I have always wondered why we consider that galaxies are drifting away and not just contracting at a particle level so that locally there is no visible contraction. I mean, if gravity consumes energy, that would make sense that all particles shrink simultaneously, no?

  • $\begingroup$ Why do you think that gravity consumes energy? $\endgroup$ – G. Smith Apr 27 '19 at 11:36
  • $\begingroup$ The question is rather why do we not assume forces like gravity and electric charge consume anything. But you can safely ignore this part of my question, and I will erase it if it generates more concerns. My question is about the undistinguishability between expansion somewhere and contraction of the rest, since their relative volumes keep the same ratio. So wit $\endgroup$ – Exocytosis Apr 28 '19 at 3:14
  • $\begingroup$ So this is a matter of interpretation of GR that does not suppose any change of it, just like you may have your own interpretation of QM. $\endgroup$ – Exocytosis Apr 28 '19 at 3:15
  • $\begingroup$ This is not an interpretation of GR that physicists accept. Your ideas are far out of the mainstream. In the currently accepted cosmological theory, the universe is expanding everywhere, uniformly, faster and faster. $\endgroup$ – G. Smith Apr 28 '19 at 14:37

An expanding universe can be formulated as a solution to Einsteins theory of general relativity, which remains very successful until today. At the same time I don't know about any scientific theory of shrinking matter.

  • $\begingroup$ As I wrote above, this is not in contradiction with GR. This is merely a choice of interpretation. $\endgroup$ – Exocytosis Apr 28 '19 at 3:17
  • $\begingroup$ @Exocytosis I don't think it is. Just now I don't even see how your "interpretation" even fits the measured data of galaxies being accelerated away from us the farther away they are. How du xou explain the redshift of photons (which are not counted as matter). $\endgroup$ – Paul Apr 28 '19 at 7:11
  • $\begingroup$ @Exocytosis If I understand you right, you are trying to apply the fact that if sth is getting bigger than sth else and the observer being one of them not being able to distinguish between him getting smaller or the other sth getting bigger. But it is not that easy when the other sth is space itself. We are not watching space from outside, but we are watching the effect of space expanding all around us. I think there is no observational equivalence between an expanding universe and matter getting smaller. $\endgroup$ – Paul Apr 28 '19 at 7:17
  • $\begingroup$ Let me ask you this: say you have two objects, A and B. Both are able to measure each other size but only using their own size as a comparison. They are unable to determine a change in their own size, by definition. Now, B observes that A is getting bigger. How does it know it is not getting smaller? Same thing about expansion/drift. If galaxies got isotropically smaller, they would observe an apparent increase in distance and photons emitted in the far past would have been emitted by bigger particles, and thus have a longer wavelength. $\endgroup$ – Exocytosis Apr 28 '19 at 7:27
  • $\begingroup$ Didnt see your second comment in time. What would be the reason of the absence of an equivalence? $\endgroup$ – Exocytosis Apr 28 '19 at 7:37

I have always wondered why we consider that galaxies are drifting away and not just contracting at a particle level

Two good reasons:

1) General Relativity, the "formula" for gravity, is highly unlikely to result in a stable universe, or "flat". It is much (infinitely?) more likely that it is expanding or contracting. The fact that we seemingly observe it expanding strongly suggests that we are indeed seeing it actually expanding.

2) There is no analogous physics in Quantum Mechanics, the "formula" for matter, that suggests the distance between particles changes over time. This could happen, but it would require changes to fundamental values that we've measured to the 25th decimal place and do not appear to be changing. It would also leave a mark in history if these changed, which we could notice through telescopes, and we see no sign of this.

So basically we expect to see this happening because of (1) and don't expect it in (2) and that's enough for most people :-)

  • $\begingroup$ You got it wrong, I never said the universe is flat. I am saying that you cannot tell if you are getting bigger (expansion) than me, or if actually I am the one getting smaller unless you use some external ruler, because the ratio is the same. $\endgroup$ – Exocytosis Apr 28 '19 at 3:09
  • $\begingroup$ I think you have read something into my reply that is not there. My point is that universal expansion is physically possible and expected. In contrast, "local contraction" is both physically unexplainable and not expected. $\endgroup$ – Maury Markowitz Apr 28 '19 at 12:43
  • $\begingroup$ @Exocytosis The “external ruler” is the one provided by the speed of light. GR is not about “your size” relative to “my size”. It is about absolute distance in spacetime. The “relativity” in General Relativity is not about relative spacetime distances, as you seem to think. The spacetime distance between two events is the same for all observers. $\endgroup$ – G. Smith Apr 28 '19 at 14:44

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