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].
The article has a section about what it can be used for, but only mentions the uses of the IAEA.
Of course I am biased because I work in the field, but the by far most wide reaching application of Cherenkov radiation is in the detection of high energy particles, particularly in astrophysics.
- Imaging Atmospheric Cherenkov telescopes detect the Cherenkov radiation emmited in the atmosphere when a high energy cosmic ray or gamma ray creates an air shower
- Water Cherenkov Detectors detect Cherenkov light when the secondary particles of these air showers reach water tanks on the ground
- Neutrino telescopes like kamiokande, Icecube and km3net detect Cherenkov radiation in water or ice produced by secondary particles produced by the rare interactions of Neutrinos in their detector volumes
Modern, high energy astrophysics is all about detecting different kinds of Cherenkov radiation and the reconstructing the original particle properties.
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.
I dont think there can be intuition for this. How can one even imagine an electron "flying" through space, there is no measurable thing that travels, we can only see the disturbance of the surrounding space caused by it. But what is the it, does it maybe live in other dimension not accessible to us, or is there no it at all ? I gave up as a layman trying to understand any of this. Sure, there is math and you can somehow make some mental models involving 3d graphics and all, but that is not actually what is going on down there.
> 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. (Usually. Someone else brought up solitons.) 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.
> 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:
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.
Love this, good point!
It really shows how difficult it is to give an easy explanation that generalizes well...
So, although quite abstract, the fundamental propagation of changes in the EM field at the speed of causality is a good enough approximation?
> 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.
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.
> In water photons travel at say 200,000km a second.
It really depends on the energy of the photons. There is "dispersion". It's the same effect that causes a prism to split white light into different wavelengths.
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.
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.
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."
I think a problem is that because of historical reasons the speed of light is used interchangeably to something much more fundamental - the maximum speed at which information can propagate in space. Which is of course the speed of light in a vacuum but a better approach is the inverse - light in a vacuum moves at the maximum speed possible in our universe.
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.
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
> 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.
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".
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"
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...
>> The title (likely intentionally) is misleading,
That is because that is not the title...the title is: "What is Cherenkov Radiation?"
He obviously meant the title of the HN post? Why not straight up say that the article title is different and avoid the passive aggressiveness.
The article has a section about what it can be used for, but only mentions the uses of the IAEA.
Of course I am biased because I work in the field, but the by far most wide reaching application of Cherenkov radiation is in the detection of high energy particles, particularly in astrophysics.
- Imaging Atmospheric Cherenkov telescopes detect the Cherenkov radiation emmited in the atmosphere when a high energy cosmic ray or gamma ray creates an air shower - Water Cherenkov Detectors detect Cherenkov light when the secondary particles of these air showers reach water tanks on the ground - Neutrino telescopes like kamiokande, Icecube and km3net detect Cherenkov radiation in water or ice produced by secondary particles produced by the rare interactions of Neutrinos in their detector volumes
Modern, high energy astrophysics is all about detecting different kinds of Cherenkov radiation and the reconstructing the original particle properties.
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.
I dont think there can be intuition for this. How can one even imagine an electron "flying" through space, there is no measurable thing that travels, we can only see the disturbance of the surrounding space caused by it. But what is the it, does it maybe live in other dimension not accessible to us, or is there no it at all ? I gave up as a layman trying to understand any of this. Sure, there is math and you can somehow make some mental models involving 3d graphics and all, but that is not actually what is going on down there.
> 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. (Usually. Someone else brought up solitons.) 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.
Love this, good point! It really shows how difficult it is to give an easy explanation that generalizes well... So, although quite abstract, the fundamental propagation of changes in the EM field at the speed of causality is a good enough approximation?
> 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.
> In water photons travel at say 200,000km a second.
It really depends on the energy of the photons. There is "dispersion". It's the same effect that causes a prism to split white light into different wavelengths.
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."
EDIT:
Got it, faster than light IN THAT MEDIUM.
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.
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.
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)
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.
I think a problem is that because of historical reasons the speed of light is used interchangeably to something much more fundamental - the maximum speed at which information can propagate in space. Which is of course the speed of light in a vacuum but a better approach is the inverse - light in a vacuum moves at the maximum speed possible in our universe.
In water!
"In water" is the "In mice" equivalent for physics.
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_...
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
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
> 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".
I have that question as well!
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.
Kind of, with trickery.
https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLig...
Not a science guy per se, is this blue the same blue in the radioactive accident in Goiânia’s?
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"
Completely normal phenomenon
Core seems fine to me!