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Reading about photons

  1. Apr 6, 2006 #1
    As I was reading about photons, Some of the information was puzzeling for like; photons have no mass or that they dont have a kinetic energy.

    I understood having a zero mass at rest but when traveing with the speed of light the photons should gain a mass ( I think ) because of 2 universal evidences 1) The space-time curvature of a body such as a star causes the gravitational lensing and should the light have a mass to be affected by the mass of the sun? 2) The light is completely trapped in a black hole because of its extreamly intense gravity or the space-time wrapture so should it have a mass to be effected by the gravity?

    The next was about the Kinetic energy of light but if I understand the problem with the mass I will sure solve this one myself :smile:

    I'll be really thankfull for your answers.

    To make much more clear what I mean is that; If and if photons gain a mass due to their speed they probably have a kinetic energy too. right?
    If so than E= (2ke/c^2)c^2 which means E=2ke which again means:
    E=2(mv^2/2) where v=c so E= mc^2 And this shows that the mass is directly proportional to its speed and that the mass is variable at least in case of Electromegnatic rays...??

    Now I'm 15 years old and I haven't yet studied the other types of masses expressed in many of the posts. so I would thankfull for using simple forms of explanation, THANKYOU.......:smile:
    Last edited: Apr 6, 2006
  2. jcsd
  3. Apr 6, 2006 #2


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    i am loathe to answer, because even if i quote nearly verbatim from my 30 year old Modern Physics text (Beiser), what i would say would be contradicted by many here. or, at least, they would say that the perspective of such is more complicated or contrived or just not as "right" as the perspective that is in vogue now. so i'll leave it to someone else.
  4. Apr 6, 2006 #3


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    It sounds like the OP has worked hard to confuse himself.

    Photons have energy, and momentum. What they don't have is mass (invariant mass).

    (Do photons have kinetic energy? It's hard to say, but they definitely have energy! That's the important point).

    The fact that photons don't have mass also does not mean that photons do not interact with gravity. The idea that "mass causes gravity" is an old idea from Newtonian gravity. In GR, gravity couples to energy, not mass. To be more precise, the source of gravity in GR is the stress-energy tensor - not mass.
  5. Apr 6, 2006 #4
    have a look at the answer given by rbj at the thread photon mass i have started. i consider that it is the clearest approach to the problem.
  6. Apr 7, 2006 #5
    mubashirmansoor .... you remind me of myself and pervect has the right idea.

    use this tool as well as take a riding on reading ....


    http://arxiv.org/PS_cache/cond-mat/pdf/0503/0503313.pdf [Broken]


    pervect ..... that is beautiful. Stress energy tensor nice 'new' stuff.... I kept thinking "entanglement." But this cross over is a round about method I have been trying to arrange for quite some time. Thank you for the theoretical description.

    and your right again Photon's do not exhibit the properties of "invariant mass" but "yes" they have mass and can share it's spin in associations

    feshbach resonance ?!?!

    again thank you very much
    Last edited by a moderator: May 2, 2017
  7. Apr 7, 2006 #6
    Toist, Thankyou for the links. The links are perfect but some how not related to wht I asked...:)
    Last edited: Apr 8, 2006
  8. Apr 7, 2006 #7
    ??? It's the Einstein field equations!

    The first paper was completely off-topic, as far as I was aware. SLE? Conformal Field Theory?

    Is anyone else confused?
  9. Apr 7, 2006 #8


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    <ZapperZ raises hand>

    I just thought we'd see where this ends when the blind leads the blind....

  10. Apr 7, 2006 #9


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    Well, to be a bit more serious


    talks about the stress-energy tensor. The basic idea is really not all that hard, one combines energy density (a scalar), momentum density (a 3-vector), and pressure (a 3x3 matrix) into a big 4x4 matrix, called the stress-energy tensor.
  11. Apr 7, 2006 #10
    Incidentally, would you say the matrix IS the tensor? I'd say it's the tensor expressed in a certain basis... I'm not being pedantic, just checking to make sure my notions aren't wrong.
  12. Apr 8, 2006 #11


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    It's probably best to say that the matrix is a representation of the tensor.

    The main point is that a lot of things contribute to gravity in GR - energy, momentum, and pressure. Mass per se, however, is not one of those things that appear explicitly in Einstein's field equations for gravity.

    Mass indirectly contributes to gravity, of course, through it's contribution to energy, which is explicitly part of the stress-energy tensor.
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