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Gravitational redshift

Cause : A reduction in the decompression of space* over distance (due to the presence of mass).

Mainstream theory: Photons are emitted in a region of higher gravity with Energy A, and received in a region of lower gravity with Energy A, where Energy A is lower than the Energy B that would have been emitted by a similar event in a region of lower gravity.

Evidence: There is plenty of experimental evidence to support this theory.

Cosmological redshift

Cause: A reduction in the decompression of space* over time (due to universal expansion).

Mainstream Theory: Photons were emitted in the distant past with Energy B and received in the present with lower Energy A, where B is the energy that would be emitted from a similar event in the present.

Evidence: No hard evidence that I am aware of. Some may argue that CMB was emitted as ultraviolet radiation and we now see it as microwave radiation. I agree, but if we are being consistent with the hard evidence we have about gravitational redshift, then we should expect that time is moving much more quickly today than it was when the CMB was emitted, just as it moves more quicky in regions of lower gravity. So yesterday's ultraviolet wave is today's microwave - with the same energy.

*I use the word space for brevity, but really mean the universal metric through which light travels.

As far as I can see, apart from their causes, the only difference between cosmological redshift and gravitational redshift is that one happens over space and the other happens over time. So why is the mainstream view on cosmological redshift inconsistent with the mainstream view on gravitational redshift, for which there is ample supporting evidence?

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  • $\begingroup$ Both of these effects can be computed in the same way from the same equation. It is true that, in the usual coordinate systems, one of them involves a change in $g_{tt}$ (temporal component) while the other involves a change in $g_{ii}$ (spatial component). $\endgroup$ – knzhou Jan 13 at 11:49
  • $\begingroup$ Of course, you can change your coordinates so that both are due to $g_{tt}$, I suppose, or both due to $g_{ii}$, or really anything else. But it really doesn't matter. You can solve a problem in more than one coordinate system. So it is unclear what "inconsistency" you're referring to. $\endgroup$ – knzhou Jan 13 at 11:51
  • $\begingroup$ @knzhou I'm referring to the difference I highlighted in bold. In the one case we say that the energy of the photon remained constant, in the other case we say it reduced. $\endgroup$ – Alan Gee Jan 13 at 15:44
  • $\begingroup$ what you describe as redshift is not a shift, if the energy does not change. Do you have a link for the statement? $\endgroup$ – anna v Jan 13 at 15:53
  • $\begingroup$ @anna v a shift in frequency is not necessarily an energy change if time is running faster, So my question can also be phrased as 'why do we think time dilation occurs due to gravity but not due to (opposite of)expansion?' $\endgroup$ – Alan Gee Jan 13 at 16:02
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Consider a garden pond with evenly spaced pebbles across it.

A frog and a fish start out at one end and cross it. The frog with a constant hop rate across the pebbles, the fish at a constant swim speed. They reach the other side at the same time. In this static situation there is little to choose between the two creatures because they both get the same job done in the same time.

Now the owner of the pond decides to extend it a little, and after doing so, adjusts the spacing of the existing pebbles to make it look pretty again. The frog and the fish repeat their journey, and this time the frog gets to the other side first. That's because the frog had the same number of pebbles to hop, at a constant hop rate, while the fish had further to swim.

When Physicists analyse light from distant galaxies, they assume that it travelled through space and time like the fish through an expanding pond. Gravitational time dilation, on the other hand, indicates that it travels more like the frog.

I know that when light travels through space that has been warped by the presence of mass its speed varies accordingly. Yet I am expected to believe that when light passes through space that has been warped by universal expansion, its speed remains constant. Correct me if I'm wrong, but for me that smacks of inconsistency.

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  • $\begingroup$ If you would like to verify that the current paradigm is self-consistent, you're free to get any textbook on general relativity and verify the equations yourself. We're not hiding anything -- the book will even help you evaluate it. The problem, as I stated in the comments above, is that the math-free intuition one uses can be quite unreliable, changing depending on the reference frame. In practice, in GR physicists often calculate things rigorously first, and only afterward drape some intuitive words around the final result. $\endgroup$ – knzhou Feb 27 at 2:40
  • $\begingroup$ @knzhou I am not basing anything on intuition. I am basing it on what I know about light, and I know a lot about light. I refuse to blindly accept things just because most people do. You are welcome to GR with all of its abstractions, I will focus on simple truths. $\endgroup$ – Alan Gee Feb 27 at 8:04
  • $\begingroup$ And from where did you get that information about light? Light is quite unintuitive. Electrical engineers can spend a lifetime studying its subtleties. And you're sure you know everything about it without even knowing any of the math? $\endgroup$ – knzhou Feb 27 at 11:23
  • $\begingroup$ What I do know is, that by stubbornly insisting that the speed of light can never vary, you are hiding the majority of the meaning of $$E = mc^2$$ which as far as I am concerned is like self inflicted harm. It will probably take someone with far more clout than me to convince people of this, but it will happen. $\endgroup$ – Alan Gee Feb 27 at 18:20
  • $\begingroup$ What are you actually proposing? If you're proposing an alternative theory that makes alternative predictions, go find out what they are, and make sure they're not already observed to be false. If you're proposing a different way of looking at the same theory, nobody really cares unless you can explain why your way is more useful. Right now you are saying that you really really really like one particular classical fact, and will take any interpretation that preserves it at the cost of ruining everything else. You can choose to do that but you can't choose to make others do it. $\endgroup$ – knzhou Feb 27 at 18:44

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