[nuccy]

The atmosphere is also used as a detection medium for Imaging Atmospheric Cherenkov Telescopes (IACTs) [1]. Very high energy (>tens GeV) photons, electrons or nuclons hit atoms in the atmosphere and produce a cascade of secondary particles, which move faster than the speed of light in the air emitting Cherenkov light [2]. Flashes of this light are lasting for few nanoseconds, so the cameras have nanosecond sampling time and are based on photo multipliers.

There are many such telescopes around the world: MAGIC (2 telescopes), FACT (1), HESS (5), Veritas (4). The biggest assembly of such telescopes is currently under construction - Cherenkov Telescole Array [3] with hundred of telescopes in Paranal, Chile and 20 in La Palma, Canary Islands. One large telescope with a mirror of 23m diameter is already operating [4].

[1] https://en.m.wikipedia.org/wiki/IACT

[2] https://en.m.wikipedia.org/wiki/Air_shower_(physics)

[3] https://m.youtube.com/watch?v=5gRHFQP_SjU

[4] https://www.cta-observatory.org/lst-1_inauguration/

[javawizard]

This is fascinating, but:

> which move faster than the speed of light

Citation needed. I was of the understanding that this is impossible, and your first two links provide no evidence to the contrary.

[throwaway_pdp09]

> which move faster than the speed of light in the air emitting Cherenkov light

Key words "in the air". As they can't go faster than C in this medium[0], they are forced to slow down, and emit this lost kinetic energy as radiation - my understanding.

Something a bit better, from https://en.wikipedia.org/wiki/Cherenkov_radiation

Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium. A classic example of Cherenkov radiation is the characteristic blue glow of an underwater nuclear reactor.

[0] I realise this seems contradictory - they can't but for a short time they are - I don't understand that either.

[nuccy]

Exactly, speed of light in the vacuum (c) is the hard limit, only mass-less objects can move that fast (e.g. photons). The refraction index (n) tells you how fast light moves in a medium. It is defined as n=c/v, where v is the speed of light in the medium. That's why (and also because of internal reflections) the speed of light in the fiber optics is not quite reaching c [0], and with inter-satellite communication Starlink may provide lower latency than the one using landline fiber optics channels. For air refraction index is very close to 1 [1], but not quite, at 10km altitude n=1.0001, which is 99.99% of c. For example a proton with an energy of 1TeV (Tera electron-volt, 1x10^12 eV) is moving 99.999956% speed of light in vacuum [2], and we see protons up to 5x10^19 eV reaching 99.99999999999999999998% of c [3]. Even "low" energy protons (tens of GeV) hitting molecules of air can kick out electrons, which, due to being lighter, can move faster than the initial protons and emit Cherenkov radiation.

[0] https://www.quora.com/What-is-precisely-the-speed-of-light-i...

[1] https://www.researchgate.net/profile/Dmytro_Vasylyev/publica...

[2] https://uspas.fnal.gov/materials/10MIT/Review_of_Relativity....

[3] https://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit

[sdflhasjd]

Crucial detail:

> which move faster than the speed of light in the air

The speed of light in air is slightly less than the speed of light in a vacuum (c)

Nothing is going faster than c