So, my friend the other day brought up an interesting topic, does physics need to be the same everywhere? He asked, how do we know the universe doesn't, say, simplify itself, far away from us? For example, look at a far away star, shining in the night sky. How do we know all the chemical processes to make that star emit light, are really happening? What if the universe filters it out, makes the process simple, and just creates photons out of nothing and shoots them towards us? Sort of like a video game rendering, it doesn't need as much power for things you don't see. Is there any way to disprove this?
closed as off-topic by StephenG, Buzz, Ben Crowell, ZeroTheHero, Aaron Stevens Dec 26 '18 at 23:29
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In a strong sense, this is a philosophical question. It would only become a strictly physical question if we were to go to every place in the universe and test our laws of physics and see if they hold.
Insofar as it's a philosophical problem, it is very closely related to The Problem of Induction, in the sense that just because our physical laws seem to work everywhere we look, how do we know that they hold when we extrapolate to all spacetime points (far past/future; spatially far)? Well, in short there is no real way to justify this within say $5\sigma$ or anything like that, but that doesn't mean that we are completely in the dark.
For instance, we are given hints by the universe. Such hints include the apparent uniformity of the CMB, or the fact that we can make accurate predictions about, orbiting bodies, gravitational lensing, and other far-away (and spatially diverse) astronomical phenomena. This reinforces our assumption that the same laws of physics govern the motion of bodies, no matter where we are in the universe. Does this mean that we know therefore that there is no corner of the universe where, for some reason, say, the laws of electromagnetism just completely change? No, it doesn't. But as we continue to make successful predictions no matter where we look, we are less inclined to believe that there exists such pathological pockets of the universe.
See Do the laws of physics work everywhere in the universe? in particular my answer invoking Noether's theorem.
If the laws of physics aren't the same everywhere in the universe, then that implies a violation of conservation of linear momentum. Since we measure conservation of linear momentum to very good precision, this implies that if the laws of physics vary with position, they do so very slowly. The furthest objects might not obey the same laws then, but the closer ones (such as other stars in the Milky Way), and the fact that what works to describe these close objects also seem to work for the further objects imply that the laws of physics are the same everywhere in the universe.
Astronomers and physicists have been searching for such differences for as long as those fields of study have existed, and found no evidence for those differences. Here is a short list of what those differences might look like.
The inner workings of stars and how those workings evolve over the lifetime of any given star are determined by a short list of fundamental laws and can be studied from afar by collecting the light given off by a star using a telescope, and then breaking down that light into its fundamental spectrum using a spectrograph connected to the telescope. If a distant star was operating according to a different set of physical laws, then the difference would probably show up in its spectrum. No such differences have shown up which cannot be satisfactorily accounted for on the basis of the size, age, and time of birth of the particular star under study.
In those cases where the stars are too far away to study individually, the same sort of trick can be exploited to study entire galaxies. In this case, the results are the same: the galaxy spectra do not suggest that the laws of physics are any different in any corner of the universe we care to look, no matter how far away.
Supernova events, as detected and recorded by a variety of different instruments here on earth and in orbit, furnish the opportunity to look for differences in other physical laws and fundamental constants of nature. Excellent data on this was collected from Supernova 1987A, which happened close enough to us to enable detailed analysis, and no evidence was found of any differences between the known values of those constants as measured here and those which were back-calculated from those observations.
The overall conclusion remains: the laws of physics and constants of nature seem to be the same as they are on earth, everywhere we can see out into the very distant parts of the universe. If they were different, it would be the stuff from which Nobel prizes would be earned.