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

  • 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 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?


    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.

    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.

    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.

    @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.

    Note that if the laws of physics would be different from the known laws of physics far away but in such a way that the laws of physics change in an analytic way as a function of position, then this means that the laws of physics as they are valid right here would be different and knowing the correct laws of physics would allow us to describe the difference in the approximate laws of physics as observed here and in far away regions.

    @Sazzad Hissain Khan: we can't be sure. But as long as we don't have a compelling reason to think the physics are different in other places, we have to work under the hypothesis that they are, otherwise its only speculation. Einstein could prove his theory by explaining with extreme precision observations that were impossible according to Newtonian physics. Young Earth creationists often posit that the speed of light decreased over time to explain how the universe can be 6000 years old while we can see stars millions of light years away, but they have no evidence that it is in fact the case.

    @armand I agree with you. Young Earth creationists claim sounds absurd to me. From my empirical perspective I am just trying to understand the philosophical aspects of science especially in that concern. Anyway, thanks for your time. I think I got the answer.

    Isn't the definition of 'our universe' simply all the stuff around us that obeys the same rules

    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.

    The physics of today is inadequate to explain the inside a black hole.

    @RossMillikan it seems a non sequitur. ‘but we can measure and test relativity here on earth’. I agree. My point was different. Just like Einstein refined the Newtonian laws considering velocity aspect, thus how can we be sure that some other laws can’t have scope to refine Einsteinian laws considering other aspect (which we don’t know)? Until then we are confidently using that laws to calculate other unknowns of the universe. I just raised question about such big confidence, nothing else. However, I agree, thats how science works. Heard of Occams razor for the first time and got more insight.

    Which laws though? If a given set of laws is not universally applicable, then they aren't really laws, and we would seek a generalization that is universally applicable, which we would then call "the physical laws of the universe", like relativity generalized classical physics.

  • James K

    James K Correct answer

    2 years ago

    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.


    After accounting for redshift, spectral emission lines remain unchanged by distance. This implies that the fine-structure constant is unchanged by distance: https://en.wikipedia.org/wiki/Fine_structure

    Lets bring a hypothesis, a start at a very distant part of the universe which in reality emmits 500% more light compare to what we observed by our equations because the physical laws was different at the point it emits and hence the luminosity decreased. How can we disprove that hypothesis? @Stranger

    @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

    @planetmaker I have to voice the usual Humean skepticism. You seem to be saying that nature is uniform because we observe its uniformity. But observable uniformity only implies that that uniformity extends to the unobserved if we assume that nature is uniform. Because the logic is circular, we don't get to apply Occam's razor. Better to say that we take it on faith that nature is uniform. Or, if you've spent a lot of time reading Kant (not me!), you might say that the uniformity of nature is an inextricable feature of the thinkable world (as opposed to the physical world).

    @planetmaker why it wouldn’t be a positive claim that the physical laws work uniformly at the distant part of the universe even without any single empirical evidence then? Also why don’t scientists apply the same arguments and confirm that the un observable universe has the same principle?

    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.

    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

    In reality, science tries to find things which are useful to do science. All these "what if"s aren't relevant because they aren't useful.

    @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?

    @IMil look here the point is not about observability instead knowledge. If we can assume that the laws applicable for what we have no knowledge by the argument “simplest solution needs to be considered” then why we can’t apply same principle to something non observable (in other words, completely unknown)?

    @SazzadHissainKhan Isn't that what s/he said? We assume the unknown things are as simple as possible. We can't send someone over to Alpha Centauri to measure the gravity there, but we assume it works the same as it does on Earth, and all the measurements we do from Earth seem to support that theory.

    @SazzadHissainKhan: You can feel free to make any hypothesis you want. But what distinguishes a scientific hypothesis from a nonscientific one is that a scientific hypothesis is *disprovable*, meaning that you can in principle design an experiment based on observation of the physical which will distinguish between truth and falsity of the hypothesis. So as soon as you say "the point is not about observability", you have left the realm of science and entered, I dunno, the world of science fiction?

    @LeeMosher, yes I agree scientific hypotheses require falsifiability. Now if you think deeply all current hypotheses on observable universe in reality makes falsifiability model which is dependent to totally unknown (as I already pointed out far reality is actually unknown in post). Then whats problem to make hypotheses on non observable universe? In fact many scientific hypotheses on non observable stuffs, i.e. multiverses etc. What are the distinction points? That question might not come if science could make local these laws as hypotheses for distant universe, but they described as theory.

    @SazzadHissainKhan's comment seems a bit hard to understand; I will try to rewrite it as well as I can, so that this interesting discussion can continue. Here goes: // Yes, I agree that scientific theories need to be falsifiable. However, if you examine the current state of astrophysics in depth, you will see that even falsifiable theories that use data from the 'observable' universe [presumably to extrapolate to 'unobservable' regions] use models that depend on the unknown, since 'reality' is by definition not attainable. So how are hypotheses on the 'non-observable' universe different? ...

    ... The last sentence is quite a poser, but I'll try anyway. // The question might not arise if physical models could be applied to the distant [i.e. 'non-observable'] universe; however, they are "described as theory" [perhaps he sees 'theory' as armchair guessing?].

    Now my reply to @SazzadHissainKhan's comment. You seem to be mixing physics and philosophy here. Physics is concerned with facts that can, in principle, be verified by others. The notion of reality as a hidden 'true' or 'as is' world was postulated by Descartes, Kant and Hegel, but it never lost its metaphysical trait and graduated to a verifiable theory or even hypothesis. Your question is about physical laws. The difference between falsifiable laws or hypotheses of physics and 'reality' (_really_ unknown) is moot. Be a little more humble. "We don't know; we only guess." (Raimund Popper)

    @Timm first of all thanks a lot for translating my obscure comment into a sensible form which was exactly what I meant. Sorry for my bad nonnative language. I clearly understood your reply and I totally agree with you. I just tried to express the reason of this post as a reply to someone’s question. However, after reading the answers and comments its now clear to me. I also believe its humble to say ‘we don’t know’ instead of justification with vague arguments. Thanks you again.

    Remarkably, after this long comment thread, it seems that we have returned to my actual answer. Read the first three words of my answer!

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Content dated before 7/24/2021 11:53 AM