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Why don't photons possess mass?

  1. Aug 22, 2004 #1
    Yea, noob question here, if e=mc^2, and mass and energy is the same thing, why is a photon massless when it has energy?
  2. jcsd
  3. Aug 22, 2004 #2


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    Photons always travel at the speed of light and have 0 rest mass. For gravitational purposes, the energy acts like mass.
  4. Aug 22, 2004 #3


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    Cos the full formula is [itex]E^2 = m^2c^4 + p^2c^2[/itex], whic gives you the energy of a particle in an inertial frame. Photons don't have mass but they do have momentum (p), so they have energy.
  5. Aug 22, 2004 #4


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    Furthermore, photons carry EM radiation, and if they had mass EM radiation would have a longitudinal component. But no such component can be detected to ever increasing levels of accuracy. So the photon's mass has to be less than a number starting with a decimal and followed by dozens of zeros before a significant digit, and that's the UPPER LIMIT. So the theoretical requirement that it have zero mass is confirmed in experments up to the limit of experimental accuracy achievable.
  6. Aug 22, 2004 #5
    how would you "measure" the mass of a photon? to test it w
  7. Aug 23, 2004 #6
    so are you saying that photons do possess mass but it is just extremely miniscule?
  8. Aug 23, 2004 #7
    E=mC^2 gives us the relation between the mass and energy. according to it mass and eenergy are interchangeble, not that mass always posses energy or anything having energy always have mass.
  9. Aug 23, 2004 #8


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    NO! That's why I put the stuff about an upper bound in caps. Theory says the photon has no mass, and experiments have found no mass, but being experiments they have a margin of error, and that margin is an extremely tiny number. Also whenever they figure out how to do a better measurment, the margin always goes down. All the experiments are consistent with the statement that the mass is zero.
  10. Aug 23, 2004 #9


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    As jcsd said, the complete relation has also a term for momentum.
  11. Aug 24, 2004 #10
    so, the energy comes from their momentum instead of their mass? but isn't p=mv, and if mass is 0, momentum is 0. Or is there another formula for it?

  12. Aug 24, 2004 #11
    Photons of course have momentum and the momentum is due to the mass, but still the photons do not have any mass this is because,the energy that the photon had, changed in mass. But neither the change in the mass nor in the energy could be noticed, according to the hysenberg's uncertainty principle
    dm*de=h/2*pi (transformation of the standerd equation)
  13. Aug 24, 2004 #12

    If you are saying that the Photons contains EM radiations then i think you are seriously missconcepted about photon. In fact photon itself is a radiation as photons were introduced to explains the particle character of the EM waves.
  14. Aug 24, 2004 #13


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    This is a FAQ in relativity. The m in E=mc^2 was time ago the "relativistic mass". The m in the "complete" formula is the "rest mass". Modernly people prefers to use only the latter, to avoid this kind of misunderstandings.
  15. Aug 26, 2004 #14
    For details on this subject/FAQ see the thread named "Mass of Light" in "General Physics". The same conversation is going on there.

    As I've often reminded folks here, many people still refer to "relativistic mass" (aka inertial mass) simply as "mass".

    E.g. the newest modern text that I have is Cosmological Physics, by John A. Peacock. When he speaks of mass in the first chapter he is not refering to rest mass. E.g. he refers to T00/c2 as mass density where, as you know, T00 is energy density.

    The misunderstanding that arises on topics like this is due to a lack of knowledge on the subject. It is not due to "poor" or outdated knowledge. Ignoring it can still lead to misunderstandings. Only complete knowledge and understanding will lead to a minimum of misunderstandings.

  16. Aug 26, 2004 #15
    The most beautiful story about the photon mass can be found in Feynman's lecture on gravitation. I am not going to try to reproduce Feynman's great style. Basically, he is telling the story of a game, where another famous physicist asked him to prove that the photon has no mass. Feynman answered he is willing to play, under the condition : "give me an upper bound, I'll answer. After that, you are not allowed to change the bound !" Of course, the other physicist cheats, and gives Feynman four or five small and smaller bounds.

    Anyway, I am not home so I don't have the book with me, I could not even copy the two or three pages if I wanted to. Really, try to find the book. Feynman derives Einstein equations from the assumption "spin 2 massles", as if we did not know about gravitation, but understand QFT already.
  17. Aug 26, 2004 #16
    Photons are massless according to Higgs theory because they do not interact with the Higgs field.Protons and electrons do.The Higgs field has not been proven to exist bu t even if it does not it would seem reasonable that a photon is massless because it does not interact with whatever field does allow protons and electrons to have mass.
  18. Aug 27, 2004 #17
    interesting, so the answer to this actually lies in the higgs field? any reason as to why the photon don't react with the higgs field???
  19. Aug 27, 2004 #18
    I don't know a physical mechanism but someone on here might have a mathematical reason!
  20. Aug 27, 2004 #19
    It's a wave; do waves have mass? You could say they are mass, but they are just particles exchanging velocity (as in all waves?); as written by another scientist here at this forum, mass is caused by particles in movement, and there is really no other solution to the problem. Try to explain it yourself.

    By the way: p=mv, so sure, photons are mass
    Last edited: Aug 27, 2004
  21. Aug 27, 2004 #20
    The energy and momentum of a photon are respectively [tex]hf[/tex] and [tex]\frac{h}{\lambda}=\frac{hf}{c}[/tex]. Putting this into the equation [tex]E^2 = m^2c^4 + p^2c^2[/tex] you will find m=0.
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