GOING QUANTUM WITH
CHEMISTRY’S STRUCTURES

Figure 1. Left-hand side: cubane; right-hand side image: styrene. Images from Wikipedia.

The problem of molecular structure concerns how to get from our eight carbon atoms and eight hydrogen atoms to these different complex shapes and arrangements of atoms. Quite surprisingly, there is no consensus as to how this happens quantum mechanically. In particular, there is nothing in standard quantum theory that tells us whether we should expect to end up with styrene, cubane, or any other combination of the available atoms. In sum: what is currently unavailable is a physical story of how we get from all the different possible molecular structures to the particular cubane or styrene we observe in experiment.

Much better known than the problem of molecular structure is the measurement problem of quantum mechanics. This is commonly introduced with the example of Schrödinger’s cat. This tortured feline is held in a box with a vial of poison hooked up to a quantum system that is in a quantum combination known as a ‘superposition’. According to one of the basic rules of quantum mechanics, the cat should then end up in a superposition of being alive and being dead at the same time. This is, of course, nonsensical, and hence we have the measurement problem! What physicists need to solve this problem is a story about how we get from quantum superpositions to our cat being either alive or dead, but not both.

Figure 2. Schrödinger’s cat. Image from Wikipedia.

The problem of molecular structure and the measurement problem are pivotal to our understanding of how the world is fundamentally pieced together. If you want to understand how you breathe, or how the energy from your food can be used to power your body, or how materials combust, you will need to understand how atoms are arranged into molecules. Likewise, the quantum processes that are involved in everything from how the sun is powered to how your phone computes rely on the principle of superposition. In both cases, the science works but the scientists are in the dark as to exactly why it works, and how we can get from the basic building blocks to the physical and chemical complexity we find in the world.

Such profound questions require revolutionary answers. In our article, we do not defend any particular answer. Nevertheless, we argue that whichever solution is offered to the quantum-mechanical measurement problem, it will also apply to chemistry’s problem of molecular structure. We claim that the problem of molecular structure is a special case of the measurement problem. To illustrate this, we look at three of the most discussed putative resolutions to the measurement problem: the many-worlds interpretation, de Broglie–Bohm theory, and spontaneous collapse theories. We show that each of these solutions to the measurement problem are also solutions to the problem of molecular structure.

Consider, for example, the many worlds interpretation of quantum mechanics: The basic idea is that the process of measuring our quantum system leads to a literal splitting of worlds. In the Schrödinger’s cat experiment described above, rather than a nonsensical superposition—one cat both alive and dead—we end up with two cats in two different worlds, one cat alive and the other dead. While this might sound absurd, it’s probably the most fully worked out (realist) way of making sense of all our observations: cats are alive and cats are dead, but we only ever see alive or dead cats because we also split into two versions of ourselves, in two different worlds! Meanwhile, microscopic quantum systems can be in strange quantum superpositions in the same world.

In our article, we argue that the many-worlds solution can also solve the problem of molecular structure. Recall that we were after a story about how we could get from our collection of atoms to cubane and styrene. In the context of the many-worlds interpretation, physicists provide just such a story: a story of how world-splitting occurs. This tells us that, in certain circumstances, our eight carbon atoms and eight hydrogen atoms can be viewed as a microscopic superposition, and that we end up with multiple worlds: some with cubane, some with styrene, and some with other molecular structures. Each time the right kind of interaction occurs, a new world-splitting takes place, and each splitting leads to different molecules in each world. Therefore, the story of world-splitting is part of the story of how we get the wondrous plenitude of different kinds of molecules we observe all around us.

Of course, this is not the only way one can solve the problem of molecular structure. Any of the available solutions to the measurement problem can explain why quantum mechanics describes molecular structure the way it does. The take-home message is that the problem of molecular structure just is a special case of the measurement problem and, as such, is open to the same array of solutions.

This conclusion is neither obvious nor uncontroversial. In fact, many would find this conclusion unacceptable. This is, in part, because the problem of molecular structure has persistently been invoked to block the theoretical reduction of chemistry to physics. The idea is that if physics is unable to explain the structure of molecules, then chemistry attains a kind of autonomy from physics. If our claims are right, then physics has the resources to account for this central chemical concept. That does not imply, however, that chemists should lay down their tools—even if physics can, in principle, make sense of molecular structure, in practice, chemistry still very much stands apart.

One particularly interesting consequence of our analysis concerns isolated molecules. It’s standard practice in chemistry to imagine molecules as in the figures above: isolated entities alone in space, with perfectly well-defined molecular structures. However, if molecular structure comes about as we suggest, then this is called into question. For example, according to the many worlds interpretation, molecules that are isolated from their environment would, in general, exist as superpositions of a great many distinct molecular structures. On this view, there would be no facts at all about the distances between the atoms of such molecules until they come into contact with their environment, and no diagrams could be drawn to represent such structure accurately. This could explain why chemistry and quantum mechanics differ so starkly in their ability to identify structure: chemistry primarily works in non-isolated contexts, where structure is well defined, whereas quantum mechanics starts by describing isolated systems.

Science will always surprise us, and this is definitely not the end of the story! Nevertheless, it is worth taking a moment to reflect on the idea that problems of chemistry can be informed by looking at what physics tells us about the world. This does not undermine the importance of chemistry in understanding nature—rather, it shows that there is an intriguing interchange between the two disciplines that can offer fascinating insights into the nature and structure of things.