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Don't you know that photons are MATTER?

  1. May 21, 2003 #1
    Quote originally by Alexander from religion forum.

    "Don't you know that photons are MATTER? Being bosons they obey different statistics than fermions (electrons, protons), rather than that have same properties other particles have - they are wavy (as all particles), have spin, momentum, energy. What makes you think that photons are not matter?

    By the way, if you place about 3x1035 of green photons in a massless box, the box will acquire mass 1 kg. (Both inertial mass and gravitational mass)."

    Is this true? The last that I read from books published as late as 2002, photons were still considered quantum packets of energy with zero rest mass and traveled at the speed of light, not matter which has mass and therefore cannot travel at the speed of light.
    Last edited by a moderator: Feb 5, 2013
  2. jcsd
  3. May 21, 2003 #2
    Through Quantum Mechanics we find that they can be both matter and a wave. So they can have mass and still travel at C as a wave.
  4. May 21, 2003 #3


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    Re: Photons

    Part of the discussion is simply about how contemporary scientists use words. ("matter"(nonzero rest mass) vs. "electromagnetic radiation"(zero rest mass) "mass"(inertia at rest))

    Part of the discussion is substantitive, not merely semantic.

    As mainstream physicists talk (Tom confirmed this in another thread) "mass" means rest mass. Photons do not have mass. They are not ordinarily described as matter. Alexander is using words in an eccentric personal way if he calls them matter.

    On the other hand putting energy into a box certainly increases its inertia! A good deal of the inertia of the sun, for example, is due to the radiation energy in it and the kinetic energy----beyond what you would get if you took all the particles out and weighed them separately.

    This is an unintuitive kink in physicists language. A photon has zero mass, but if you put it in a box the box will then have more mass. There is an intuitive failure of the "additivity" picture. that is just how it is---how conventional physics language has evolved.

    If you focus on energy, everything will be all right verbally. The equation E = mc^2 only applies to bodies at rest (as Tom's recommended Special Relativity text states very clearly).
    You can add the energy of the light to the energy of the box and get a new energy for the box and since the box is at rest you can measure its inertia and also use E = mc^2 to calculate its new inertia and get the right answer.

    You just cant add inertias (as if the light were a material) because then you get the wrong answer.

    Now there is the substantive question. It would be good to check Alexander's statement that the inertia conferred by
    3E35 green photons is a kilo. (they dont have inertia but putting them in the box, which is at rest, gives the box that much)
    In natural units (to make calculating quick) the typical energy for green photons is 2E-28
    so the energy of 3E35 photons would be 6E7, or 60E6
    This is 60 x 22 grams or 1300 grams. So if Alexander said that
    2E35 green photons contribute 1000 grams to the box then he is to be congratulated! He is only a bit on the low side----just fine for back of envelope.
    Maybe his greens are less energetic and more yellowish than
    the ones I am thinking of! :smile:

    The substantitive question is probably more important than the verbal one. If he wants to call light a form of matter (which ordinary working physicists would not) then just let him---he probably doesnt do any harm by having his own verbal habits.
  5. May 21, 2003 #4


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    But what of "gravitational mass"? We've gone 'round about this before in the Forums, but does the extra inertia of the box cause it to have a stronger gravitational pull? This question is at the center of all the "Relative Black Holes" discussions; does relativistic mass have gravitational influence?
  6. May 21, 2003 #5


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    there is a considerable effort being made in the physics community to discourage the use of "relativistic mass" as a concept----einstein himself decided against it and contemporaries from Tom of PF to John Baez of Usenet have deprecated its use

    please don't say "does relativistic mass have gravitational influence?"

    But the real physics question is DOES ENERGY EXERT A GRAVITATIONAL PULL? And the answer is absolutely yes!
    The Einstein equation is not based on mass but on energy.
    The righthand side of

    Gmu,nu = 8pi Tmu,nu

    is essentially an energy density

    For sure having light in the box contributes to its
    gravitational attractiveness! Light is energy, so it has to.
    Mass is not at the root of gravity so bringing in the mass issue
    is sort of a red herring, gets things out of focus.
  7. May 21, 2003 #6


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    And there is a considerable effort being made in the physics community to *EN*courage the use of "relativistic mass" as a concept.

    re - "The Einstein equation is not based on mass but on energy." - It;s based on the Equivalence of mass and energy.

    The righthand side of

    Gmu,nu = 8pi Tmu,nu

    is essentially an energy density

    Not quite - The right hand side is a statement regarding relativistic mass - but in its most general form. You happened to use units for which c = 1 so you've hidden the mass-energy equivalence. The right side boilds down to being equivalent to M= E/c^2

    Look in MTW - When they introduce the stress-energy-momentumThey state quite specifically that energy = mass

  8. May 21, 2003 #7
    By the way, box don't even need to absorb photons.

    If you put 3x1033 green photons in a mirrored box, the box will still accure same 1 kg of gravitational and inertial mass.

    Easy to prove mathematically.
  9. May 22, 2003 #8
    Re: Re: Photons

    Photons can't have "rest" mass because they don't exist at rest.

    Yet they have both ineria and gravity.
  10. May 22, 2003 #9


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    Re: Re: Re: Photons

    Why don't they exist? Where did you get this idea from? Not that I haven't seen/heard it before nor do I neccesarily disagree with it.

  11. May 22, 2003 #10
    From experiment.
  12. May 22, 2003 #11


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    Sorrry. When I read that originally I misunderstood. I thought that you mean that photon's don't exist.

    However there is at least one Nobel Prize winner who claims that photon's don't exist, i.e. Willis Lamb

  13. May 23, 2003 #12
    You misunderstood. What I heard from him (in person) was: "We don't know what exactly is a photon."
  14. May 23, 2003 #13


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    It would be very hard form me to misunderstand what he wrote.

    From "Anti-photon," W.E. Lamb, Applied Physics B 60, 77-84 (1995)
    {He goes on to that effect, i.e. "comedy of errors," etc}

    How did I misunderstand that?

    He has a comments online at


    I find that last part kinda weird. If it makes sense in at least one case then how can he say there is no such thing!?

  15. May 24, 2003 #14
    What he means is that a photon simply is a mathematical object which can not be equalized with a billiard ball except for rare cases.
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