What would you say physics is? What is physics?

Some people object to certain things physicist try to do, dismissing it as not real physics. I'd like to say, physics is describing nature / the world using mathematics ( + the experimental method). But then there are a number of things, such as mathematical ecology, that certain people wouldn't accept as physics.

So it seems to me, physics, as seen even by some physicists, is merely defined by tradition: the traditional topics that "physicists" study. There's no concise way to define it.

Still, it's worth mentioning that it's physicists who usually have the skill to work with mathematical models, so even those who do e.g. mathematical ecology often turn out to have a training in physics.

The whole thing becomes a not so philosophical issue e.g. when it comes to approving thesis topics.

Well, there is pure physics (theoretical and experimental) and applied physics (all the rest if it using physical theories), whereas mathematical ecology uses mathematical techiques from physics but not its theories. The models there are soft, not based on physical laws.

@Arnold That is a good point. Physics has the ambition to go down to the lowest level. Explain the motion of particle with mechanics, and explain how a fluid behaves using the particles its made of, etc. There's a full chain down to the lowest level. But there isn't a full chain when you look at how many rabbits are there on the meadow. There might be some regularities, e.g. Lotka Volterra, but there's no full chain from the rabbit to the Schroedinger equation. At least in practice. :-)

It is like in math. Mathematicians work after their degree often in software engineering because they are good at programming and abstract thinking, not because what is done there is interesting mathematics.

@ArnoldNeumaier I disagree there. Using these methods is not more trivial in "other fields" than in "physics". It's not the same as going from research to application.