Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Cherenkov Radiation

iaea.org

Post reply on HN
Print view View original

61–70 of 100 posts

Re: Cherenkov Radiation

#61

Post by smueller1234 »

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…

Cherenkov radiation is also a key mechanism behind ultra high energy cosmic ray (UHECR) detectors. UHECR are understood to be nuclei, not gamma rays. For ground based detectors, light tight water tanks are fitted with photo multipliers to capture the flash of cherenkov light even secondary particles pass through the water.

Additionally, they can use telescopes pointed at the atmosphere, using the atmosphere as a calorimeter, basically. The primary signal those telescopes look for is fluorescence, but when the direction of travel points at the telescope, cherenkov light far outstrips it in brightness, so it has to be included in the event reconstruction.

Most prominent contemporary example is the Pierre Auger Observatory https://auger.org

Re: Cherenkov Radiation

#62

Post by pasquinelli »

Betelbuddy wrote: post #41
mmmattt wrote: post #36

Earlier quoted context omitted.

He obviously meant the title of the HN post? Why not straight up say that the article title is different and avoid the passive aggressiveness.

I did not meant it in that way...and I would bet, most did not interpret it as such...the passive aggressiveness is coming from you. I meant it that the poster, changed the original title, what is a kind of editorializing. I thought of calling on the mods to edit it back, but judged that would be a kind of... semi-passive aggressiveness.

> the passive aggressiveness is coming from you.

could you explain this? the comment you're replying to, how is it passive-aggressive? seems more regular-aggressive to me.

Re: Cherenkov Radiation

#63

Post by molyss »

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…

I don't think the title is that misleading.

We talk about the sonic boom as something that happens when we travel faster than the speed of sound. We don’t specify “in the same medium you’re traveling in”, and noone reasonable thinks “it’s really hard to travel faster than the sound in carbon”.

You’ve added “speed of light” in your reading and concluded that’s what the OP wanted to imply.

I think that was unfair, and distracted from the rest of your comment.

I, for one, didn’t know what the cherenkov radiation is, and when reading the title thought “oh, so it’s a sonic boom for light? That’s so cool!”

Re: Cherenkov Radiation

#64

Post by tekla »

deadlast2 wrote: post #56
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…

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.

>numerous experiments which challenge this premise.

Citation needed

Re: Cherenkov Radiation

#65

Post by simonebrunozzi »

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…

Your comment is a good reminder of why I love the HN community so much. Having acccess to knowledge in specific domains, like in this case, is truly a gem.

And thanks!

Re: Cherenkov Radiation

#66

Post by cbolton »

yayachiken wrote: post #22
cbolton wrote: 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…

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 I meant).

But still I think saying "the primary wave gets absolutely and completely delayed" is not helpful. Using a wave description as in Feynman's lecture[1] is more enlightening: the incoming wave travels at "full speed" through the medium, but doing so it interacts with atoms such that they emit an additional wave, and the sum is a slower wave.

You can say it's the same since there's only one electric field in space and so the only "real wave" is the sum of all effects. But I find it quite helpful to think that one of the components in this sum is the original wave traveling at the speed of light in vacuum, also in the space occupied by the medium.

[1] https://www.feynmanlectures.caltech.edu/I_31.html

Re: Cherenkov Radiation

#67

Post by rbanffy »

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...

Re: Cherenkov Radiation

#68

Post by 04rob »

molyss wrote: post #63
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…

I don't think the title is that misleading. We talk about the sonic boom as something that happens when we travel faster than the speed of sound. We don’t specify “in the same medium you’re traveling in”, and noone reasonable thinks “it’s really hard to travel faster than the sound in carbon”. You’ve added “speed of light” in your reading and concluded that’s what the OP wanted to imply. I think that was unfair, and…

One difference is sound can only travel through a medium, so that bit is always implied. Not so with light.

Re: Cherenkov Radiation

#69

Post by DoNotMindMe »

prathje wrote: post #19
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…

In a medium, this can create a "phase kickback" which creates a combined wave that appears to travel slower than the original one. The kickback is just the result of multiple EM changes propagating, i.e. the photons interact with the material, re-emitting photons. 3Blue1Brown has a beautiful animation for this phase kickback here: https://youtube.com/shorts/XIW-2ykgVPI?si=PJWiAC2BO7_xP0S6

FYI/PSA: If you leave the tracking portion of the youtube link (e.g. after the '?') it is trivial to track back to the youtube account of the person who clicked the 'share' icon. Always best to remove the '?si=...' part before publicly sharing.

Thanks for the link though, love 3b1b.

Re: Cherenkov Radiation

#70

Post by Sharlin »

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!
Post reply on HN