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Why does low frequency light appear to travel faster than high frequency light?

  1. Sep 5, 2012 #1
    I understand that new findings from the 'magic' telescope, suggest that higher frequency light arrives earlier than low frequency light. The results have been taken from a light source that is very far away. apparently this shows that spacetime is not uniform and that higher frequency light follows the minute countours closer than low frequency light and thus takes longer to propergate.

    if this is true, then why dosent light follow spacetime precisely, regardless of its wavelength?
  2. jcsd
  3. Sep 5, 2012 #2
    It would be nice if you could include a link to some reference.

    Usually one assumes that electromagnetic waves follow null geodesics. If the curvature varies significantly in the distance scale of one photon wavelength, then this assumption is poor and you would expect that low and high-frequency waves would follow different paths.
  4. Sep 5, 2012 #3


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    No. To cite the manuscript in question (http://arxiv.org/abs/0708.2889):"We cannot exclude, however, the possibility that the delay we find, which is significant beyond the 95% C.L., is due to some energy-dependent effect at the source."

    If one wants to model that effect as a property of space-time, one ends up with a slightly energy-dependent vacuum refractive index. However, it is not clear that the experimental finding points towards such an explanation.
  5. Sep 5, 2012 #4
    ok, I guess my question is more related to how exactly light propagates through space. Why exactly do photons of higher energy levels follow the distortions of spacetime more accurately than lower energy level photons?

    Please bear in mind my knowledge is fairly limited, I apologise if my questions seem obvious. I am trying to visualise a lightwave travelling through space.

    this seems like it answers my question, although I need a more detailed explination, something I can visualise.

    Again, this answer assumes I know more than I do, what are null geodesisc? I need to know how spacetime and the photon are interacting. or even in this definition what spacetime is and what a photon is, and how they relate to oneanother. My understanding of spacetime is limited to length contraction and time dilation, I am currently seeing spacetime as all pervading, how can a wave of electromagnetic energy only follow certain parts of spacetime? Do I need to visualise the photon as a particle following the contours of a fabric, like a bumpy road? Or light as a three dimensional wave, pulsing from a source, through a medium that is spacetime? Or neither of these?
  6. Sep 5, 2012 #5
    Could an explanation also be that dust and free hydrogen in space is slightly more transparent for these higher frequencies observed than for the lower frequencies? (I'm skeptical, because light's interaction with cosmic dust never seems to be the solution for any cosmological problem, but I thought I'd throw it out there.)
  7. Sep 5, 2012 #6
    The only theory with any experimental evidence to support it that can calculate this (GR), says that the energy momentum tensor is what causes spacetime curvature. This means that photons with higher energy change the spacetime curvature more than low energy ones. This effect is extremely small. I don't think that this translates to your statement that "high energy photons follow distortions more accurately".

    I believe the only theory that predicts differing arrival times for photons of differing energy is LQG, but I am no expert.
  8. Sep 5, 2012 #7
  9. Sep 6, 2012 #8
    I believe that's true. I was kinda thinking about that.

    EDIT: Here's a nice little pdf on that

    LQG predicts high energy photons to travel slower, since they "see" the microscopic nature of space-time better, and basically, are slowed down by it. Low frequency light doesn't really pick up on this micro-structure of spacetime, and so moves faster across it. To quote the pdf:

    peterspencers: in the title of this thread, you state that low frequency light travels faster than high frequency, yet in your OP, you said higher frequencies arrive earlier. Which is it?
    Last edited: Sep 6, 2012
  10. Sep 6, 2012 #9
    Yep, apologies, it wont let me edit it, but your right, it should read, 'later'

    How exactly do the higher energy photons 'see' as you put it, spacetime better?
  11. Sep 6, 2012 #10
    We think that spacetime, apart from being this continuous, smooth structure, must have very tiny fluctuations on the Planck scale in order to accommodate the Uncertainty Principle. Low frequency light, like radio waves, have long wavelengths, often on the order of a meter. Relative to that wavelength, space-time is totally smooth: there is no microscopic structure of which to speak, because the relevant scale for a radio wavelength is so much bigger than any quantum gravity scale. The relatively trivial interaction of long wavelengths with space time could then be considered a classical realm of a more general theory. On the opposite end of things, a gamma ray photon has an incredibly small wavelength, one which exists on a scale somewhat closer to the scale where quantum gravitational effects become relevant. This would be getting into the quantum realm of gravity.

    Imagine two cars: one normal sized and one very tiny, like a millimeter across. Both cars have the potential to reach the same speed and are traveling on the same road, but the bigger car (or in the physics case: the light with the big wavelength) has big tires that hardly interact at all with the small bumps and grooves on the road: the road appears to be very flat and smooth to a driver of that car, so it rolls along smoothly. The other car is much smaller, and the size of its tires is comparable to the size of these tiny bumps and imperfections in the road, this makes it much harder for the tiny car to drive over this road, and though both cars would be going to same speed on a perfectly smooth road, the normal sized car should beat out the little one.

    This is an exaggerated analogy, as spacetime fluctuations are very small even compared to gamma wavelengths, but the idea being that given enough traveling distance, low frequency, long wavelength light should be able to beat out high frequency light, small wavelength light.
  12. Sep 6, 2012 #11
    Transparency? I can't see how - but perhaps you meant difference in refractive index (dispersion)? In that case (hydrogen behaving like glass), higher frequencies should arrive later than lower ones. Hmm, that does match the corrected OP...
  13. Sep 6, 2012 #12
    Yeah, that's what I meant.
  14. Sep 6, 2012 #13

    The reference that you cite does not claim that light is traveling at different speeds at different wavelengths. It refers to a delay that may not have anything to do with the speed of light in a vacuum.

    To cite the manuscript in question (http://arxiv.org/abs/0708.2889):"We [Broken] cannot exclude, however, the possibility that the delay we find, which is significant beyond the 95% C.L., is due to some energy-dependent effect at the source."

    "At the source" means that the delay occurs inside the source, before the light has entered the vacuum. The source may be emitting light at different frequencies at different times. This is a highly important "however".

    How about a three dimensional wave, pulsing INSIDE the source, through a highly dense medium that is not a vacuum? Maybe the medium inside the source is highly dispersive, like flint glass. Maybe the light is diffracting off something very much like a diffraction grating. That would also cause a delay that has nothing to do with space time.
    I don't know one way or the other. It is an interesting article.
    Last edited by a moderator: May 6, 2017
  15. Sep 8, 2012 #14
    This is the same as the explanation given by Dr. Robert Wagner http://www.mpp.mpg.de/~rwagner/ (working at the 'magic' telescope) in the BBC horizon documentary I posted. It implies a physical interaction between light and spacetime, implying they are seperate physical things. I really dont get on with this explanation and would appreciate if someone could give me an exact description of the physics behind this. I know its not necessarily whats actually going on, however, the physicists at the 'magic' telescope are considering it a possible explanation for there results. I would like to know how this is theoretically possible?
    Last edited: Sep 8, 2012
  16. Sep 8, 2012 #15
    It sounds like what he means is, "Photons have a nonzero rest mass." If this is true, then I don't understand why he doesn't say it just like that.
    The current theory says that a photon has a rest mass of zero. If that is true, then all photons in a vacuum would travel at the same speed irrespective of their energy. If one proved that photons in a vacuum with different energies traveled at different speeds, then one could conclude that the rest mass of a photon is not zero. This sounds to me that the scientists in this study found that photons with different energies traveled at different speeds.
    The statement that space time and light interacted sounds equivalent to saying that the rest mass was not zero. By the law of equivalence, a greater mass means a greater gravitational field. A greater gravitational field means that the spacetime is warped. Thus, photons of different energy are interacting with the spacetime differently.
    I don't know why Wagner has to bring in space time when the concept of rest mass is well known among physicists. He may be hypothesizing that more laymen are exposed to the concept of space time then to the concept of rest mass. So he describes things in terms of space time.
    If so, then I think he should have taken the opportunity to explain "rest mass" to the interested layman. That way, there won't be confusion when the results are verified if they ever are verified.
  17. Sep 8, 2012 #16


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    Because that's not a plausible explanation. The effect of a small rest mass would only be seen at low energies, comparable to the mass itself. What he's observing is a dispersion in the GeV range.
  18. Sep 8, 2012 #17
    I dont think Dr Wagner was referring to the non zero rest mass.

    He and Dr Giovanni Amelino-Camelia http://en.wikipedia.org/wiki/Giovanni_Amelino-Camelia were offering the hypothesis that the results could indicate the existence of QG and that spacetime had a consistiency analogous to 'cappacino foam' and that photons of shorter wavelengths were able to traverse the naturally inconsistent nature of spacetime more accurately; leading to longer relative travel time.

    It is this explanation that I am having difficulty grasping. I understand the principle behind a photon having a non zero rest mass and how this could warp space time to varying degrees around photons of different energies; leading to differnt travel times.

    And I can also grasp the idea of quantum gravity, but what baffles me, is if QG is real, then why should this effect different wavelengths of light in different ways?

    the explanations to which I am refering...... http://www.bbc.co.uk/iplayer/episode/b01mmrc0/Horizon_20122013_How_Small_is_the_Universe/ from approximately 40 miunites in..
    Last edited: Sep 8, 2012
  19. Sep 8, 2012 #18


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    Vacuum fluctuations provide one mechanism to explain this. If a photon coincides with an energy fluctuation then the energy density in a small region of space might be enough to change into ( say) and electron and anti-electron. This state lasts for a very short time before the e+ and e- anihilate each other, recovering the photon. This slows the photon. The probability of a low energy photon having this interaction is much lower than that for a gamma or X-ray. The probability is so low that it is unlikely to happen many times, even in 14 billion years.
    Last edited: Sep 8, 2012
  20. Sep 9, 2012 #19
    Ok, thankyou very much for that explanation, I do follow the logic in what you have said. Is this potential phenomenon due to QG? and also, is it fair for me to conclude that the explanation given about high energy photons more accurately following the distortions of spacetime.... complete nonsense? It seems like the given explanation (in the documentary) is so far removed from any of the possible reasons for the telescopes results (given by you guys in the forum) that the physicists there wouldnt even consider it as even a crude metaphor.

    I get the feeling the physics behind there explanation has not yet been picked up by anybody here yet........ has anybody actually watched the programme to which I am referring? I have posted it multiple times in the above posts
  21. Sep 9, 2012 #20
    To which "recent" MAGIC result are they referring to? Is it this one?

    Note that there are more recent measurements performed by Fermi-LAT, which on the contrary showed no energy dependence of the speed of light from GRB sources. This is considered a major blow to Lorentz violating QG theories:
    Abdo et al, http://arxiv.org/abs/0908.1832

    Even Amelino-Camelio wrote in: "Astrophysics: Burst of support for relativity". Nature 462 (7271): 291–292.
    Thus as far as I can see, the mainstream consensus is that the speed of light doesn't depend on its energy or frequency. And as of yet, there is no experiment that refutes this.
    Last edited: Sep 9, 2012
  22. Sep 10, 2012 #21
    I tried watching that program you posted, but it doesn't seem to be available outside the UK =/
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