For example: theory of relativity and law of gravity.
I googled this but answer wasn’t clear to me.
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Let’s look at gravity specifically here, since it is an interesting case.
You can drop something and watch what happens: it moves downward, assuming you are on Earth. That’s just an observation of the phenomena. If you got very specific in your observations, you could say: it moves at a specific velocity towards the center of the Earth. Neat.
Now you could turn that last observation into something of a “law” if you wanted: it would just be the equation you’d use to calculate exactly what that phenomena would be. We usually only elevate these to “laws” if they prove exceeding useful or historically important.
Note that a “law” (or any other equation of this sort) would _not_ tell you _why_ it happens. It would just tell you that it _does_ happen. So Boyle’s law is simply a statement that the “that the pressure of a given mass of an ideal gas is inversely proportional to its volume at a constant temperature.” It doesn’t tell you why this is the case (to explain _why_ required a lot more work). Hooke’s law is simply “the strain in a solid is proportional to the applied stress within the elastic limit of that solid.”
OK. So _why_ does our original phenomena (you drop something, it goes downward) happen? That’s where the theory comes in. Does it happen because, as Aristotle believed,that whatever you dropped is made up of “earth” and/or “water” elements, and these elements naturally move towards the ground? Or does it happen because, as Newton held, there is an invisible attractive force called gravitation that anything with mass emits? Or, does it happen because, as Einstein argued, mass is warping the structure of space-time, making a movement towards the center of Earth’s gravitational mass the shortest distance in space-time? Each of these are theories. They are _why_ explanations.
(I teach history of science at a STEM school, and I always delight in asking students, after explaining Aristotle’s theory of gravitation, what the modern answer is. They _always_ recite Newton’s answer, because that is what is intuitive to them. I congratulate them on being 100 years behind the times.)
For these theories to be very useful, we need to be able to distinguish between one or all of them being right or wrong. Einstein’s theory, for example, has some slightly different predictions about how gravity works than Newton’s, and we can (and have) used instruments to confirm that reality meets Einstein’s approach better than it does Newton’s (Newton’s works fine for a certain approximation). That doesn’t mean Einstein’s theory is true, but it does mean it is probably more true (or more useful) than Newton’s theory. It is possible that in the future we will have a theory that better explains gravity even better than Einstein’s theory (there are some things that it does not explain quite right, so there is probably more needed).
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