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How do scientists know that distant parts of the universe obey the physical laws exactly as we observe around us?

The question might look a bit odd but I am really stuck on my head. We know, scientists (with tools) explored physically only our solar system and some parts of our galaxy which is really a tiny part of the observable universe. And now they are constantly using these knowledge base along with 'tested physical laws' to measure the properties of distant parts of our universe.

For example, we tested and found the speed of the light is constant within our local periphery many times (within our Earth and Space around Earth). But yet we presume that the speed of the light is constant even at the farthest part of our universe. Certainly we did not test it in the other distant part of the universe because we have no way until now. Not only light but also the other physical properties like, luminosity, gravity, and etc related physical laws are agreed upon based on tests within our solar system. And based on these laws we deduced the properties of other parts of the universe (i.e. age, distance, mass, luminosity of stars in millions/billions light years away).

My question is, how do we know that these physical laws which we tested within a tiny area of the universe are consistently working in the distant parts of it? Is there any probability that the distant part of our universe obeys physical laws differently and our prediction based on applied physical laws gave us an unreal illusion of the actual reality, yet consistently?

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    $\begingroup$ A useful phrase to google on is local position invariance (LPI). It's not true that we can test whether the speed of light is the same in distant regions of the universe. This is the way it is sometimes described in popularizations, but it's wrong. What is really being tested is the fine structure constant. It's only possible to test for LPI of a physical constant if that constant is unitless. $\endgroup$
    – user15381
    Commented Sep 27, 2020 at 21:54
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    $\begingroup$ 1/2 This is an important philosophy of science issue. The thing is, nothing prevents anyone from posing the hypothesis that physical laws are different in other parts of the universe. As you say, because we haven't been there yet, it's impossible to prove or disprove. Problem is, by posing this hypothesis we would give up all possibility of understanding what we see when observing there: if anything goes, there is no point in trying to make sense of the astronomical phenomenons we do observe. $\endgroup$
    – armand
    Commented Sep 28, 2020 at 6:07
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    $\begingroup$ 2/2 It has happened that observation of events far away led us to reconsider how things work here. For example, Newtonian physics worked on earth but could not explain the orbit of Mercury. Then the theory of relativity came and managed to explain how gravity works differently near the giganornous mass of the Sun, while still being consistent with what we observe on Earth and thus extended the Newtonian theory. $\endgroup$
    – armand
    Commented Sep 28, 2020 at 6:12
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    $\begingroup$ @armand my point is same. Just like we found Newtonian laws did not work outside our Earth periphery and we needed to update with Einsteinian laws. Now with same argument how we can be sure that Einstienian laws work in the whole Universe uniformly until we reach to the farthest part. Pretty much a deviation of Occams razor theory. $\endgroup$
    – Sazzad Hissain Khan
    Commented Sep 28, 2020 at 6:21
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    $\begingroup$ Your next to last comment is incorrect. It is true that Newton's laws were an approximation to relativity, but we can measure and test relativity here on earth. We do it all the time in particle accelerators and GPS satellites. The only things we cannot test are those where we cannot create the proper conditions. In this context, that is extreme density over a large region. We have done it with gravitational redshift experiments and Gravity Probe B. $\endgroup$
    – Ross Millikan
    Commented Sep 29, 2020 at 2:55

3 Answers 3

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We don't know in general but to the extent we can measure, the laws seem to be the same, even if conditions are not.

For example radioactive decay: We know how fast various elements decay, and we can observe the results of radioactive decay in distant supernovae. The conclusion is that, for at least some elements, the rate of radioactive decay is the same on Earth as it is in distant supernovae.

After accounting for redshift, spectral emission lines remain unchanged by distance. This implies that the fine-structure constant is indeed constant.

Distant galaxies have gravitational fields, and interactions between galaxies proceeds in the same way in distant galaxies as it does in local ones. Eventually, the justification is philosophical: There is no observational reason to believe gravity behaves differently in distant parts of the universe, and so we believe that it does not,

In the extreme conditions of the early universe, some physical laws were different. For example, instead of distinct electromagnetic and weak fields, there was a single Electroweak field. But this can be described as single "law" with the electromagnetic and weak interactions being just the low energy approximation of the electroweak interaction.

So if it were discovered that Gravity (for example) was working differently in distant parts of the universe, but that there was a consistent pattern or rule for how it varied, then that would simply become the new theory of gravity (with general relativity becoming only the local approximation to this new law).

There is a more fundamental assumption: that the behaviour of matter and energy in the universe can be modelled by "laws". There are no angels dancing on pinheads. The justification for this is strictly in the realm of philosophy.

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    $\begingroup$ After accounting for redshift, spectral emission lines remain unchanged by distance. This implies that the fine-structure constant is unchanged by distance: en.wikipedia.org/wiki/Fine_structure $\endgroup$
    – Wayfaring Stranger
    Commented Sep 27, 2020 at 14:59
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    $\begingroup$ @SazzadHissainKhan we don't have to disprove any and every hypothesis. A hypothesis which makes unusual claims needs means to positively verify its predictions. There simply is no way to disprove my hypothesis that in a far far part of the universe a planet exists where dragons rule and atoms weigh twice what they do here. Yet there simply is no indication that this hypothesis should be true as all we see seems to confirm that the universe behaves the same as here. Occams razor applies: the simplest solution is assumed in the absence of proof to the contrary $\endgroup$
    – planetmaker
    Commented Sep 27, 2020 at 18:44
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    $\begingroup$ I think this ultimately comes down to Occam's Razor. We assume uniformity because it's simpler, and we haven't detected anything that contradicts it yet. $\endgroup$
    – Barmar
    Commented Sep 27, 2020 at 20:47
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    $\begingroup$ Science tries to find things which are not known, and thus also which contradict current theories. But as long as there is not proof to the contrary, the simplest solution is to be assumed. A theory never is reality but only a description of realiy - the simplest possible one. And if something unexplicable with current theory is found, we adjust theory to explain that fact, too - of course that may and will mean that the overall picture gets more complicated. But little point in making it more complicated than necessary to explain all observations $\endgroup$
    – planetmaker
    Commented Sep 27, 2020 at 21:22
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    $\begingroup$ @SazzadHissainKhan "why don’t scientists apply the same arguments and confirm that the unobservable universe has the same principle" - how exactly do you propose to confirm anything about something unobservable? $\endgroup$
    – IMil
    Commented Sep 27, 2020 at 23:16
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See also: Do the laws of physics work everywhere in the universe?

Noether's theorem, in the context of this question, states that:

If the laws of physics do not vary with position, then linear momentum is conserved (and vice versa).

Therefore if we observe conservation of momentum (which we do with exquisite precision) then we do not expect the laws of physics to vary with position. In other words, distant parts of the universe obey the same physical laws.

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    $\begingroup$ The way it is stated here, this is a non sequitur. "If A then B" does not imply "If B then A". $\endgroup$
    – Guntram Blohm
    Commented Sep 28, 2020 at 5:26
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    $\begingroup$ @Allure The post you cited: "We don't expect physical theories to be true in any absolute sense of formal logic. We expect them to be good approximations under certain conditions." I find this quite confusing. Within a theory, formal logic has to apply - at least ideally -, just because it is a science. But the theories themselves do not claim to yield perfect descriptions of the entire would, just useful approximations within a certain context. And yes, out of this context, they may yield false statements about reality. $\endgroup$
    – rexkogitans
    Commented Sep 28, 2020 at 8:29
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    $\begingroup$ Related: physics.stackexchange.com/questions/24596/… $\endgroup$
    – jpa
    Commented Sep 28, 2020 at 10:28
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    $\begingroup$ Noether's theorem is a biimplication -- symmetry implies conserved quantity and conserved quantity implies symmetry. You have only written half of the theorem. $\endgroup$
    – Eric Towers
    Commented Sep 29, 2020 at 4:30
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    $\begingroup$ @user253751 : Then LIGO ring-downs would be wildly different. And exoplanets would be much harder to detect because their orbits would not be periodic. And globular clusters would not be stable. And Hubble shift would not be spherically symmetric and radially smoothly increasing. And the population of type Ia supernovae would not be as uniform as it is. And pulsar periods would not be stable. And ... And ... And ... We have literally millions of observations, any one of which could reject extra-solar conservation of momentum, and all have failed to do so. $\endgroup$
    – Eric Towers
    Commented Sep 29, 2020 at 4:38
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The measurements of all astronomical manifestation are currently the same in all directions of the sky, rather than variable by direction and distance.

The CMB temperature is nearly constant with with anisotropy which has been very well studied, there is a dipole variance which indicates that that we are moving at 370 km per second relative to the CMB, travelling with the Galaxy.

Physicists tried to find evidence that gives credence to your suggestions, but the CMB implies that the big bang was the same, 13 billion light years in every Direction.

The same atoms seem to be emitting the same wavelengths, the quasars are the same, the galaxies are the same locally, far away and with dipole variations, the star numbers and distributions are the same, the supernovae are the same. The laws of physics are the same, same hydrogen lines and same rotation speeds.

The visible universe is the invariant, although you can be right about 130 or 500 trillion light years away, there may be some variation in in an unobservable distance of the universe, only that it's beyond what we can see.

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