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I have always thought of light as a point-like object in space, with the EM field strengths dependent on time.

However, stretching of space would not affect a point-like object.

Thus, I assume that a photon occupies a non-zero-sized space, it has different parts located at different places, and the expansion of space pulls the different parts of a photon apart, so that the photon becomes bigger.

We usually visualize EM waves as 2 sinuses, one for the electric, one for the magnetic field. If a single photon is spread out in space, it has different parts all over the place, this depiction is not correct, as it only tracks 1 part of the photon. It would be more correct, to add at least 1 spatial dimension to the depiction, and along the spatial axis, draw a section, or a fuzzy section, to show that the wave is spread out in space.

point-like in time:

E
^
|  .-.       .-
| /   \     /
|/     \   /
|       ._.
+-------------> time

band-like in space:

E
^
|  ..--..        ..--
| //    \\      //
|//      \\    //
|         ..__..
+-------------> x

QUESTION:

Am I correct in these assumptions?

If not, how can expansion of space affect the frequency of light?

Zoltan K.
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2 Answers2

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This is a completely classical effect, so there is no need to talk about photons. (And in any case, photons are not really pointlike, since they behave according to wave-particle duality.)

Cosmological expansion acts as though space itself is expanding. (The mathematical expression of this in general relativity doesn't actually express it that way, but it's a good enough conceptual framework for most purposes.) When space expands by a factor $a$, the wavelength of an electromagnetic wave expands by the same factor.

At a slightly more rigorous level, you can model two successive wavefronts by two null geodesics. The proper distance between these (i.e., the distance measured by an observer at rest relative to the Hubble flow) scales with $a$.

  • Maybe I am missing the point here. Does your answer mean that an EM wave occupies a non-infinitezimal volume in space, and as the space expands, that volume expands, too? Or you refer to a description of an EM wave, where it depends on something else, than time? (I checked the corresponding solution for the Maxwell equations, but I did not find anything helpful. Maybe I just did not comprehend the useful parts.) I don't get, why are you talking about 2 wavefronts? Please excuse my confusion! :) – Zoltan K. Jul 03 '19 at 21:14
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It is the relativistic doppler effect.Imagine a source emmiting waves with a costant frequency .Now we start moving the source relative to you. Let's take the period of time before the second emission and after the first emission.Since the source is moving relative to you, its position in space will change . And when the time comes for the second emission the distance between the waves will be different than if the source wasnt moving relative to you.So you believe that the frequency of the source is different than its actual frequency.

Now when the space its expanding , its like the galaxy from where the light comes from is moving away from you!

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    It's possible to conceptualize cosmological redshifts as kinematic Doppler shifts, but that doesn't really work ultimately, because GR doesn't have a frame of reference big enough to include both the emission and observation: https://physics.stackexchange.com/questions/458854/how-do-frames-of-reference-work-in-general-relativity-and-are-they-described-by –  Jul 03 '19 at 18:12
  • ???What do you mean? –  Jul 03 '19 at 18:35
  • This is experimentally proven wrong answer.

    We know that the light is more redshifted if the source is farther away. This means that something has happened on_the_way, gradually. The way you describe the "normal" doppler-effect acts when the wave reaches the observer.

    Even more, even though the universe expands faster then light, the speed of the stuff moving around in it does not acquire faster then light movement speed... the rate of change of distance between 2 points can be attributed to movement and space expansion, and these are different.

     – Zoltan K. Jul 03 '19 at 21:06
  • Hmmm....The universe doesnt expand faster than light.Nothing can travel faster than light.If the universe was expanding faster than light we would not see the stars. –  Jul 03 '19 at 21:09
  • We don't see all the stars. There will be a time, when we will only see the stars in out galaxy, because everything else will be getting away from us too fast. Every minute, there are galaxies, from where light will never reach us. That is the horizon, where the distance between us and another point separates faster than the speed of light. – Zoltan K. Jul 04 '19 at 00:00
  • The statement "nothing can move faster the the speed of light" is false. Information is cannot travel faster. The known laws of physics allow for traveling faster then the speed of light, but nothing can travel at exactly the speed of light in vacuum. Check the special relativity equations for details. Of course, in matter, you can travel faster then the actual light, and then you have the pretty Cherenkov radiation. What is important here, the expansion of the universe is not movement, so the speed of light is irrelevant. – Zoltan K. Jul 04 '19 at 00:03
  • Easy explanation: https://www.youtube.com/watch?v=vIJTwYOZrGU – Zoltan K. Jul 04 '19 at 00:07
  • Gravitational waves travel at the speed of light so the expansion of the universe from dark energy cannot travel faster than light. –  Jul 04 '19 at 01:56
  • Special relativity doesnt allow anything to travel faster than light.Massless particles travel at the speed of light and mass particles travel below the speed of light. –  Jul 04 '19 at 01:57
  • If the universe was expanding faster than light that would mean it would expand faster than the speed of gluons so in the end all atoms will be broken apart. –  Jul 04 '19 at 01:58
  • The expansion of the universe is information saying that the space-time becomes flat so it cannot exceed the speed of light. –  Jul 04 '19 at 02:00
  • And what you said the Cherenkov radiation , it is just normal radiation with a frequency bigger than any light,its propagation speed is still lower than c. –  Jul 04 '19 at 02:03
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    @user234867 Please see https://en.wikipedia.org/wiki/Hubble%27s_law and https://en.wikipedia.org/wiki/Cosmological_horizon Special Relativity forbids objects with mass from traveling through space at lightspeed or faster. But space itself can expand faster than lightspeed. The expansion speed between 2 points is (approximately) proportional to the distance between them. But this expansion is only noticeable on the very large scale. At smaller scales the expansion is dominated by gravity. – PM 2Ring Jul 04 '19 at 09:56