Fundamental physics has become complicated over the past century, at least in public perception. With many flashy new ideas like ‘strings’ and ‘multiverses’ being proposed, it is difficult to separate what we think we actually know, from hypotheses, or even conjectures.
Physics is the natural science that seeks to explain our objective reality using a small set of laws that govern it. There is much packed within this pithy definition, that we can go over with a fine-toothed comb.
First and foremost, physics is a study of reality. Its goal is to observe reality very carefully and look for patterns that may be codified into fundamental laws. Its method is experiment — start with a hypothesis of what such a fundamental law might be, design an experiment whose anticipated outcome is best explained by the hypothesis, and then perform the experiment to test the prediction. Good experiments ensure that if they are clearly unsuccessful, the hypothesis may be safely discarded. On the contrary, a successful experiment does not prove the hypothesis, it only increases the likelihood that the hypothesis is correct. The history of science is rife with examples of better explanations superseding good ones.
We seldom know if a particular law is ‘fundamental’. Many laws that we deemed fundamental turned out to be special cases of something even simpler. But the creed of physics is parsimony — to reduce reality into as few laws as possible that merit explanation. We can explain the movements of all the planets in the sky when we understand the laws of gravity. This ‘law of parsimony’ is colloquially known as Occam’s Razor.
Second, the reality studied by physics must objectively exist. Unless we agree that something is real, in the sense that it can be subject to experiment, it doesn’t have a place of study in physics. For instance, hallucinations generated by the human brain are not part of our objective reality, though one is free to study the behavior of the brain (this is neuroscience, not physics). The study of consciousness is not a part of physics, unless there is evidence that the behavior of higher organisms cannot be adequately explained by known physical laws (we have no such evidence).
Occasionally, a theory may predict the existence of hitherto unheard of phenomena as side-effects. If these phenomena are later discovered, they lend further credence to the theory. For instance, black holes were predicted by general relativity, and experimentally detected later on. But for a hypothesis to be scientifically useful, it must be falsifiable — it must offer an experiment whose outcome is capable of refuting the theory.
It’s worth noting at this point that there are several popular science conjectures that are decidedly unscientific. Any variant of the ‘multiverse’ conjecture — the idea that there are other so-called universes that no conceivable experiment can detect — fails the test of falsifiability, and has no place in science. The ‘mathematical universe hypothesis’ — the claim that the universe is mathematics — falls into the same category of pseudoscience, as it doesn’t explain reality as we know it. Yet another idea that is unscientific is the claim that the universe is a gigantic simulation — no experiment has ever been proposed that would distinguish a simulated universe from a real one. If you find people making tall claims that are unsupported by experimental evidence, you would be right to be skeptical.
Finally, a subtle yet important assumption in physics is that our reality is governed by laws that remain the same over time. There is a tenet here that there is order underlying the chaos of reality. The idea that our reality is dictated by these unchanging laws is deeply incompatible with the idea of an omnipresent and omniscient deity making decisions on a whim. If there were experimental evidence of such a deity, the laws governing the deity would then merit further explanation. The laws of physics are all there is to reality, and the quest of physics is to discover them. It’s turtles all the way down.
The static nature of laws is a consequence of the law of parsimony. If a certain law changes over time or is different in different parts of the universe, there must be a higher law that explains the conditions under which it varies. If you keep discovering these higher laws, what you eventually end up with must be permanent and unchanging.