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Cherenkov Radiation

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Re: Cherenkov Radiation

#21

Post by dguest »

intrasight wrote: post #9

> 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?

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".

Re: Cherenkov Radiation

#22

Post by yayachiken »

cbolton wrote: post #12
chinathrow wrote: post #7

> 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 o…

Yeah I didn't find that helpful. What I remember from Feynman's lectures is that photons still travel at "full speed" c between atoms, but if you look at the global progression of light as photons get absorbed then emitted it progresses slower than c.

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. photons) and the math comes out the right way for the primary wave if you assume that only some of these wave packets get phase-shifted, and add all elementary waves together afterwards.

But still, it's photons as "wave packets" that influence the whole system, not photons as independent particles that either bounce on something or don't.

Re: Cherenkov Radiation

#23

Post by zhivota »

georgemcbay wrote: post #11
chinathrow wrote: post #7

> 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 o…

They didn't word that very well. Would have been clearer if they said "However, in other mediums (like water), particles can potentially move faster than light does in that same medium."

Ah, so it's not faster than c, it's faster than light's speed inside the medium. This makes a lot more sense.

Re: Cherenkov Radiation

#24

Post by chinathrow »

hdgvhicv wrote: post #10
chinathrow wrote: post #7

> 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 o…

In 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.

Thanks - 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."

Re: Cherenkov Radiation

#25

Post by chinathrow »

chinathrow wrote: post #24
hdgvhicv wrote: post #10

Earlier quoted context omitted.

In 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.

Thanks - 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."

EDIT:

Got it, faster than light IN THAT MEDIUM.

Re: Cherenkov Radiation

#26

Post by Betelbuddy »

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…

>> The title (likely intentionally) is misleading,

That is because that is not the title...the title is: "What is Cherenkov Radiation?"

Re: Cherenkov Radiation

#27

Post by spwa4 »

prathje wrote: post #15

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 m…

> Single EM waves propagate with exactly this speed ...

Yes ... and no. Or we should say yes, but not necessarily forward. What about circular? What about ball shaped? What about a vortex? Any valid soliton is a solution and a single wave. Which means this is not just possible in water, but also in the electromagnetic field:

https://www.youtube.com/watch?v=909o_kbCdFgll

Circular (as in 2d circular in 3d space) propagation in EM waves is like 2 waves in exactly the same location and direction, with opposite rotation along the axis. Which could be extremely useful since they propagate like single waves. In other words, there is a non-circular spectrum ... AND a circular spectrum. So, if we modify all radios we have double the spectrum.

Re: Cherenkov Radiation

#29

Post by Betelbuddy »

chinathrow wrote: post #25
chinathrow wrote: post #24

Earlier quoted context omitted.

Thanks - 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."

EDIT: Got it, faster than light IN THAT MEDIUM.

Yes ...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...

Re: Cherenkov Radiation

#30

Post by gste »

This 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

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