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There are many principles in physics. Two examples are conservation of energy and Pauli exclusion principle.

How do we know that such principles are true? What if there occur exceptional events that we are yet to detect? Isn't it possible that we might be restricting ourselves from exploring further by taking them for granted?

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    $\begingroup$ Don't know why the -1, I think this question is legitimate. In general, a principle is something that is either predicted, or observed and then introduced in a theory in order to describe correctly phenomena. In the former case, if it is predicted by a theory within which we have a great confidence, we assume it to be at least a good approximation of what really happens. Conservation of energy comes from.the Noether theorem, and PEP from the anti-commutative nature of half-integer spin particles. $\endgroup$ Sep 17, 2021 at 2:50
  • $\begingroup$ @JeanbaptisteRoux Please post answers as answers, not as comments. Comments should only be used to ask clarifying questions from the OP or to suggest ways to improve the post. $\endgroup$ Sep 17, 2021 at 2:51
  • $\begingroup$ Have a look at my answer here physics.stackexchange.com/q/517914 . Principles, laws and postulates are distillates of experimental observations, and are used as extra axioms picking up solutions from the differential equations used in modeling physics. $\endgroup$
    – anna v
    Sep 17, 2021 at 4:23
  • $\begingroup$ I agree that closing it as opinion based is not a good idea. On the other hand, I consider @dough_h‘s answer perfectly valid. I wonder what makes you think it’s not acceptable. $\endgroup$ Sep 17, 2021 at 5:02
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    $\begingroup$ I'm not voting to reopen though, as there are still issues with this question. It needs way more clarity and focus. Not all physical principles are verified in the same way. You give two examples that are very different, and then you just ask about physical principles in general. I suggest focusing on a single principle. Unfortunately, answers to the less focused answer have already been made. So I suggest making a new post that is better. Editing the question now would invalidate existing answers. $\endgroup$ Sep 17, 2021 at 10:29

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They are not taken for granted and any true scientific law/principle must be falsifiable. That is, it must be structured so that it is possible to find contradictory evidence. If such contradictory evidence can be verified, by multiple experiments, the validity and/or applicable domain of the law or principle must be questioned.

For this reason, scientific principles are always tentative to some extent. But something like conservation of energy which continues to explain phenomena is considered to be supported by so much evidence it "must" be true. But you can never prove it absolutely.

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  • $\begingroup$ [a principle] which continues to explain phenomena is considered to be supported by so much evidence it "must" be true. Is it possible that they are not explaining the actual case but we are bending the solutions to fit into the principle/postulate? $\endgroup$
    – Xfce4
    Sep 17, 2021 at 6:28
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If we found convincing evidence that a system of half-integer spin particles did not obey fermionic statistics (the immediate origin of the Pauli exclusion principle), that would indicate that one or more of the assumptions of the spin-statistics theorem is not valid.

This would be tremendously exciting! The spin-statistics theorem has very few underlying assumptions, all of which are believed to be generally true in mainstream physics. Evidence to the contrary would be frankly Earth-shattering.

However, there is no indication of this. On the contrary, the spin-statistics theorem is well-validated by every experiment ever performed. In the absence of compelling motivation, therefore, it makes little sense to spend an exorbitant amount of time or money to investigate a question that by all accounts appears to have been answered for 100 years.

By the phrasing of your question, it sounds like you are under the impression that the physics community is closed off to the idea; it would be more accurate to say that it takes a very compelling, specific idea to win enough favor to be funded, and without any specific reason to think that the Pauli exclusion principle should fail and in the face of a mountain of evidence to the contrary, that would be a difficult proposal to sell.

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We know they are true because nature follows them. If we are wrong, then we run into problems. The $\tau$-$\theta$ puzzle is an example. It was assumed parity is conserved (since it always had been), thus the apparent existence of two particles with the same mass, each decaying into opposite parity final states was, indeed, a puzzle. It took some time to propose there was but one particle; however, the weak interaction could violate parity.

Likewise, the hadron zoo. Since the neutron and proton were assumed to be fundamental, it was difficult to explain the spectrum of baryons and mesons. Of course, the quark does it beautifully, but it violated the principle that all particles have a charge that is an integer multiple of $e$, and the principle that elementary particles could be free particles. The story is told, along with other musing about your question, here: https://www.youtube.com/watch?v=jVKyFoKoWS0&t=1s .

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