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Does Light Have Mass?

  1. Aug 20, 2008 #1
    I get confused because i am told it has mass but if it has mass wouldnt the laws be different and we would feel the weight of the sun light when we go outside?
     
  2. jcsd
  3. Aug 20, 2008 #2

    Dale

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    Hi Champion, welcome to PF.

    There have been experiments designed to answer exactly this question. All of them to date are consistent with the idea that the photon is massless.
     
  4. Aug 20, 2008 #3
    light as particle is massless
     
  5. Aug 20, 2008 #4
    what do you mean as particles? it take's other forms?
     
  6. Aug 21, 2008 #5
  7. Aug 21, 2008 #6

    HallsofIvy

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    Yes, light acts both as a particle and as a wave. In fact, if you get small enough (the size of elementary particles) the very notion of "particle" or "wave" loses meaning.

    By the way, your original argument, "we would feel the weight of the sun light when we go outside?" is invalid. If light had a very low weight, we wouldn't feel it. Do you feel the weight of the air?
     
  8. Aug 21, 2008 #7
    so if it doesnt have mass why can it not escape from a black hole? why does it experience gravity?
     
  9. Aug 21, 2008 #8
    Wouldn't we lose the "feel" of air for a different reason? I can feel it when I drink from a straw. lol

    Light has enough mass to feel the effects of a gravitational field and also exerts its own gravitational attraction, so it must have mass.
    But, by definition, it can't have any rest mass.

    At rest it would be massless, but it can't be at rest. At the speed of light, it shows the properties one would expect from mass.
     
  10. Aug 21, 2008 #9

    HallsofIvy

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    No, light does not have mass. In the theory of relativity, "gravity" is a property of space around a massive object. Anything moving around a massive object has its trajectory different from a straight line whether it has mass or not.

    I should warn you that you will stir up a nest of hornets if you refer to "mass" as other than "rest mass". Light has energy, not mass.
     
  11. Aug 21, 2008 #10
    How do hornets have mass? I can't feel them.

    So, mass always has energy so that total energy is concerved, but kinetic energy doesn't have mass? Isn't that just arguing semantics?




    (Let it be known that I had no ill will when poking the hornet's next with a stick...just bored)
     
  12. Aug 21, 2008 #11
    light cannot escape a BH because the escape velocity is >C. alternatively, the photon cannot escape from BH because spacetime is warped to the extent that a straight line cannot extend past the EH, but is rather curved back upon itself.

    light does not have measureable mass, per se, but as per E=MC2, the amount of energy of a given photon is equivalent to a fixed amount of mass.
     
  13. Aug 21, 2008 #12

    Dale

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    Hmm, I am quite skeptical of this claim based on my own youthful experiences with bees and wasps.

    No, it is not semantics. I would highly recommend you look into the standard relativistic concept of the four-momentum. In geometric terms you can think of the mass of a particle as being the length (Minkowski norm) of its four-momentum vector, and the energy is just one component of the vector. So the distinction between the two concepts is not just semantic.

    The other common usage of the word mass is "relativistic mass". This is the hornet's nest refered to earlier. The concept of "relativistic mass" is deprecated by most modern physicists precisely because there is no distinction (other than semantics) between "relativistic mass" and energy. So it is more clear to use the term energy when refering to "relativistic mass" and reserve the term mass to refer strictly to the invariant rest mass.
     
  14. Aug 21, 2008 #13

    Dale

    Staff: Mentor

    Think about this a little carefully. What does a satellite's orbital path depend on? So in the limit as m->0, what would you expect?
     
  15. Aug 21, 2008 #14
    Does this mean pain is analagous to mass? I never really felt them until they announced their presence in the unsavory way they're known to do.


    The other common usage of the word mass is "relativistic mass". This is the hornet's nest refered to earlier. The concept of "relativistic mass" is deprecated by most modern physicists precisely because there is no distinction (other than semantics) between "relativistic mass" and energy. So it is more clear to use the term energy when refering to "relativistic mass" and reserve the term mass to refer strictly to the invariant rest mass.[/QUOTE]

    Exactly, semantics. The terms hold more meaning and clarity when the term "mass" is limited to invariant rest mass.....




    All in fun, I just wanted to see what responses I'd get. Agitated assertions of one's point tend to bring out more definite answers. (An agitated "authority" also tends to talk down to the "moron" with use of simple analogies....which helps this particular moron gain a better understanding) lol
     
  16. Aug 21, 2008 #15
    Mass is structured energy and as light is also a form of structured energy it will show mass-like characteristics. However the wave structure of light dictates that the mass effect alternates between a photon and an anti-photon and so at the end of the day you end up with zero mass.
     
  17. Aug 21, 2008 #16

    Mentz114

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    Matter, which makes up everything with mass, is very different from light. Arguing about whether light has mass is a dead end because it makes no difference to anything whatever you position you take.

    Assigning matter-like properties to light in an attempt to understand it analogously with matter is also pointless. Maxwell's equations tell all.
     
  18. Aug 21, 2008 #17
    Can light have a frequency so that E = M?

    I am guessing the answer would be yes; energy of any finite frequency can satisfy that equation as long as the time of exposure is sufficient.

    Then let me be more clear;

    If we assume that a maximum frequency of light exists (denoted by the planck length); What if the observer was to accelerate towards the source of this light of maximum frequency?

    Energy of the light must increase with the increase relative velocity, but the light's frequency can't go any higher. Would it be viable to assume that light packets condense into mass and decrease in velocity and in frequency since that is the only way energy can be conserved in this situation?
     
  19. Aug 21, 2008 #18
    if it doesn't have mass then why does it bend in gravitational field?

    hmmm....light cannot escape a black hole because the light is not bounce back so we can see it? what we see is what is not absorbed?
     
  20. Aug 21, 2008 #19

    jtbell

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  21. Aug 22, 2008 #20

    atyy

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    The excuse given in the post for not using the relativistic mass is that it is speed dependent. However, light always travels at the speed of light, so it happens to be the only sort of particle for which you might think the relativistic mass is also invariant, and therefore useful to put in a table. However, looking at the definition of relativistic mass in that post, if v=c, then the relativistic mass contains zero in the denominator. If a particle with rest mass greater than zero travels at the speed of light, its relativistic mass is infinite, which is nonsensical. One way to save the formula is to say that only particles with zero rest mass are allowed to travel at the speed of light. If a particle travels at the speed of light and its rest mass is zero, then its relativistic mass is indefinite (still nonsensical, but not as nonsensical as an infinite mass). I like this idea that light has some sort of indefinite mass, because then it makes sense that it should be attracted by gravity.

    Strictly speaking, the special theory of relativity is not compatible with gravity. The indefinite mass of a photon is just a heuristic to see that a proper theory of gravity should predict the deflection of light. However, it is actually possible to construct coherent modifications of the special theory in which gravity doesn't bend light. So ultimately, that gravity bends light is based on experiment, not intellectual necessity.
     
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