In: Physics
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.
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.