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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?
  22. Jul 19, 2003 #21


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    another clarification if you please...
    from my understanding, the instantaneous information transfer of quantum entanglement does not involve tunneling through space, since collapsing the wave function of particle A will simultaneously collapse the wave funcion of B regardless of distance. Tunneling seems inadequate to describe this phenomena unless there is some way in which A and B are already directly connected through just such sort of a tunnel through space.
  23. Jul 19, 2003 #22
    Ok, i'm sorry if I confused you a bit there, I understand quantum wave/particle duality and entanglement but didn't want to go into the gory details above since I felt that the simple explination showed the possible applications for it to send signals faster than light. Anyways, The neutrons witch do have mass do not exceed C, C is the constant speed an EM wave will travel at through a vacuum. The constant for light traveling in diffrent mediums such as water is know, and it is known that neutrons flung out from nuclear reactions exceed this speed often enough for the water to emit a bluish glow.
    Last edited by a moderator: Jul 19, 2003
  24. Jul 19, 2003 #23


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    oops, thanks for the clarification. I see now that what you said initially made sense.
    As for the entaglement thing... I wouldn't mind listening to the gory details if you wouldn't mind telling them.
  25. Jul 19, 2003 #24

    I wanted to correct some of the statements others have made and answer a few of the questions that came up in response to particles travelling faster than c. Some of these answer have already been clarified by Peter_C. Note that I am NOT answering any of these questions based on a "guess" or a cool dream I had or an Isaac Asimov book. If I am unsure, I will let you know and not mislead anyone. Also, please correct me if I am wrong.

    "a type of theoretical particle that can travel faster than c"

    These particles are indeed theoretical, in the sense that someone conceived of them simply to make physics look more symmetrical. These particles are called "tachyons" and have NEGATIVE MASS. Note that this hypothetical negative mass is DIFFERENT from the negative EFFECTIVE mass you define in a crystal lattice for electrons and holes. I will stress that, to the best of my knowledge, there is NO THEORETICAL EVIDENCE to support their existence.

    "quantum entanglement being interpretted as an instance in which information travels faster than c"

    The relevant theory explaining this is called EPR (Einstein-Podolsky-Rosen). Please visit this link (I will could consider it a reliable source). Read the article VERY CAREFULLY and at least twice before coming to any conclusions.


    Here is a quote:

    "a detailed analysis of the EPR scenario shows that quantum mechanics violates locality without violating causality, because no information can be transmitted using quantum entanglement"


    "Cerenkov radiation violating causality - i.e. sending info. faster than c"

    As Peter_C explained, Cerenkov radiation does not travel faster than c, it travels faster than the speed of light in a dielectric medium. This is analogous to a sound wave travelling faster to the speed of sound in a gas. Approximately, the speed of light in a medium is given by c/n, where n > 1 (lets not talk about meta-materials). Therefore, the speed of light in a medium is LESS than the speed of light in vacuo.

    In addition, quantum tunelling has nothing to do with quantum entanglement, other than the fact that they are both separate effects of QM.

    "how is it possible for a mass-possessing neutron to exceed c"

    Neutrons never exceed the speed of light in vacuo. They can, however, exceed the "local speed of light" in a medium. For perhaps a better explanation than what I have provided, see, once again, Wikipedia:


    "the gory details of EPR theory and quantum entanglement"

    A suggestion: not many people are qualified to give you the gory details. Make sure the person you are talking to is qualified to answer tough questions on this. Even simple questions rely on careful interpretations. Don't be shy to ask them if they are qualified if you are looking for a straight answer. It is VERY EASY to misinterpret results. I am NOT qualified to answer questions on EPR, I have only seen a handful of lectures on the topic. Keep in mind that people like Einstein and Bell (not A. G. Bell) argued over the interpretation.
  26. Jul 19, 2003 #25
    Re: Corrections

    Actualy tachyons have negative mass squared. Specifically in the equation;


    the parameter m2, which has the dimensions of a component of four momentum has a negative sign. A less confusing description might be to say that they have a "rest momentum" as opposed to a rest energy for ordinary matter.

    The Higgs particle is a spinless tachyon, and in the form of pseudo particles they have be used to describe ferromagnetism.

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