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].
I did not meant it in that way...and I would bet, most did not interpret it as such...the passive aggressiveness is coming from you.
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.
Your comment is a good reminder of why I love the HN community so much. Having acccess to knowledge in specific domains, like in this case, is truly a gem.
Maybe the coolest (heh) application of Cherenkov radiation is neutrino telescopes like IceCube, detecting tiny flashes of light in cubic kilometers of ultra-clear Antarctic ice, caused by vanishingly rare interactions between neutrinos and ordinary matter – and to exclude non-neutrino interactions, they look down, using the entire Earth as a shield guaranteed to stop anything that’s not a neutrino!
While I knew that IceCube existed it didn't occur to me until know that I have no idea how they filter out collisions from particles that comes from above or sideways.
Is the resulting flash of light not omnidirectional?
Is it polarized differently?
One of the amazing things about Cherenkov radiation is that it is not omnidirectional. There are processes (fluorescence, or scintillation, for example) which result in isotropic emission of light from materials. But Cherenkov light is emitted along a conical surface about the axis of the charged particle's direction of travel. As a sister comment said, muons tend to move linearly, so this light stacks up in a particular direction. Electrons, particularly at lower energy, scatter more, and that stacking gets washed out a bit.
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.
Right, so without knowing much about physics I'm guessing that momentum is preserved and IceCube works a little bit like a giant cloud chamber but looks for trails of Cherenkov radiation rather than an ionized particle trail.
The cone of Cherenkov radiation produces arcs of "bright pixels" in the detector.
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.
I believe they reconstruct the paths simply based on the timing of detection events. Muons created by neutrino interactions travel more or less linearly and create a "bow shock" of Cherenkov radiation; it's not a single flash of light, just like a sonic boom is not a single bang. The directions of electron and tau neutrinos are more difficult to reconstruct because the associated particles tend to scatter and bounce around.
Cherenkov radiation is also a key mechanism behind ultra high energy cosmic ray (UHECR) detectors. UHECR are understood to be nuclei, not gamma rays. For ground based detectors, light tight water tanks are fitted with photo multipliers to capture the flash of cherenkov light even secondary particles pass through the water.
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
Not just gamma rays, cosmic rays too, although they filter the UV and are targeting nitrogen fluorescence and catch the cerenkov overlap. I’m sure there’s many people here who have worked on them.
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!”
Nothing can travel faster than light through vacuum, so that bit is implied as well.
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.
Arguably there's a lot of people who don't know nothing is faster than light in a vacuum.
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.
Yeah, but what is a medium? Every material is basically empty space with an atom here and there. So there is not a hard boundary for a particle to instantaneously slow down from speed of light in the air to speed of light in water. Some of them will travel a bit deep before hitting their first H or O or electron, or some salt ion. Anyway, it won't travel faster than c.
It's funny because I was reading about Cherenkov radiation just yesterday and the same statement was made (although with a proper clarification afterwards).
Why do you believe that like if you are measuring it maybe you are doing it wrong. Seems to me that we need this light fastest X to hold up a load of physics even when there has been recently numerous experiments which challenge this premise.
A fun fact about Cherenkov radiation is that research on making efficient Cherenkov detectors in the 1960s led to the development of optical principles still used to design illumination systems and solar concentrators today.
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
I worked for a little bit with a group making optical sieves, this stuff is mindblowing to a non-trained-in-the-art outsider. I can easily see losing a couple of decades in that rabbit hole.
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 then reconstructing the original particle properties.
As an aside, open core research reactors that glow are a very cool marketing tool for STEM. I went to a college with an open-core TRIGA run by undergraduates that loved to pulse it. It never failed to awe high school students.
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! (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.
> 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.
FYI/PSA: If you leave the tracking portion of the youtube link (e.g. after the '?') it is trivial to track back to the youtube account of the person who clicked the 'share' icon. Always best to remove the '?si=...' part before publicly sharing.
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.
> 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?
The issue is many kinds of solutions exist, but aren't common in different fields. On a guitar string, waves are 1d waves in a 1d world. In water everyone knows only standard waves. Which are 1d solitons in a 2d "world". In the electromag field everyone only knows light, which are 2d solitons in a 3d world. Electrons have an axis of rotation that does not point in any spatial or temporal direction.
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.
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)
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.
Looking it up I was actually remembering from Feynman's QED book (path integral approach). I don't have it at hand but I think it's a description based on photon particles. Anyway I agree my description with photon travel "between atoms", emission and absorption was quite bad (especially if you think absorption and emission as slow incoherent processes instead of a general way of describing interactions which is what I meant).
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."
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.
Neutrinos are neutral particles, they do not carry an electromagnetic charge. They thus do not cause any Cherenkov light themselves.
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.
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...
> 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.
I've no idea if they still do it, or if this was an option open to the general public or if it was a special thing for our group, but once upon a time, in my early teens, I got to tour NIST's test reactor in Boulder, Colorado.
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.
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
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
Several comments here mention that nothings goes faster than light in a vacuum, which is right.
But...
Putting aside all considerations of causality, if a particle was to go faster than light, it would also emit a vacuum Cherenkov radiation, as this particle would go faster than light. That would be a kind of supersonic bang. Some theories about this say that when a particle going faster than light, it loses its energy and emits photons.
IANAP (yes, I am not a physicist), but I would love to hear a theory about how those FTL particles could be detected if they were to exist, and what could be the observation, probably coming the this vacuum Cherenkov effect. Again, I know this is against all physics, but the theory would be cool!
I am willing to be wrong on this, as I’m not a physicist and I’m going from memory. But I thought that nothing could travel faster than light because spacetime is interconnected. So as something travels “faster” it is simply moving more through spatial dimensions and less through time dimension. Light travels completely through spatial dimensions, leaving no movement through time, which is why nothing can travel “faster” than that.
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.
All the ones I see are reactors through water shielding, in which case you’d be fine.
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.
Yes, "too" close by definition would be a problem. But even a meter of water between you and the source absorbs 99.994% of the radiation (down to 6 thousandths of a percent vs in air, or by a factor of about 1/16000).
Technically, no. However there are metamaterials in which certain wavelengths exhibit a negative index of refraction, which cause light to behave in some ways similarly to if it were moving faster.
> 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. In order to regain equilibrium, those atoms release photons – the types of particles that compose visible light, creating a “shock-wave” of visible light.
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?
Sonic boom doesn't explain what causes sound only how it piles up.
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.
Very shortly summarized, only if the particle is faster than the local speed of light, you get constructive interference between many atoms that were polarized by the moving particle.
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:
> 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".
> but there are other particles that don’t slow down as much and end up moving faster than light
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.
How is it possible for particles (with mass) to exceed the speed of (massless) photons in a medium?
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?
The photons don't slow down. It's the phase velocity that slows down because the electrons in the medium rearrange themselves in the presence of electromagnetic radiation, causing interference reducing the phase velocity of a wave. This is why the dielectric constant is related to the polarizability of a medium.
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 :).
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.
That is interesting. But then what is the explanation? Medium is not really anything concrete, we call that a collection of atoms isn't it?
By default there is only one medium, the vacuum of space, in which there can be various densities of "stuff", particles/atoms/molecules.
So then, it becomes some sort of quantum thing, a probability of photons bumping/being captured/emitted or?
Simplifying from memory, light propagation in medium can be treated on two levels (some phenomena require the 2nd, quantum, level):
- 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.
Thank you, that is very interesting!
I remember seeing something about absorption/emission and seemed to make some logical sense. But indeed if emission is in a random direction then transparent materials wouldn't make sense.
I had learned about Cherenkov Radiation and its characteristic blue color at university. When, a couple of years later, the university had finished building a new research reactor, they had an open house day with guided tours. Of course I’d take one!
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
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.
I see the passive aggressiveness only in your response, not the parent.
I did not meant it in that way...and I would bet, most did not interpret it as such...the passive aggressiveness is coming from you.
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.
I don't think there's anything 'passive' about specifically asking for what you think should be done!
> the passive aggressiveness is coming from you.
could you explain this? the comment you're replying to, how is it passive-aggressive? seems more regular-aggressive to me.
It came of kinda passive aggressive to me, to be honest!
Your comment is a good reminder of why I love the HN community so much. Having acccess to knowledge in specific domains, like in this case, is truly a gem.
And thanks!
Maybe the coolest (heh) application of Cherenkov radiation is neutrino telescopes like IceCube, detecting tiny flashes of light in cubic kilometers of ultra-clear Antarctic ice, caused by vanishingly rare interactions between neutrinos and ordinary matter – and to exclude non-neutrino interactions, they look down, using the entire Earth as a shield guaranteed to stop anything that’s not a neutrino!
While I knew that IceCube existed it didn't occur to me until know that I have no idea how they filter out collisions from particles that comes from above or sideways. Is the resulting flash of light not omnidirectional? Is it polarized differently?
One of the amazing things about Cherenkov radiation is that it is not omnidirectional. There are processes (fluorescence, or scintillation, for example) which result in isotropic emission of light from materials. But Cherenkov light is emitted along a conical surface about the axis of the charged particle's direction of travel. As a sister comment said, muons tend to move linearly, so this light stacks up in a particular direction. Electrons, particularly at lower energy, scatter more, and that stacking gets washed out a bit.
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.
Right, so without knowing much about physics I'm guessing that momentum is preserved and IceCube works a little bit like a giant cloud chamber but looks for trails of Cherenkov radiation rather than an ionized particle trail.
The cone of Cherenkov radiation produces arcs of "bright pixels" in the detector.
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.
I believe they reconstruct the paths simply based on the timing of detection events. Muons created by neutrino interactions travel more or less linearly and create a "bow shock" of Cherenkov radiation; it's not a single flash of light, just like a sonic boom is not a single bang. The directions of electron and tau neutrinos are more difficult to reconstruct because the associated particles tend to scatter and bounce around.
Cherenkov radiation is also a key mechanism behind ultra high energy cosmic ray (UHECR) detectors. UHECR are understood to be nuclei, not gamma rays. For ground based detectors, light tight water tanks are fitted with photo multipliers to capture the flash of cherenkov light even secondary particles pass through the water.
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
Not just gamma rays, cosmic rays too, although they filter the UV and are targeting nitrogen fluorescence and catch the cerenkov overlap. I’m sure there’s many people here who have worked on them.
I worked on HiRes/Telescope Array in Utah (Not CTA).
I don't think the title is that misleading.
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!”
One difference is sound can only travel through a medium, so that bit is always implied. Not so with light.
Nothing can travel faster than light through vacuum, so that bit is implied as well.
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.
Arguably there's a lot of people who don't know nothing is faster than light in a vacuum.
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.
Yeah, but what is a medium? Every material is basically empty space with an atom here and there. So there is not a hard boundary for a particle to instantaneously slow down from speed of light in the air to speed of light in water. Some of them will travel a bit deep before hitting their first H or O or electron, or some salt ion. Anyway, it won't travel faster than c.
It's funny because I was reading about Cherenkov radiation just yesterday and the same statement was made (although with a proper clarification afterwards).
-1 for Pedantic, but +1 for cool links to telescopes
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Why do you believe that like if you are measuring it maybe you are doing it wrong. Seems to me that we need this light fastest X to hold up a load of physics even when there has been recently numerous experiments which challenge this premise.
> there has been recently numerous experiments which challenge this premise
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?
>numerous experiments which challenge this premise.
Citation needed
A fun fact about Cherenkov radiation is that research on making efficient Cherenkov detectors in the 1960s led to the development of optical principles still used to design illumination systems and solar concentrators today.
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
I worked for a little bit with a group making optical sieves, this stuff is mindblowing to a non-trained-in-the-art outsider. I can easily see losing a couple of decades in that rabbit hole.
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 then reconstructing the original particle properties.
It's also used in radiation oncology for real-time dose verification: https://doi.org/10.1016/j.prro.2025.09.004.
There is even a company selling Cherenkov imager for surface guided dose visualization: https://visionrt.com/applications-for-sgrt/cherenkov-imaging...
Exactly! I visited HESS in Namibia last week, so Cherenkov telescopes are on the top of my mind right now.
As an aside, open core research reactors that glow are a very cool marketing tool for STEM. I went to a college with an open-core TRIGA run by undergraduates that loved to pulse it. It never failed to awe high school students.
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! (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.
> 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
FYI/PSA: If you leave the tracking portion of the youtube link (e.g. after the '?') it is trivial to track back to the youtube account of the person who clicked the 'share' icon. Always best to remove the '?si=...' part before publicly sharing.
Thanks for the link though, love 3b1b.
Thanks!
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.
> 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?
The issue is many kinds of solutions exist, but aren't common in different fields. On a guitar string, waves are 1d waves in a 1d world. In water everyone knows only standard waves. Which are 1d solitons in a 2d "world". In the electromag field everyone only knows light, which are 2d solitons in a 3d world. Electrons have an axis of rotation that does not point in any spatial or temporal direction.
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?
> 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.
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)
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.
Looking it up I was actually remembering from Feynman's QED book (path integral approach). I don't have it at hand but I think it's a description based on photon particles. Anyway I agree my description with photon travel "between atoms", emission and absorption was quite bad (especially if you think absorption and emission as slow incoherent processes instead of a general way of describing interactions which is what I meant).
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.
[1] https://www.feynmanlectures.caltech.edu/I_31.html
> with no part getting ahead
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.
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 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.
Neutrinos are neutral particles, they do not carry an electromagnetic charge. They thus do not cause any Cherenkov light themselves.
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.
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...
> 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.
I've no idea if they still do it, or if this was an option open to the general public or if it was a special thing for our group, but once upon a time, in my early teens, I got to tour NIST's test reactor in Boulder, Colorado.
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.
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
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
Several comments here mention that nothings goes faster than light in a vacuum, which is right. But... Putting aside all considerations of causality, if a particle was to go faster than light, it would also emit a vacuum Cherenkov radiation, as this particle would go faster than light. That would be a kind of supersonic bang. Some theories about this say that when a particle going faster than light, it loses its energy and emits photons. IANAP (yes, I am not a physicist), but I would love to hear a theory about how those FTL particles could be detected if they were to exist, and what could be the observation, probably coming the this vacuum Cherenkov effect. Again, I know this is against all physics, but the theory would be cool!
I am willing to be wrong on this, as I’m not a physicist and I’m going from memory. But I thought that nothing could travel faster than light because spacetime is interconnected. So as something travels “faster” it is simply moving more through spatial dimensions and less through time dimension. Light travels completely through spatial dimensions, leaving no movement through time, which is why nothing can travel “faster” than that.
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.
A general rule of thumb is that if something is glowing blue or has a blue halo around it, you should run in the opposite 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...
Short of LINAC irradiator ride-through videos on YouTube, are there any photos of this effect in air from a point source?
All the ones I see are reactors through water shielding, in which case you’d be fine.
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.
Astronauts have reported having difficulty sleeping due to cosmic-ray-induced Cherenkov flashes in their eye bulbs.
The best comment on those videos:
“Worst Ride Ever”
Or unless it's in water, in which case you're actually probably fine.
If you swim too close to it, you might as well not bother to surface.
Yes, "too" close by definition would be a problem. But even a meter of water between you and the source absorbs 99.994% of the radiation (down to 6 thousandths of a percent vs in air, or by a factor of about 1/16000).
In that case, I'm afraid your phone possibly won't function
Well… it might be a Nokia 2260
I wonder if there could be something other than vacuum, in which light would travel faster.
Technically, no. However there are metamaterials in which certain wavelengths exhibit a negative index of refraction, which cause light to behave in some ways similarly to if it were moving faster.
https://en.wikipedia.org/wiki/Negative-index_metamaterial
Kind of, with trickery.
https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLig...
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.
> 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. In order to regain equilibrium, those atoms release photons – the types of particles that compose visible light, creating a “shock-wave” of visible light.
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?
I think a common analogy is that this is a bit like the sonic boom when something travels faster than sound in that medium.
Sonic boom doesn't explain what causes sound only how it piles up.
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?
Very shortly summarized, only if the particle is faster than the local speed of light, you get constructive interference between many atoms that were polarized by the moving particle.
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...
Thank you. That's a very good explanation.
> 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!
> but there are other particles that don’t slow down as much and end up moving faster than light
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.
How is it possible for particles (with mass) to exceed the speed of (massless) photons in a medium?
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?
The photons don't slow down. It's the phase velocity that slows down because the electrons in the medium rearrange themselves in the presence of electromagnetic radiation, causing interference reducing the phase velocity of a wave. This is why the dielectric constant is related to the polarizability of a medium.
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 :).
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.
Light still propagates at c inside a medium; the apparent slowdown is just phase kickback from re-emission.
We know light is not being absorbed and re-emitted, because re-emission sends light in a random direction.
That is interesting. But then what is the explanation? Medium is not really anything concrete, we call that a collection of atoms isn't it? By default there is only one medium, the vacuum of space, in which there can be various densities of "stuff", particles/atoms/molecules. So then, it becomes some sort of quantum thing, a probability of photons bumping/being captured/emitted or?
Simplifying from memory, light propagation in medium can be treated on two levels (some phenomena require the 2nd, quantum, level):
- 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.
Thank you, that is very interesting! I remember seeing something about absorption/emission and seemed to make some logical sense. But indeed if emission is in a random direction then transparent materials wouldn't make sense.
To be clear nothing is traveling faster than C.
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.
Can Cherenkov radiation carry information? If it can does this mean information can travel faster than light? Question from a physics newbie
Cherenkov radiation is UV to visible blue light, it travels at the speed of light in the medium, which is lower than the speed of light in vacuum.
Only the charged particle that induces the emission travels faster than the local speed of light.
it's only faster in a medium, which technically isn't even light but rather electron interactions.
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
I had learned about Cherenkov Radiation and its characteristic blue color at university. When, a couple of years later, the university had finished building a new research reactor, they had an open house day with guided tours. Of course I’d take one!
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
The Cherenkov effect, completely normal phenomenon, it can happen with minimal radiation. All you need, I’ve been told, is 3.6 Roentgens.
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"
Great analysis. The systematic approach to this problem is well-thought-out.
Completely normal phenomenon
It can happen with minimal radiation
Core seems fine to me!
obligatory Demon Core blue light reference: https://en.wikipedia.org/wiki/Demon_core#In_popular_culture
“I’m not saying it is tachyons… but…”