almost no one taking the intro courses really understands it the first time through, and neither did he.

It was also weird to know that then as it still is today, the only wave function that can be solved is single atom hydrogen.

Comment by dekhn 4 days ago

I used to struggle with quantum physics- in the same way many others do- in that the model is not intuitive to people who think in terms of billiard-ball physics. But, I think it's clear at this point that all the observable data from well-run experiments supports QM better than the alternatives. IN particular, the observation of matter waves and double-slit experiment on atoms were what finally made me accept it. Entanglement doesn't make any sense to me, but I'm assured it can't be used to transmit information faster than the speed of light, so I can live with it :)

Comment by immmmmm 5 days ago

I loved the book QED: The Strange Theory of Light and Matter by Feynman as a child. Funny enough I always saw QM from a path integral PoV, a decade before I got to learn the formalism

Comment by atemerev 5 days ago

When I learned about causal sets program and algebraic quantum field theory, I finally (somewhat) understood quantum physics.

Tl;dr spacetime does not exist as a global background where things are "placed", it is not ontic. "Things" are first, and spacetime is derived from interaction statistics of "things". This eliminates nearly all naive paradoxes like double slit etc. Now, these "things" are somewhat complicated and non-intuitive (e.g. there are no individual "particles", they are a limit of discrete distributions), but that's a good start.

Comment by cgio 5 days ago

Sorkin is great. I think causal sets make some things simpler but other more mysterious. I also found t’ Hooft’s automata and Spekkens toy illuminating in different ways.

Comment by atemerev 5 days ago

"Was", unfortunately. He died this September. What a loss, he earned a Nobel long ago.

Comment by cgio 4 days ago

I am so sorry to hear this. I enjoyed visiting his web page July, when I came upon a couple of emails he had there, which were eye opening as they made some of the philosophical thoughts behind causal sets much clearer. A huge loss, but he did contribute amazing work.

Comment by sylware 4 days ago

There is nothing to 'understand': the statistical models of the underlaying reality are far from our higher scale 'emergent' and classic models our brains is used to.

When the "measurement"/"interaction" has a catastrophic :) impact on what you want to understand, won't be easy to "understand" anything.

On the bright side, the universe did throw at us some nibbles through that 'catastrophic' fog which can be "useful".

I think, it is important to teach the mathematical emergence of our classic physics from quantum physics (well... not gravity ofc).

Another important thing to teach kids: the difference between models and reality. We were lucky this emergent physics was not chaotic and have had strong predictability and has been very stable... well we would not exist otherwise.

Comment by anon291 4 days ago

Wave function collapse is a misnomer. We now know that weak measurements do not collapse the wave function but rather drives the probability amplitudes of the states towards an eigenvalue. For those familiar with linear algebra recall that power iteration of a matrix with a random vector will yield the matrices eigen vector given enough iterations. Collapse is similar to this phenomenon but non deterministic.

My theory: The reason we perceive collapse as irreversible is because the computational complexity of reversing the equations is very high. Especially as the state converges towards an eigenvalue of the measurement operator, it becomes more difficult to reverse. This is similar to how a hash function is reversible in principle but not practice. My two cents.

Many worlds and such are just pseudo religious nonsense.

Comment by goatlover 2 days ago

MWI just takes the math as saying the entire world is entangled in a massive superposition (the universal wave equation), but decoherence means there are different branches in that superposition that look classical to any observers in those branches, because coherence has been suppressed by all the environmental entaglements.

I don't know what that has to do with religion or nonsense. Everett and physicists, mathematicians and philosophers since then of a like mind thought the math applied to the entire universe, and the was no classical cutoff where the wave function "collapsed" upon a measurement. Everett was providing a deterministic interpretation competing with the Copenhagen one, which he thought was insufficient.

Comment by kgwgk 1 day ago

> there are different branches

If by different branches you mean something like « now there is one, now there are two » it seems that branching is no less mysterious than collapse.

Comment by goatlover 1 day ago

Branching comes from decoherence where quantum entanglements spread to the environment so the coherence of the entangled environment is suppressed and we only observe what looks like a classical world unless we isolate systems to see their coherence. MWI is just saying that entanglement does not stop at measurement, and that decoherence spreads it to the universe.

Comment by kgwgk 1 day ago

So there are "different branches" as in "one branch now, two different branches later" or not? (Or one gazillion branches now, one gazillion branches plus one later. The point is whether the "different branches" can be counted or not.)

Comment by diedyesterday 3 days ago

Yes, there is no "collapse". Just intentional (in the case of measurement) "narrowing" of the wavefunction (=state in the basis of some operator)or a tighter distribution of eigenvalues, driven by the specific experiment setup (e.g. we set up the experiment to measure momentum or angular momentum along-Z or ...)

Comment by lkm0 5 days ago

"IS QUANTUM PHYSICS CORRECT AND COMPLETE?" is reductive, there are more quantum denominations: Copenhagen, QBism, RQM, consistent histories, Many-worlds, Bohmian mechanics, and various spontaneous collapse people. I might've missed a few.

Comment by rf_physics 4 days ago

This makes me wonder if we could also argue that there were multiple denominations of classical mechanics. Newton's laws, Hamiltonians and Lagrangians come to mind. I recall that people including Newton were also unsettled by Newton's law of gravity ('action at a distance')[1] and tried to come up with more 'intuitive' theories. The one where the law of attraction arises from being bombarded by particles from the universe (screening) was an amusing one that doesn't work due to thermal effects [2].

[1] https://en.wikipedia.org/wiki/Action_at_a_distance [2] https://en.wikipedia.org/wiki/Mechanical_explanations_of_gra...

Comment by SJC_Hacker 4 days ago

> The one where the law of attraction arises from being bombarded by particles from the universe (screening) was an amusing one that doesn't work due to thermal effects [2]

But isn’t that how EM actually works ?

The photon mediates the electromagnetic force

Comment by Synaesthesia 5 days ago

Yes this is a particular interest of mine, the various interpretations of QM. Most physicists subscribe to the Copenhagen interpretation, but I still find it has philosophical quandaries. He mentioned many-worlds. Bohmian mechanics has been shown to be inadequate.

Blokhinstev's ensemble interpretation, which I like, is unattractive to many.

Comment by sampo 5 days ago

Situation: There are 12 major interpretations of quantum mechanics.

Quantum Bayesianism (QBism): We need to eliminate the interpretational conundrums that have beset quantum theory. A quantum state should not be thought to offer a description or representation of physical reality – instead it represents the degrees of belief an agent has about the possible outcomes of measurements.

Situation: There are 13 major interpretations of quantum mechanics.

https://xkcd.com/927/

Comment by spolitry 4 days ago

A Bohmian Rhapsody, you say?

Comment by elcid60 3 days ago

Not even Zeno's arrow paradox is understood (solved), best take is Smooth infinitesimal analysis (SDG; Lawvere 1980, Kock 1981, Bell's Primer).

Comment by tsunamifury 4 days ago

It’s easier the accept quantum physics if you first

1) Disregard many worlds as a purely remaining mathematical outcome of bounded probability rather than discrete reality. (No 1 thing exists just 1ish things but ultimately that doesn’t mean there are many versions of any give 1)

2) just accept you can’t find the “stuff” of reality. You simply measure increasingly small probability domains which interact with other probability domains

3) time is the sequential working out of probability of anything existing and other things that might exist co defining themselves.

4) space is just the interaction of these probabilities working themselves out into a stable projection.

Comment by frumiousirc 5 days ago

I take issue with "understand". I agree QM can not be understood in terms of everyday experience. One must build experience, language and proper metaphor for the understanding. For example, in the intro Ross uses the example of explaining a waterfall to someone who never saw it. He says that can be done but he relies on implicitly the listener knowing what people on Earth may take for granted. That understanding of a waterfall can only happen if the listener already has some understanding of fluid flow and gravity. We do, but say a newly found alien friend from a dry environment that had never experienced fluid flow would not have the mental building blocks to understand it. Feynman gave a famous interview where he said, essentially, "no, I can't explain it to you because you are too ignorant" (he said it more politely). Going the other direction, Physics steeped enough in quantum theory do understand QM enough to have meaningful discussions free (at times) of rigorous math. They do this because they have a shared language, vocabulary, metaphor. Sure, they must rely on math and experiment to ground that understanding in truth but I guarantee they do not communicate understanding (solely) in math and measurement data.

Comment by dahart 4 days ago

That’s not the meaning of “understand” that was being used. This article points out that nobody understands QM well enough, not even the experts. (Do note the author is an expert!) By “I Don’t Understand”, what he actually means is that QM is incomplete in the same way that Newtonian Mechanics is incomplete; it doesn’t explain physics well enough to understand or explain what’s really there. This is, of course, why there’s an ongoing debate among the experts about what QM even means and how to interpret it.

Comment by spolitry 4 days ago

What interview was that?

The famous interview I know is when he said:

"I really can't do a good job, any job, of explaining magnetic force in terms of something else you're more familiar with, because I don't understand it in terms of anything else that you're more familiar with"

His point was that fundamental forces cannot be understood as "like" everyday objects, because we don't "understand" everyday objects except as applications of fundamental forces. Using those things to explain fundamental forces would just be circular. He can tell you that a stretching rubber band is due to the electric field, but he can't then turn around and "explain" that the electric field is like a stretching rubber band! Fundamental forces just are, and you understand them by studying them.

Comment by frumiousirc 4 days ago

Yes, this sounds like the interview that I was remembering.

>Fundamental forces just are, and you understand them by studying them.

This is largely the point I was trying to make. One must understand QM using non-everyday ideas developed with math and measurement. But once those ideas are held, similes/metaphor (sometimes actual isomorphs) can be useful parts of that understanding and its communication between those that share the basis for that understanding.

Comment by throwaway093020 4 days ago

A useful analogy might be understanding quantum mechanics versus understanding a computer. In the early days of computing, an engineer might plausibly have known every component in a computer, understood how these components worked, and understood why each one was there. As computers got more complicated, no longer could anyone understand the entire system on that level. But still, as an individual engineer, you can understand the system at a high-level, and for each lower-level component, you can have an approximate idea of why it's there, what it does, and how to learn it in more detail if you ever have to. Any "why" question that you might have about it has a straightforward answer that can be stated in a few sentences.

With quantum mechanics, there's a lot that you have to accept when you learn the subject without understanding why. Why complex numbers? Why infinite-dimensional Hilbert spaces? Why non-commutative algebra? You can kind-of, with great effort, make arguments that all of these are at least reasonable. But nobody really understands "why" at a level beyond "because the theory works".

Comment by 453yuh46 4 days ago

You can imagine waterfall of sand - the fluidity here comes from all those small sand particles moving simultaneously.

>>>Physics steeped enough in quantum theory do understand QM enough to have meaningful discussions free (at times) of rigorous math.

That sounds a lot like gate keeping and has no actual backing - to understand QM you still need to understand Physics, as mechanics of both of them are explaining each other - the scale is the only difference.

Comment by gus_massa 4 days ago

The idea is that you can discuss using hand waving while drinking coffee, but after that you must write the complete mathematical calculation, so there is no gate keeping.

Most of the times the formal version with equations agree with the hand waving, but sometimes you have nasty surprises and must learn to hand wave better.

Comment by soliax 5 days ago

[dead]

Comment by charcircuit 5 days ago

Why can AI generate pictures of waterfalls if it has never lived on Earth?

Comment by chistev 5 days ago

"I think I can safely say that nobody understands quantum mechanics."

- Richard Feynman

Comment by stratos123 5 days ago

Whether this was even true in Feynman's time, it's much less true now.

Comment by tim333 5 days ago

With the meaning that Feynman meant it was true then and now. From QED:

>The next reason that you might think you do not understand what I am telling you is, while I am describing to you how Nature works, you won't understand why Nature works that way. But you see, nobody understands that. I can't explain why Nature behaves in this peculiar way.

Comment by Viliam1234 3 days ago

Heresy! Feynman was the last Prophet of Physics, no one can ever figure out the things he didn't know. If people could discover things after Feynman, those things could get into textbooks, and then mere students could know things that Feynman did not -- which is clearly absurd.

Comment by ahartmetz 5 days ago

I also like "If you think you understand quantum mechanics, you don't understand quantum mechanics" which is supposedly also from Feynman.

Comment by chistev 5 days ago

I think it's one of those famous quotes that was never said. The one I quoted was what he apparently said.

Comment by card_zero 4 days ago

Similar to a quote from Niels Bohr, "those who are not shocked when they first come across quantum theory cannot possibly have understood it".

Comment by atemerev 5 days ago

Quantum _mechanics_ cannot be understood, it is full of paradoxes. It contradicts classical EM and special relativity, among other things. A toy theory that was an important step to what came later.

Feynman's own QED is a part of QFT, but this is no longer quantum mechanics. Special relativity and quantum physics are reconciled since then. Now, we only need to reconcile GR and QFT, but this leap was so far beyond our attempts to scale it. Yet, QED/QFT/SM can be "understood" as there are fewer obvious paradoxes that plagued QM.

Comment by rramadass 5 days ago

Author: Prof. Douglas Ross of Southampton University - https://www.southampton.ac.uk/~doug/

Comment by rramadass 4 days ago

Addendum:

The author also wrote a sequel; I DO Understand Relativity - https://news.ycombinator.com/item?id=50220891

Comment by tialaramex 5 days ago

And in case it wasn't already obvious, that URL indicates Doug was at the University for a long time. The University stopped letting even its staff pick a username years ago, maybe even decades ago and the default assigned usernames are... a bit dull.

Comment by rramadass 4 days ago

Your colleague i presume?

He also wrote another similar popular article on Relativity, I DO Understand Relativity - https://news.ycombinator.com/item?id=50220891

With article titles like these, i just had to share it on HN!

Somebody should work on the above two webpages to make them more readable with html+pdf formats. That would make these more accessible to the "interested layman".

Comment by tialaramex 3 days ago

> Your colleague i presume?

Yes but it's a very large institution. Thousands of staff, tens of thousands of students across a bunch of campuses.

Comment by cjfd 4 days ago

Regarding questions of what how measurement relates to schrodinger time evolution: https://arxiv.org/abs/quant-ph/0503009 . I think it is absolutely astonishing that many physicists do not know this.

Comment by tsunamifury 4 days ago

Many worlds is a mathematical construct of our inability to measure specific dimensions hyper locally and an artifact of that. Which for some reason the entire scientific community mistakes as a real observation of reality.

Just because at a quantum scale everything becomes potential doesn’t mean all potential has been manifested.

It baffles me that so many serious scientists don’t understand this basic tenet.

Comment by bananamogul 4 days ago

"I have used many photos and drawings which I have found on the Internet, and in many cases the source of such photos and drawings is not clear. I have assumed that images which appear on the Internet are in the public domain and do not need to be individually referenced."

That's a pretty bold assumption.

Comment by CamperBob2 4 days ago

I've always said (and been duly downvoted for it), "Don't want people to take it and reuse it? Don't put it on the Internet."

Sadly, being right about something like that does not, in fact, automatically entitle me to founders' shares in OpenAI or Anthropic. But I'll keep preaching it, nonetheless.

Comment by ktallett 5 days ago

The key aspects of quantum physics can be explained very simply. The issue is going in depth without those basics. I often find if someone doesn't get Quantum Physics, it's that source or you need the steps explaining more in-depth. Often sources assume your base knowledge and that is where many fall flat.

Comment by polotics 5 days ago

me neither! How does the Einstein Rosen Podosky Paradox work? can the unknown that's being collapsed not be a pre-determined value that therefore is not traveling faster than the speed of light?

Comment by superluserdo 5 days ago

>can the unknown that's being collapsed not be a pre-determined value that therefore is not traveling faster than the speed of light?

This is known as a "hidden variable theory". People will tell you that a local (things can only affect their immediate surroundings) version of the theory is impossible because it would be a violation of Bell's inequality, but there is a loophole that requires accepting the idea of "superdeterminism". This is the idea that where what measurement a scientist chooses to measure is predetermined and correlated with what the particle being measured is going to do.

So if a particle lands on a screen according to distribution p when going through a double slit, but with distribution p' when one of the slits is measured at, conventional QM says that the measurement "collapses the wave function". But a superdeterministic local hidden variable theory would imply that the choice of whether to measure at the slit or not was already set in stone at the same time that the particle "decided" which path to take to land on the screen, more or less.

Comment by beloch 5 days ago

If you split up an entangled pair of particles and measure them in a the same 50/50 basis simultaneously (sort of like a coin flip) at distantly removed places, the results will be individually random but perfectly correlated when compared with each other. It seems natural to assume that something here was pre-determined. i.e. There's was some kind of hidden information that we haven't found a way to measure yet. The way in which the coin-tosses were going to pan out was determined when the pair of particles were entangled.

Quantum physics makes predictions that suggest this isn't so. You can design experiments (e.g. A Bell inequality test) in which quantum physics predicts correlations that go beyond what would be possible with some hidden local information attached to the entangled particles that we just don't know about. Experiments don't prove theories to be correct. That's not how science works. However, experimental tests of Bell inequalities have pretty conclusively shown that local hidden variable theories are inadequate to explain what's going on.

Yes, it appears what Einstein called "spooky action at a distance" seems to propagate close to instantly. It's important to appreciate that you can't transmit information this way though. We can only make sense of the quantum correlations after communicating other information at classical speeds. Sometimes you see quantum entanglement devices allowing FTL communication in sci-fi, but that's pure horse hockey.

That all being said, I don't understand quantum physics either.

Comment by lkm0 5 days ago

One of the "coping mechanisms" is to say that information cannot travel faster than light, even if entanglement is propagated instantaneously and measurement acts faster than light. If Alice and Bob share an entangled pair of spins and Alice measures it, Bob has gained no information until Alice communicates (sublight speed) her result. No new information is shared at measurement time. This is known as the no-communication theorem.

Comment by antihipocrat 4 days ago

I don't think that's correct? Alice knows immediately what Bob's result is going to be as soon as she measure's it. The travel faster than light part that many people (including Einstein) take/took issue with includes this as an effect. More generally put is that if spin is in a superposition until measurement collapses it into a known quantity, then the result of the collapse itself is somehow propagated to the other entangled particle instantaneously.

Comment by lkm0 3 days ago

Yes, Alice knows what Bob's measurement is going to be, but only if they agreed beforehand on what state to prepare ("let's make the |00> + |11> state"). In that case no information traveled because what Alice learned came from the previous discussion (which presumably happened in the break room, sublight speed). If they prepared a random state, then there is no way for Alice to know what the second spin is only using measurements on the first.

Comment by antihipocrat 2 days ago

So the decision shared with each other at sub-lightspeed prior to measurement is actually the hidden variable everyone is getting hung up on? If this is correct then the superposition is just the cat in the box, and 'superposition' is just that the state is unknown until measured?

Comment by lkm0 2 days ago

Hidden variables are more about finding a way to derandomize quantum physics. The state |00> + |11> can collapse to |00> or |11> when measured. It appears to be that this randomness is baked into reality (God, meet dice, etc), and many people find this a bit distasteful, because it means measurements propagate faster than light (even if Alice and Bob can't communicate that info). If it turns out the theory is incomplete and there are missing/hidden variables, then it means whether it was |00> or |11> was determined from the start, before the measurement was made, and there's no faster-than-light interaction. Experiments around Bell's inequality prove that hidden variables don't exist, unless we start getting into scenarios that are as weird as entanglement itself. So the cat is truly alive and dead at the same time, and superposition is physically happening; this property is called quantum contextuality and is related to Bell/etc but still another discussion.

The important distinction in this case is about FTL existing but not being able to exploit it for FTL communication.

Comment by antihipocrat 2 days ago

Thanks for taking the time to explain it further. This is a topic that keeps me up thinking about, not because a fundamental randomness is distasteful, but because it doesn't seem to match with what we see, and I'm wondering if we are getting caught up in describing laboratory/pure math isolated states rather than what actually occurs in nature.

In reality the entangled particle will interact with something almost instantly thus collapsing it. The cat is actually dead or alive from the perspective of the microscopic things in the box. A world never witnessed or measured by a being doesn't collapse it's entire history of interactions of every subatomic particle, atom, molecule and emergent property from the start of the universe to 'now' instantaneously upon being viewed. The photons from that world have been hitting earth continuously since earth existed after all.

Comment by lkm0 1 day ago

No worries, explaining helps my understanding as well.

The new thing you're mentioning is a common observation about observers: if there are two cats in the box, should they be |dead dead> + |alive alive> or is one measuring the other and they've collapsed to a classical state? This depends on a person being outside the box; the scenario is called Wigner's friend. It's been used to show many other unintuitive aspects of quantum physics.

Comment by 4 days ago

Comment by _heimdall 5 days ago

Unpopular opinion I think, but I still believe the physics world was wrong to chase the Copenhagen interpretation.

The math does indeed work, and many experiments and practical uses seem to show that there is something to quantum broadly. But to throw determinism out the window and believe that quantum is fundamentally probabilistic, and that one can never really run scientific experiments because observing state collapses superposition, has always seemed asinine to me.

I could be completely wrong, but it's always seemed obvious to me that the need for probabilities in quantum is a clear indicator that something is missing in the theory. I'm never surprised when a person can't understand quantum as its described today.

Comment by lefra 5 days ago

It's completely counterintuitive, but that's how it works experimentally. For example:

Measure the spin of electrons in direction X (that's done with a weirdly shaped magnet). You'll get either +1 or -1 in a conveniently chosen unit system. That's already weird, you'll never measure +0.5 for example. Keep only the +1 electrons. You can measure them again and again, you'll always get +1 in the X direction.

Now measure those electrons in the Y direction, you'll also get either +1 or -1.

But now, if you measure in the X direction again, you'll get +1 or -1 with 50% probability; although before measuring in the Y direction, these electrons always had spin +1 in the X direction.

Comment by _heimdall 5 days ago

Oh I get that experiments prove out. What has always been unintuitive to me to the point of seeming wrong is that said spin is random and lives in a probability cloud that is probabilistic only until we observe it.

The math works and the experiments work, but it feels like a broken clock situation. It happens to correlate well, but the understanding and models are missing something in the actual underlying physics that, if understood, would throw the whole randomness part out the window.

Comment by lefra 4 days ago

> [the] spin is random and lives in a probability cloud that is probabilistic only until we observe it.

My mental model of it is kinda reversed. The maths assign a specific, deterministic state to each electron between each measurement. However, the "left" state is equal to "(up + down) / sqrt(2)". It then follows that when we measure an electron that is in the "left" state along the vertical direction, we get 50% up and 50% down: the probabilistic behavior happens when measuring, which "forces" an electron that was in the "left" state to become either "up" or "down".

I agree though that it would be less frustrating if there was some property of the electrons that would allow to predict in which state it'll collapse.

Comment by bluepnume 4 days ago

This is called the "measurement problem" and nobody has a good answer to it.

Comment by empiricus 4 days ago

Probability cloud? we would be lucky :) it lives in the complex square root of probability distribution. why would nature even do this

Comment by karussell 5 days ago

It has been shown that you can keep determinism (Bohm etc), but what follows is not necessary something you'll find more intuitive or cleaner or more logical.

Comment by _heimdall 5 days ago

I haven't been sold by pilot-wave theory, adding hidden variables to attempt to get rid of randomness feels like a cheat rather that a realistic view of how reality would work.

Unrelated, but I also wish scientists properly used the term "theory".

Comment by spolitry 4 days ago

Reality is in no way constrained by what you feel is realistic.

Comment by _heimdall 4 days ago

It also is not confined by what theoretical math works on paper.

Comment by stevenhuang 4 days ago

Why should it be asinine? The universe is under no obligation to be understandable to humans.

Comment by _heimdall 4 days ago

Completely agree. I'm happy to accept that I won't understand it, I take issue with the idea that some believe they do understand it and that the universe is based on random probability that can't be measure or scientifically measured without ruining the test.

Comment by maxdo 4 days ago

No one on earth understands it perfectly.

Comment by everyone 4 days ago

Imo it's a bunch of rules and if u follow them you get accurate answers. What more is there to understand? It seems very unlikely that aspects of the universe more removed from our macro-scale existence would make sense or be comprehensible to beings whose minds evolved for hunter-gathering and social dynamics in Pleistocene Africa.

Comment by tbrownaw 4 days ago

There's a section at the start that talks about what "understand" actually means.

It cannot be usefully analogized to things you can experience (or even to other things you might already understand).

Comment by MetroWind 2 days ago

Nobody understands quantum mechanics.

Comment by dtj1123 5 days ago

We understand quantum mechanics well enough to build machines that depend almost entirely on quantum principles.

It sounds all superior and mysterious to quote Feynman here, but the fact is many people absolutely do understand quantum mechanics.

Comment by jasode 5 days ago

>It sounds all superior and mysterious to quote Feynman here,

It isn't about being superior, mysterious, or clever. The following "nobody understands" type of quips and thought experiments are different ways of saying our intuitions don't help us understand it :

- "God does not play dice with the universe.", Albert Einstein

- "Shut up and calculate", N. David Mermin

- "Schrödinger's cat" being both alive and dead

- various interpretations/analogies explaining quantum physics leading to absurdities or contradictions

Yes, humanity has harnessed quantum mechanics with mathematic frameworks and repeatable experiments to make useful things... but that's a different layer of "understanding" than the deeper levels in the above quotes.

"Nobody understands what gravity is." ... Wait, if nobody "understands" gravity, how did we send people to the moon?!?

Because those are different usages of the word "understand".

Comment by TheOtherHobbes 5 days ago

"Understand" means "Have a reasonable metaphor for."

Our monkey minds evolved to throw rocks at birds, and our metaphors are based on concepts like position, velocity, and temperature. We added first-level abstractions like energy and cause-and-effect, then built higher level abstractions on those basic concepts.

QM clearly doesn't work like that. It's probabilistic, possibly non-local, possibly subjective, possibly a holographic projection of some underlying reality that is nothing like this one, and its relationship with causality is slippery.

We have a mathematical UI for it, but that's not at all the same thing as understanding the machinery on the other side of the UI.

Comment by rramadass 4 days ago

A very sharp and succinct statement of the problem!

Comment by Razengan 5 days ago

Or we understand just enough to make things and instruments that only work on (and up to) the layer that we understand.

Ancient humans could build boats 5000 years ago and expand throughout the planet, but they didn't know about water being made up of hydrogen+oxygen, or the tides being caused by the moon, etc.

Their calculations served them on the layer they operated on.

Comment by lloeki 5 days ago

> the tides being caused by the moon

Mostly because people around the mediterranean sea didn't have to care, tides being so small there. Yet the Greek had fairly advanced models of tide schedules.

Pretty sure peoples that were experiencing bigger tides and depended on it e.g for fishing figured out the relationship with the moon pretty quickly, whatever esoteric reason they may have plastered upon it.

Comment by HarHarVeryFunny 4 days ago

I think the disconnect here is the use of the word "understand". What Feynman presumably meant was really "interpret" not "understand", and theories are not made to be interpreted - they are made to make experimental predictions.

Many people of course, Feynman obviously included, understand the theory in the sense of knowing how to correctly apply it to make predictions.

Comment by mettamage 5 days ago

Can’t we say the same about LLMs? Yet interpretability of them is rough.

We can build black boxes and exploit the behavior of what we see.

But we can’t always go to deeper and deeper layers of understanding.

QM is not a black boxes we built but one could perhaps say it’s a black box that we see.

Comment by 453yuh46 4 days ago

No, I do not claim I am expert at understanding at how LLMs operate, but there is a huge difference - we can measure changes in models and even identify what part of model is used - it is clearly a niche thing to do and not particularly interesting to many people(and not to me as well, unless those LLMs will be able to replicate brains - in future, then I would be biting my nails that I have ignored the basics), but there is no such issue on dissecting and identifying how bits are shuffled around, unlike with QM.

For QM the main issue is that we are barely able to see results of that black box and have to make guesses even about observable results, because what we see is result of particle long gone and the result has been a mess of paths that it took. And basically we are operating at identifying single particle, while clearly the path that particle took was defined by other particles and "gravity"? from interacting with other particles bumping it around. I'm even less knowledgeable when it comes to QM terminology, but I get impression that most of the commenters here simply are parroting things they have read or being taught without actually trying to understand and taking into account all the unresolved problems in QM, which is why so many of them sounds so confident is my guess. Because knowing limitations of my brain I would be not so confident of anything, only because of massive load of unresolved issues that QM has.

Comment by uecker 5 days ago

If one does understand quantum mechanics well enough to use it, then one also understands that there are aspects of it we do not fully understand.

Comment by dtj1123 5 days ago

You can make an identical argument with respect to classical physics, e.g. outstanding issues in fluid dynamics or statistical mechanics.

Nobody takes this to mean that classical physics is generally incomprehensible.

Comment by uecker 4 days ago

The difference is that the part we do not understand about QM is rather fundamental to its interpretation, while in classical physics it is more a problem of understanding its consequences in complex scenarios.

Comment by 453yuh46 4 days ago

Listen, people were able to make calendars by observing skies and assuming that Sun was orbiting Earth. It is not exactly the same, but very much how people understand QM - they do not need to known mechanics, but observing end results is enough to "understand" how it functions.

Also billions of people understand how to operate mobile phones, but "understanding how they operate" would be rather stretch and even the biggest specialists of QM have understanding only on how to interpret end results in QM - at best and that is basically it.

Quite frankly, I'm confused about these bold statements. I assume there seems to be some knowledge background of what you are saying - it very much reminds me what stupid strawman arguments I had in high school with my schoolmates then as a part of learning process, but rather far from actual wisdom.

Comment by tim333 5 days ago

We understand part of it but not how it works.

Comment by 5 days ago

Comment by icantevenhold 5 days ago

Forgive my ignorance but which machines depend almost entirely on quantum principles?

Comment by CamperBob2 3 days ago

When you ask what time it is, the answer ultimately comes from an atomic clock that uses quantum state selection. You could argue that any sort of fluorescent lamp or LED is quantum-driven, based on bandgap thresholds or ionization of gas. Same for anything involving solar PV electricity generation.

The example the author gives, transistors that somehow rely on quantum tunneling to produce gain, is not a good example at all. In fact it's downright wrong. The essay could have been titled "I don't understand transistors."

Comment by dtj1123 3 days ago

Google or ask an LLM about the technological application of solid state physics or many body quantum physics.

Comment by folbec 5 days ago

The processor of the computer you are writing your question on is using tunnelling transistors.

Comment by TheOtherHobbes 5 days ago

Modern processors are designed to minimise quantum effects, because phenomena like tunnelling create inefficient leakage currents.

Comment by gus_massa 4 days ago

All the idea of valence and conduction bands depends on quantum effects. The holes are quantum effects, the electrons are not plain electrons and in particular they have an effective mass that is not equal to the mass of a plain electron traveling in vacuum.

Anyway, I prefer LEDs as an example.

Comment by jamesrcole 4 days ago

does building those machines require us to fully understand quantum mechanics?

Comment by boltzmann64 5 days ago

Saying I don't understand Quantum Mechanics is like saying, "I don't understand how attention/transformers explain the human brain."

It doesn't. It's model. All models are wrong (including quantum mechanics and attention) but some are useful. You understand the domain of the model, the problems it can solve, the math behind the models and use the model to make predictions. If you are competent at linear algebra and functional analysis, you can do quantum mechanics.

If you make the error of believing that quantum mechanics is ontological god's honest truth of nature, you will end up with blunders like using it to explain consciousness like Prof Penrose. His math was impeccable, but he was making a category error.

Comment by kitd 5 days ago

Don't take the title too literally. It's an article aimed at lay people, using a typical lay person's expression as the title.

Comment by caaqil 4 days ago

The trick to a top HN comment is to basically reflexively react to the title with no regard for the substance of the relevant article. Amusingly, pointing out this self-evident truth is itself frowned upon and more offensive than the described trick, according to the sacred text.

Comment by fsh 5 days ago

I disagree. The human brain is difficult to understand due to the extreme complexity. Quantum mechanics is weird even for simple arrangements of a few building blocks such as the double-slit experiment or Bell tests.

Comment by croniev 4 days ago

Are you implying that on a basic level or for simple arrangements transformers do explain what happens in the brain? If so, I would be curious to hear how.

Comment by lefra 5 days ago

Quantum physics is a theory, as in "as far as we can tell, this is an exact mathematical description of the world".

Models are simplifications/approximations made on purpose, as in "I know this is wrong on some level, but it's close enough for my use case", or "this makes the maths solvable".

Comment by codethief 4 days ago

As someone with a degree in physics and math, I find the line you're trying to draw rather blurry.

> Quantum physics is a theory, as in "as far as we can tell, this is an exact mathematical description of the world".

Is it?

If by "mathematical exactness" you merely mean we have somehow been able to produce some numbers that later on we measured in experiments, then yes. (Though we haven't been able to produce such numbers in all cases, see e.g. [0].)

I my book, though, mathematical exactness means your model is mathematically consistent and is applied with the necessary mathematical rigor.

Quantum mechanics and quantum field theory, as they are commonly taught in universities, are everything but. They are a bunch of concepts and recipes that, albeit successful, are held together loosely with duct tape and lots of hand waving to make you ignore the logical gaps & jumps and the fact that many of the "mathematical" objects physicists refer to don't even exist[1]!

[0]: https://www.scientificamerican.com/article/this-simple-exper...

[1]: Haag's Theorem, Aizenman–Fröhlich Triviality Theorem, etc.

Comment by Sharlin 4 days ago

I’m not sure most physicists would make that specific distinction between "theory" and "model". We know QM is wrong or at the very least incomplete as long as there’s no theory of quantum gravity. Every theory, like every model, has a domain of applicability and gives wrong or even nonsensical answers when applied outside its domain.

What a theory is, is more like a model (or maybe meta-model) with a particularly broad scope or high explanatory value, often a mathematical framework that gives a common foundation and theoretical justification for multiple experimentally-derived models.

Comment by DiogenesKynikos 4 days ago

This is really a misunderstanding of what quantum mechanics is. It's not a particular theory. It's a category of theories that fit into a certain framework.

Quantum electrodynamics is a quantum theory. Quantum chromodynamics is a quantum theory. String theory is a quantum theory. I'm sure there is a correct quantum theory of gravity - we just don't know what it is yet.

Comment by sigmoid10 5 days ago

This is an important distinction. Quantum mechanics as a framework is the correct description of reality as far as we can measure (excluding some loopholes like superdeterminism). This is as close to the foundation or "god" or whatever you want to call it as it gets. If you look at specific quantum model descriptions (like the Schrödinger equation), we know they are not complete. Even relativistically correct Dirac equation is still not the full picture, because we don't fully understand how gravity fits into this on short length scales. But the current principles like Bell's theorem exist independently with all their implications on our reality and they will not go away, no matter what else we discover at the bottom.

Comment by 453yuh46 5 days ago

>>>Quantum physics is a theory, as in "as far as we can tell, this is an exact mathematical description of the world".

... which "we think" is functioning that way not to mention that theory itself might be wrong and be replaced as most of the supporters of quantum theory doesn't seem to look on it as something that explain mechanics of it, but rather how magic of "unexplainable" quantum mechanics works.

Comment by nchagnet 5 days ago

Eh I would disagree on such a sharp distinction. Both quantum mechanics and general relativity are theories, and we know that both of them are not accurate descriptions at all scales.

Heck your descriptions of a model is also what we would call an effective theory (i.e. a description valid for a specific range of parameters/scale).

I don't think there's a commonly agreed difference between model and theory and I sure have heard many physicists use them interchangeably when it suits them (and I'm guilty of it myself). Perhaps theory feels 'wider', as in quantum theory encompasses models for many quantum systems, but that's as far as I'd go.

Comment by marginalia_nu 5 days ago

It all reminds me of my favorite Asimov quote:

> [...] when people thought the Earth was flat, they were wrong. When people thought the Earth was spherical, they were wrong. But if you think that thinking the Earth is spherical is just as wrong as thinking the Earth is flat, then your view is wronger than both of them put together.

Comment by SJC_Hacker 4 days ago

QM is accurate at scale. This is the correspondence principle. It’s just not useful at scale

Comment by glenstein 4 days ago

I think the vast majority of physicists have been perfectly able to avoid Penrose's error. I would also think the lesson would be simply to strive to avoid making errors or the kind he did, not necessarily to sacrifice realism about physical objects just to avoid the possibility of someone's mistake. Plenty of people, in fact I would say many more people abuse the observer effect in QM to make crackpot claims about consciousness. But it doesn't mean the observer effect is wrong.

I also think you can be a realist without having to make the jump to ontological irreducibility. You can say that our latest theory is a better approximation than the previous one and the previous one was an approximation as well.

Comment by thatsabadlook 5 days ago

Somehow hacker news makes the top comment about some weird unrelated ai take. So unrelated. The take mostly doesn't make sense either.

Comment by dahart 4 days ago

One problem with your analogy is that transformers do not even attempt to model the human brain, while Quantum Mechanics does attempt to model real physics.

> All models are wrong

That famous quote could use attribution: https://en.wikipedia.org/wiki/All_models_are_wrong

> If you make the error of believing that quantum mechanics is ontological god’s honest truth of nature

This is why science & experiment exists. The author holds no such belief, and the paper makes that very clear. That is what he actually meant by “I Don’t Understand Quantum Physics”. Jump to the very last section, “Is Quantum Theory Correct and Complete?” He’s pointing out the confusing and contradictory nature of the model so far, and saying that Quantum is incomplete and needs work. He is not making any category error.

Comment by goodmythical 4 days ago

>That famous quote could use attribution

It could, but doesn't need it.

That models are not direct truth is just logical. If there were such a thing that were indistinguishable from the direct truth, it would be the direct truth, not some other thing called a model.

This is why I've never understood the "revalatory" nature of Godel's incompleteness. Of course any axiomatic system must not contain all truths. Axiomatic systems are models and models can't contain all truth. If an axiomatic system contained all truth it would just be all truth rather than some other thing called an axiomatic system.

The only case in which Godel's argument surprises is the case in which one has a fundamental misunderstanding of what a model can and cannot be, which I suppose must have been more common in his time, but I was really confused when my highschool teacher went to great lengths to explain that a model cannot contain the whole truth because...of course it can't?

Comment by dahart 4 days ago

> of course it can’t?

If obvious, can you prove it without reading Gödel’s proof or anyone else’s?

The important (2nd) part of Gödel’s proof is not that a model can’t contain all truth, it’s that a model cannot prove itself true, which is a far stronger statement than the intuition you describe. Of course, belief that your own intuition is independent of history is also highly questionable; you were raised and educated in an environment in which people were already aware of Gödel. It’s like saying the number zero is obvious (which it is now) despite the fact that it took thousands of years before it was used formally as a number. There was a time when it wasn’t obvious to the smartest people alive.

Note that formal axiomatic systems aren’t physical systems; what Gödel was talking about and what George Box was talking about are related but somewhat different concepts. It is obvious in physics that a parabolic ballistic trajectory model doesn’t account for air turbulence. It’s not at all obvious that all of math itself is guaranteed to produce contradictions and cannot prove itself, especially if you’re allowed to change math or use any system you want.

Comment by robotresearcher 4 days ago

If I present a novel, unique axiomatic system, with no intent that it refers to anything in the world, what is it a model of?

Comment by sega_sai 4 days ago

What a strange statement. Quantum mechanics is a physical model of the world. Attention and transformers are not. They can be construed into one in principle, but they are not models of the world themselves.

Comment by alansaber 4 days ago

Yes, but even with a model, you should aim for some serious intuition as to how and why it works.

Penrose has fallen into the same logical fallacy that many older, retired theoretical physicists have (notable einstein?).

Comment by DiogenesKynikos 5 days ago

There are only a few things that I think are exactly correct in physics, and quantum mechanics is one of them, alongside Relativity.

Quantum mechanics is a framework for how theories are built, not a specific theory, and it's really all or nothing. There's not much between the quantum and classical ways of viewing the world.

Comment by kergonath 5 days ago

Both cannot be “exactly correct” at the same time. And most likely, neither are.

Comment by DiogenesKynikos 5 days ago

Indeed. The classical view is wrong, and the quantum view is correct.

But it just turns out that there's a limit in which the quantum world looks approximately classical.

Comment by TuringTest 4 days ago

I think the GP meant that quantum theory and relativity are incompatible and thus both cannot be exactly correct at the same time. Not referring to quantum vs classical.

Comment by DiogenesKynikos 4 days ago

Quantum mechanics and General Relativity[0] are not fundamentally incompatible. It's just that the most straightforward approach to incorporating gravity into the Standard Model fails. In fact, we know that it is possible to construct quantum theories of gravity that reduce to General Relativity in the classical limit. String Theory is one such theory. We just don't know if it's the correct theory.

0. Special Relativity is already baked into the Standard Model, which is a quantum theory. It's General Relativity that causes problems.

Comment by kergonath 4 days ago

Indeed. I know it’s a lazy way out because I don’t have to assume anything about the actual correctness of either, given that they are famously incompatible in the details.

But as a physicist I have no illusion that any of our theories are actually true. They are all models that help us making sense of an unimaginably complex universe.

Comment by jamesrcole 4 days ago

> exactly correct in physics, and quantum mechanics is one of them, alongside Relativity

what about gravity? If we don't know the correct account of that, how can we know those are exactly correct?

Comment by DiogenesKynikos 4 days ago

First, we know for certain that pretty much everything other than gravity is described by quantum theories. It's almost an impossibility to merge quantum theories with non-quantum theories in a logically coherent manner, so either there exists a quantum theory of gravity (which we just haven't discovered yet), or the rest of physics is not really quantum mechanical. But there's a strong theorem (Bell's inequality) that rules out non-quantum explanations of experiments we can conduct in the lab, unless you invoke truly exotic explanations that are much weirder and implausible than quantum mechanics.

So we really are stuck with quantum mechanics, and there must be a quantum theory of gravity. In fact, there are actually quantum theories of gravity (including String Theory), but we don't know whether they're the correct theory that describes our universe.

Comment by 453yuh46 4 days ago

>>>There are only a few things that I think are exactly correct in physics, and quantum mechanics is one of them, alongside Relativity.

...unless I have understood it otherwise, but quantum mechanics is still an approximation of things we simply are unable to measure individually - mostly because of speed and have to resort to guessing that approximation of that limited time period, which is still too long. It would be the same as trying to guess position of a ship during voyage across the ocean and taking the smallest measurable time period as 100 years, in which the ship has been already sunk out of existence, while smaller time period would have given us better picture and quantum mechanics that we are able to use has exactly that issue - we are unable to measure and record particles at smaller time intervals. At best we can see trajectories of particles long gone.

Also, this book is not agreeing with your statement, to me which sounds like Religion(because you have no explanation at all!) and should not be part of Science.

Comment by truculent 5 days ago

> It doesn't. It's model. All models are wrong (including quantum mechanics and attention) but some are useful.

Sure, but what if you don’t understand the model?

Comment by 4 days ago

Comment by paid_dot_expert 4 days ago

Your username checks out. Time to shine?

Comment by paulnpace 5 days ago

This deserves a [pdf] in the title.

Comment by bozdemir 5 days ago

So do the rest of us.

Comment by phpnodejspwn 5 days ago

2 things to understand about science:

It's useful, not metaphysically real. (Hot take btw) This is called Instrumentalism. The belief that it's real is called Scientific Realism. However over and over again, science can't cover edge cases, and we find better fitting models. The Copernician revolution is a good example of this dichotomy.

2. Academia basically forces you to believe the doctrine or you fail the class. From philosophy to physics to sociology. If you read the philosopher survey, you will find trendy things being quite high. It's a religion in itself.

Bonus 3: Philosophical Pragmatism and Instrumentalism generally solves this, but is intellectually a dead end. There is a reason analytic philosophy continues despite gaping holes in linguistics (late Wittgenstein) that ruin it. But at the same time, best fits might be missing a new paradigm. I wish I could tell people to ask AI to explain Hegel to understand, but I think people need to read it themselves.

Tl;dr: quantum mechanics is the best we have. It's definitely wrong, but it's the most useful we have.

Comment by gus_massa 5 days ago

> 2. Academia basically forces you to believe the doctrine or you fail the class.

Yes, but if you can make a reliable experiment that disagree with the doctrine, it will change, even if everyone hates the idea. In particular https://en.wikipedia.org/wiki/Wu_experiment

Comment by layer8 4 days ago

I disagree. Science is what we do to find out what is the case, i.e. real. The fact that it also has useful applications is directly tied to the extent to which the findings are real. It works because it describes something real.

Comment by 1718627440 4 days ago

Things being might be real in a specific point in spacetime, but that they are real is independent of time. However what Science "claims to be real"[0] is dependent of time. You can't know that the realness is the final understanding. The closest you can come to believing things are real, is that you can't think of an experiment that falsifies them. But you can't really know for sure, that falsifying is impossible. How do you solve that? I don't have that problem, because I agree with your parent, but this is what I thinks contradicts your understanding of Science.

[0] I personally don't think Science concerns itself whether things are real, only whether the result of model and realness match, but that's what we are discussing here.

Comment by phpnodejspwn 4 days ago

This is what they teach in school.

If you read late Wittgenstein or Karl Popper, you will find that science is a useful simplification. Rorty is boring and not useful, but he kills any sort of Realism.

The 3 gigantic holes:

1. It's all language. (Wittgenstein)

2. The edges of systems do not connect with each other.

3. There are exceptions to everything.

I know that might seem a bit generic, but these are the major flaws in the philosophy of science.

I was a true believer too.

If you ever want to learn a ton about science, read Popper. He tortures himself, but he's also scientific meta.

Comment by HarHarVeryFunny 4 days ago

I guess that depends on what you mean by "real". I've always taken the scientific method to be about creating theories that correctly predict experimental outcomes, which is to say that utility is primary.

I guess if/when we discover a theory of everything then we could claim "this is real/reality", but what would that even mean other than modelling reality/outcomes with 100% fidelity?

Comment by noduerme 5 days ago

My question is, if I just refuse to believe in quantum physics, that makes it go away and/or be inapplicable to me, right?

Uh oh, some people have no sense of humor =\

Comment by soliax 5 days ago

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Comment by turbobutton 5 days ago

[dead]

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Comment by mkesper 5 days ago

(2018)

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Comment by CH-Abhinav 4 days ago

2018 ragebait?

Comment by zkmon 4 days ago

>> but we have to live with this strange idea if we accept that Quantum Physics describes Nature correctly.

That depends on how you define nature, world and universe. For me nature is defined by things that can be perceived by human senses.

Comment by 1718627440 4 days ago

So an effect that kills are humans wouldn't be real, as long as it kills faster than you can perceive. Are things predating humans not real?

Comment by zkmon 4 days ago

Any physical thing that doesn't relate itself to a spatial location or doesn't honor space, distances, time, causality - does not belong the real world that is defined by the same artifacts.

Comment by colinb 4 days ago

So, not gravity or radio waves?