The title (likely intentionally) is misleading, it should say "travelling faster than light in a medium". Nothing here travels faster than light in vacuum.

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...

A fun fact about Cherenkov radiation is that research on making efficient Cherenkov detectors in the 1960s led to the development of optical principles still used to design illumination systems and solar concentrators today.

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

The article has a section about what it can be used for, but only mentions the uses of the IAEA.

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.

Several comments here mention that nothings goes faster than light in a vacuum, which is right. But... Putting aside all considerations of causality, if a particle was to go faster than light, it would also emit a vacuum Cherenkov radiation, as this particle would go faster than light. That would be a kind of supersonic bang. Some theories about this say that when a particle going faster than light, it loses its energy and emits photons. IANAP (yes, I am not a physicist), but I would love to hear a theory about how those FTL particles could be detected if they were to exist, and what could be the observation, probably coming the this vacuum Cherenkov effect. Again, I know this is against all physics, but the theory would be cool!

It took me a long time to develop an intuition for light and electromagnetic wave propagation, and I’m still working on it.

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.

> How can something travel faster than light?

> 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.

A general rule of thumb is that if something is glowing blue or has a blue halo around it, you should run in the opposite direction.

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...

To be precise, what we call the “speed of light” is the limiting speed at which information and causal effects can propagate through spacetime. In vacuum, it coincides with the propagation speed of photons, i.e. of light. In other media or under certain conditions, however, light can propagate at a speed lower than , without changing the fundamental limit imposed by relativity. So "speed of light" used to denote is a bit misleading

In water!

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

I wonder if there could be something other than vacuum, in which light would travel faster.

> When charged particles moving faster than light travel in, for example, water, they perturb the energy equilibrium of the atoms that are in their way.

Why? How good an analogy is a sonic boom?

I think a problem is that because of historical reasons the speed of light is used interchangeably to something much more fundamental - the maximum speed at which information can propagate in space. Which is of course the speed of light in a vacuum but a better approach is the inverse - light in a vacuum moves at the maximum speed possible in our universe.

Light still propagates at c inside a medium; the apparent slowdown is just phase kickback from re-emission.

The Cherenkov effect, completely normal phenomenon, it can happen with minimal radiation. All you need, I’ve been told, is 3.6 Roentgens.

This reminds me of prescientific explanations of the sun and stars

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

I had learned about Cherenkov Radiation and its characteristic blue color at university. When, a couple of years later, the university had finished building a new research reactor, they had an open house day with guided tours. Of course I’d take one!

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

Great analysis. The systematic approach to this problem is well-thought-out.

Not a science guy per se, is this blue the same blue in the radioactive accident in Goiânia’s?

To be clear nothing is traveling faster than C.

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.

> When charged particles moving faster than light travel in, for example, water, they perturb the energy equilibrium of the atoms that are in their way. In order to regain equilibrium, those atoms release photons – the types of particles that compose visible light, creating a “shock-wave” of visible light.

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 there are other particles that don’t slow down as much and end up moving faster than light

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.

Completely normal phenomenon

“I’m not saying it is tachyons… but…”