What’s the Difference Between Physics and Chemistry?

An off-line question from someone at Seed:

Fundamentally, what is the difference between chemistry and physics?

There are a bunch of different ways to try to explain the dividing lines between disciplines. My take on this particular question is that there’s a whole hierarchy of (sub)fields, based on what level of abstraction you work at. The question really has to do with what you consider the fundamental building block of the systems you study.

At the most fundamental level, you have particle physics and high-energy nuclear physics, which sees everything in terms of quarks and leptons, which are put together to form mesons and hadrons, including the protons and neutrons that we’re used to.

The next level up would be low-energy nuclear physics, which deals with protons and neutrons as the essential building blocks, and looks at how they’re put together to make nuclei. They don’t discard the quark model of nucleons, but it would be calculationally intractable to deal with the individual quarks, so they treat protons and neutrons as given (more or less), and look at how they are arranged.

Next up is atomic physics, which takes nuclei and electrons as given, and looks at how they’re put together to form atoms. We don’t really worry that much about how the protons and neutrons are arranged in the nucleus, save for where that affects how the electrons are arranged (in things like the hyperfine structure of atoms, which depends on the nuclear spin).

Next is where you start to make the transition between physics and chemistry. This is the level of the overlapping fields of molecular physics and physical chemistry, which takes atoms as the essential particles and looks at how they fit together to make simple molecules. They don’t worry about the nuclei at all, really, and only a little bit about the electrons. It’s a tricky division to make, but if I had to make a stab at defining the essential difference between molecular physics and small-molecule chemistry, I would say that the physics side is mostly concerned with how small molecules are put together and how they stay together, while chemists are more interested in how small molecules react with each other and swap pieces back and forth.

The next level is what most people think of when you say “chemistry,” which is dealing with complex molecules. Here, the fundamental entities are groups of atoms– hydroxyl this and ester that and sulfide and azide and all the rest. They look at how small molecules are put together to form large ones.

From here, you’ve got two different branches, but the same scale-based hierarchy continues. If you consider big molecules as your basic units, and look at how they combine in small numbers to make more complicated structures, then you’re getting into biochemistry. The next level is cell biology, then you get into the study of whole organisms, and eventually into neuroscience and psychology and on into things that aren’t really science any more.

On the other branch, if you pack enough molecules of the same type together, it goes back to being physics, in the condensed matter/ solid state regime. There, you treat huge numbers of nuclei and electrons in a statistical sort of way– you take the bulk structure as a given, and ask how the electrons are, on average, distributed through the system. The fundamental units in this case are collections of vast numbers of atoms and electrons.

And, of course, if you go to a large enough condensed matter system, it just becomes mechanics, in which you treat huge agglomerations of atoms and molecules as solid objects, and look at their motion in response to bulk forces from other huge agglomerations of atoms. When the solid objects become big enough, it becomes astronomy, and then cosmology.

That’s my personal Grand Unified Theory of the sciences, anyway.