You probably know that water is H2O, some of you might also know that the two hydrogen atoms meet the oxygen atom at an angle – approximately 104.5°. These are facts about molecular structure and they are at the heart of chemistry. This essay is about a particular issue – known as the ‘problem of molecular structure’ – that besets chemists’ explanation of how molecular structure comes about. What I’ll argue here, somewhat controversially, is that this problem is not peculiar to chemistry, but that it is an instance of the measurement problem of quantum mechanics, made famous by Schrödinger’s cat.
To get started, I need to give you a couple more examples of molecular structure. If we take 8 carbon atoms and 8 hydrogen atoms, it turns out that there are a few different molecules that they can make, depending on their molecular structure. So they could make up cubane, the cube-like molecule below, with black carbon atoms and white hydrogen atoms. Or, the very same atoms could be rearranged to make styrene, with its hexagon of 6 carbon atoms, two extra carbons hanging off the side, and hydrogens connected to the vertices.
Cubane, Image by By Jynto (more from this user) - Own work. This image was created with Discovery Studio Visualizer., CC0, https://commons.wikimedia.org/w/index.php?curid=45932737 // Styrene, Image by By Ben Mills - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=18332175
The problem of molecular structure concerns how we get from our 8 carbon atoms and 8 hydrogen atoms to these different complex shapes and arrangements of atoms. And, surprisingly, there’s no consensus among chemists as to how this happens. In particular, there’s nothing that tells us whether we should expect to end up with styrene, cubane or any other combination of the available atoms. In sum: what chemists need, and what they don’t currently have, is a story of how we get from all the different possible molecular structures to the particular cubane or styrene we find 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 that’s hooked up to a quantum system that is in a quantum combination – known as a `superposition’ – of two different states. 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 either being alive, or being dead, but not both.
Schrödinger's cat, Image by By Dhatfield - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4279886
Hopefully you should now have got an inkling as to the similarity between the two problems. Chemists need a story to say how we get from 8 carbon and 8 hydrogen atoms to either cubane or styrene but not both, and physicists need a story to say how we get from quantum superpositions to Schrödinger’s cat being either dead or alive but not both. I’ll claim that the same story can satisfy both chemists’ and physicists’ needs.
Now these problems are both 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. And, in this case, one of the most plausible candidates is, indeed, transformative of our conception of reality. Since Hugh Everett III first proposed it in 1957, the many worlds interpretation of quantum mechanics has captured imaginations, but in the last few decades it has become one of the most popular solutions to the measurement problem of quantum mechanics.
The basic idea is that the process of measuring our quantum system leads to a literal splitting of worlds. This guarantees that, while microscopic systems are in quantum superpositions, cats are only ever dead or alive. If the device is rigged up as above, then rather than ending up with a nonsensical superposition of a single cat being both alive and dead, we end up with two cats in different worlds, one that’s alive, and the other that’s dead. While this might sound absurd, it’s the only fully worked out way of making sense of all our observations: cats are alive, and cats are dead, but in different worlds, and we only ever see alive or dead cats because we split in two as well! Meanwhile, microscopic quantum systems can be in strange quantum superpositions in the same world.
Intriguingly, the many worlds solution can also solve the problem of molecular structure.
Intriguingly, the many worlds solution can also solve the problem of molecular structure. Recall that the chemists were after a story about how we could get from our collection of atoms to cubane and styrene; the physicists have provided just such a story – the story of how the world splitting occurs. If one pays careful attention to this story it tells us that, in certain circumstances, our 8 carbon atoms and 8 hydrogen atoms should 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 may end up with different molecules in each world. Therefore, the story of world splitting is the story of how we get the wondrous plenitude of different kinds of molecules we observe all around us.
Science will always surprise us, and this might not be the end of the story … but it’s definitely worth taking a moment to reflect on the idea that the physics and chemistry that accounts for everything around us may well have clues to other worlds!
Based on ‘The Problem of Molecular Structure Just Is the Measurement Problem’, co-authored with Vanessa Seifert, forthcoming in The British Journal for the Philosophy of Science
Alexander Franklin is a Lecturer in philosophy at King's College London, UK. (photo credit: Matt Lincoln Photography)