What exactly is a law of physics? Suppose, for an hypothetical example, that high-energy light travels ever-so-faster than low-energy light. Then it would turn out that in fact light does not always travel in the same speed in a vacuum, so that would be a false law. So, my question really is, does the term "law of physics" refer to our current knowledge, or to the actual laws, whatever they may be?
Science only gives explanations of what we observe, and how to predict future actions. They are not explanations of the 'thing' in itself. D'Arcy Wentworth Thompson says in his book On Growth and Form (p 288):
For as Newton said, to tell us that a thing "is endowed with an occult specific quality, By which it acts and produces manifest effects, is to tell us nothing; but to derive two or three general principles of motion [author's footnote] from phenomena would be a very great step in philosophy, though the causes of those principles were not yet discovered."
Footnote: This is the old philosophic axiom writ large; Ignorato motu, ignoratur natura, which again is but an adaption of Aristotle phrase [Greek] as equivalent to the "Efficient Cause". Fitzgerald holds that "all explanations consist in a description of underlying motions" (Scientific Writings, 1902, p 385); and Oliver Lodge remarked, "You can move Matter; it is the only thing you can do to it."
Science is not absolute. We adjust or find a better explanation or theory to describe interactions of matter and forces every day - no physical theories are absolute as none solve the question of the "Efficient Cause". The 'Efficient Cause' is the realm of philosophy not science.
Mathematics is the language of science, a language we use to understand the workings of the universe. The universe can be written in the language of mathematics, it explains how events in the universe seem to occur from our perspective. It does not explain what the universe is. In the book Quantum Physics and Ultimate Reality: Mystical Writings of Great Physicists by Michael Green, Wolfgang Pauli is quoted :
...a mathematical formula can never tell us what a thing is, but only how it behaves; it can only specify an object through its properties. And these are unlikely to coincide in toto with the properties of any single microscopic object of our everyday life.
[And Arthur Eddington:]
For example, we may admire the triumph of patience of the mathematician in predicting so closely the positions of the moon, but aesthetically the lunar theory is atrocious; it is obvious that the moon and the mathematician use different methods of finding the lunar orbit...But now we realise that science has nothing to say as to the intrinsic nature of the atom. The physical atom is, like everything else in physics, a schedule of pointer readings...
...matter is something that Mr. X knows. Let us see how it goes: This is the potential that was derived from the interval that was measured by the scale that was made from the matter that Mr. X knows. Next question: What is Mr. X? Well it happens that physics is not at all anxious to pursue the question: What is Mr. X? It is not disposed to admit that its elaborate structure of a physical universe is "The House that Mr. X built."...matter, in some indirect way, comes within the purview of Mr. X's mind is not a fact of any utility for a theoretical scheme of physics. We cannot embody it in a differential equation. It is ignored, and the physical properties of matter and other entities are expressed in their linkages in the cycle. And you can see how by the ingenious device of the cycle physics secures for itself a self-contained domain for study with no loose ends projecting into the unknown. All other physical definitions have the same kind of interlocking. Electrical force is defined as something which causes motion of an electric charge; an electric charge is something that exerts something that produces motion of something that exerts something that produces...ad infinitum.
Science and philosophy dwell in different realms, they are not opposed to each other or overlapping as some try to frame in arguments, they are complimentary. Science deals with matter and energy and their interaction or collocation. It explains how, never why. And both are as pointed out by D'Arcy Thompson only properties of three dimensional space; and as the others point out, only pointer readings as to how we interpret the universe and not 'the thing in itself' or 'Efficient Cause'.
The physicist and historian and philosopher of physics Pierre Duhem defines—in Aim & Structure of Physical Theory pt. 2, ch. 5 ("Physical Law"), p. 168—a physical law as
a symbolic relation whose application to concrete reality requires that a whole group of laws be known and accepted.
In that chapter, he shows that
- The Laws of Physics Are Symbolic Relations
- A Law of Physics Is, Properly Speaking, neither True nor False but Approximate
- Every Law of Physics Is Provisional and Relative because It Is Approximate
- Every Physical Law Is Provisional because It Is Symbolic
- The Laws of Physics Are More Detailed than the Laws of Common Sense
The Physical System of St. Thomas ch. 10 "Physical Laws" by G.M. Cornoldi, S.J., gives a broader definition:
Law is a rule and measure of operations and law must proceed from reason
cf. Summa Theologica I-II q. 90 "Of the essence of law"
Also: "3.3 How the laws of nature lie (or at least engage in mental reservation)" of Ed Feser's Aristotle’s Revenge: The Metaphysical Foundations of Physical and Biological Science
In practice, the term "law of physics" refers to things we already know to be wrong more often than not.
For instance, Newton's law of gravitation is wrong, it has been superseded by general relativity in terms of "correctness" for more than a hundred years now, yet no one has stopped calling Newton's law of gravitation a law, we still teach it in schools and textbooks, and it makes for reasonably decent simulations of solar systems in various computer games. That the Rayleigh-Jeans law cannot possibly be correct was pretty much known at the time it was conceived (see ultraviolet catastrophe). There are plenty of more examples.
Laws of physics, are not meant to be statements about some unchanging universal truths about the world. They are models, or the building blocks of models, enabling prediction and simulation of real-world systems to a reasonable precision. When you want to crank the precision up, or change the situation to something not well-modeled by the law you were using, you switch laws (e.g. Newtonian gravity -> General Relativity). Often there are competing descriptions of a physical situation in terms of different laws, and neither of them is "wrong" - they just look at different scales, at different things, with different accuracy. For instance, the strong nuclear force is "fundamentally" mediated by gluon particles, but in the context of atomic nuclei it is often useful to think of the residual nuclear force as being mediated by pions.
You can find some related discussion on the purpose of "wrong" laws on physics.SE, e.g. here.
Since it seems pertinent, I rush in with an observation by Stephen Jay Gould:
In science, “fact” can only mean “confirmed to such a degree that it would be perverse to withhold provisional assent.” ("Evolution as Fact and Theory", pp. 254–55)
I'm fond of that quote because every word counts, and because it makes clear that physicists are not after truth, but instead quantitative rules of thumb for dealing with the behaviour of the universe. The most useful such rules are those we have repeatedly tried to falsify by observation, but which have nevertheless proven reliable (either universally so, or in a well-defined set of limited circumstances).
Truth we leave to the mathematicians, who are welcome to it.