The reductionist zeitgeist of physics cannot “explain” chemistry any more than “entropy” explains the inexorable march of life from birth to death. It’s important to understand what we mean when we say that physics cannot explain chemistry. Physics of course accounts for chemistry in the trite sense that molecules are composed of atoms. But then physics also “accounts for” human behavior since the brain is ultimately composed of atoms too. Yet we have no clue how to get from atoms to things like jealousy and musical creativity. When we say that A explains B, it usually means there is an unbroken and logical thread of continuity connecting A to B by way of which the properties of A are manifestly demonstrated in B. This physics cannot do even in the highly reductionist realm of chemistry, let alone in “higher” realms like neuroscience and sociology. These days emergence has become a fashionable word that’s often thrown around to describe any kind of complexity, but the emergence of chemical and biological properties that cannot be deduced from their underlying physics is in fact quite real.
There are several reasons why the reductionist approach in science doesn’t always work, but one of the most important ones was alluded to by the physicist and writer Jeremy Bernstein in a Wall Street Journal review of a biography of George Gamow and Max Delbruck:
Some sciences are more unruly than others. Here's a parable to illustrate what I mean. Imagine that when the first life form appeared there was a super intelligent freak. If this freak had had a complete knowledge of the laws of physics, what could it have predicted? Quite a lot. All atomic nuclei consist of neutrons and protons, and the number of protons determines each element's chemical nature. Knowing this, the freak could have predicted all the elements that could possibly exist, along with their respective characteristics. Suppose that it also knew all the laws of biology, including the "central dogma," which explains how genes are expressed as proteins. Even so, it could not have predicted the existence of giraffes, nor even the fact that my brother and I share only half our genes. Both of these are evolutionary accidents. If it had not been for random mutation there would be no giraffes, and my brother and I might have shared all our genes, as male bumblebees do. Biology is not like physics.
This paragraph succinctly drills down to one of the fundamental limitations of physics-based reductionism and it's a point that applies to chemistry as well. It's a very important one. The problem is that reductionism cannot account for the role of historical contingency and accident. Even if an all-powerful being could account for all biological scenarios emerging from an initial state of the universe, it could never tell us why one particular scenario is preferred over others. As Bernstein says, evolutionary accidents by definition cannot be predicted from starting conditions because they depend on chance and opportunity.
In addition function can never be uniquely derived from reductionism even if structure is. For instance in his book "Reinventing the Sacred", the complexity theorist Stuart Kauffman makes a powerful argument that even if one could derive the structure of the human heart from string theory in principle, string theory would never tell us that its most important function is to pump blood. The function of biological organs arose as an adaptive consequence of the countless unpredictable constraints that molded them during evolution. In addition the evolution of both structure and function was a mix-and-match process that depended as much on chance encounters as on strict adaptation. All this can never be captured in a reductionist worldview.
The same principle applies to chemistry. Evolution has fashioned many unique molecules that underpin life’s machinery. The question facing many chemists and especially chemists working on the origins of life is, why this particular molecule and not that one? Here are some more specific conundrums: Why are there only twenty amino acids, why are there alpha amino acids instead of beta or gamma amino acids (which have extra carbon atoms), why is amino acid stereochemistry (molecular “handedness”) L while sugar stereochemistry is D, why does DNA consist of a very specific set of four nucleotides and no other, why did nature choose phosphates in the construction of so many important biomolecules (the chemist Frank Westheimer comes close to answering this question), why does a given protein fold into only one unique functional structure, why is water is the only solvent known to sustain life, and in general why are the myriad small and large molecules of life what they are. In retrospect of course one could provide several arguments for the existence of these molecules based on stability, function and structure but there is no way to predict these parameters prospectively.
The fact is that an all-powerful, super-reductionist freak would have been useless in accounting for the unique existence of life’s chemical precursors. This is because there is nothing in the nature of these molecules which dictates that their presence should have been uniquely determined. For instance we now know from chemical studies that beta and gamma amino acids can also fold into the kind of helix and sheets motifs that are ubiquitous for alpha amino acids. They also have other favorable properties like chemical diversity which might have made them better building blocks compared to alpha amino acids. Yet for some reason they were discarded during evolution. Why? We could come up with several arguments. For instance because of their floppiness, maybe the higher order versions had to pay an unacceptable entropic penalty that could not compensate for their folding propensity. Or maybe a reaction called the Strecker reaction that is thought to produce alpha amino acids could never be superseded by beta amino acid-forming chemical reactions. Or perhaps alpha amino acids shield hydrophobic or water-hating side chains much better than their longer chain counterparts. These are all cogent reasons, and yet I am sure we could find an equal number of arguments against alpha amino acids if we searched hard enough. The truth is that the ultimate failure to find an explanation for the existence of alpha amino acids is a powerful reminder of the importance that chance and circumstance played in the evolution of both biomolecules as well as living organisms. Reductionism does not help us in tracing a path through this random, probabilistic landscape of evolution. The identities of life’s fundamental building blocks were shaped by chance followed by Darwinian natural selection.
This role of contingency and accident is one of the most important reasons why the reduction of chemistry and biology to physics will not work. Even if reductionism could provide us a list of all possible scenarios in chemical and biological evolution, it could never tell us which one would be preferred and for what reason. This is yet another reason why chemistry and biology are not physics.
This is a revised and updated version of a post on The Curious Wavefunction blog.