Physics and Chemistry: Are they one and the same?

I read this excellent editorial in The New York Times about Einstein’s famous equation:

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And it kind of struck me that physics and chemistry are really the same thing. It seems that when you’re talking about physics, you’re also talking about chemistry and vice-versa.

I’m posting this to GQ, but it might end up being more suited for IMHO.

The way I learned it in high school was: if it’s green: it’s biology. If it stinks: it’s chemistry. If it doesn’t work: it’s physics.

Chemistry is certainly dependent on the laws of physics for almost everything. However, you are making a reductionist argument. That is not always the best way to organize inofrmation. For example, cell biology can be explained by chemistry which in turn can be explained by physics. Physics tends to be near the root of much of many sciences as well as engineering and others. However, we don’t expect a biologist to worry about all the competing theories in quantum mechanics when studying part of a cell membrane.

It would be a poor idea to start lumping all of these higher level things into physics. The sciences, as they are defined now, explain things at a given level. It is not necessary to explain something at its lowest level most of the time.

One example of this comes from physics. Newtonian physics was found to be invalid after the theory of relativity was published. The theory of relativity complicated things greatly but it only has a significant effect under certain conditions. Newtonian physics is still used for most real world physics applications.

Indeed, biology is nothing more than particle physics in action. Read about the work of Professor John Hopfield at Princeton for an example.

I’ve often thought that the scientific disciplines as they are usually defined are partly artificial constructs that help us humans divide up the sum total of all that is potentially knowable into manageable chunks.

If you want to understand math, you need only study math.

If you want to understand physics, you need to know some math, and study physics.

If you want want to understand chemistry, you need to know some physics (and the math underlying it), and study chemistry.

If you want to understand biology, you need to know some chemistry (and some physics and some math), and study biology.

So to the OP, when you’re talking about chemistry, there is necessarily some physics involved. But the inverse is not true – there’s a lot in the realm of physics that has nothing at all to do with chemistry.

Gary T… best explanation… nice work!

An old one I first saw when I was a wee undergraduate, [mumble] years ago:

Biologists think they’re biochemists.
Biochemists think they’re chemists.
Chemists think they’re physical chemists.
Physical chemists think they’re physicists.
Physicists think they’re God.
God thinks He’s a mathematician.

My favorite:

Biology is really just chemistry.
Chemistry is really just physics.
Physics is really just math.
Math is really just logic.
Logic is really just philosophy.
Philosophy is really just psychology.
Psychology is really just biology.

On a more serious note, though, it’s just a matter of knowing what questions to ask. Given a small number of particles, you can talk about the position of each of the particles as a function of time, or at least their state as a function of time. If you can answer that, then that’s the sort of question you ask, in which case you’re doing mechanics. If you have trillions of particles, then you can’t determine the position of every particle as a function of time, so you don’t ask that question. But you can talk about things like the temperature of the system of particles, or the pressure, so those are the questions you ask. This might get you thermodynamics or chemistry. But then when you start getting very complicated chemicals, in complexly-varying proportions or arrangements, as in a living organism, the tools of chemistry start becomming inadequate, so you can’t practically list every chemical reaction that’s going on in the system. But now you can start talking about things like natural selection.

In principle, if you had perfect computing capability, you could derive all of evolutionary genetics from the laws of fundamental physics. But our computing ability isn’t perfect, so we have to use different sets of scientific tools at different levels of complexity.

As a HS chem teacher, I tell kids that the big difference is the caring. Chemists just don’t care that much about the physics of it all, if it’s not helping make something useful. They get paid to make useful stuff like paint, glue, medicine. If you’re just going on about the quantum numbers because you like them, you’re probably a physicist.

Didn’t Feynman discuss this a bit in the intro of Volume 1 of his Lectures on Physics? Might be worth a look next time you’re at the library.

In Quantum Mechanics, you can model the behavior of a Hydrogen atom down to the finest detail. This tells you things such as energy levels in the atom, and the emission spectrum and so on.

You can then approximate the behavior of Helium atoms. My QM professor led us through this procedure, and we ended up with an equation that was much more complex than for Hydrogen, and one of the terms he added was this: “+ {horror}”. That term is so ugly it can’t be calculated. For Helium, it’s a very small effect. For other atoms, it gets even uglier.

The study of this equation including that horrible term is called Chemistry.

Wouldn’t green be botany? Biology has eyes.

Biology is the study of all living things. Plants are alive.

The way I look at:

Chemistry is to physics is what ecology is to cellular biology.

Chemists are into molecules and compounds and how they interact with one another and their environment.

Physics are into atoms and subatomic particles and how they interact with one another and their environment.

Ecologists are into populations, communities, and ecosystems and how they interact with one another and their environment.

Cellular biologists are into cells and organelles (the components of cells) and how they interact with one another and their environment.
It is indeed reductionistic to state that chemistry is just physics. With any complex system–whether it’s biological or physical in nature–you will find emergent properties that develop as you ascend up the system’s organizational hiearchy. These properties, by definiton, are not predictable by studying the underlying components (cells, proteins, atoms, subatom particles, etc.) that form them. In other words, a molecule is more than the sum of its parts (atoms) since it behaves in ways that individual atoms cannot. Knowing the number of quarks in a hydrogen atom is not going to help you understand why sodium hydroxide and hydrochloric acid shouldn’t be mixed.

“All science is either physics or stamp collecting.”

Ernest Rutherford

The divisions within science are for our benefit, but that doesn’t mean they are wholly (or even largely) arbitrary. There are, as has been mentioned, levels of complexity beyond which we cannot cope even in principle: We can’t reduce fluid dynamics to mechanics because chaotic systems aren’t amenable to being made into determinsitic ones. The complexity is too great and our knowledge is always too limited. We need new cognitive tools, a new language, to deal with how water gushes out of a pipe or how storm fronts move across a continent. Going from physics to chemistry is exactly analgous, as is going from chemistry to biology and biology to psychiatry.

(It’s interesting to think that we can’t get a really good weather forecast for the same reason we can’t absolutely know our thoughts by measuring neurochemistry and neuron firings.)

Even at a base physical level, the Universe isn’t deterministic. Quantum Mechanics precludes us from attaining the Newtonian dream of coming up with a perfectly mechanized clockwork Universe, in which all actions are predetermined by initial conditions. So reducing things to mechanics, even if it were possible, isn’t going to make our science completely certain anyway.

And what category did he win his Nobel Prize in…


There is a discipline called biophysics which bridges the divide between biology and physics. Individual cells can, at the level of protein molecules, ionophores, and whatnot, respond to the movement of single electrons, or the presence of minute electric or magnetic fields, or the passage of a single photon. This is where physics pushes up against biology and makes itself noticed.

And yeah, it was my major in college.

Which is then either physics or stamp collecting. (Actually, I’ve always heard the quote as butterfly collecting, but the sentiment is the same.)