Dismiss Notice
Join Physics Forums Today!

Why can't a photon exceed the speed of light?

  1. Sep 22, 2007 #1
    Since it does not have mass?
  2. jcsd
  3. Sep 22, 2007 #2
    All massless objects have to move at exactly the speed of light to have real energy.
  4. Sep 22, 2007 #3


    User Avatar

    photons don't have rest mass. and they cannot have rest mass precisely because they move at speed c for any observer.
  5. Sep 22, 2007 #4
    A photon can't travel at less than the speed of light, c, either (including trajectories within refractive materials). That a photon travels at c only (a postulate of relativity), and that any entity moving at the speed of light has zero rest mass are both observational truths.
  6. Sep 23, 2007 #5
    However, are there these postulated particles (tachyons) which supposedly travel and velocities >c.

    Allow me to refer you to the wikipedia article.

  7. Sep 23, 2007 #6


    User Avatar

    Staff: Mentor

    Light by definition travels at the speed of light, not more and not less (except when passing though transparent material where it interacts with the local EM field).

    No one knows 'why'. We only know from experiments that light and other massless objects travel at the speed of light.

    Nature is what is it is - independent of our observations and models. The challenge for us is to understand it, without necessarily knowing why.
  8. Sep 23, 2007 #7
    why does an apple fall down?
  9. Sep 23, 2007 #8


    User Avatar
    Science Advisor
    Homework Helper

    An apple gets attracted because of gravitational force, just like a photon will get attracted through it. Back on-topic?
  10. Sep 23, 2007 #9
    i read a bunch of articles on how some researchers overcame the speed of light, but only using photons, not something that has mass. Also, in the issue of Scientific American just a month ago (i think), there was an article on how some woman succeeded in slowing down photons by orders of magnitude...
    so your responses confuse me
  11. Sep 23, 2007 #10
    I think another postulate of relativity is also that nothing can exceed the speed of light.

    But there are "things" that travel faster than c.

    Does that mean that relativity is somehow a flawed theory?
  12. Sep 23, 2007 #11


    User Avatar
    Science Advisor

    There are no known phenomena that can transmit information, matter or energy faster than c. Certain other "things" such as wavecrests or shadows can move faster than c, but this doesn't violate that rule (you could never exploit these to send a signal faster than light, for example).
  13. Sep 23, 2007 #12


    User Avatar
    Science Advisor

    Maybe you're thinking of this? If so see the discussion on that thread, most physicists would say that this experiment cannot actually transmit information faster than light and so is not a violation of relativity.
    Photons can travel slower when moving through a medium like water, it's only the speed of light in a vacuum which can't be exceeded (and in a medium, you can roughly imagine that photons are slowed down because they are repeatedly absorbed and reemitted by the particles that make up the medium, although this is an oversimplification). The "c" in relativity's equations always refers to the speed of light in a vacuum.
  14. Sep 23, 2007 #13
    Your example is about how, not the why.
    I think the why Astronuc was speaking of is: behind the gravitational force itself, why does masses create a gravitational field, why does matter attract, why does charges interact ?
    Well, i wont say it better, the challenge here is understanding the universe, to maybe just hint at an answear to why it works that way.

    Correct me if I am wrong.
  15. Sep 23, 2007 #14
    Not in any bit.
    The theory says nothing with real rest mass can travel at c or higher.
    The tachyons are hypothetical particles that have imaginary mass, hense no contradiction.

    Correct me if I am wrong.
    Last edited: Sep 23, 2007
  16. Sep 23, 2007 #15
    It is not a postulate. This follows from the two postulates if you consider any finite system with real mass.
  17. Sep 23, 2007 #16


    User Avatar

    i didn't think that the postulates of relativity mention photons. what the postulates of special relativity say is that [itex]c = 1/\sqrt{\epsilon_0 \mu_0}[/itex], the wavespeed of light always is the same for every inertial observer. but, just to not be mistaken, i am not advocating that photons as particles do not move at the wavespeed c and then as a consequence, must have zero rest mass. i think that such is the case.

    i dunno how they measure such, but there is published lit that puts an upper limit of invariant mass for photons that is not zero.

    Tachyons, if they exist, are not photons.

    and there was some published lit somewhere (can't find the reference other than http://www.math.ucr.edu/home/baez/physics/ParticleAndNuclear/photonMass.html ) that puts upper limits of the photon rest mass at something like 10-17 eV/c2 or 10-27 eV/c2 (implying that photons may move a little slower than c, but not enough that you would see much of a difference for light that travels the radius of the observable universe). but just because there are these experimental upper limits, does not imply that photons have rest mass at all, only if they might, they would have mass less than such limits.
    Last edited: Sep 24, 2007
  18. Sep 23, 2007 #17
    How fast does gravity travel?
  19. Sep 23, 2007 #18


    User Avatar

    it's s'posed to be (according to GR) the same c.
  20. Sep 24, 2007 #19
    Photons have no mass in the classical sense (called rest mass). But remember that energy and mass are just different forms of the same thing, so they do have mass due to their energies. We do observe something called light pressure or electromagnetic pressure. When light hits an object, it feels a small force or pressure – this would imply that they do have inertia. A photon cannot slow down – the speed of light is constant. If it were to “slow down”, space and time around it would become warped so that it was still traveling 3 X 10^8 m/s. As for a force acting on it, since photons don’t have a rest mass and they don’t have a charge, there aren’t any forces that can act on them in order to change their motion (that we know of). They are affected by curved space-time, but this only changes the direction, not the speed. The only way they can be “stopped” is if they are absorbed into something and the energy is transformed from kinetic into heat (radiation) or electrical (photoelectric effect) energy.

Have something to add?

Similar Discussions: Why can't a photon exceed the speed of light?