Cherenkov Radiation - traveling faster than light
40 points - today at 8:42 AM
SourceIt 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.
JumpCrisscross
today at 10:12 AM
> 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)
Speed of causality is a way more intuitive term. Massless particles travel at the speed of causation in a vacuum. Not necessarily in a medium. Trying to work backwards from speed of light to gravity propagating is tortured; understanding that gravity can't cause an effect faster than causality itself is more direct.
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
> Single EM waves propagate with exactly this speed ...
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:
https://www.youtube.com/watch?v=909o_kbCdFgll
Circular (as in 2d circular in 3d space) propagation in EM waves is like 2 waves in exactly the same location and direction, with opposite rotation along the axis. Which could be extremely useful since they propagate like single waves. In other words, there is a non-circular spectrum ... AND a circular spectrum. So, if we modify all radios we have double the spectrum.
This reminds me of prescientific explanations of the sun and stars
Like the best thing we have to remark on is the fact it is blue when this is probably the least remarkable thing about it
chinathrow
today at 9:28 AM
>
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.
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
today at 10:00 AM
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."
chinathrow
today at 10:00 AM
EDIT:
Got it, faster than light IN THAT MEDIUM.
Betelbuddy
today at 10:07 AM
Yes ...it deserves to be flagged...This is the type of article we would never waste time with at the Vulcan Academy of Science. But you guys there at the Star Trek Academy, always had looser standards...
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
today at 9:57 AM
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 once you passed sufficient matter.
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
today at 9:37 AM
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
today at 9:36 AM
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."
Ah, so it's not faster than c, it's faster than light's speed inside the medium. This makes a lot more sense.
In water!
"In water" is the "In mice" equivalent for physics.
sigmoid10
today at 9:28 AM
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
today at 9:40 AM
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
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
today at 9:35 AM
> 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?
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
today at 9:38 AM
I have that question as well!
HPsquared
today at 9:24 AM
I wonder if there could be something other than vacuum, in which light would travel faster.
Probably not, to get light to move faster, you don't need a new medium - you just need less of the universe getting in its way.
Not a science guy per se, is this blue the same blue in the radioactive accident in Goiânia’s?
voidUpdate
today at 10:11 AM
According to the all-knowing Wikipedia, "The exact mechanism by which the blue light was generated was not known at the time the IAEA report of the incident was written, though it was thought to be either ionized air glow, fluorescence, or Cherenkov radiation associated with the absorption of moisture by the source; a similar blue light was observed in 1988 at Oak Ridge National Laboratory in the United States during the disencapsulation of a caesium-137 source"
HelloUsername
today at 9:47 AM
Completely normal phenomenon
sweswas2
today at 10:06 AM
Core seems fine to me!