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Another way of asking the question:

Suppose there are two locations A and B, and the distance between A and B is infinite. Suppose there are two observers, one at each location, and finally assume that initially the laws of physics are the same in both locations.

If the laws of physics suddenly changed at location A s.t observer A is able to get different results when conducting the same experiments as observer B, would observer B eventually detect a change in the laws of physics at his location? My initial impression would be no, because I assume that any changes to reality at location A would never "reach" location B, but I could be wrong.

Chidi
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    If the laws of physics suddenly changed… In mainstream physics, the laws of physics do not change. – G. Smith May 29 '21 at 05:55
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    In the current standard model of cosmology, the universe might be spatially infinite, and if it is, the laws of physics are assumed to be the same everywhere. – G. Smith May 29 '21 at 05:58
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    We have no evidence that the fundamental laws of physics have ever changed, or that they differ in various parts of the universe. But the vacuum state and thus the effective laws may have changed. For example, in the early universe the various forces may have been unified, but no longer appear to be. – G. Smith May 29 '21 at 06:08
  • What Dr Smith said. OTOH, if false vacuum decay happens it could radically change some physical paramaters. If the decay spread from some point at the speed of light it would take infinite time to traverse infinite distance. Ah, I see he's just mentioned the vacuum state. :) – PM 2Ring May 29 '21 at 06:08
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    But at the fundamental level of quantum mechanics and relativity, things didn’t change. – G. Smith May 29 '21 at 06:09
  • @G.Smith Okay, if the laws of physics do not change, then the experiments of both observers would always yield the same result. Which means that, if the laws of physics at both locations were different from each other to begin with, then they would remain different, right? – Chidi May 29 '21 at 06:16
  • @Chidi Laws of physics at both locations cannot be different to begin with . Laws of physics are the same all over the universe – silverrahul May 29 '21 at 06:18
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    If the fundamental laws were different in different parts of the universe, then we would want to have a theory about how they differed, and that metatheory would need to be the same everywhere to have any explanatory power. Then the laws we previously thought were fundamental would no longer be considered so. Etc. Fortunately we are not in this situation! – G. Smith May 29 '21 at 06:21
  • If any change could affect something at an infinite distance it would have to have infinite speed and would thus affect everywhere instantly. – Adrian Howard May 29 '21 at 06:25
  • @Chidi G smith is right. If the laws that we consider fundamental were different in different parts of the universe, then they would no longer be considered fundamental laws and we would have to come up with other " more fundamental laws " which would describe why the laws we previously considered fundamental are different in different parts. This new fundamental law would be the same in all parts. If it is not, then we would have to come up with even more fundamental laws and so on and so forth – silverrahul May 29 '21 at 06:29
  • @silverrahul The universe is not infinite in size, so the fact that the laws of physics hold everywhere within it does not really answer the question. With that being said, am I correct in assuming that you mean that a fundamental law must necessarily hold at any possible location, no matter how far away it is? – Chidi May 29 '21 at 06:31
  • @Chidi Yes, a fundamental law must necessarily hold at any possible location, no matter how far away it is. If it does not hold, then it is not a fundamental law – silverrahul May 29 '21 at 06:35
  • @silverrahul Is there any way to prove this? Some sort of mathematical statement that shows that a given fundamental law does not depend on location, which holds even when infinite distances are considered? Or any papers or research you can direct me to? – Chidi May 29 '21 at 06:43
  • The only way of proof is experiment. But such an experiment of checking whether the law holds at every point in the universe cannot be done. So we just assume it to be true untill it is proven false, which is not a bad way to go about life i guess – Sidarth May 29 '21 at 06:46
  • @Chidi If you want really rigorous mathematical proofs , try looking up Noether's theorem, space translation invariance or translational symmetry – silverrahul May 29 '21 at 06:47
  • @silverrahul Alright, I will look those up, thank you – Chidi May 29 '21 at 06:48

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I am afraid your questions is more or less meaningless, for several reasons:

(a) two locations cannot be an infinite distance apart. Even in an infinite universe, the distance between any two given locations must be finite. The term "infinite universe" simply means that there is no upper bound on the set of finite distances between locations in that universe.

(b) we have never observed a change in the fundamental laws of physics (although our understanding of them has changed over time). So we have no idea how such a change might propagate through space. Maybe its propagation would be limited by the speed of light - but on the other hand that is one of the fundamental laws of physics that might change ...

(c) as pointed out in the comments, even if we did observe an apparent change in one of the laws of physics, then we would simply say that our understanding of that law had been incomplete, and we would seek to incorporate the new observational data into a wider, more general law. For example, if we found that gravity acts differently from how we expect when we look at very large or very small scales (and this is an active area of research right now) then we would have to modify general relativity to accommodate that new data. So, almost by definition, the fundamental laws of physics cannot change because if they did they would not be laws.

(d) our observational data about the universe is limited by the size of the observable universe. We have no way of ever knowing what the universe is like far outside of this region (or even whether the universe is or is not infinite in extent). The best we can do is assume that the region of the universe that we can observe is representative of the whole, and the same laws of physics apply everywhere in the universe - since even if they did not, it is not clear how that could possibly affect us.

gandalf61
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  • "We have no way of ever knowing what the universe is like far outside this region" - I feel like this is the crux of my question though. If we have no way of knowing this, does that not prove that at a point sufficiently far away from us, a change in reality would go undetected? Or is the phrase "a location beyond the observable universe" inherently meaningless? – Chidi May 29 '21 at 07:08
  • @Chidi I should have said “we have no way of ever knowing ... according to our current understanding of the laws of physics”. This qualification applies to all statements in physics. But if you postulate an unspecified change in the laws of physics then all bets are off. Maybe the change could take effect at the same time (whatever that might mean) throughout the whole universe. Our minds are subject to the laws of physics too, so maybe they do change, but in such a way that we cannot remember they have changed .... – gandalf61 May 29 '21 at 08:59
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Here is another way to think of this.

We look out into deep space with our telescopes, make observations, and then ask ourselves if the fundamental constants of nature that we measure here on earth are the same on opposite sides of the observable universe, as represented by what our telescopes can tell us.

Note however that there was a time in the distant past when points on opposite sides of the observable universe were practically on top of one another: during the big bang. Our question then becomes: during the big bang, did the fundamental constants of nature vary from point to point on vanishing small distance scales, so that when the universe gets expanded out to its present size over 13 billion years, different regions of the universe far apart from one another possess different values of those constants?

The best models developed so far to describe the overall characteristics of the observable universe at its current age in terms of what went on during the big bang succeed in getting the picture right with no assumptions that the constants of nature were different in different regions of the big bang while it was happening. This argues against the hypothesis that different regions of our universe today have different values of the fundamental constants, or different types of physics altogether.

niels nielsen
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It is impossible to give an authoritative answer to your hypothetical question, since without knowing what changes occurred to the laws of physics at A, and why those changes occurred, how could we know their consequences?

Marco Ocram
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