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Cherenkov Radiation

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Re: Cherenkov Radiation

#81

Post by aceazzameen »

chucksmash wrote: post #71
04rob wrote: post #68

Earlier quoted context omitted.

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.

Re: Cherenkov Radiation

#82

Post by petsfed »

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.

Re: Cherenkov Radiation

#83

Post by 542354234235 »

GlobalFrog wrote: post #79

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 ener…

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.

Re: Cherenkov Radiation

#84

Post by [deleted] »

cbolton wrote: post #66
yayachiken wrote: post #22

Earlier quoted context omitted.

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. pho…

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…

[deleted]

Re: Cherenkov Radiation

#85

Post by prpl »

nuccy wrote: post #3

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 Atm…

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).

Re: Cherenkov Radiation

#86

Post by maxnoe »

scotty79 wrote: post #53
mytailorisrich wrote: post #49

Earlier quoted context omitted.

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...

Re: Cherenkov Radiation

#87

Post by maxnoe »

xattt wrote: post #74
rbanffy wrote: post #67

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?

Astronauts have reported having difficulty sleeping due to cosmic-ray-induced Cherenkov flashes in their eye bulbs.

Re: Cherenkov Radiation

#88

Post by RubberbandSoul »

Sharlin wrote: post #70
nuccy wrote: post #3

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 Atm…

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?

Re: Cherenkov Radiation

#90

Post by Sharlin »

RubberbandSoul wrote: post #88
Sharlin wrote: post #70

Earlier quoted context omitted.

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?

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.
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