| ▲ | Cherenkov Radiation - traveling faster than light(iaea.org) |
| 37 points by andsoitis an hour ago | 23 comments |
| |
|
| ▲ | nuccy 38 minutes ago | parent | next [-] |
| 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... |
|
| ▲ | prathje 21 minutes ago | parent | prev | next [-] |
| 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! 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. |
| |
| ▲ | prathje 14 minutes ago | parent [-] | | In a medium, this can create a "phase kickback" which creates a combined wave that appears to travel slower than the original one. The kickback is just the result of multiple EM changes propagating, i.e. the photons interact with the material, re-emitting photons. 3Blue1Brown has a beautiful animation for this phase kickback here: https://youtube.com/shorts/XIW-2ykgVPI?si=PJWiAC2BO7_xP0S6 |
|
|
| ▲ | chinathrow 32 minutes ago | parent | prev | next [-] |
| >
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. |
| |
| ▲ | hdgvhicv 26 minutes ago | parent | next [-] | | In water photons travel at say 200,000km a second. Neutrinos travel at nearly 300,000km a second. That’s causes a blue glow. Which is how neutrino detectors work. | | |
| ▲ | chinathrow a minute ago | parent [-] | | Thanks - but I fell over this sentence: > 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." |
| |
| ▲ | cbolton 24 minutes ago | parent | prev | next [-] | | Yeah I didn't find that helpful. What I remember from Feynman's lectures is that photons still travel at "full speed" c between atoms, but if you look at the global progression of light as photons get absorbed then emitted it progresses slower than c. | | |
| ▲ | yayachiken 3 minutes ago | parent [-] | | You cannot treat light as particles in that scenario. The primary wave gets absolutely and completely delayed, with no part getting ahead. It's not some photons doing something with a certain probability and then causing a macroscopic effect once the probability goes towards 1. 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. |
| |
| ▲ | adaml_623 23 minutes ago | parent | prev | next [-] | | How can something travel faster than light? 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) | |
| ▲ | georgemcbay 24 minutes ago | parent | prev [-] | | They didn't word that very well. 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." | | |
| ▲ | zhivota 3 minutes ago | parent [-] | | Ah, so it's not faster than c, it's faster than light's speed inside the medium. This makes a lot more sense. |
|
|
|
| ▲ | baxtr 40 minutes ago | parent | prev | next [-] |
| In water! "In water" is the "In mice" equivalent for physics. |
| |
| ▲ | sigmoid10 33 minutes ago | parent | next [-] | | Technically it's any medium. The lower the refractive index, the closer the particle needs to travel to the speed of light in vacuum. But you can for example measure Cherenkov Radiation in the air (where n~=1.0003 or 99.97% of c) from highly energetic cosmic rays. https://en.wikipedia.org/wiki/Imaging_atmospheric_Cherenkov_... | |
| ▲ | adaml_623 20 minutes ago | parent | prev [-] | | Not really equivalent because physics can model the difference between "in water" and vacuum quite well. Definitely far better than biologists understand mice and humans |
|
|
| ▲ | fbn79 20 minutes ago | parent | prev | next [-] |
| 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 |
|
| ▲ | intrasight 26 minutes ago | parent | prev | next [-] |
| > 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? |
| |
| ▲ | dguest 9 minutes ago | parent | next [-] | | It's exactly 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". | |
| ▲ | adaml_623 22 minutes ago | parent | prev [-] | | I have that question as well! |
|
|
| ▲ | HPsquared 36 minutes ago | parent | prev | next [-] |
| I wonder if there could be something other than vacuum, in which light would travel faster. |
| |
|
| ▲ | freitzzz 20 minutes ago | parent | prev | next [-] |
| Not a science guy per se, is this blue the same blue in the radioactive accident in Goiânia’s? |
|
| ▲ | HelloUsername 13 minutes ago | parent | prev [-] |
| Completely normal phenomenon |
