Also spelled Čerenkov
August 3, 2012 6:40 AM   Subscribe

What happens when charged particles travel faster than the speed of light of the medium they're in? Cherenkov Radiation. Like this. Or this. Not this. More like this. Certainly not this. But definitely this or this or this. Not this.
posted by griphus (26 comments total) 12 users marked this as a favorite
 
And from 0:49 forward you can get an explanation for it at the best-kept secret in Southeast Portland.
posted by gingerest at 6:47 AM on August 3, 2012 [1 favorite]


It looks an awful lot like what happens when you put a gin and tonic (really just the tonic) under UV light. In fact that's how they shot the reactor scenes in K-19.
posted by Artw at 6:55 AM on August 3, 2012 [2 favorites]


Absolutely not this.
posted by weapons-grade pandemonium at 6:56 AM on August 3, 2012


travel faster than the speed of light

Er, no. It's when it travels faster than the phase velocity in the dielectric, not when it travels faster than the normal speed of a photon between that material. That is, it's the speed at which the wave peaks move, not the speed at which the photons move.
posted by atbash at 7:07 AM on August 3, 2012 [4 favorites]


Allowing scientists to build experiments like this to measure neutrino oscillations and look for supernovae and proton decay.
posted by lozierj at 7:08 AM on August 3, 2012


atbash, how should I rephrase the sentence in the FPP for accuracy? I'll ask the mods to change it, but I don't want to get the terminology wrong.
posted by griphus at 7:13 AM on August 3, 2012


Metafilter: Er, no.
posted by weapons-grade pandemonium at 7:18 AM on August 3, 2012 [2 favorites]


Wikipedia is phrased more or less the same, FWIW. I suspect it's the link placement highlighting "faster than the speed of light" that's problematic.
posted by Artw at 7:22 AM on August 3, 2012


how should I rephrase the sentence in the FPP for accuracy?

I think it's fine. Physicists talk about this phenomenon using those terms all of the time. Phase velocity vs. group velocity is really only for periodic or dispersive media.
posted by lozierj at 7:22 AM on August 3, 2012


I would just say faster than the speed of light in a medium. Yes, the photons are always moving at the speed of light in a vacuum, but it's clear what you mean, and even we physicists are not that pedantic.
posted by physicsmatt at 7:47 AM on August 3, 2012


Yeah, I think artw is probably right that it's the end of the link placement, which makes it seem to highlight part of the phrase.
posted by atbash at 8:03 AM on August 3, 2012


[Nudged the link per poster request. Carry on at substrate-appropriate speeds.]
posted by cortex (staff) at 8:12 AM on August 3, 2012 [4 favorites]


Physicist 1: Why the chicken cross the road?

Physicist 2: I don't know, why?

Physicist 1: Well, I can show you, but I'll need a perfectly spherical chicken and an infinite plane in a absolute vaccuum.
posted by blue_beetle at 8:44 AM on August 3, 2012 [2 favorites]


wait! don't forget..it's also like this.
posted by sexyrobot at 8:58 AM on August 3, 2012 [1 favorite]


Great post, though I hated the background music in the first link—what, kids these days can't follow scientific exposition without it? Anyway, it was neat seeing those flashes and hearing the reactions of onlookers. Pedantic LH note: in Russian, it's cherenKOV (stress on the last syllable), and Russians call it Vavilov-Cherenkov radiation (Cherenkov was working in Vavilov's lab when the phenomenon was discovered).

(Now I want to see Repo Man again, dammit.)
posted by languagehat at 9:30 AM on August 3, 2012 [2 favorites]


Awww...

Thanks, griphus.

My favorite kind of radiation.

You shouldn't have...
posted by PROD_TPSL at 9:34 AM on August 3, 2012


... not when it travels faster than the normal speed of a photon

Tachyonic Cerenkov radiation?
posted by benito.strauss at 9:48 AM on August 3, 2012


Are tachyons even a thing anymore?

I always liked the idea of faster-than-light, backward-in-time particles but I don't think they've been part of physics for a while.
posted by Artw at 9:52 AM on August 3, 2012


My favorite bit about Cherenkov radiation is the part where the Apollo astronauts had it happening INSIDE THEIR EYES.
posted by ckape at 9:52 AM on August 3, 2012


It was hushed up, but they all got superpowers after that.
posted by Artw at 9:54 AM on August 3, 2012


...they couldn't fly or lift especially heavy objects, but wherever they went, it was always too late -- they'd seen it all.
posted by ROU_Xenophobe at 10:51 AM on August 3, 2012 [3 favorites]


A decelerating charged particle will emit photons, called bremsstrahlung, or braking radiation, and Cherenkov radiation certainly has a braking effect, since its energy ultimately comes from the kinetic energy of the charged particle, though apparently accounting for only on the order of about 1/5000 of that kinetic energy.

But it's not clear to me whether Cherenkov radiation could be seen entirely or partially as the visible slice of the bremsstrahlung, which has a continuous spectrum with a minimum wavelength in the X-ray range for relativistic particles (limited by the kinetic energy of the particle) but no maximum, though looing at it like that might help explain the fact that it always seems to have the same bluish color by portraying Cherenkov radiation as an almost always similarly-shaped slice of the tail of a much larger spectrum, with the only exceptions being the very brief moment in the period of deceleration of a particle when the minimum bremsstrahlung wavelength passes through the visible spectrum.
posted by jamjam at 12:48 PM on August 3, 2012


Turns out Pavel Cherenkov is related to Pavel Chekov. I did not know that.
posted by Twang at 1:19 PM on August 3, 2012


Jamjam,
Not quite, though Cherenkov and Bremsstralung are related, as they are both caused by the acceleration of charged particles. However, bremsstralung is coming from the (de)accelerated charge directly, while cherenkov sort of a second-order effect coming from the motion of the charges in the medium perturbed by the moving charge.

You are correct that Cherenkov is a continuous spectrum, and we are only seeing a tiny part of that. It's increasing at short wavelengths (high energy), which is why we see it as blue (though you have to fold in the response function of the human eye to get this apparently, since it should really appear purple/violet. It's always interesting to me to see how non-trivial the transformation between "true color" (wavelength) and "perceived color" is). I've heard that Cherenkov light from glass is actually purple due to the different index of refraction (and probably a different dependence on wavelength of the index of refraction), but I can't seem to confirm this after cursory googling.

I did get to see Cherenkov light from a nuclear reactor once, and I can confirm that it is awesome. Alas, I did not gain any superpowers.
posted by physicsmatt at 1:47 PM on August 3, 2012 [1 favorite]


Artw,
At least in my subfield, we'll still refer to tachyons, but only in the context of a particle that appears to have a negative mass squared. This is typically a sign that you're attempting to work in a theory which has an unstable potential, meaning that you're not actually working with something that describes the Universe "as is." What should happen is that the 'tachyonic' field will move to a stable equilibrium with positive mass squared.

The type of tachyon you're thinking of, a real particle moving faster than light, doesn't really come up. Not that they're necessarily impossible as far as I know, but there's no evidence for them and a lot of problems making a consistent theory with them, so people in my line of work ignore them.
posted by physicsmatt at 2:01 PM on August 3, 2012 [1 favorite]


I approve of this great post.

I also approve of the phrasing "faster than the speed of light in the medium." Among other reasons, I think (though I could be misremembering) that Cherenkov emission can be described completely by classical electromagnetism, no photons required.
posted by fantabulous timewaster at 2:34 PM on August 3, 2012


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