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There’s a variety of common definitions:
These definitions are “dictionary definitions”; besides actually being in dictionaries, they explain by enumerating all the various parts of physics, which isn’t necessarily a good way to understand the subject itself. Our purpose here is not to understand these constituent parts, but to rather get an intuition on what physics attempts to do.
Every given definition—not limited to these dictionary definitions, but every definition—is a bit different, because physics is a constantly evolving field. In the words of Feynman, there is an “expanding frontier of ignorance” in the sciences - every step forward is just another view into what we do not know. Even within the sciences, it’s difficult to divide and define the pursuit of knowledge in discrete divisions; it seems as though every day there is a new variety of scientist created with an ever-blurring spectrum in each discipline.
So---let’s start with a (hopefully) easier question: what is science?
Science is the human endeavor to explain the world. It’s an attempt at assembling knowledge in a manner that explains, formalizes, and predicts.
Your immediate reaction to this statement likely follows along the lines of that doesn’t really mean anything! I agree; the statement alone elucidates next to nothing on what science really is. If science is the pursuit of explanation, what exactly do we count as “the world” that we’re explaining? What do we define as “explained” and “not explained”? What’s the point of defining this, anyways? This ambiguity culminates into a look at the demarcation problem: distinguishing what is and isn’t science.
Categorization and distinction are fundamentally philosophical concepts. Attempting to define an abstract concept like “science” or “physics” is difficult, because they are general terms that encompass a large body of thought categorized under a name such as “science”. As subject matter continues to generalize, it becomes increasingly challenging to formulate a boundary on the body of thought we’d like to define.
What to take from this ambiguity is that these definitions aren’t clear because they are undecided; it’s easy to know what isn’t science, but it’s hard to say if some things definitely are. The definition above isn’t exactly right, but it’s a pretty good approximation—much like a lot of science itself.
To provide a general definition, physics is a conceptualization of the observable world into mathematical structures. It formalizes the behavior of the universe with mathematics, making useful predictions utilized at the base of nearly every industry and scientific discipline imaginable. An advancement in physics is an advancement in human progress itself!
It’s not perfect—far from it. The laws we propose and view as fact are simply approximations; relationships between observables that we’ve found to be true, to the best of our knowledge. But our progressive honing of approximations is possibly the most significant human pursuit imaginable; perhaps the best definition for physics is something we must move forward.