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Mass of the photon

  1. May 30, 2015 #1
    according to einstine energy and mass both are euivalent it means mass can be converted into energy and energy can be converted into mass.
    it means photon is a kind of energy particle with velocity is equal to velocity of light if we condense the energy of photon the we will get the mass of it.
    but photons are mass less whyy??
  2. jcsd
  3. May 30, 2015 #2
    I'll try and answer it to the degree which I think I understand it. The idea is that mass actually has 2 parts. The first part is what general population would call mass. This is intrinsic mass which exists regardless of motion. The second part is mass increase due to motion. It seems that as a body increases speed it's total mass increases. This makes sense because according to relativity, the speed of light is the maximum speed. The mechanism to make sure this speed limitation is not violated is that the total mass necessarily needs to increase with speed. A photon can only exist when it is moving at the speed of light in a vacuum (or slower than the speed of light in other medium). Thus a photon at rest is not possible. Because a photon at rest is no longer a photon, it necessarily cannot have intrinsic mass.
  4. May 30, 2015 #3
    We can describe total energy of particle ##E^2=(pc)^2+(mc^2)^2##
    Then photon has no mass so photon energy will be ##E^2=(pc)^2##
  5. May 30, 2015 #4


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    Staff: Mentor

    For the last several decades, most physicists have also called this "the mass" of an object.

    For the first few decades after Einstein put forth his relativity theory, most physicists also used this notion of mass. It started to fall out of favor around 1950, I think. Even Einstein in his later years said that people should use the intrinsic mass (a.k.a. invariant mass or "rest mass") and not the speed-dependent "relativistic mass." When I was a graduate student in the late 1970s and early 1980s in experimental particle physics, where we worked routinely with highly relativistic particles, none of us used "relativistic mass."

    Nowadays "relativistic mass" appears only in popularizations of relativity, and in introductory-level textbooks that haven't been updated.
  6. May 30, 2015 #5
    So of relativistic mass it's not a phenomenon that physicists use what is the mechanism to insure the speed of light is not violated? Another words, why does added energy to a body in motion not continue to increase speed at the same rate?
  7. May 30, 2015 #6
    That given energy will increase speed and increase in speed will increase object energy.You are right If you add energy on moving object speed will increase.
  8. May 30, 2015 #7
    It will not increase at the same rate. Thus a Joule of energy added to a body for 1 second may give the object an increase of speed of 500 miles per hour but if we then add another Joule for another full second the next increase in speed will be less than the 500 mile per hour increase that we obtained the first time. A great way to explain this is to assume that the mass of the object must have increased with the first increase in speed. Thus relativistic mass explains this phenomenon and you previously mentioned that that relativistic mass is not used by physicists. If that is this case I am curious as to what physicists use to explain this phenomenon.
  9. May 30, 2015 #8


    Staff: Mentor

    First, relativistic mass is an outdated concept:

    But beyond that Einstein's famous equation E=MC^2 does not say energy and mass are equivalent - it says mass is a form of energy - not energy is a form of mass. It is a different type of energy to kinetic energy, electrical energy, chemical energy, thermodynamic energy etc etc. They are all forms of energy but they are not mass, nor is mass any of those forms of energy. Since energy is conserved in principle it is possible to convert from one to the other but they are not the same.

    Why this is was sorted out by a very great mathematician, Emmy Noether, in a justifiably famous theorem, Noethers Theorem, she proved in response to a query by Hilbert about Einsteins theory:

    Using that, proving E=MC^2 is a snap, and strikingly elegant:

    Just as an aside there is a discussion at the moment on what energy is:

    I gave the modern answer, basically the one I outlined here, but for some reason it didn't garner much support - don't know why.

    Last edited: May 30, 2015
  10. May 30, 2015 #9
    Thanks for the details
  11. May 31, 2015 #10
    thanxx guysss.
  12. May 31, 2015 #11


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    Staff: Mentor

    This is discussed in our FAQ section at https://www.physicsforums.com/threads/do-photons-have-mass.511175/ [Broken]

    Be aware that when you ask a question without bothering to look at the FAQ section or the other threads, you project the impression that you aren't really all that serious about learning. If it's not important enough to you to spend five minutes thinking about it, you shouldn't expect other people to spend a lot of time working on giving you a good answer either.
    Last edited by a moderator: May 7, 2017
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