Cherenkov Radiation: When Particles Break Light's Local Speed Limit

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.

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.

I've no idea if they still do it, or if this was an option open to the general public or if it was a special thing for our group, but once upon a time, in my early teens, I got to tour NIST's test reactor in Boulder, Colorado.

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.

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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.

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

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!