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I understand that "laws of physics" is a bit of a misleading term since all they really are is just us applying a logical statement about observed physical phenomena in a way that allows us to predict or understand said physical phenomena.

That said, what my question is getting at is whether there are any laws of physics that "hold at all levels?" The idea of the "invariance of physical laws" is, and has been, a key notion for developing new theories and furthering understanding of phenomena. But for a lot of given laws, there seems to be some system or situation, in which the laws must be modified or corrected in order to hold, or is simply not applicable. I'm not concerned with the numerical accuracy of physical laws, which seems to be the focus of similar questions on this forum.

An example of this would be Newton's laws of motion, which break down at the quantum scale (despite having analogous principles) or at relativistic speeds. Maxwell's equations have been described as the "solutions" to electromagnetic theory, but they are only correct up to the point of treating magnetism as an unexplained phenomenon and not the result of relativity.

Are there any laws that, with current and modern understanding, are always true (I'm not saying that they couldn't be found to not completely the case in the future, but we believe they are currently)?

The only one that I could believe fits this description, is the second law of thermodynamics, in that "entropy never decreases." We have entropies for vastly different systems at vastly different scales, from quantum systems to human scales, to galaxies and black holes. However, this may be a little handwavy, as entropy has different definitions for different levels (Von Neumann entropy and the entropy of a black hole for example) and my understanding is that they do not translate from one another.

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    $\begingroup$ maybe my answer here is relevant physics.stackexchange.com/q/259076 $\endgroup$
    – anna v
    Sep 2, 2023 at 19:46
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    $\begingroup$ Magnetism is not the result of relativity. $\endgroup$ Sep 2, 2023 at 20:03
  • $\begingroup$ "I'm not saying that they couldn't be found to not completely the case in the future, but we believe they are currently" - whether or not you (as a scientist/physicist) believe that a "law" holds true depends on your philosophical view about the world and (natural) science in particular. $\endgroup$ Sep 2, 2023 at 20:05
  • $\begingroup$ Physics is all approximations. Has conservation if momentum ever been measured exactly? No $\endgroup$
    – user376578
    Sep 3, 2023 at 19:06
  • $\begingroup$ @TobiasFünke I would suggest you check out this answer as a starting place: physics.stackexchange.com/a/65392/286423 and do some further reading on the matter if that is your stance. $\endgroup$
    – YaGoi Root
    Feb 25 at 0:02

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The stable foundations of physics are experiments and observations. Galileo experimented with gravity and interpreted his results as a universal downward acceleration. Newton replaced that with a force between masses. Einstein came up with a geometric interpretation. But through that evolution, the experimental results remain.

I would agree with you in the sense that the most fundamental observation in physics is that past and future are qualitatively different things. The second law of thermodynamics is an abstract expression of this, but as we have seen, the abstractions shift as we learn more.

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