Cherenkov Radiation

BTW there are special types of telescopes used to observe gamma rays - they cannot see gamma ray directly but observe a flash of Cherenkov light of a cascade of charged particles created when gamma ray hits atoms in the atmosphere. Those telescopes are Imaging Atmospheric Cherenkov Telescopes [1].

1. https://en.wikipedia.org/wiki/MAGIC_(telescope) or https://en.wikipedia.org/wiki/VERITAS or https://en.wikipedia.org/wiki/High_Energy_Stereoscopic_Syste... or https://en.wikipedia.org/wiki/Cherenkov_Telescope_Array_Obse...

The late professor Roland Winston worked on this problem and discovered a geometry that could concentrate the light from a diffuse source like Cherenkov radiation to a detector with near-ideal performance ( https://doi.org/10.1063/1.1720428 ). It turns out that efficiently transferring light from diffuse sources has applications far beyond detecting Cherenkov radiation, so Winston founded the field of Nonimaging Optics and spent much of the rest of his carreer on developing the foundations of the field and on bringing together a community of scientists who would work on carrying the field forwards.

I do research in this field myself, and I find the optics and principles behind it endlessly fascinating

Of course I am biased because I work in the field, but the by far most wide reaching application of Cherenkov radiation is in the detection of high energy particles, particularly in astrophysics.

- Imaging Atmospheric Cherenkov telescopes detect the Cherenkov radiation emmited in the atmosphere when a high energy cosmic ray or gamma ray creates an air shower

- Water Cherenkov Detectors detect Cherenkov light when the secondary particles of these air showers reach water tanks on the ground

- Neutrino telescopes like kamiokande, Icecube and km3net detect Cherenkov radiation in water or ice produced by secondary particles produced by the rare interactions of Neutrinos in their detector volumes

Modern, high energy astrophysics is all about detecting different kinds of Cherenkov radiation and then reconstructing the original particle properties.

Fundamentally, changes in the EM field propagate always with the speed of light in a vacuum, i.e., c (also known as the speed of causality). Single EM waves propagate with exactly this speed and they do not magically slow down in a medium... they propagate happily at speed c! (FYI EM waves are more complicated like this and involve electric and magnetic fields evolving together).

But since EM radiation interacts with matter and this interaction itself changes the EM field again it results in more EM waves that propagate also at c. Hence, they propagate together and the net result be constructive or deconstructive as well as anything in between. If they have different frequencies, they can also create "interference" patterns or pulse envelopes that seem to propagate slower and even faster than c.

No doubt that the causes and effects are not easy to understand but always thinking in terms of changes in the EM field ALWAYS propagating at c helped me.

At one point in the tour, they turned on the reactor, while we stood along the edges of the pool it was immersed in. Literally all that separated us from the magic of fission was about 5-6 meters of water. I still remember the electric blue glow of the Cherenkov radiation. Even with decades of life and experience and education between now and then, its hard to describe the psychic impact of observing with my own eyes something that I had heretofore understood to be impossible. Something akin to seeing Narnia through the wardrobe for the first time.

I was already into physics at that time (I had shadowed a sibling for a day at the University of Washington, and got to attend a lecture about nuclear fission a few years before in the physics-for-liberal-arts-majors course she was taking at that time), but this was quite something else. All of that to say, I already understood that dragons exist, in a manner of speech, but there's a difference between understanding it and feeling one's breath on your face.

> Nothing can travel faster than the speed of light in a vacuum. However, in other mediums, particles can potentially move faster than light. For instance, while in water, light would instantly slow down to 75% of its normal speed, but there are other particles that don’t slow down as much and end up moving faster than light. Whenever that happens, a blue or violet glow occurs.

After reading this answer, I was not any wiser.

"In water" is the "In mice" equivalent for physics.

Unless you saw it from really close, in which case it's too late and you should probably relax, sit down, have a drink, call your loved ones...

I don’t mean in terms of the speed of light in a vacuum. Just seems weird that something with mass could outpace something without… Why are the photons so slow?

Why? How good an analogy is a sonic boom?

Like the best thing we have to remark on is the fact it is blue when this is probably the least remarkable thing about it

The new reactor was of the swimming pool type, and seen from the wraparound gallery above, you could easily mistake it for one. Except for the blue shimmer in the water.

Remembering my studies, my head went hot and cold. Hadn’t they said the reactor wasn’t operational yet? Or had I just assumed, because of the open day? ub So I hesitantly approached our guide and asked about the blue light, to which he answered in the most casual way you can imagine:

"Oh, that’s because of the Cherenkov Radiation."

Pause.

Laughter. Seeing the doubts in my eyes he had just been messing with me, and they had deliberately installed blue lights there to make the experience more realistic for the open day

In a medium light slows down. Particles in the same medium can travel faster than the slowed down light. But nothing ever breaks the barrier of C.

That's like the vaguest description of anything ever. Is physics a stealth startup?

Why does it specifically happen when particles travel faster than light in a given medium? There's no glow for particles moving slower?

But if they say that light is fastest in vacuum, slower elsewhere, why can they then say that other energy variants would move faster? They'd still be objectively slower than light in vacuum. This is like saying my bicycle is faster than a Ferrari if the latter is stuck in mud or a pit.

Edit: Just noticed that others such as u/nuccy also pointed that out. Agreed. The title is wrong.