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Anybody got a light

  1. Jun 9, 2008 #1
    anybody got a "light"

    So i have been thinking about this for a while and researched a fair amount... I understand from the maxwell equations we find that light is pure energy and has matter-like characteristics such as momentum but it in fact does not have mass. Light is energy and energy has characteristics of matter because matter and energy are forms of the same thing.

    So in a vacuum nothing with mass can move faster than the speed of light. I have heard that in nuclear reactors there are particles with mass (very little mass but still mass) that can move faster than the speed of light THROUGH WATER which creates a "sonic boom" of light in a sense. I wonder how this is possible that light with no mass has more resistance when moving through water than something with mass.

    Just interested in some input... im sorry if my wording isnt clear... i find that sometimes when i post here people tear apart my wording. If you want me to clarify... let me know.

    Hit me with your best shot!
  2. jcsd
  3. Jun 9, 2008 #2
  4. Jun 9, 2008 #3
    That is a very interesting question. I'm curious to what poeple on here will say or answer. I can't wait to get started with college and at least get to the point of understanding what you guys are talking about half of the time!
  5. Jun 9, 2008 #4

    Doc Al

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    Staff: Mentor

    What exactly is your question? Relativity prohibits massive particles from traveling faster than c (the speed of light in a vacuum). In Cerenkov radiation, massive particles travel faster than the speed of light in some media, which is less than c. This is perfectly consistent with relativity, if that's what you're getting at.
  6. Jun 9, 2008 #5
    yes i understand that point... i already stated this in my question...

    if you have a large object falling through the air it will have more air resistance than a smaller, more aerodynamic object...

    my question is... how can light, with no mass and no surface area (i would assume), have more "water resistance" than an object with mass/surface area?
    Last edited: Jun 9, 2008
  7. Jun 9, 2008 #6

    Doc Al

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    So really your question has nothing to do with Cerenkov radiation; you are just asking how is it possible that light goes slower in some medium? Perhaps our FAQ entry might help: https://www.physicsforums.com/showpost.php?p=899393&postcount=4" [Broken]

    (Thinking in terms of macroscopic "air resistance", such as an object might experience, won't help much.)
    Last edited by a moderator: May 3, 2017
  8. Jun 9, 2008 #7
    I don't think Shamrock is questionng if light travels slower in a medium. He is just questioning why particles with mass can travel faster than light through a medium when light is supposed to be the fastest thing around which is a reasonable question. I think you are right to point that a medium that appears solid to us on a macroscopic scale, looks like mostly empty space to an atomic particle. A small particle passing through water is like a rocket moving through a galaxy of stars. The rocket is very unlikely to accidently hit a star because they are so far apart (if we ignore gravity.) I think someone once said if the size of the nucleus of an atom in a solid is represented by a football then the nucleus of the next nearest atom in the solid is about a football field away (or more). A ball kicked randomly in any direction is very unlikely to hit another ball when they are spaced that far apart. Keeping to loose analogies, a photon wave packet is not localised like a massive particle and its electromagnetic field interacts with the electons of atoms in the medium around it, so it is slowed down without having to make a direct hit with any atom in the medium. It is a bit like pushing a bar magnet through a coil of wire connected to a light bulb. The light bulb lights up, but the energy to light up the bulb comes from the motion of the magnet which is resisted without the magnet making any physical contact. A non magnetic bar (representing a particle) would pass through the coil without being resisted. Of course this is a very loose analogy which should not be taken at all literally, but I hope it gives an intuitive feel for what is happening.
    Last edited by a moderator: May 3, 2017
  9. Jun 9, 2008 #8
    Its incorrect to think of light as pure energy. The term "pure energy" is not a term that is defined or has meaning in physics. You can give it a meaning if you'd like of course but physicists wince when they hear the term "pure energy."
    That depends on the definition of the term "mass." Proper mass? No. Inertial mass (aka relativistic mass)? Yes.
    Regardless of the mass nothing can travel faster than the speed of light.
    I have heard that in nuclear reactors there are particles with mass (very little mass but still mass) that can move faster than the speed of light THROUGH WATER which creates a "sonic boom" of light in a sense. I wonder how this is possible that light with no mass has more resistance when moving through water than something with mass.


  10. Jun 9, 2008 #9
    i wonder if a sound wave carries momentum.
  11. Jun 10, 2008 #10
    kev: thanks for the reply... what you said makes alot of sense but im not sure if it answers my question, if anyone can... if a solid or liquid looks like mostly empty space to a particle wouldnt it look the same to light... and wouldnt the light travelling through it have no interaction with the particles it is passing by while the particle might. I would think this would slow the particle while the light would just keep on moving at constant velocity. maybe it has to do with certain wavelengths of light slowing down?

    pmb phy: sorry for not using completely correct terms... i believe what i mean is light is energy with zero resting mass.

    granpa: i believe it does since sound must move through a medium
  12. Jun 10, 2008 #11
    Light moves the speed it moves through a medium because it interacts with the medium electromagnetically. You can think of atoms continually absorbing and re-emitting the light, causing a wave to propagate. On the other hand, a charged particle can move quite a distance through the medium before it collides (large mean free path). While it is moving, it decelerates, causing the radiation. But it's speed is still greater than the speed of the radiation (for a while).

    Think of a bullet moving through the atmosphere at supersonic speed. The speed of the bullet and the speed of sound are limited by different processes.
  13. Jun 10, 2008 #12
    Phonons do.
  14. Jun 10, 2008 #13
    thanks ibrits... thats a pretty good explanation... im wondering if you can explain why the light interacts electromagnetically where light has no charge but a particle usual will have one
  15. Jun 10, 2008 #14
    I shouldn't've written it that way. While in the medium, light is a "waving" of the medium, not of the vacuum. The medium has a preferred Lorentz frame, so don't think of it as a free particle. For instance, a moving medium will have different indices of refraction in different directions. It is an excitation of the dielectric. On the other hand, the electron is still an electron, and acts like an electron with "stuff" around it.
  16. Jun 10, 2008 #15
    because its an electromagnetic wave? the divergence may be zero but the curl is not.
  17. Jun 10, 2008 #16
    interesting stuff... thanks for the input
  18. Jun 12, 2008 #17
    a quick question... you said light is a waving of the medium. So are you saying that when light passes through a medium it moves in a sense kind of like sound does? light can travel in a vacuum but sound cannot...
  19. Jun 12, 2008 #18
    When light moves in the vacuum, it is the vacuum Maxwell's equations that we solve. In a medium, the permittivity and permeability become altered due to the presence of the dielectric, and the speed of light in the medium is reduced. Since only the vacuum speed is the same in all reference frames, you can see that there are frames in which the speed inside the medium can take on any value < c.

    The permittivity, for instance, comes about because as the electromagnetic wave passes through the medium, it polarizes the atoms which in turn set up their own electromagnetic wave. The light that propagates is then a sum of the two effects. For linear materials, the polarization is proportional to the original wave, and light behaves just like it does in vacuum, albeit slower. For non-linear materials, Maxwell's equations become a mess.

    I should stress that you can treat light in a medium by solving Maxwell's equations "in vacuum with sources", so it is really different from sound. However, the medium plays an important role here (because of polarization), which is analogous to propagation of sound.
  20. Jun 12, 2008 #19
    No problemo! Its best to ask what you're interested in without worrying about being precise. Hopefully the responses you get here will help clarify things, even in areas where you didn't expect. To say that light "is" energy is to consider light and energy to be the same thing. That'd be like saying that a moving electron is energy because it has kinetic energy. Its best to think of a photon has having energy. To be precise one should consider the photon has having all kinetic energy (and, of course, zero rest energy).
    Unlike sound waves, electromagnetic waves are not a "waving" of a medium. In fact an EM wave can propagate in the complete absense of any medium, i.e. in a vacuum. What is being propagated is a disturbance in the electric and magnetic fields. In the case of a EM wave in a vacuum in the otherwise absence of an EM field there are propagating electric and magnetic fields. There is no real medium to speak of. It would be incorrect to think of the medium as being the EM field since the EM field itself can be said to be what is actually moving. What is literally happening is that there is a time varying value of the electromagnetic field as function of position. This gives rise to a propagating disturbance. The precise definition and meaning is given mathematically in terms of the solution to the wave equation (which is derived from Maxwell's equations).

  21. Jun 12, 2008 #20
    Mmm... dunno how you managed to butcher my username like that. I blame the fonts =).
    Anyway, I gave clarification in my last post. The point is that electromagnetic radiation in vacuum doesn't require a medium except a spacetime to propagate on. But the OP asked about light in a medium and the origin of Cherenkov radiation. Therefore my reply was framed in terms of this context, and the medium in which light moves is very much important. Think of radio waves attenuated in a plasma. Sure, the plasma oscillation *isn't* the radio wave, but heavily influences it. Now, in a normal dielectric this influence is linear, so the light is in a sense locked to the polarization of the medium (evidenced by the change in the permittivity).
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