Cherenkov Radiation

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Cherenkov Radiation

Postby Event Horizon on March 14th, 2018, 5:49 am 

When a particle approaches lightspeed, photons & Muons emit Cherenkov radiation, which is a kind of blue-green colour. Because leptons have no other components to their structure, I'm wondering what is being converted into said radiation.
Presumably they have to shed all mass to be able to attain lightspeed or near lightspeed. But the only mass is the lepton itself, it has no internal particles. It seems it radiates its mass away, but wouldn't that cause it to cease to exist?
Also, does Cherenkov radiation interact with things? I'm presuming it consists of photons given it has a colour we can see? It appears to be a low energy radiation as I've never heard of anyone having radiation damage because of it.
Is there any use for Cherenkov radiation?

I'm no physicist, I can't do the math. I hope my questions aren't entirely rubbish. I apologise for my ignorance.
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Re: Cherenkov Radiation

Postby Braininvat on March 14th, 2018, 10:07 am 

Several new threads started. Give them time to ferment. :-)
Last edited by Braininvat on March 14th, 2018, 11:42 am, edited 1 time in total.
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Re: Cherenkov Radiation

Postby JMP1958 on March 14th, 2018, 11:22 am 

Event Horizon » March 14th, 2018, 2:49 am wrote:When a particle approaches lightspeed, photons & Muons emit Cherenkov radiation, which is a kind of blue-green colour. Because leptons have no other components to their structure, I'm wondering what is being converted into said radiation.
Presumably they have to shed all mass to be able to attain lightspeed or near lightspeed. But the only mass is the lepton itself, it has no internal particles. It seems it radiates its mass away, but wouldn't that cause it to cease to exist?
Also, does Cherenkov radiation interact with things? I'm presuming it consists of photons given it has a colour we can see? It appears to be a low energy radiation as I've never heard of anyone having radiation damage because of it.
Is there any use for Cherenkov radiation?

I'm no physicist, I can't do the math. I hope my questions aren't entirely rubbish. I apologise for my ignorance.


Cherenkov radiation is generated when a charged particle travels through a medium at a speed that is greater than the speed light travels through that medium. (The restriction against exceeding light speed only applies to exceeding the speed of light in a vacuum.) Such a particle is always emitting some electromagnetic radiation due to its interaction with the medium and this comes at the cost of the particle's velocity. If the particle is moving faster than light for the medium, this EMF can form something that is the equivalent of the sonic boom generated by aircraft exceeding the speed of sound. This is the Cheronkov radiation.

It is highly directional and you can determine the speed of the emitting particle by measuring the angle at which the Cheronkov radiation is emitted.

It also is emitted mostly in the blue to low ultra-violet range of the spectrum. Here's an image of Cheronkov radiation emitted from The Reed research reactor

Image
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Re: Cherenkov Radiation

Postby Braininvat on March 14th, 2018, 11:31 am 

The "sonic boom" analogy is the one I've always heard. It's helpful to understand what phase velocity is, as that is the velocity that the particle is exceeding, and that helps to picture how the "optic boom" happens.
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Re: Cherenkov Radiation

Postby someguy1 on March 14th, 2018, 12:11 pm 

It's a beautiful strange blue glow. I saw it once. Experimental reactor at UC Berkeley, I was in a class and we went to see it. I still remember it, it was a fascinating sight. The reactor eventually got shut down out of "nuclear" fears. Silly, there was no danger from this small apparatus. Anti-science at one of the great universities, or maybe it was the local politicians.
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Re: Cherenkov Radiation

Postby Event Horizon on March 14th, 2018, 6:51 pm 

Um, the reactor core is not in a vacuum. Nor the pond. So how are the Muons exceeding C going through the same medium? They have no internal particles to sacrifice as photons that I know of (and to be fair, I don't know much), to sacrifice. So is it the medium and not the Muon that is reacting?
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Re: Cherenkov Radiation

Postby JMP1958 on March 15th, 2018, 11:32 am 

Event Horizon » March 14th, 2018, 3:51 pm wrote:Um, the reactor core is not in a vacuum. Nor the pond. So how are the Muons exceeding C going through the same medium? They have no internal particles to sacrifice as photons that I know of (and to be fair, I don't know much), to sacrifice. So is it the medium and not the Muon that is reacting?


The muons never exceed c. c is the the speed of light in a vacuum. They exceed c/n, where n is the refractive index of the medium they are passing through. For example, the IOR for water is 1.33, thus the speed light travels through water is 75% of that which it travels through water.

If a muon travels at greater than 0.75c while passing through water, then it is outpacing light traveling through the water, but still is traveling below the speed limit of c.

The interaction with the medium will cause the muons to slow down as they travel. When ever a charged particle undergoes a change of velocity, energy lost is emitted as electromagnetic radiation. One example is called bremsstrahlung or "braking radiation", which is produced when charges undergo random velocity changes.

This is how an x-ray tube works. High speed electrons are fired at a target and are decelerated by the impact, which cause them to produce bremsstrahlung in the x-ray range. The electrons do not give up any of themselves to produce the photons it comes from their Kinetic energy loss.

A more everyday example is a hot object cooling by the emission of infrared radiation. The atoms of the hot object are vibrating back and forth which is a form of acceleration. The charged particles of the atoms thus give off radiation, which removes energy from the vibrations slowing them down, and the object cools.
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Re: Cherenkov Radiation

Postby Braininvat on March 15th, 2018, 12:17 pm 

Though I could not begin to explain this with the clarity of JMP, I will just add this:

It's helpful to think about fields. As a charged particle travels, it disrupts the local electromagnetic field in its medium. In particular, the medium becomes electrically polarized by the particle's electric field. If the particle travels slowly then the disturbance elastically relaxes back to mechanical equilibrium as the particle passes. When the particle is traveling fast enough, however, the limited response speed of the medium means that a disturbance is left in the wake of the particle, and the energy contained in this disturbance radiates as a coherent shockwave.

Consider a fisherman gently slipping a lure into the water, doing it so slowly that the lure just slips in between the water molecules without much disruption. Then consider someone hurling a stone into the water and the ensuing wave that radiates outward from where it enters the water.

They have no internal particles to sacrifice as photons that I know of (and to be fair, I don't know much), to sacrifice.
- EH

A particle doesn't have to give up an internal particle in order to be able to give up momentum to a medium. (as JMP makes clear) The rock in my example doesn't have to give up a chunk of itself to make a ripple across the pond.
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Re: Cherenkov Radiation

Postby Event Horizon on March 15th, 2018, 2:31 pm 

Ah, thanks guys. I get it now I think. Could it be be analogous to a speeding car that has to brake hard, resulting in the brakes getting hot and radiating thermal energy as a result?
My reason for the OP is because I'm trying to discover whether the inflation event caused an EM shockwave, which primed spacetime so the universe had something to expand into. I was wondering how to discover this, and looking for clues. I had hoped that Cherenkov radiation might have been emitted, giving me something to search for.
Even if it had been caused, it would likely still be travelling at 40xC with nothing to slow it down. Had Cherenkov radiation been produced - and I no longer think it was, it would give the universe a blue glow if it were possible to theoretically stand outside the universe and look back. I think that is a whole other thread.
I have a number of threads running, and I don't really want to start another one just yet. I end up thinking too much, and it wrecks my sleep cycle.
If however you feel it's pertinent, and you want to explore this, please post an op anyway. You guys can explain things much better than I can.
Many thanks.
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