A philosopher argues quantum mechanics still can't actually explain basic chemistry

[Kane]

Philosopher of science Olimpia Lombardi argues that despite Paul Dirac's famous 1929 claim that quantum mechanics contains everything needed to explain the whole of chemistry. The two fields remain genuinely incompatible at a technical level, not just practically difficult to reconcile, but conceptually in tension in ways that haven't actually been resolved.

The core problem is what she calls the symmetry problem. A pure quantum mechanical description of a molecule, based purely on the Coulomb forces between charged particles, is symmetric, it doesn't change under rotation, reflection, or swapping identical nuclei. But real molecules, as chemistry actually describes them, often have asymmetric structures that are essential to their chemical behavior, hydrogen chloride's asymmetric charge distribution is what actually explains its acidity, for instance. Quantum mechanics, taken on its own terms, simply doesn't predict that asymmetry should exist.

This shows up most sharply with chiral molecules, pairs of compounds that are mirror images of each other but can't be superimposed, the same way your left and right hand can't be perfectly overlapped. Thalidomide is the grim real world example, one mirror image form worked as an effective sedative while the other caused severe birth defects, with an estimated 10,000 to 100,000 affected children worldwide. Chemists can reliably explain and control which specific chiral form gets produced, but a pure quantum description of the molecule can't actually predict why one specific geometric arrangement exists in nature rather than the infinite number of other arrangements the quantum state is equally compatible with.

The usual fix, arguing that a molecule's environment breaks the underlying symmetry. Runs into its own problem, if the environment is made of similarly symmetric molecules, it can't actually break the symmetry either, so the argument has to appeal to a specifically asymmetric environment, which just pushes the same unresolved question back a level rather than actually answering it.

Lombardi's own proposed way out is a genuinely pluralist view of science. That chemistry and physics can each be internally coherent and practically successful within their own explanatory frameworks without physics necessarily being the more fundamental, all encompassing theory chemistry ultimately reduces to, a position that resists the popular assumption that a sufficiently complete physics would eventually explain literally everything

Chemistry is not explained by quantum mechanics | Olimpia Lombardi » IAI TViai.tv

[TealBear]

Adding to this, most working chemists probably don't think about this problem day to day since the Born-Oppenheimer approximation gets them practically useful results.

But Lombardi is specifically pointing out that practical usefulness and genuine theoretical explanation are two different things

[Backprop Depot]

My honest first reaction was surprise that this is still a particularly open philosophical question. Assumed quantum chemistry had this fully sorted out decades ago given how routinely it gets used in actual chemistry education

[Josh_79]

Seems like this connects to a much bigger question about reductionism in science generally.

If chemistry can't be fully reduced to physics, that raises real questions about whether biology can be fully reduced to chemistry, or psychology to neuroscience

[Foundry16]

The Dirac quote from 1929 aging this poorly is a great reminder that even brilliant physicists can be overconfident about how cleanly their own field's success generalizes to explaining everything else.

Small but real thing