A plan to turn the atmosphere into an enormous sensor • Hacker News Board

Re: A plan to turn the atmosphere into an enormous sensor

#11

Post by throwaway_pdp09 » Wed, Jun 17, 2020, 8:32 AM UTC

javawizard wrote: post #9

nuccy wrote: post #8

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 t…

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.

> 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.

Re: A plan to turn the atmosphere into an enormous sensor

#12

Post by nuccy » Wed, Jun 17, 2020, 10:19 AM UTC

throwaway_pdp09 wrote: post #11

javawizard wrote: post #9

Earlier quoted context omitted.

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.

> 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 th…

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

Re: A plan to turn the atmosphere into an enormous sensor

#13

Post by [deleted] » Wed, Jun 17, 2020, 10:36 AM UTC

nuccy wrote: post #12

throwaway_pdp09 wrote: post #11

Earlier quoted context omitted.

> 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 th…

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 communicati…

[deleted]