7 ms·
Cherenkov Radiation
- baxtr 6d agoIn water! "In water" is the "In mice" equivalent for physics.
- sigmoid10 6d agoTechnically 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_telescope https://en.wikipedia.org/wiki/Imaging_atmospheric_Cherenkov_...
- adaml_623 6d agoNot really equivalent because physics can model the difference between "in water" and vacuum quite well. Definitely far better than biologists understand mice and humans
- nuccy 6d agoThe 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) https://en.wikipedia.org/wiki/MAGIC_(telescope) or https://en.wikipedia.org/wiki/VERITAS https://en.wikipedia.org/wiki/VERITAS or https://en.wikipedia.org/wiki/High_Energy_Stereoscopic_System https://en.wikipedia.org/wiki/High_Energy_Stereoscopic_Syste... or https://en.wikipedia.org/wiki/Cherenkov_Telescope_Array_Observatory https://en.wikipedia.org/wiki/Cherenkov_Telescope_Array_Obse...
- Betelbuddy 5d ago>> The title (likely intentionally) is misleading, That is because that is not the title...the title is: "What is Cherenkov Radiation?"
- mmmattt 5d agoHe obviously meant the title of the HN post? Why not straight up say that the article title is different and avoid the passive aggressiveness.
- Betelbuddy 5d agoI 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.
- jefftk 5d agoI don't think there's anything 'passive' about specifically asking for what you think should be done!
- JosephRedfern 5d agoIt came of kinda passive aggressive to me, to be honest!
- pasquinelli 5d ago> 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.
- subscribed 4d agoI see the passive aggressiveness only in your response, not the parent.
- HPsquared 6d agoI wonder if there could be something other than vacuum, in which light would travel faster.
- anon48293 6d agoKind of, with trickery. https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html https://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLig...
- ttyyzz 6d agoProbably 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.
- aidenn0 5d agoTechnically, 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 https://en.wikipedia.org/wiki/Negative-index_metamaterial
- chinathrow 6d ago> How can something travel faster than light? > Nothing can travel faster than the speed of light in a vacuum. However, in other mediums, particles can potentially move faster than light. For instance, while in water, light would instantly slow down to 75% of its normal speed, but there are other particles that don’t slow down as much and end up moving faster than light. Whenever that happens, a blue or violet glow occurs. After reading this answer, I was not any wiser.
- hdgvhicv 6d agoIn water photons travel at say 200,000km a second. Neutrinos travel at nearly 300,000km a second. That’s causes a blue glow. Which is how neutrino detectors work.
- chinathrow 5d agoThanks - but I fell over this sentence: > but there are other particles that don’t slow down as much and end up moving faster than light. Not slowing down as much I can understand but shouldn't it read as "but there are other particles that don’t slow down as much OR EVEN end up moving faster than light."
- chinathrow 5d agoEDIT: Got it, faster than light IN THAT MEDIUM.
- Betelbuddy 5d agoYes ...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...
- pfdietz 5d ago> 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.
- intrasight 6d ago> 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?
- adaml_623 6d agoI have that question as well!
- dguest 6d agoIt'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".
- prathje 6d agoIt 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.
- prathje 6d agoIn 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 https://youtube.com/shorts/XIW-2ykgVPI?si=PJWiAC2BO7_xP0S6
- DoNotMindMe 5d agoFYI/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.
- prathje 5d agoThanks!
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- fbn79 6d agoTo 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
- freitzzz 6d agoNot a science guy per se, is this blue the same blue in the radioactive accident in Goiânia’s?
- voidUpdate 5d agoAccording 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"
- deleted 5d ago[deleted]
- HelloUsername 6d agoCompletely normal phenomenon
- gste 5d agoThis 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
- margorczynski 5d agoI 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.
- maxnoe 5d agoThe 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.
- fnands 5d agoExactly! I visited HESS in Namibia last week, so Cherenkov telescopes are on the top of my mind right now.
- nxobject 4d agoAs 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.
- fdekerm 4d agoIt's also used in radiation oncology for real-time dose verification: https://doi.org/10.1016/j.prro.2025.09.004 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/ https://visionrt.com/applications-for-sgrt/cherenkov-imaging...
- weinzierl 5d agoI 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
- bit_rot73 5d agoLight still propagates at c inside a medium; the apparent slowdown is just phase kickback from re-emission.
- post-it 5d agoWe know light is not being absorbed and re-emitted, because re-emission sends light in a random direction.
- loopies 5d agoThat 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?
- gattr 5d agoSimplifying 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.
- loopies 5d agoThank 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.
- shevy-java 5d ago> 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.
- fad_fusion_law 5d agoGreat analysis. The systematic approach to this problem is well-thought-out.
- scotty79 5d ago> 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?
- mytailorisrich 5d agoI think a common analogy is that this is a bit like the sonic boom when something travels faster than sound in that medium.
- scotty79 5d agoSonic 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?
- maxnoe 5d agoVery 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/File:Cherenkov_radiation-animation.gif https://en.wikipedia.org/wiki/Cherenkov_radiation#/media/Fil...
- scotty79 5d agoThank you. That's a very good explanation.
- hakonjdjohnsen 5d agoA 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 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
- jacquesm 5d agoI 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.
- mag7269 5d agoThe Cherenkov effect, completely normal phenomenon, it can happen with minimal radiation. All you need, I’ve been told, is 3.6 Roentgens.
- threethirtytwo 5d agoTo 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.
- jimmcslim 5d ago“I’m not saying it is tachyons… but…”
- rbanffy 5d agoA 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...
- toolsmax 5d agoIn that case, I'm afraid your phone possibly won't function
- rbanffy 4d agoWell… it might be a Nokia 2260
- delecti 5d agoOr unless it's in water, in which case you're actually probably fine.
- rbanffy 4d agoIf you swim too close to it, you might as well not bother to surface.
- delecti 4d agoYes, "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).
- xattt 5d agoShort of LINAC irradiator ride-through videos on YouTube, are there any photos of this effect in air from a point source?
- lazide 5d ago
- GlobalFrog 5d agoSeveral 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!
- 542354234235 5d agoI 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.
- petsfed 5d agoI'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.
- shabaduu 5d agoobligatory Demon Core blue light reference: https://en.wikipedia.org/wiki/Demon_core#In_popular_culture https://en.wikipedia.org/wiki/Demon_core#In_popular_culture
- lowestprimate 5d agoCan Cherenkov radiation carry information? If it can does this mean information can travel faster than light? Question from a physics newbie
- teravor 5d agoit's only faster in a medium, which technically isn't even light but rather electron interactions.
- maxnoe 5d agoCherenkov 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.
- BobbyTables2 5d agoHow 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?
- cozzyd 5d agoThe 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 :).