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What is Cherenkov Radiation?

  1. Nov 13, 2006 #1
    Ok, so I've heard of Cherenkov Radiation and its azure bluish glow but how is it created? What experiments produce it?
     
  2. jcsd
  3. Nov 13, 2006 #2
    It is a "sonic boom" of sorts of light. It is produced when a particle is temporarily moving faster than it is supposed to in a medium.

    The Super-K neutrino experiment uses it to detect neutrinos. Nuclear reactors also glow due to CR.

    Wikipedia has a nice summary on it.
    http://en.wikipedia.org/wiki/Cherenkov_radiation
     
  4. Nov 13, 2006 #3

    jtbell

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    What speed is a particle supposed to move at, in a medium? :confused:

    Cherenkov radiation occurs when a particle moves faster than the speed of light in that medium, i.e. when v(particle) > c/n.

    Of course, v(particle) < c must also be true.
     
  5. Nov 13, 2006 #4
    Sorry -- coffee hadn't kicked in yet, so "supposed to" was a poor choice. I was trying to say "allowed to" -- for example a particle going through water should only be able to travel at ~0.75c, but will create CR if it is going faster.

    *going to refill my cup right now*
     
  6. Nov 13, 2006 #5
    Heh, ya I was also wondering about the 'supposed' term, its vagueness gave me a good chuckle.

    SO THEN, yes I knew that cherenkov radiation is produced when a particle exceeds the maximum speed of light in its medium, but how does this occur? What mechanism(s) are at work that can 'pump' that particle beyond the medium's speed limit? Also, I am guessing that the CR light is the energy difference between the particle's speed and the maximum speed of light in the medium, is this true?
     
  7. Nov 14, 2006 #6

    jtbell

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    The medium doesn't have a speed limit that's different from the universal speed limit of c. The fact that light as a classical electromagnetic wave travels slower than c in a medium doesn't mean that the speed limit has been modified. Note that individual photons still travel at c inside the medium. They just don't travel very far before they are absorbed by the medium, which then radiates "new" photons.
     
  8. Nov 14, 2006 #7

    vanesch

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    Yes, this is an often stated misunderstanding.

    The universal speed limit, no matter where, is: c, which happens to be also the speed of light in vacuum. That doesn't mean that a particle cannot "go faster than light". It only means that a particle cannot have a higher velocity than the number c, and given that light in vacuum has the speed c, well, no particle can go faster than light in vacuum.
    But given that light in matter doesn't have the speed c, but rather the speed c/n, well, then nothing stops a particle (which still has upper speed limit c), to go faster than light in matter.

    Although I'm pushing things a bit, you could consider that "the speed of light" has nothing a priori to do with the "maximum allowed speed of things in general". There turns out to be a "maximum allowed speed of things in general", and that's given by a speed which we call "c".

    Next, you should consider that light, in vacuum, is the stuff that "speeds as fast as it is allowed to", so in vacuum, light has velocity c.
    In matter, light simply doesn't "speed as fast as it is allowed to", it goes slower, due to a collective interaction with the matter.
    So there, it can be "overtaken" by other stuff.
     
  9. Nov 15, 2006 #8

    Right, the speed is always C between absorbtion/emissions, but these abosorbtion/emissions take time, so on aggregate the speed appears slowed down from a classical perspective. I think its misleading to say the 'universal speed limit, no matter where, is C'. Space it self expands at speeds much greater than C, so you really need to say particles cannot exceed C.

    What particle(s) exceed C/n in a medium of water that produce CR? Alpha particles, beta particles, neutrons? I'm guessing neutrinos would because they barely interact with anything at all and therefore they're very unlikely to be absorbed/emitted by, well, anything. Otherwise the SuperKamiokade and SNO would detect wayyyyy more than they do.
     
  10. Nov 15, 2006 #9

    vanesch

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    Ok, things transporting energy and momentum (whether these are particles, field disturbances or other stuff), can, LOCALLY, not exceed c, in any local reference frame.

    The problem is that those hadronic particles would interact strongly with the water before producing much CR (that's the principle of a hadronic calorimeter). Neutral particles will not generate any CR. What usually produces CR in copious amounts around water-based nuclear reactors, are secondary electrons resulting from energetic gamma radiation through the photo-electric or more often the Compton effect.

    That said, I'm talking about CR in water. There's also atmospheric CR, and I'm less well acquainted with it.
     
  11. Nov 15, 2006 #10
    Does that mean that if a generate a neutron and a photon at the same time and pass them through a medium I will see the neutron first? Could I then say that the neutron travelled faster than the speed of light.

    Also if I could encode the neutron with information could I get that information faster then light transmitted information?
     
  12. Nov 15, 2006 #11
    I'm fairly sure I read somewhere that in principle a neutrino telescope will see a supernova a few seconds before a light based telescope will. While space is almost a perfect vacuum, it's not perfect and over distances measuring tens of thousands of light years light will interact ever so slightly with the thin amount of gas between the supernova and us and be slowed down just as it is in water (slowed down in the sense as mentioned in this thread) but neutrinos, being annoyingly uninteracting, will just almost all just stream through space without noticing.
    You need to be precise. You can say that the neutron moved faster than light's speed in that medium, you cannot say it moved faster than c, light's speed in a perfect vacuum.
    Causality is about c, not c/n (using the equation jtbell mentioned).
     
  13. Nov 16, 2006 #12
    Very interesting, do you remember where you read about seeing the neutrinos before light?
     
  14. Nov 16, 2006 #13

    jtbell

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    I thought the light from a supernova was delayed by its passage through the intervening layers of the dense stellar atmosphere, not interstellar space. The light and neutrinos are created deep within the star and it takes time for the light to make its way to the surface, so to speak, whereas the neutrinos just zip right on out.
     
    Last edited: Nov 16, 2006
  15. Nov 16, 2006 #14
    You can't see neutrinos with a telescope. Because of their non-interacting nature, it requires large detectors made of water or steel to catch a few hundred a year. In Super-K for example, the CR comes from the products of the collision between a neutrino and a nucleus.

    What you might be thinking of is an X-ray telescope. The process of a supernova produces a cascade of X-rays a few minutes before the visible light is formed and starts travelling. So the X-ray burst is our warning to turn visible light telescopes to a region of the sky.
     
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