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For example, Einstein postulated that the speed of light, c, is constant in all inertial frames of reference.

Bohr postulated that electrons go around the atom in orbits and that there are pre-defined orbits, and that moving from a higher orbit to a lower orbit makes releases energy.

I bet there are many other postulates that we take for granted.

Therefore I've a question, how can we claim something violates some physical law, when so many physical laws have been postulated?

All it takes is to disprove one of the postulates and suddenly a lot of things break down because of the claims that are based on the postulates.

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Yes, any human belief in a physical law started as someone's guess about reality. But so does every belief. A child guesses that objects are still there even when they can't be seen, and so on. So if someone thinks it's a law of physics that parity is conserved, and then we see that it isn't, then the "law" is once again revealed as a guess, and a wrong guess too... It's important philosophically to realize that everything can be doubted, but it's also important to be pragmatic about how you employ this realization. Most "laws" are backed up by a lot of evidence. –  Mitchell Porter Sep 22 '12 at 22:43
    
Its all about terminology. Law in physics is simply some FACT or IDEA or HYPOTHESIS which is OBSERVED and "VERIFIED" to be "true" for a long period of time and in many different situations. Nothing more nothing less - so its a statistics if you want. –  Asphir Dom Sep 23 '12 at 8:57
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You can claim anything you want, it doesn't mean your claim is correct, or that it's useful.

I'm guessing that what you meant to ask about are statements of the form "X is impossible because it violates the following law(s) of physics: ..." or "Y has to be true because of these laws of physics: ..." or so on. In statements of this sort, the laws cited have been supported by many very precise experiments. So the statement wouldn't be invalidated just because someone's postulate turned out to be wrong. It could be invalidated if all those experiments (in some cases, thousands) turned out to be wrong, but you can imagine just how unlikely that is.

If someone did make a statement of that sort based on "laws" that were merely postulated, without any convincing reason to believe them to be true, then nobody would (or at least should) take it seriously. That is the sort of behavior that, if pursued with enough conviction in the absence of evidence, can get you labeled a crackpot. In real science we do not take our postulates for granted.

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I think that you may not quite understand the nature of a postulate in the context of science.

When a certain regularity is observed, it is natural to postulate a principle underlying and explaining this regularity.

After a certain amount of evidence accumulates, a postulate may rise to the status of "physical law", i.e., a principle that would require extraordinarily unambiguous evidence to the contrary to plausibly bring it into question.

Nonetheless, were unambiguous evidence discovered that contradicted a physical law, it would simply be the case that there would be lots of scientists and graduate students that would happily submit grant proposals for funding research to discover the more fundamental principle that both explains the agreement and the discrepancy with the previous "law".

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