# What are the main branches of physics?

I am basically listing out all topics in science (natural science) and was looking at branches of physics.

Wikipedia says that physics can broadly be classified into following: - Classical Mechanics, Relativity, Quantum Theory, Thermodynamics and Electromagnetism.
On what basis is this done?
Can we classify physics on the basis of what are we studying. E.g. how particles move is Mechanics, what makes them move is Electromagnetism and gravity, what happens at microscopic level is Quantum Theory ... something like this.

What you have listed are theories or subjects in physics.

When I think of "branches of physics" I think more of sub-fields that professionals work in. Like condensed matter physics, high energy particle physics, cosmology, quantum optics and information, biological physics, etc. Each of these sub-fields draws on multiple theories and subjects in physics.

Pythagorean
Gold Member
There is probably more than one way to classify topics.

In Classical Mechanics we basically learned to use differential equations to express particle motion, generally starting with Newton's 2nd or conservation. It can even involve electrodynamics and gravity, just as long as the focus is on particle motion via acting forces.

When I was graduating my physics program, they actually took all the wave study from classical mechanics, EnM, and QM, and put them in one class called wave mechanics, and took all the mathematics and put them in a mathematical physic courses (whereas when I took classes, classical mechanics and EM and QM were each two semester classes).

EnM can all reduced to Maxwell's Equations, which can be reduced to the d'Alembertian, so that justifies making it a single subject... but you would also see some special relativity and classical mechanics in the class.

QM is obviously separable given the completely new conceptual approach (observables are now operators, discretization, etc.) Also a lot of new math compared to classical. (Now need some probability and linear algebra).

Nonlinear dynamics I tend to think of as branch of modern physics (alongside GR and QM).

Computational physics is more about the approach; it involves systems where there is no analytical solutions, requiring computation to find numerical solutions. Can be applied to any of the other topics.

After googling a bit, I realised that classifying physics is very subjective. Different people categorize on different basis.
As you said, it can also be categorized according to sub-fields professional work. Similarly I think you can divide it in many other ways.

EDIT: I see that Pythagorean posted the same thing before me

If you were to classify physics based on what we are actually trying to find, then what would be the topics? For eg:- If we are trying to find how particles move then it would be Mechanics, when particles move then it would include the forces i.e. gravity and electromagnetism ... and similarly.
So based on this criteria what would be the topics?

I like to divide my subject based on the motive. Wikipedia and other sources usually categorizes on the basis of historical discoveries.