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Whether light has mass? if yes what is the reason behind it?

  1. Aug 5, 2010 #1
    whether light has mass?if yes what is the reason behind it?
     
  2. jcsd
  3. Aug 5, 2010 #2
    Re: light

    Light (photons) has no rest mass. But it has relativistic mass and momentum.
    More details for questions can be seen in this link.
    http://crib.corepower.com:8080/~relfaq/light_mass.html [Broken]
     
    Last edited by a moderator: May 4, 2017
  4. Aug 5, 2010 #3
    Re: light

    To my knowledge, a photon does not have mass. If it had mass it could not travel at the speed of light. This is because it would take an infinite amount of energy to accelerate it to that velocity. And if a massive object traveled at the speed of light it would have an infinite amount of mass.
     
  5. Aug 5, 2010 #4
    Re: light

    "Light (photons) has no rest mass. But it has relativistic mass and momentum.
    More details for questions can be seen in this link.
    http://crib.corepower.com:8080/~relfaq/light_mass.html" [Broken]

    This paper is wrong, photons are massles bosons and momentum cannot be applied nor can be describe as having momentum. You could say they have magnitude but not momentum.
     
    Last edited by a moderator: May 4, 2017
  6. Aug 6, 2010 #5
    Re: light

    Light can be used to cause objects to move. In other words, they impart momentum to objects. Thus, if we are to believe the law of conservation of momentum, we have to assume that the initial momentum of the photon was non-zero.
     
  7. Aug 6, 2010 #6
    Re: light

    Particles in QFT are irreducible representations of the proper orthocronous Lorentz group. The momentum [tex]p^{\mu}[/tex] is the generator of space-time translations. Proper orthocronous Lorents transformations leave invariant the square [tex]p^{2}=p^{\mu}p_{\mu}[/tex] and the sign of energy [tex]p^{0}[/tex]. Only representations with [tex]p^{2}\geq 0[/tex] exist in Nature (as far as I know). The ones with [tex]p^{2}=m^{2}>0[/tex] are massive particles, the ones with [tex]p^{2}=0[/tex] are massless particles, like photons. For theese, [tex]p^{\mu}=(\hbar\omega,\hbar\mathbf{k})[/tex], and the relation [tex]p^{2}=0[/tex] implies [tex]\omega=|\mathbf{k}|[/tex], that is, photons travel at the speed of light c (=1 in my units). Things that go at c cannot have mass.
     
  8. Aug 6, 2010 #7
    Re: light

    light is wave ,it has no mass
     
  9. Aug 6, 2010 #8
    Re: light

    The electron is also a wave, and it has mass.
     
  10. Aug 7, 2010 #9
    Re: light

    Why photons can not be described as having momentum?
     
    Last edited by a moderator: May 4, 2017
  11. Aug 7, 2010 #10
  12. Aug 27, 2010 #11
    Re: light

    but what would you like to say about the photoelectric effect?Light there acts as an impulse.....i suppose impulse is created by only those bodies with mass
     
  13. Sep 20, 2010 #12
    Re: light

    Light is energy, and as E=mc2 says, energy and mass each carry a "punch"

    Also, light has no mass, but does have momentum. Momentum for zero-mass particles relates to their energy, not mass. Energy of a massless particle equals planck's constant x frequency.
    Momentum of a massless particle equals that same (Planck,s constant x frequency) divided by speed of light.
    p=(hf)/c
     
  14. Sep 20, 2010 #13

    Drakkith

    User Avatar

    Staff: Mentor

    Re: light

    Light has no rest mass, but it does have energy. I'm not sure whether that energy is in the form of magnitude, or momentum, or whatever. Whats the difference in magnitude and momentum in this context anyways?
     
  15. Sep 20, 2010 #14
    Re: light

    In classical electrodynamics light carries momentum and energy. You can't really talk about photons because the theory produces waves however it is still a field theory and relevant to QM.

    In this sense, light has no rest mass, waves don't have a mass in classical electrodynamics, however light can carry energy and momentum. As a result of light waves carrying momentum we get radiation pressure. Radiation pressure is something that can physically be measured supports momentum carried by light.

    It's very important. Which way is the momentum "flowing"? The direction is actually given by a tensor, horribly named the Maxwell stress tensor.
     
    Last edited: Sep 20, 2010
  16. Sep 20, 2010 #15

    Drakkith

    User Avatar

    Staff: Mentor

    Re: light

    I'm not sure i understand what your saying. Is the momentum of the wave the amount of energy it carries? Is the magnitude the direction?
     
  17. Sep 21, 2010 #16
    Re: light

    The energy an EM wave carries is Planck's constant x the wave's frequency E=hf

    So yes, the momentum derives from the energy of the wave. Momentum p =(hf)/c
    or momentum = wave's energy divided by light speed.

    I am no scientist, but do love physics. All I could find of magnitude is that it is a scalar value (or so I understood it).
     
  18. Sep 21, 2010 #17
    Re: light

    So when you say, is the magnitude the direction, I think (?) an EM wave exists in all directions until it interacts. If so, I don't know if "direction" applies here?
     
  19. Sep 21, 2010 #18
    Re: light

    Magnitude and direction are the two quantities that define a vector. So no, the magnitude is NOT the direction.
     
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