I know why you’re here.
You know that quantum mechanics is an extraordinarily successful scientific theory, on which much of our modern, tech-obsessed lifestyles depend, from smartphones to streaming to satellites. You also know that it is completely mad. Its discovery forced open the window on all those comfortable notions we had gathered about physical reality from our naïve interpretation of Isaac Newton’s laws of motion, and unceremoniously shoved them out. Although quantum mechanics quite obviously works, it appears to leave us chasing ghosts and phantoms, particles that are waves and waves that are particles, cats that are at once both alive and dead, lots of seemingly spooky goings-on, and a desperate desire to lie down quietly in a darkened room.
But, hold on. If we’re prepared to be a little more specific about what we mean when we talk about ‘reality’ and a little more circumspect about how we think a scientific theory might represent such a reality, then all the mystery goes away.
I’m not kidding. I have a bit of a reputation as the kind of guy you might find in the kitchen at parties; the kind who spoils all the fun, bursting the bubbles of excitable mystery and urban myth (what Americans sometimes call ‘woo’) with a cold scepticism and a calculating rationality. Spock, not Kirk (or McCoy). One commentator recently called me ‘depressingly sane’.* This is a badge I’m happy to wear with pride. There are many popular books you can buy about the weirdness and the ‘woo’ of quantum mechanics. This isn’t one of them.
And in any case that’s not why you’re here.
But—let’s be absolutely clear—a book that says, ‘Honestly, there is no mystery’ would not only be a bit dull and uninteresting (no matter how well it was written), it would also be completely untrue. For sure we can rid ourselves of all the mystery in quantum mechanics but only by abandoning any hope of deepening our understanding of nature. We must become content to use the quantum representation simply as a way to perform calculations and make predictions, and we must resist the temptation to ask: But how does nature actually do that? And there lies the rub: for what is the purpose of a scientific theory if not to aid our understanding of the physical world?
Let’s be under no illusions. The choice we face is a philosophical one. There is absolutely nothing scientifically wrong with a depressingly sane interpretation of quantum mechanics in which there is no mystery. If we choose instead to pull on the loose thread we are inevitably obliged to take the quantum representation at face value, and interpret its concepts rather more literally. Surprise, surprise. The fabric unravels to give us all those things about the quantum world that we find utterly baffling, and we’re right back where we started.
My purpose in this book is (hopefully) not to spoil your fun, but to try to explain what it is about quantum mechanics that forces us to confront this kind of choice, and why this is entirely philosophical in nature. Making different choices leads to different interpretations or even modifications of the quantum representation and its concepts, in what I call (with acknowledgements to George R. R. Martin) the game of theories.
Part I opens with a brief summary of everything you might need to know about quantum mechanics, which should hopefully set you up for what follows. I will then tell you about the rules of the game, based on a pragmatic but perfectly reasonable understanding of what we mean by ‘reality’, and the kinds of things we can hope to learn from a scientific representation of this. Part I concludes with Albert Einstein’s great debate with Niels Bohr in the late 1920s and early 1930s, and the emergence of the anti-realist Copenhagen interpretation, which admirably sets the scene.
We will then go on in Part II to look at various attempts to play the game, from the legacy of Copenhagen, through relational quantum mechanics, to interpretations based on quantum ‘information’. We will look at attempts to redefine quantum probability, by reformulating the axioms of quantum mechanics, introducing the notion of consistent histories, and quantum Bayesianism. We then turn our attention to realist interpretations based on the idea that quantum mechanics is a statistical theory. These include hidden variable theories of local (Bell’s inequality), and ‘crypto’ non-local (Leggett’s inequality) varieties.
Experimental evidence gathered over the past forty years or so comes down pretty firmly against local and crypto non-local hidden variables. So we turn to interpretations based on non-local hidden variables (such as so-called ‘pilot wave’ theories) or we try to fix problems associated with the ‘collapse of the wavefunction’ by introducing new physical mechanisms, including a possible role for human consciousness. We conclude with the notion that the wavefunction is real but doesn’t collapse, which leads to many worlds and the multiverse.
If you will indulge me, through all of this I will make use of a no doubt overfanciful analogy or metaphor.1 This is based on the notion that the game of theories involves navigating the ‘Ship of Science’ on the perilous ‘Sea of Representation’. Yes, I’ve obviously read too many fantasy novels.
We sail the ship back and forth between two shores. These are the deceptively welcoming, soft, sandy beaches of Metaphysical Reality and the broken, rocky, and often inhospitable shores of Empirical Reality. The former are shaped by our abstract imaginings, free-wheeling creativity, personal values and prejudices, and a variety of sometimes pretty mundane things we’re obliged to accept without proof in order to do any kind of science at all. These become translated into one or more metaphysical preconceptions, which summarize how we think or even come to believe reality should be. These are beliefs that, by their nature, are not supported by empirical evidence. So, if you prefer you could think of these preconceptions as intuitions or even articles of faith, echoing one of my favourite Einstein quotes: ‘I have no better expression than the term “religious” for this trust in the rational character of reality and in its being accessible, to some extent, to human reason.’2
Within the sea I have charted two grave dangers. The rock shoal of Scylla lies close to the shores of Empirical Reality. It is a rather empty instrumentalism, perfectly empirically adequate but devoid of any real physical insight and understanding. Charybdis lies close to the beaches of Metaphysical Reality. It is a whirlpool of wild, unconstrained metaphysical nonsense. The challenge to theorists is to discover safe passage across the Sea of Representation. In Quantum Reality I want to explain why this has proven so darn difficult, and why I have a very bad feeling about it.
So, welcome. You’re here because you want some answers. Please take a seat and make yourself comfortable, and I’ll go and put the kettle on.