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Massless photons is a hard concept to grasp

  1. Jul 19, 2007 #1
    how can an elementary particle like a photon be massless? How can physicist measure and detect a photon if the don't know its inherent properties? I know a photon travels at the speed of light , but wouldn't you have to know a particle mass , in order to know its speed? Or are physicists referring to a particle who's mass is extreemely close, to zero, but nonetheless the particle still has a nonzero mass when physicists refer to a massless photon?
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  3. Jul 19, 2007 #2


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    My experience is that laymen mistake "mass" with "substance".
  4. Jul 19, 2007 #3
    Can you not speak with someone even though you don't know his/her name, age, sex, ... or mass !? :tongue2:
    Speed is length divided by time, independent of mass.
    In our current models, the photon is theoretically zero, exactly, not close to. Now, from a pragmatic point of view, nobody can ever say "I have measured zero". You can only say "I have measured zero within [some] experimental errors" which is the same as saying that "the result is less than [some] experimental errors". So we will never have an experimental proof that the photon mass is exactly zero. It close to zero enough, so that for all purpose, we can set it to zero.
    Last edited: Jul 19, 2007
  5. Jul 19, 2007 #4
    Yes , but you would need to know the mass of a particle during a collision between two or more particles known as the momentum equation in order to calculate the speed of a final partilce. and if you divide by zero mass, the particle has infinited speed which is impossible since no particle can travel faster than the speed of light.
  6. Jul 19, 2007 #5
    Actually the photon momentum is given by

    p = [tex]\bar{h}[/tex]k

    Which is completely independent of its mass and tells us that photon momentum is based on its energy or wavelength depending on how you want to interpret it.

    not the classical p = mv equation.

    I hope this helps clear up your confusion.

    Welcome to the wonderful world of non-classical physics.
    Last edited: Jul 19, 2007
  7. Jul 19, 2007 #6
    I don't understand.

    I have a detector, an electron beam is directed to a certain target. Whenever one electron induces a collision in the target, and other recoil particles are produced (s.a., a photon), the first thing I do is to reconstruct my electron. Then I do know when the collision occured as well as where it occured (at the level of the target at a time which I know very precisely because the beam is very well under control).

    Now if I see in my detectors a hit not corresponding to any charged particle track, I have a photon candidate. To ensure that it is actually a photon (and not, say, a neutron) one of the means I use is to calculate its speed. Since I actually do that, I have a very hard time figuring out what your objection can possibly be.
  8. Jul 19, 2007 #7
    The photon's charge is probably known with a better precision that the masses of any other particle.

    Within a very small uncertainty, we can assume it is zero.
    So it is wrong to assume that the mass of the photon is unknown: we know it is zero within a very small uncertainty.

    Does anyone know the upper limit on the mass of the photon, and how it was established?
    The same kind of data are also well known for the neutrinos today.
    It would be funny to compare the available data for photon and neutrino and the methods involved.
    I guess the photon is easier to measure! :rolleyes:

    In the same line of thought, I read recently a page about the charge of the photon:

    New limit placed on photon charge

    The charge of the photon is also very well known: it is zero within an extremely small uncertainty.
    Last edited: Jul 19, 2007
  9. Jul 19, 2007 #8


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    also, there is some dispute or difference in semantic about using the term "massless" for photons without any qualification. photons have mass:

    [tex] m = \frac{E}{c^2} = \frac{h \nu}{c^2} [/tex]

    but no rest mass (or "invariant mass")

    [tex] m_0 = m \sqrt{1 - \frac{v^2}{c^2}} [/tex]

    because their velocity is [itex] v = c [/itex] .
  10. Jul 19, 2007 #9
    The particle data group indicates :
    • [tex]m_{\gamma} < 6 \times 10^{-17}[/tex] eV (from Ryutov (Plasma Physics Control Fusion 39 (1997) A73) : magnetohydrodynamics argument concerning survival of the Sun's field to the radius of the Earth's orbit
    • [tex]m_{\gamma} < 7 \times 10^{-19}[/tex] eV (from Luo et al. Phys.Rev.Lett.90:081801,2003) : determination of a limit on [tex]\mu^2A < 1.1 \times 10^{-11}[/tex] T/m (with [tex]\mu^{-1}[/tex] = characteristic length for photon mass; [tex]A[/tex] =ambient vector potential) however Goldhaber argue that because plasma current effects are neglected, the Luo limit does not provide the best available limit on [tex]\mu^{2}A[/tex] nor a reliable limit at all on [tex]\mu[/tex]. The reason is that the [tex]A[/tex] associated with cluster magnetic fields could become arbitrarily small in plasma voids, whose existence would be compatible with present knowledge. Luo reply that fields of distant clusters are not accurately mapped, but assert that a zero A is unlikely given what we know about the magnetic field in our galaxy.
    See references at the link to particle data group.
  11. Jul 20, 2007 #10
    I don't see why there's a problem with massless particles. I can only guess that there's a problem with your initial assumptions of what mass is.

    Treat mass as the manifestation of an as-yet unknown gauge field. (Dodgy terminology here as I know nothing about QFT, but I'm going to hazard a guess that a gauge field is what I'm talking about...). A photon is simply a quantum that doesn't interact with the mass-field force carriers - in precisely the same way that Z bosons don't interact with the electromagnetic field.

    What I am getting at is that it should not be unreasonable for a particle to have no mass if you treat mass in the same way as the other properties of matter.
  12. Jul 20, 2007 #11


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    You are not that far off. In fact, if the Standard Model is correct, ALL fundamental particles are actually massless! So intrinsically, mass isn't an inherent property. It is the coupling to the Higgs fields that gives many particles a property that we call "mass". Particles such as photons and possibly the "gravitons" do not couple to this field (at least not in the lower order interactions - a high energy theorist insisted I say that last time I asked him about this).

  13. Jul 20, 2007 #12
    Excellent, thanks. That's what I was trying to communicate, but I wasn't aware that the Higgs mechanism was considered 'fact' or as-good-as.
  14. Jul 20, 2007 #13


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    Although, as pointed out by ZapperZ, it's true that in the Standard model masses arise from the so-called Yukawa couplings of the particles with the Higgs field, I have a problem with introducing the mass this way. The problem is that there is no fundamental reason to say that the mass must be presented as arising from the interaction with some "mass field carrier". By that I mean that one may write down perfectly legitimate theories of scalar particles or spin 1/2 particles that are massless without invoking any "mass-carrier field". Mass is then simply a property of the field that says that energy is required to create excitations of the field even in the infinite wavelength limit.
  15. Jul 20, 2007 #14


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    There are, I'm sure, variations to this. I could also cite the effective mass in straightforward condensed matter system, where the electrons in your conductor do not actually have the same "mass" as the electronic bare mass, and how "holes", which are vacuum vacancies, can actually acquire mass simply via their self-energy interactions.

    We simply can't account for everything that people prefer.

  16. Jul 20, 2007 #15
    I was talking to a professional physicist a month or so ago who told me about a recent conference he attended. He said that only about half the attendees felt that Higgs was the right concept, and viewed it instead as some kind of flaw in the Standard Model.
  17. Jul 20, 2007 #16


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    That is nothing new. For example, if you read C.N. Yang's reflection on the whole LHC and the search for the Higgs, this is what he had to say:

    There are many physicists who will be excited if the Higgs is found. However, I know of many more that would be ecstatic if it isn't. People may not believe it, but this is how physics is done, and how "discoveries" are made when something unexpected happens.

  18. Jul 20, 2007 #17
    The last talk by J. Ellis I attended, he clearly stated the same opinion : it is clear that it would be much more interesting if the Higgs is not found. I even felt that he expected something else would be found. That was just my biased impression however. :smile:

    It would however be a diplomatic challenge to recover polician's confidence if we do not find (at least) one Higgs boson.
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