Posted by andsoitis 1 hour ago
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...
That is because that is not the title...the title is: "What is Cherenkov Radiation?"
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!
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.
Yes ... and no. Or we should say yes, but not necessarily forward. What about circular? What about ball shaped? What about a vortex. Any valid soliton is a solution and a single wave. Which means this is not just possible in water, but also in the electromagnetic field:
3Blue1Brown has a beautiful animation for this phase kickback here: https://youtube.com/shorts/XIW-2ykgVPI?si=PJWiAC2BO7_xP0S6
> 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.
> but there are other particles that don’t slow down as much and end up moving faster than light.
Not slowing down as much I can understand but shouldn't it read as
"but there are other particles that don’t slow down as much OR EVEN end up moving faster than light."
Got it, faster than light IN THAT MEDIUM.
What Feynman does (where this confusion comes from) is that you can look at discrete wave packets (i.e. photons) and the math comes out the right way for the primary wave if you assume that only some of these wave packets get phase-shifted, and add all elementary waves together afterwards.
But still, it's photons as "wave packets" that influence the whole system, not photons as independent particles that either bounce on something or don't.
Answer: Light slows down when going through water or air or gas. It's only in a vacuum that light travels at 'c' (from Einstein's equation). And it's that speed c that is a limit due to relativity.
But the exciting thing is that when you're not in a vacuum particles can be traveling faster than the local speed of light (maybe 75% c). And that process of a particle zipping along gives off Cherenkov radiation.
I think of it as the light equivalent of a supersonic shockwave and sonic boom. Faster than sound gives noise. Faster than light gives light (or other electro magnetic radiation)
(People with more knowledge might say the sonic boom analogy is very inaccurate but not sure)
Would have been clearer if they said "However, in other mediums (like water), particles can potentially move faster than light does in that same medium."
"In water" is the "In mice" equivalent for physics.
https://en.wikipedia.org/wiki/Imaging_atmospheric_Cherenkov_...
Why? How good an analogy is a sonic boom?
You can release a party balloon and it will create pressure disturbances as it moves to the top of the room, which theoretically you could measure. It's just not very loud.
Similarly, a charged particle passing through anything at any speed creates a disturbance, it's just not very easy to pick up on until it breaks the speed of "sound".
https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLig...