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Photons are massless?

  1. Jul 4, 2008 #1
    I do not understand how photons are massless.

    I understand why they need to be massless, because if they had mass, you could accelerate them a little more and they would be going faster than the speed of light, but as a massless particle, you can't accelerate them, so that makes sense.

    Here are the reasons I cant grip the idea that they are massless:

    The first reason is because I thought they hold energy, and if they contain energy, can't you consider that a small amount of mass? (E=mc2)

    Second, Photons can't escape blackholes, and are effected by gravity. I thought only things with mass were under the effect of gravity.

    Thanks, Zach
  2. jcsd
  3. Jul 4, 2008 #2


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    Please start by reading our FAQ in the General Physics forum.

  4. Jul 4, 2008 #3


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    Quick answers:
    Having an amount of energy equivalent to an amount of mass is not the same thing as having mass.
    No. Gravity is not a force, it is the curvature of spacetime. Photons, like everything else, try to follow the shortest path through spacetime, which, near a gravitational source, is curved.

    But, as Zz points out, the FAQ answers these questions better.
  5. Jul 5, 2008 #4


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    Why would having mass make them faster than c? It would make them harder to accelerate.
  6. Jul 5, 2008 #5
    This is a terminology snafu that confused me for a long time too. In the equation E=mc^2, "m" refers to "effective mass" and E refers to "effective energy". Unfortunately, when physicists use the letter "m" or the word "mass" in almost any other context, they mean "rest mass" and they assume you know this (despite the fact that no one ever says it out loud).

    "rest mass" and "rest energy" mean the mass/energy that an object has specifically in the reference frame in which it is standing still. "effective mass" and "effective energy" mean the mass/energy that an object has in any particular reference frame, including the extra energy it has because it is moving. Note that almost no one ever says "effective energy", they just assume you know that "energy" usually means "effective energy" (*grumble*).

    So photons have zero rest mass, but if they are moving relative to you, they have some effective mass due to the fact that they are moving.

    Then you haven't met relativity. In classical physics, only mass is affected by gravity. In relativity, everything with any effective mass (which means everything with energy, which, I think, is everything) is affected by gravity. In fact, effective mass is in general the "real" mass in relativity. The only reason to talk about rest mass is to be able to talk about some value that doesn't depend on our frame of reference, that is, an "invariant".

  7. Jul 5, 2008 #6
    Yes, thank you. That was helpful. So if something has rest energy, it does not mean it has rest mass. E=mc2 is like a one way street, it works in looking at mass as energy, but not in looking at energy as mass, correct?
  8. Jul 5, 2008 #7
    Not at all. That wouldn't even make sense, since equality is transitive. a=b always means b=a.

    If something has rest energy, its rest mass is described exactly by [tex]m = {E \over c^2}[/tex]. Photons have zero rest energy and zero rest mass. They only have effective mass and effective energy. That is, they only have mass/energy because they are moving relative to us.

    EDIT: In fact, you might want to reread my earlier post. I wasn't trying to draw a distinction between mass and energy (there is no distinction except terminology), I was trying to draw a distinction between "rest" and "effective". The central problem was that you had rest mass and effective mass mixed up.
  9. Jul 6, 2008 #8
    ohh ok. Wow, so...that is bizzare. Photons have no rest energy, yet they have effective energy because they are moving. This is strange to me because it seems that because a wave of nothing is moving at the speed of light, it has effective energy. I mean I know its not nothing, its just not...mass or energy. So, light is like something completely unique from mass and energy that just happens to carry energy, correct?
  10. Jul 6, 2008 #9
    Hmmm... I don't know exactly how to answer that, but I think you're getting the idea. It certainly has mass/energy from our point of view (after all, it is moving from our point of view), but it is completely different from something that has rest mass. Actually, it's fair to say that there are two categories of objects in the universe:

    1. things that move at the speed of light and have no rest mass, and
    2. things that move at some speed less than the speed of light and do have rest mass.

    Nothing can move at the speed of light if it does have rest mass and nothing can move less than the speed of light (and still be meaningful to us) if it has no rest mass. Photons, gluons, and perhaps gravitons (if they exist) are in the massless, speed-of-light category, and everything else (I think) is in the massy, less-than-speed-of-light category.
  11. Jul 8, 2008 #10


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    I ask this question. I thought the FAQ explained it quite well. There is another equation showing that their rest mass is 0.
  12. Jul 12, 2008 #11
    dave answered it perfectly,photons ARE massless, gravity curves space-time EVERYTHING mass (any mass) energy (any energy) is subsequently effected.
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