Cherenkov Radiation
iaea.org
iaea.org
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?"
I meant it that the poster, changed the original title, what is a kind of editorializing. I thought of calling on the mods to edit it back, but judged that would be a kind of... semi-passive aggressiveness.
could you explain this? the comment you're replying to, how is it passive-aggressive? seems more regular-aggressive to me.
And thanks!
That directionality is very useful. The directionality of a muon or electron produced from a neutrino interaction is correlated with the direction of the incident neutrino. That's interesting for the astrophysical kind of questions that IceCube is addressing, and it's also useful in other contexts (background rejection for fundamental physics searches, monitoring of nuclear reactors via neutrinos, ...).
Cherenkov radiation is indeed polarized, but to my knowledge that hasn't ever been made use of.
Tangentially: there's also some really cool work being done on detectors which utilize both Cherenkov and scintillation light at the same time, exploiting the directional (and other) differences to extract the most information possible out of each event.
There are very good illustrations of this if you search for "Super-Kamiokande Cherenkov ring." That's a Japanese neutrino telescope with a bunch of huge photomultipliers lining the walls of a giant underground cistern filled with water. Ice Cube has similar light detectors, probably from the same Japanese manufacturer, that were lowered on strings into wells melted into ice.
Additionally, they can use telescopes pointed at the atmosphere, using the atmosphere as a calorimeter, basically. The primary signal those telescopes look for is fluorescence, but when the direction of travel points at the telescope, cherenkov light far outstrips it in brightness, so it has to be included in the event reconstruction.
Most prominent contemporary example is the Pierre Auger Observatory https://auger.org
I worked on HiRes/Telescope Array in Utah (Not CTA).
We talk about the sonic boom as something that happens when we travel faster than the speed of sound. We don’t specify “in the same medium you’re traveling in”, and noone reasonable thinks “it’s really hard to travel faster than the sound in carbon”.
You’ve added “speed of light” in your reading and concluded that’s what the OP wanted to imply.
I think that was unfair, and distracted from the rest of your comment.
I, for one, didn’t know what the cherenkov radiation is, and when reading the title thought “oh, so it’s a sonic boom for light? That’s so cool!”
I disagree with their quibble about the title being intentionally misleading but, meh, I've picked stranger technically correct hills to die on, the rest of their comment is interesting, it's fine.
Because of science fiction, and lack of education in much of the world (looking at you USA), the title can easily make certain people think we discovered new science that allows us to travel faster than light. Especially for folks who only look at headlines.
The whole thing is a silly quibble. But sensationalized headlines get better engagement, which we can all see here.
I've seen Sabine Hossenfelder mentioning some papers but without the gory details.
But she also notably said:
>> A lot of research [in] the foundations of physics is now pseudocience. It hasn’t followed the scientific method for decades.
What experiments you're talking about?
Citation needed
The late professor Roland Winston worked on this problem and discovered a geometry that could concentrate the light from a diffuse source like Cherenkov radiation to a detector with near-ideal performance ( https://doi.org/10.1063/1.1720428 ). It turns out that efficiently transferring light from diffuse sources has applications far beyond detecting Cherenkov radiation, so Winston founded the field of Nonimaging Optics and spent much of the rest of his carreer on developing the foundations of the field and on bringing together a community of scientists who would work on carrying the field forwards.
I do research in this field myself, and I find the optics and principles behind it endlessly fascinating
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 then reconstructing the original particle properties.
There is even a company selling Cherenkov imager for surface guided dose visualization: https://visionrt.com/applications-for-sgrt/cherenkov-imaging...
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! (FYI EM waves are more complicated like this and involve electric and magnetic fields evolving together).
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.
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.
3Blue1Brown has a beautiful animation for this phase kickback here: https://youtube.com/shorts/XIW-2ykgVPI?si=PJWiAC2BO7_xP0S6
Thanks for the link though, love 3b1b.
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.
So everybody thinks these are far more different than they really are.
But the wonder is that water has "light rays", 1d solitons in a 3d world (ie. under water). Dolphins love making them and playing with them. It's possible to make them with your hand in a pool, it's just pretty hard, but with a bit of practice. You need to hold your hand flat above the water, splash down, then retract your hand hard and get out of the way. The faster you retract your hand the further they'll go (but you need time to get out of the way). If you do it right a ring of water bubbles will go into a straight line several meters. Dolphins make them go hundreds of meters, and play with them, seemingly for fun. They aim them at eachother and pass them along (these don't have the same geometric structure as light rays, just the same movement. Except that they trap bubbles and so they "fall upward", especially when they slow down)
Water has "particle-antiparticle soliton pairs". You stick your hand, held flat, 90 degrees to the surface, half submerged, and you move it through the water, parallel to the surface. You do it right, 2 vortices will leave and move through the water. You can see circular shadows move over the bottom of the pool (because of the dimple shape where the vortex meets the surface). These you can probably get to the other side of a quiet pool if you try hard.
According to my math, it should be possible to make a version of this where the vortices rotate around each other. And you should be able to make any even number of vortices (like 4, 6, 8, or 800 for that matter) but I've never done even 2 rotating, nor have I been able to make 4 move together.
Water has 2d solitons in 2d as well, but you can't make just them. If you make 2 waves intersect at exactly 90 degrees, every so often the intersection point will "leave" on it's own. A "hill" on the water will start moving through the water, and you'll swear to God something is moving below the water, but there's nothing there but the wave. But this is almost impossible. You can try in a huge pool (ie. no reflections), and nobody but you in there.
It's weird to think about the properties water has that the electric field does not appear to have. For instance, water has a surface, which reflects solitons. Is there a surface in the electric field somewhere? Would it be a mirror in space, that is not just a perfect reflector of light rays but of matter too?
> 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.
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)
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.
But still I think saying "the primary wave gets absolutely and completely delayed" is not helpful. Using a wave description as in Feynman's lecture[1] is more enlightening: the incoming wave travels at "full speed" through the medium, but doing so it interacts with atoms such that they emit an additional wave, and the sum is a slower wave.
You can say it's the same since there's only one electric field in space and so the only "real wave" is the sum of all effects. But I find it quite helpful to think that one of the components in this sum is the original wave traveling at the speed of light in vacuum, also in the space occupied by the medium.
I don't think that can be strictly true. No matter how dense the material is some photons have a chance to get through unimpeded through something like tunelling. Practically unlikely but mathematically possible.
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."
What Neutrino detector measure is the Cherenkov light emitted by secondary particles that are created when a Neutrino interacts with the matter inside the detector.
E.g. a muon Neutrino reacts with a water molecule producing a muon, which is a charged particle and thus emits Cherenkov light.
> 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.
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.
At one point in the tour, they turned on the reactor, while we stood along the edges of the pool it was immersed in. Literally all that separated us from the magic of fission was about 5-6 meters of water. I still remember the electric blue glow of the Cherenkov radiation. Even with decades of life and experience and education between now and then, its hard to describe the psychic impact of observing with my own eyes something that I had heretofore understood to be impossible. Something akin to seeing Narnia through the wardrobe for the first time.
I was already into physics at that time (I had shadowed a sibling for a day at the University of Washington, and got to attend a lecture about nuclear fission a few years before in the physics-for-liberal-arts-majors course she was taking at that time), but this was quite something else. All of that to say, I already understood that dragons exist, in a manner of speech, but there's a difference between understanding it and feeling one's breath on your face.
"In water" is the "In mice" equivalent for physics.
https://en.wikipedia.org/wiki/Imaging_atmospheric_Cherenkov_...
Like if I am walking North-East, I can change direction and travel in a more northerly direction or more easterly direction, but if I’m traveling North, I can’t change directions to travel any more in the northerly direction. I’m already traveling 100% in the northerly direction.
Unless you saw it from really close, in which case it's too late and you should probably relax, sit down, have a drink, call your loved ones...
If you see a noticeable cherenkov effect from something that close that wasn’t heavily shielded by something like water, your camera sensor will likely be destroyed and you’d be dead pretty soon.
“Worst Ride Ever”
https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLig...
That's like the vaguest description of anything ever. Is physics a stealth startup?
Why does it specifically happen when particles travel faster than light in a given medium? There's no glow for particles moving slower?
Similarily I don't see how it explains the glow. Photons get generated regardless of whether they pile up or not. It's a consequence of particles bumping into atoms not the whatever speed of light might be in this medium.
How is piling up important?
For a slower than light particle, you also get emission, but it is completely random and thus does not give the well defined emission in a cone of Cherenkov radiation.
For a faster than light particle, the spherical suddenly line up to form a cone:
https://en.wikipedia.org/wiki/Cherenkov_radiation#/media/Fil...
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".
But if they say that light is fastest in vacuum, slower elsewhere, why can they then say that other energy variants would move faster? They'd still be objectively slower than light in vacuum. This is like saying my bicycle is faster than a Ferrari if the latter is stuck in mud or a pit.
Edit: Just noticed that others such as u/nuccy also pointed that out. Agreed. The title is wrong.
- classical (Maxwell): electrons in the medium oscillate out of phase in response to the incident wave; this results in a new wave, propagating somewhat slower
- quantum (QED): incident photon couples to matter excitation (an exciton), producing a pseudo-particle called a polariton; it has a non-zero effective mass, resulting in propagation slower than c
As mentioned above, it absolutely isn't "absorbing and re-emitting". If that was the case, transparent materials would not exist (only milky/clouded ones), as re-emission is in random direction. However, photon propagation in the Sun is just that, a (very long, counting from the core to photosphere) random walk.
I don’t mean in terms of the speed of light in a vacuum. Just seems weird that something with mass could outpace something without… Why are the photons so slow?
It's more about how fast communication can happen within the medium. The Cerenkov radiation happens because electrons can't "find out" about the charged particle coming through, creating a shock, similar to a sonic blast or the wake of a boat.
Hopefully I got this more or less right... I build detectors to look for Askarayan emission (a related phenomenon in the radio) but I'm definitely not a theorist :).
In a medium light slows down. Particles in the same medium can travel faster than the slowed down light. But nothing ever breaks the barrier of C.
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
Only the charged particle that induces the emission travels faster than the local speed of light.
The new reactor was of the swimming pool type, and seen from the wraparound gallery above, you could easily mistake it for one. Except for the blue shimmer in the water.
Remembering my studies, my head went hot and cold. Hadn’t they said the reactor wasn’t operational yet? Or had I just assumed, because of the open day? ub So I hesitantly approached our guide and asked about the blue light, to which he answered in the most casual way you can imagine:
"Oh, that’s because of the Cherenkov Radiation."
Pause.
Laughter. Seeing the doubts in my eyes he had just been messing with me, and they had deliberately installed blue lights there to make the experience more realistic for the open day