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Can only photons travel at c?

  1. Jul 13, 2003 #1
    can only photons travel at the speed of light?

    can a particle that has mass travel at c?
  2. jcsd
  3. Jul 13, 2003 #2


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    No, a particle with mass cannot move at the speed of light.
  4. Jul 13, 2003 #3
    ok, so are there any other particles that are massless, besides photons, that can travel at c?
  5. Jul 13, 2003 #4
    Neturinos, if you believe in them...
  6. Jul 13, 2003 #5
    I would hold back on any difinite answer of yes or no. I'm not comepletely convinced anymore that any particle has mass, or even that there is such a thing as a paticle at all. At least not in the way we might normally think of a particle as being.

    Hows that for covering the bases to confusion.
  7. Jul 13, 2003 #6
    what exactly is your working definition for mass
  8. Jul 15, 2003 #7

    why don't you believe neutrinos exsist?

  9. Jul 15, 2003 #8


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    Neutrinos have been found to have mass.

    The strong force carriers, gluons, are the other massless particles in the Standard Model.
  10. Jul 15, 2003 #9


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    It's not known for sure if neutrinos have a non-zero proper mass.

  11. Jul 15, 2003 #10
    Selfadjoint is correct, latest studies have found neutrinos to have mass. Which would mean it can't be going the speed of light.

  12. Jul 15, 2003 #11


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    There is at the moment no final word on whether neutrinos have mass or not (though they are considered a good candidate for the WIMPs that must make up non-baryonic dark matter). Again, if they can travel at light speed or nopt is unknown (they have even been suggested as possible candidates for tachyons).
  13. Jul 15, 2003 #12


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    Uhm, didn't they determine gravity moved at the speed of light? If so I suppose a gravity particle would as well, if there is such a thing.
  14. Jul 15, 2003 #13
    Well those results are now being disputed bya Post Doc at Cal Tech. And I would have to agree they made many many assumptions to get their results.

  15. Jul 17, 2003 #14
    is photons travel in the speed of light?what is the difinition of the speed of light? isn't the speed of the free photons?
    do photon have mass? if not, why there is the effect of the light pressure? and i think it has ,and i think the mass is the carrier of momentum and energy.
    and i think if we redifine a limit of speed(theoretical light speed?) ,the speed of photons cannot make it,and all things or particles cannot
  16. Jul 18, 2003 #15
    Many questions...

    You shouldn't think of the "speed of light" as JUST being the speed at which photons travel, I would suggest you think of it more generally. The speed of light is an upper-limit on the rate at which INFORMATION can travel, where information is being used in a broad sense.

    Although photons have absolutely zero mass, yes they do have momentum associated with them. The amount of momentum a single photon carries is related to its wavevector, or frequency, or wavelength (although talking about "frequency" / "wavelength" and "photon" simultaneously kind of mixes up its wave and particle properties simultaneously). An easy way to understand why they have ENERGY associated with them is by considering the time-average power associated with the electric and magnetic fields that self-perpetuate as an EM wave (or series of photons) travels.

    With regards to explaining why photons have momentum (LINEAR MOMENTUM), I can simply tell you that the math. tells us this is the way it must be. When you solve Schrodinger's equation using time-dependent perturbation theory for an atom in the excited state, when the atom decays to the ground state emitting a photon, the centre of mass of the atom recoils (thus to conserve momentum, the photon MUST carry linear momentum).

    In addition, photons have ANGULAR MOMENTUM. This can be seen in many optics experiments, and especially in spectroscopy (atoms obey dipole "selection rules" that would only be possible to fulfil if photons carried angular momentum).

    I am not sure how to respond to the rest of your questions...you may want to sleep on the idea of redefining the speed of light - most people like the current definition.
  17. Jul 19, 2003 #16


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    I have heard of a type of theoretical particle that travels faster than the speed of light; but only remembr vague details about it.. It is not the neutrino, some of which do have mass,; but something else.
  18. Jul 19, 2003 #17


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    As far as c being the upper limit for travel of information, what about quantum entaglement? I'm not an expert, but I believe the flavor of the concept is that when two quantum particles interact, their respective wave functions become 'attached' such that when you collapse the wave function of one by taking a measurement on it, the wave function of the other is collapsed simultaneously and <i>instantaneously</i>, regardless of distance. Therefore quantum entaglement is an example of information traveling faster than c, in fact, traveling with infinite speed since the transmission of information takes no time at all.
  19. Jul 19, 2003 #18
    You people are smarter than i, but what about the Cherenkov effect - y'know the blue glow you get with tank reactors ATL, caused by particles hitting c like a plane hits mach 1 and you get a shock wave.

    correct me if i'm wrong, but doesn't that prove that they exceed c. I belive that a similar idea is used to prove neutrinos exist,
  20. Jul 19, 2003 #19
    Remember now, we are specifically talking about the speed of light in a vacuum. It is common place for particles to travel faster than light in a certain medium. For example in nuclear pool reactors. Neutrons shoot through water faster than light can and thus emit a blue glow (in water) known as Cerenkov radiation. Information can be sent faster than light also but only by quantum physics, this has been shown by sending a music modulated microwave beam into a quantum barrier. On the other side of the barrier is an extremely sensitive detector witch detects microwave photons that tunnel through space and arrive at the other side unblocked by the barrier (usually a sheet of metal) using very large bandwith O-scopes scientists have shown that they can send nusic up to 4+ times the speed of light. Although technicly that is cheating because the photons simply travel less distance since they tunnel through space traveling less distance.

    As far as explaining why only massless particles can travel at C, it is because the faster you go, the more energy is required to acheive a higher acceleration. The curve that describes the energy required to accelerate at the same rate is exponential. The curve climbs in steepness rapidly (x^2) and you will approach infinity as to reach the top you would have to eiter have a given amount of energy exerting a velocity on the object for an infinite amount of time to reach C or you could have your energy reach infinite, climbingin speed till you are just under the speed of light. To send matter to the top where the slope of the line is undefined it takes an infinite amount of energy just to acheive C

    I would like someone to try and exert an infinite amout of anything on any object. You will see that no amount of work will let you achieve infinity :-p .

    Anyways if you take the time to work out the gamma in a relitivistic equation for acceleration to the speed of light you will see that gamma (speed at witch delta (increase of slope)changes) will climb to as high a number as you want it to, still working out the formulas with any mass will still require an infinite amount of energy or time to reach C.
    Last edited by a moderator: Jul 19, 2003
  21. Jul 19, 2003 #20


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    if it is impossible for a particle with mass to achieve the speed of light, then how is it possible for a mass-possessing neutron to exceed c?
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