In physics we call some results “laws” and some “theories.” The difference has absolutely nothing to do with our certainty in the validity of the results.
Newton’s Laws of motion are called that because they can be written as concise mathematical equations, and allof the content is there. Einstein’s Theory of special relativity is just as valid, and even contains Newton’s Laws as a special case, but the content of the theory can’t be written in simple, concise equations. There are several equations included in special relativity, but they do not represent the entire content. For example, the most important statement of the theory cannot be written in equation form at all: “The measured speed of light in a vacuum will be the same for all observers in inertial reference frames, regardless of the relative speed of their reference frame.”
Darwin’s Theory of Evolution likewise cannot be written in concise statements (mathematical or otherwise), but our certainty in its validity is no less than in Newton’s Laws.
Another important subtlety: I was careful to say that we are certain of the validity. People who don’t know better are fond of saying that Newton’s Laws are wrong. This is a fallacy. Scientific laws and theories can only be valid or not, they can never be true.
A law describes what happens, a theory explains why. The law of gravity says that if you drop an item, it will fall to the ground. The theory of relativity explains that the “fall” occurs due to the curvature of space time.
I was referring to the difference between a theory and a hypothesis.
Theorem would also be interesting to add to the mix.
In a scientific context, a hypothesis is a guess, based on current knowledge, including existing laws and theories. It explicitly leaves room to be wrong, and is intended to be tested to determine correctness (to be a valid hypothesis, it must be testable). The results of testing the hypothesis (i.e. running an experiment) may support or disprove existing laws/theories.
A theorem is something that is/can be proven from axioms (accepted/known truths). These are pretty well relegated to math and similar disciplines (e.g. computer science), that aren’t dealing with “reality,” so much as “ideas.” In the real world, a perfect right triangle can’t exist, so there’s no way to look at the representation of a triangle and prove anything about the lengths of its sides and their relations to each other, and certainly no way to extract truth that applies to all other right triangles. But in the conceptual world of math, it’s trivial to describe a perfect right triangle, and prove from simple axioms that the length of the hypotenuse is equal to the square root of the sum of the squares of the remaining two sides (the Pythagorean Theorem).
Note that while theorems are generally accepted as truth, they are still sometimes disproved - errors in proofs are possible, and even axioms can be found to be false, shaking up any theorems that were built from them.
Science can never answer “why.” In your example, the question why is just moved, from “why does it fall?” to “why does mass distort space-time?” In both cases physics just describes what happens.
But that is why it happens. Causality in most certainly something that can be discerned scientifically.
