Live data from Hacker News

Cherenkov Radiation

iaea.org

51–60 of 100 posts

Re: Cherenkov Radiation

#51

> 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.

Neutrinos are neutral particles, they do not carry an electromagnetic charge. They thus do not cause any Cherenkov light themselves.

What Neutrino detector measure is the Cherenkov light emitted by secondary particles that are created when a Neutrino interacts with the matter inside the detector.

E.g. a muon Neutrino reacts with a water molecule producing a muon, which is a charged particle and thus emits Cherenkov light.

Re: Cherenkov Radiation

#52
post #12

Earlier quoted context omitted.

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. pho…

> with no part getting ahead

I don't think that can be strictly true. No matter how dense the material is some photons have a chance to get through unimpeded through something like tunelling. Practically unlikely but mathematically possible.

Re: Cherenkov Radiation

#53

> 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. In order to regain equilibrium, those atoms release photons – the types of particles that compose visible light, creating a “shock-wave” of visible light. That's like the vaguest description of anything ever. Is physics a stealth startup? Why does it specifically hap…

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?

Re: Cherenkov Radiation

#55

Light still propagates at c inside a medium; the apparent slowdown is just phase kickback from re-emission.

We know light is not being absorbed and re-emitted, because re-emission sends light in a random direction.

Re: Cherenkov Radiation

#56
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…

Why do you believe that like if you are measuring it maybe you are doing it wrong. Seems to me that we need this light fastest X to hold up a load of physics even when there has been recently numerous experiments which challenge this premise.

Re: Cherenkov Radiation

#59
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…

Yeah, but what is a medium? Every material is basically empty space with an atom here and there. So there is not a hard boundary for a particle to instantaneously slow down from speed of light in the air to speed of light in water. Some of them will travel a bit deep before hitting their first H or O or electron, or some salt ion. Anyway, it won't travel faster than c.

Re: Cherenkov Radiation

#60
post #38
post #27

Earlier quoted context omitted.

> 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) propag…

Love this, good point! It really shows how difficult it is to give an easy explanation that generalizes well... So, although quite abstract, the fundamental propagation of changes in the EM field at the speed of causality is a good enough approximation?

The issue is many kinds of solutions exist, but aren't common in different fields. On a guitar string, waves are 1d waves in a 1d world. In water everyone knows only standard waves. Which are 1d solitons in a 2d "world". In the electromag field everyone only knows light, which are 2d solitons in a 3d world. Electrons have an axis of rotation that does not point in any spatial or temporal direction.

So everybody thinks these are far more different than they really are.

But the wonder is that water has "light rays", 1d solitons in a 3d world (ie. under water). Dolphins love making them and playing with them. It's possible to make them with your hand in a pool, it's just pretty hard, but with a bit of practice. You need to hold your hand flat above the water, splash down, then retract your hand hard and get out of the way. The faster you retract your hand the further they'll go (but you need time to get out of the way). If you do it right a ring of water bubbles will go into a straight line several meters. Dolphins make them go hundreds of meters, and play with them, seemingly for fun. They aim them at eachother and pass them along (these don't have the same geometric structure as light rays, just the same movement. Except that they trap bubbles and so they "fall upward", especially when they slow down)

Water has "particle-antiparticle soliton pairs". You stick your hand, held flat, 90 degrees to the surface, half submerged, and you move it through the water, parallel to the surface. You do it right, 2 vortices will leave and move through the water. You can see circular shadows move over the bottom of the pool (because of the dimple shape where the vortex meets the surface). These you can probably get to the other side of a quiet pool if you try hard.

According to my math, it should be possible to make a version of this where the vortices rotate around each other. And you should be able to make any even number of vortices (like 4, 6, 8, or 800 for that matter) but I've never done even 2 rotating, nor have I been able to make 4 move together.

Water has 2d solitons in 2d as well, but you can't make just them. If you make 2 waves intersect at exactly 90 degrees, every so often the intersection point will "leave" on it's own. A "hill" on the water will start moving through the water, and you'll swear to God something is moving below the water, but there's nothing there but the wave. But this is almost impossible. You can try in a huge pool (ie. no reflections), and nobody but you in there.

It's weird to think about the properties water has that the electric field does not appear to have. For instance, water has a surface, which reflects solitons. Is there a surface in the electric field somewhere? Would it be a mirror in space, that is not just a perfect reflector of light rays but of matter too?

Post reply on HN