Cherenkov Radiation: When Particles Break Light's Local Speed Limit
Cherenkov Radiation - traveling faster than light
Hacker News readers dissect the misleading title "traveling faster than light" and clarify that Cherenkov radiation occurs when a charged particle exceeds light's speed in a medium, not the cosmic speed limit c. They explain the blue glow, the sonic boom analogy, and why "speed of causality" is a better term.
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".
- nuccy
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...
- hakonjdjohnsen
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
- maxnoe
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.
- GlobalFrog
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!
- prathje
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.