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Does light slowly spread out longitudinally?

  1. Jul 13, 2015 #1
    Has there ever been any detection of a pulse of light spreading out longitudinally even by an infinitesimal amount? I'm aware of the expansion of space (~74 km/sec/Mpc), but talking about something else. So for example if our light pulse is say one second in duration, then after traveling through millions of light years of deep space, will the light pulse still be one second?
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  3. Jul 13, 2015 #2
    Inevitably, there will be delay, but it may be because the index of refraction of space is not exactly 1, because it is not a perfect vacuum. Not every part of a 1 sec long light wave will encounter the same dilute gas atoms, so there will be spreading.
  4. Jul 13, 2015 #3
    I think what you are asking, which may help answer your question, is:
    Has a photon at the front of a pulse of light ever travelled at a different velocity to one at the rear of the pulse?

    The fact they're in the same pulse doesn't really make a difference, so then perhaps an even more fundamental question is:
    Has one photon ever travelled at a different velocity to another photon?

    If you ignore space-time curvature/expansion or material interaction then the answer is of course no. If you take that into account, as the above poster says, the answer is yes.
  5. Jul 13, 2015 #4
    A light pulse emanating from a high gravitational potential , say near a star, to a lower gravitational potential, say relatively open space, will be observed as red shifted. So while it travels at 'c' it nevertheless has a longer wavelength....it's lost some kinetic energy.
  6. Jul 13, 2015 #5


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    It isn't clear what level you are talking here. You seem to be (or could be) referring to Dispersion. A single pulse (or, indeed any modulated light beam) consists of a number of sidebands (frequencies on either side of the unmodulated, monochromatic source). In any medium but empty / flat space, the speed of light would not be the same for all frequencies so a pulse could be expected to become 'spread out' longitudinally.
  7. Jul 13, 2015 #6


    Staff: Mentor

    As sophiecentaur said, it sounds like you are asking about dispersion. However, you could also be talking about redshift. It is hard to tell without more context, but dispersion is a result of the medium and redshift happens due to the geometry of spacetime.
  8. Jul 13, 2015 #7
    Thanks for the replies. Yes I think dispersion describes my question. Has this been observed experimentally? If so, then did they determine it was caused entirely by different speeds of light in the medium as mentioned by sophiecentaur.
  9. Jul 13, 2015 #8


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    Yes, every time you see a rainbow.

    Dispersion is different speeds of light, by definition, so the direct answer to your question is tautologically "yes". Perhaps you had something else in mind?
  10. Jul 13, 2015 #9
    Sure, what I had in my mind was the photons having some slight interaction with each other. I question the perfect nature of light, which brings up another question. Isn't there a probability that a photon traveling through free space will be partially absorbed by the quantum foam? Thanks
  11. Jul 14, 2015 #10
    What do you mean? A photon is as 'perfect' an elementary particle as any other.

    A pulse of light and a photon are not the same thing and may 'behave' differently. When we detect a photon, when it registers on test apparatus, it appears as a 'point' particle; When we detect such phenomena we observe their physical action at small scales takes place in discrete steps.... as in multiples of 'h'.

    From other discussions in these forums.....
    "...A photon with a perfectly known position will have an arbitrary frequency. Conversely, a photon with a perfectly known frequency will have an arbitrary position..."

    and maybe wikipedia, not sure:

    "Photons, like all quantum objects, exhibit both wave-like and particle-like properties. [but not at the same time]..The photon displays clearly wave-like phenomena such as diffraction and interference on the length scale of its wavelength. For example, a single photon passing through a double-slit experiment lands on the screen exhibiting interference phenomena but only if no measure was made at the actual slit…. To account for the particle interpretation that phenomenon is called probability distribution but behaves according to Maxwell's equations. However, experiments confirm that the photon is not a short pulse of electromagnetic radiation; it does not spread out as it propagates, nor does it divide when it encounters a beam splitter.

    Also, maybe someone could comment on whether a photon takes only a 'single' path. Seems like many possible paths would be possible via QM....however unlikely.

    QED is the quantum theory of photons; Leonard Susskind has an online video on the subject which you might enjoy. I haven't seen it in quite a while, but I recall it was insightful.

    Wikipedia notes, under QED:
    : "In technical terms, QED can be described as a perturbation theory of the electromagnetic quantum vacuum..." but I am not conversant with all that.
    Last edited: Jul 14, 2015
  12. Jul 14, 2015 #11


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    There is no such thing as "quantum foam" as you're understanding the phrase. It's a metaphor that science writers use when they're writing about a particular (but not proven) hypothesis and they don't think their audience can handle the math required for the real thing. It's an OK metaphor as far as it goes, but it's not the real thing and you can't use it as a foundation for building new ideas or a deeper understanding.

    The hypothesis that light dissipates energy in a vacuum has been tested observationally - google for "tired light" and be careful about crackpot sites - and comes up short.
  13. Jul 14, 2015 #12
    @Finny, sure, I'm just using the term "photon" to describe light. Thanks for the mention of experiments since that's what I'm looking for. I wish the science community would create an open source website dedicated to housing as many experiments as possible.

    I've always pictured light as something that travels flawlessly through free space maintaining the same size longitudinally (in reference to free space, which means no particle interaction), 100% efficient without loss. Does such a thing exist? I understand the uncertainty principle, so I'm referring to experiments that would consist of repeating the experiments millions of times over in sync. What in nature is 100% perfect? Isn't everything we look at have some dissipation, loss, dispersion? Sound waves slowly spread out longitudinally and have some loss due to the medium they travel through. As a sound travels through air it slowly heats up the air due to some loss. Is space or spacetime truly lossless and perfect? Does anyone see what I'm getting at? Rotating galaxies have loss. Electric pulses through wires have loss. The thought of a light traveling across the universe through space with only the loss from particle interaction seems beyond amazing. I think this boils down to what is space or spacetime.

    @Nugatory, "tired light" sounds fascinating. If it's studied by academic scientists, then I'm very interested in it. Perhaps this is what I'm looking for. Many thanks!
  14. Jul 14, 2015 #13
    In open space, light travels endlessly through a vacuum on a null geodesic..... It travels on as straight a line as is available w/o energy loss. It travels at a constant speed 'c'.

    Light does not lose energy unless moving among different gravitational potentials. So if you want an example where light may not move 'flawlessly', better to say, an example where light loses energy and is redshifted, it happens on every every emitting star.

    A more technical description I liked..... from PeterDonis , post #59 is here:

    "... the photon's 4-momentum gets parallel transported along its worldline [and] is not frame-dependent, so there is an invariant way of defining what it means for the photon's 4-momentum to not change. On this view, as long as the photon is moving freely (i.e., it's not in a waveguide or some other device that causes it to move on a non-geodesic worldline), it's 4-momentum *never* changes, so any change in its observed frequency *must* be due to a change in 4-velocity of the detector relative to the emitter. ..."

    This is also a reminder that the detected wavelength of light can appear shifted if there is relative motion between emitter and detector. But this is NOT commonly referred to as any kind of 'dispersion' as already discussed above.
  15. Jul 14, 2015 #14
    @Finny interesting. Is there truly zero probability of a photon in free space transforming into other particle(s) or perhaps splitting into two lower energy photons due to virtual particles? Sorry, I can't subscribe to this ideal concept of light, yet.
  16. Jul 14, 2015 #15


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    In this thread we've been talking about a flash of light, a short pulse of electromagnetic radiation - not photons.

    We have Maxwell's equations, which predict that electromagnetic radiation propagates at speed ##c## in a vacuum no matter the frequency, wavelength, and distance traveled. More complex effects appear when we're not in a vacuum (rainbows and sunsets and the blue of the sky are examples) and these are also predicted by Maxwell's equations. The experimental and observational support for this theory is overwhelming.

    Photons are not what you think they are, and they are pretty much irrelevant to this discussion. Indeed, because they aren't what you think they are, if you're trying to understand the behavior of light in terms of photons you'll end up misleading yourself.... You have to dig fairly deep into quantum mechanics before you bring photons into the picture.
  17. Jul 14, 2015 #16
    Why would you say that? Wikipedia has an entire page on quantum foam, given many peer reviewed references of expensive experiments of scientists trying to detect it. It's very well established mathematically according to QM.

    Also scientists are spending £1 billion to prove that vacuum of space is filled with particles. Their experiment will shine multiple high power laser beams into a small region of space to force these particles apart.
    Last edited: Jul 14, 2015
  18. Jul 14, 2015 #17
    You accuse me of not knowing what photons are. Basically I've been asking for experiments. So what did I say was incorrect? I think I have a decent understanding of the QM photon.
  19. Jul 14, 2015 #18
    It's surprising you're getting after me for using the word "photons" in this thread.

    WikiPedia article quotes on light:

    "EMR (electromagnetic radiation) in the visible light region consists of quanta (called photons)..."

    "In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[4][5] In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Like all types of light, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles."

    "In the quantum theory, photons are seen as wave packets of the waves described in the classical theory of Maxwell. The quantum theory was needed to explain effects even with visual light that Maxwell's classical theory could not (such as spectral lines)."
  20. Jul 14, 2015 #19


    Staff: Mentor

    They don't interact directly because neither has charge. However, if they are sufficiently high energy then they can produce an electron-positron pair and interact that way. There has to be a center of momentum frame and the pair must be sufficiently massive in that frame. So two photons moving in the same direction cannot interact this way.

    Currently the "quantum foam" is largely pop-science nonsense and where it is not pop-science nonsense it remains speculative and untested. Treat any comment about it with extreme skepticism. Such topics on PF must be based on the professional literature, not the pop sci literature.
    Last edited: Jul 14, 2015
  21. Jul 14, 2015 #20
    It's looking that way that photons interact with the vacuum. If particle pairs occur like you say, then it obviously takes energy from the photon, which would change the light.

    Wikipedia provides peer reviewed references about the quantum foam.
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