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Is light massless

  1. Sep 9, 2013 #1
    I read an article a while back, I can't seem to find it now, that stated that photons can deteriorate into smaller particles, and that photons actually have mass. Is this a senseless article, or is there scientific evidence to this happening. The article said that after a trillion years or so, light would deteriorate into lighter particles, I believe a neutrino was referenced as one of the possibilities. If this is true, then would the speed of light no longer be a constant? Would light be slowed down, and actually experience time passing? (The article said that light would experience 3 years or so of time over that trillion) How would a photon decay if say only 3 years passed for the photon. I heard about an experiment that stopped light , could this experiment stop light for 3 years, and watch the photon decay into neutrinos or perhaps some other never before seen particle. Does time pass for photons when they travel through indexes of refraction like water, or do the same rules apply? Thanks for clearing this up in advance.

    Edit: Forgot some questions, If light is not massless, than why is the speed of light measured the same no matter the speed of the detector, perhaps the speed doesn't change too much based on the detectors speed, but maybe the our measurement of the speed of light could be a little off based on the speed of the galaxy? Also I've heard light referred to as having no standing mass, what exactly is meant by this?

    Edit: Found the source http://www.livescience.com/38533-photons-may-emit-faster-than-light-particles.html
     
    Last edited: Sep 9, 2013
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  3. Sep 9, 2013 #2

    ZapperZ

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  4. Sep 9, 2013 #3

    UltrafastPED

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    Currently accepted theory holds that the photon is an elementary particle, and has no mass or charge.

    Special Relativity has as an axiom that the speed of light is the same in every inertial reference frame, from which follows the fixed speed limit designated c. If you want to learn about relativity in a careful but practical way I recommend Taylor & Wheeler "Spacetime Physics".

    Does the photon experience time? This is an apparently meaningless question for anything travelling at the speed of light! I can say that light follows the "null geodesics" of General Relativity - they have a spacetime interval of zero - that is because the spatial and temporal "distances" traveled are subtracted from each other - but as they always have the same values their difference is always zero.

    Light travelling through glass or water or air - or any other transparent form of matter - suffers a delay in the wavefront which is consistent with the idea that it slows while passing through. But at the quantum level of the photon the actual process is "coherent forward scattering" - coherent because it can propagate an image, forward because the light is transmitted in the forward direction, and otherwise it is normal quantum particle scattering - which means that the light is not absorbed and then reemitted - this process is much like Huyghen's wavelets. The photons always travel at c, but the phase interference delays the wavefront. For a good read try Feynman's "QED: The Strange Theory of Light and Matter", also available online as four 90 minute lectures.

    You were reading about some speculative theory ...
     
  5. Sep 9, 2013 #4

    D H

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    The problem is that LiveScience article. That article exemplifies bad science writing. It missed the point of and misrepresented Heeck's paper, choosing instead to focus on the woo-woo instead.
     
  6. Sep 9, 2013 #5

    jtbell

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    I think that under the modern understanding of SR, it's better to state this the other way around: SR has an axiom that there is a universal speed limit, c. Objects with zero mass must travel at exactly c, and objects with nonzero mass must travel at less than c.

    Photons have a mass that is either zero, or currently experimentally indistinguishable from zero; therefore they travel at a speed that is either exactly c, or currently experimentally indistinguishable from c.

    Any measurement has limit on its precision, which can be described as a numeric "±" value, or an error bar on a graph, or some other way. When we say "the speed of light is measured the same no matter the speed of the detector" this is understood to be followed by "within the precision of our detector(s)." It is certainly possible that the speed of light may actually vary, but by such a small amount that we haven't been able to detect it yet, either by direct measurements or by measurements of related phenomena (such as the mass of the photon).
     
    Last edited: Sep 9, 2013
  7. Sep 9, 2013 #6
    It actually stated if photons had mass, they might decay.

    It is believed the photon is massless and no mass has so far been experimentally detected.

    "If wishes were horses, then beggars would ride."
     
  8. Sep 9, 2013 #7

    D H

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    Kind of. The LiveScience article starts with the headline-grabbing title "Photons May Emit Faster-Than-Light Particles, Physicists Suggest", and opens with "The particles that make up light, photons, may live for at least 1 quintillion (1 billion multiplied by 1 billion) years, new research suggests."

    That's just wrong. That 1018 years is a lower bound, not an upper bound as suggested by that opening sentence -- and that's assuming that photons do decay. That of course is a might big "if". As has been mentioned multiple times in this thread, photons are massless per the standard model. Every experiment aimed at measuring the mass of a photon has reported a result consistent with a massless photon.

    That "faster than light particles" in the headline is also misleading. If photons decay (and hence have non-zero mass), the term "speed of light" becomes a bit of a misnomer. A better term would be "ultimate speed" or "universal speed limit". The decay products would not be faster than this ultimate speed.

    Since Heeck's article has been published in a peer reviewed journal, discussing that article is allowed per the forum rules. Let's keep the LiveScience nonsense out of the discussion.


    For those who don't have access to the published article, here's a link to a preprint version:
    http://arxiv.org/abs/1304.2821
     
  9. Sep 10, 2013 #8

    UltrafastPED

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    I like Einstein's original statements - they seem quite physical to me, and the mathematical structure can be derived from them. See http://en.wikipedia.org/wiki/Special_relativity#Postulates

    "In his initial presentation of special relativity in 1905 he expressed these postulates as:[1]

    The Principle of Relativity – The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.[1]

    The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body." (from the preface).[1] That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source."
     
  10. Sep 10, 2013 #9
    Just a little nitpicking: to avoid confusion (or reduce it, because confusion is reigning concerning this point!), it's better to say The Principle of Constant Light Speed. Commonly people understand "invariant" to mean "unaltered by a particular transformation of coordinates", while here is meant everywhere the same and unaltered by a change of the speed of the source. Invariance between inertial frames comes from the PoR.
     
  11. Sep 10, 2013 #10

    D H

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    That's now viewed as a bit archaic. A more modern view is that the observation that space-time locally looks like Newtonian space-time and that locally the universe obeys Newtonian mechanics leads to the geometric conclusion that space-time is a Riemannian manifold. That in turn says that there exists exactly one speed that must necessarily be the same to all observers. You get Newtonian space-time and Newtonian mechanics if that universally agreed upon speed is infinite. If it's finite you get Einstein's relativity. Experiments from Michelson-Morley on say that this universally agreed upon speed is finite.

    The term "speed of light" was a bit of an archaism even in Einstein's day. Maxwell had shown that light was a form of electromagnetic radiation almost half a century earlier. A more accurate term would have been "speed of electromagnetic radiation". One word for that: Yech. Post-1905, an even more accurate term would have been "ultimate speed". That describes the speed at which all massless particles travel, not just photons (assuming they truly are massless), but also gluons and the hypothetical graviton.

    Sticking with this archaic term "speed of light" to really mean "ultimate speed", if photons are not massless they don't travel at the "speed of light". They travel at some lesser speed. If photons are not massless, it no longer makes sense to talk about the speed at which photons travel as this speed now varies with the observer. It still does makes sense to talk about this ultimate speed, archaically called the speed of light.
     
  12. Sep 10, 2013 #11

    UltrafastPED

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    Since first studying Special Relativity almost fifty years ago I have always believed that the speed of light is the ultimate speed.

    I hardly think that the choice of words here changes the physics.
     
  13. Sep 10, 2013 #12

    D H

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    It does here. By believing that the speed of light is the ultimate speed you are implicitly assuming that photons are massless, and that they travel at this ultimate speed. What if photons are not massless?

    In his article, Heeck made the non-standard assumption that photons are not massless. He furthermore assumed that they might then decay (massless particles cannot decay) and then asked whether the CMBR has anything to say about this. That it does is the point of this article.
     
  14. Sep 10, 2013 #13

    UltrafastPED

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    The article is speculative ... as I mentioned before ... so maybe his conclusions follow from his premises, but there is no evidence in support. So what should I think?
     
  15. Sep 10, 2013 #14
    Speculation is...sometimes interesting, sometimes without value,
    and that this thread has run it's course??
     
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