Field of Science

The difference between chemistry and physics

I came across the following quote by the Nobel Prize winning chemist William Lipscomb (think diborane, carbonic anhydrase) with which I generally concur:

‎"Chemistry is not 'physics with less rigor'. In chemistry there are discoverable guiding principles for systems which are too complex for a "first principles" approach. The nature of chemistry is very difficult to explain to most physicists, in my experience!"
In chemistry there are emergent phenomena that cannot be simply reduced to physics. One has to think at the level of molecules and not just atoms, especially for understanding chemical reactions. This is especially true for understanding biochemical reactions. Knowing about quarks won't directly help you to understand the structure of DNA but knowing about hydrogen bonds definitely will. Of course the same caveats apply to thinking about biology as 'applied chemistry'. The fact is that every science comes with its own set of fundamental laws. These laws are strictly reducible to 'lower-level' laws in a philosophical sense, but the lower-level laws don't directly lead to the higher-level fundamental ones. Thus, an understanding of the lower-level laws, no matter how thorough, does not automatically imply an understanding of the higher-level ones.


  1. Yes, the principle known in philosophy of science as Downward Causation.

  2. I often wonder if this is strictly true. It's certainly well beyond our current ability to reduce biology to chemistry or chemistry to physics, so the snarky physicists that think that everyone should be studying their discipline are simply deluded. Still, as our knowledge grows, our abilities to reduce complex phenomena grow - given enough time, we might be able to study quarks to understand biology.

  3. @Kevin: We can see that happening already with fields like Chemical Physics & Chemical Biology already full-fledged departments at many universities.

    Though, I doubt that we might need to study quarks to understand biology. ;)

  4. @Akshat: Good point.

    And I agree, I doubt we will need to study quarks to understand biology, I just wonder if one day we will be able to.

  5. Phillip Anderson said it best in a seminal article he wrote 38 years ago (Science vol. 177 pp. 393 - 396 '72) More is Different. For further views on what happens when you move to a different scale see Robert Laughlin's book "A Different Universe". I find it incomprehensible (even after auditing a good quantum mechanics course). Both are Physics Nobelists, and both don't think much of reductionism


  6. Philip Anderson's article is marvelous. Another great book which extensively cites it and argues against reductionism is Stuart Kauffman's "Reinventing the Sacred". Some ideas in it are a little screwy and a good editor could have turned it into gold, but it's highly thought-provoking nonetheless and recommended.

    Retread, as you know, auditing QM courses does not mean anyone- even Laughlin and Feynman- really understands it!

  7. Wavefunction: I meant that even after auditing a good QM course I couldn't understand large parts of Laughlin's book -- quantum regimes, quantum protectorates etc. etc. A lot of his book has to do with the quantum Hall effect (which he one the prize for). Agree, QM at the bottom is incomprehensible in the macroscopic terms we naturally think and speak in.


  8. I of course did not mean to say that you don't understand it; I was only mirroring Feynman's quote that nobody understands it at a deep level!

  9. hiç birşey anlamadım...

  10. Readers may be interested to look into articles and books on the philosophy of chemistry where topics such as the alleged reduction of chemistry to quantum mechanics are discussed. In addition there are some philosophers who seem to believe that downward causation, mentioned by one blogger, actually happens from the chemical to the physical level. It's all based on the Born-Oppenheimer approximation and the mistaken view according to me that 'chemical structure' cannot be recovered from quantum mechanics.

    eric scerri

  11. The same in geography, f.e. physics can not directly explain the relief although it formed in accordance with these laws.

  12. Pfft, physics is just applied maths.

  13. Even without having read the books and articles about the philosophy of chemistry, it is proven by mathematicians that even when you know all the basic rules and the exact starting environment that make up together an entire world, one can not predict the results for a longer period and on larger scale. This is easily shown by Langton's Ant:

    At first, all seames well defined ans simple, but when time passes by and when the system becomes more complicated and larger, a chaotic behaviour appears. Although many mathematicians have tried to find some sort of function to prove the ant's behaviour, nothing so far seems to work, exept for the step-by-step approach. And then suddenly, after the chaotic period, some certain order emerges. The order is also not predictable when one only knows about the starting terms.

    Applied to the discussion about chemistry and physics: even although physics might once find a way to completely discribe all forces acting on a whole molecule existing of thousands of non-hydrogen atoms, it would probably be not suited for describing what happens when a large quantity of acid is mixed with a large amount of a solution of two bases.
    Not to mention reactions in biochemistry.

    chemistry student

  14. Well, this is another example of a nonexistent debate between Chemistry and Physics. I only found chemists talking about it and not a single physicist really devoting any time to it.

    Chemistry is an empirical practice rather than a deductive science. On the other hand, physics is a deductive discipline rather than an empirical one. Here is an illustration of my point.

    Take for instance the 1996 Nobel Prize in Chemistry awarded to three chemists "for their discovery of fullerenes". How it was discovered? According to this article “The serendipitous discovery took place during experiments involving a cluster beam which uses a laser to vaporize a graphite rod in a helium atmosphere to produce carbon plasmas. The research was aimed at characterizing unidentified interstellar matter.”
    Hence it was discovered by accident and not by applying any deductive process to it. It doesn't make it less relevant, but is a typical chemistry discovery, more the sub-product of another project rather than the confirmation of a theory or hypothesis aimed initially by the discoverers.

    Now take the Nobel Prize in Physics of the same year, awarded to three physicists "for their discovery of superfluidity in helium-3". They discovered this phenomenon after several years of research in superfluidity, a phenomenon whose theory was awarded in 1962 with another Nobel Prize in Physics to Lev Landau "for his pioneering theories for condensed matter, especially liquid helium". Therefore, they didn't get the result by accident. That is the role of a deductive science: to theorize first, to predict later and to confirm by measurements at the end.

    Sometimes the chemistry-vs-physics debate is focused on the alleged “complexity” of chemistry: “In chemistry there are discoverable guiding principles for systems which are too complex for a "first principles" approach.” Lipscomb says this like if it was a revelation. But everyone in physics is pretty much aware of the fact that when the number of variables exceeds a certain limit, the system itself became intractable at a “first principles” level. Those systems are studied by the laws of statistical mechanics or the many-body theory. I wonder what advances he may be done without the help of “reductionist” instruments like NMR or X-ray diffraction.

    Apparently his views on physics were greatly influenced by his doctoral advisor, Linus Pauling, who once said:
    “I recognize that many physicists are smarter than I am—most of them theoretical physicists. A lot of smart people have gone into theoretical physics, therefore the field is extremely competitive. I console myself with the thought that although they may be smarter and may be deeper thinkers than I am, I have broader interests than they have.”


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