Beyond Weird: Why Everything You Thought You Knew about Quantum Physics Is Different

by Philip Ball

It’s very peculiar that a scientific theory should demand interpretation at all. Usually in science, theory and interpretation go together in a relatively transparent way. Certainly a theory might have implications that are not obvious and need spelling out, but the basic meaning is apparent at once.

Usually in science you can answer those questions by careful observation and measurement. With quantum mechanics it is not so simple. For it is not so much a theory that one can test by observation and measurement, but a theory about what it means to observe and measure.

in the epistemic view, the wavefunction tells us what expectations to have about the outcomes of observations or measurements.

But Bohr’s distinction helps to focus us on the right question. What has changed crucially, he says, is the way we look. So instead of trying to figure out what has made the difference to the outcome in terms of ‘where the particle went’, we should be asking ‘Why does it matter how we look?’ This in turn prompts a deeper question: ‘What information have we gained in this case that we did not have in that case?’ I believe that it is this question, not ‘Which path is the particle taking?’, that may eventually lead us to a better understanding of quantum mechanics.

Not everything is knowable – because not everything is beable – at once.

even if a quantum computer does indeed require only one universe, David Deutsch’s vision of a multiplicity of worlds helped him to launch the field. Critics might dismiss his view while embracing what it produced. It’s a reminder that in science – and this is arguably truer than ever in a contested field like quantum mechanics – it’s as worthwhile for an idea to be productive as it is for it to be ‘right’.

In effect this implies that the entire universe is described by a gigantic wavefunction: as Everett called it in his thesis, the ‘universal wavefunction’. This begins as a superposition of all possible states of its constituent particles – it contains within it all possible realities. As it evolves, some of these superpositions break down, making certain realities distinct and isolated from one another. In this sense, worlds are not exactly ‘created’ by measurements; they are just separated. This is why we shouldn’t, strictly speaking, talk of the ‘splitting’ of worlds (even though Everett did), as though two have been produced from one. Rather, we should speak of the unravelling of two realities that were previously just possible futures of a single reality.

What quantum theory seems to insist is that at the fundamental level the world cannot supply clear ‘yes/no’ empirical answers to all the questions that seem at face value as though they should have one. The calm acceptance of that fact by the Copenhagen Interpretation seems to some, and with good reason, to be far too unsatisfactory and complacent. The MWI is an exuberant attempt to rescue the ‘yes/no’, albeit at the cost of admitting both of them at once. That this results in an inchoate view of macroscopic reality suggests we really can’t make our macroscopic instincts the arbiter of the situation. And that, I would argue, is the value of the Many Worlds: they close off an easy way out. It was worth admitting them in order to discover that they are a dead end. But there is no point then sitting there insisting we have found the way out. We need to go back and keep searching. Where Copenhagen seems to keep insisting ‘no, no and no’, the MWI says ‘yes, yes and yes’. And in the end, if you say everything is true, you have said nothing.

You could put it this way: what is more fundamental, a fact established by logic or a fact established by observation? Everything that looks strange about quantum mechanics stems from the incompatibility of those two options.

Wheeler developed a wonderful metaphor for illustrating this idea of a participatory realism: for showing how answers about ‘reality’ can emerge from the questions we ask in a way that is perfectly consistent, rule-bound and non-random without requiring a pre-existing ‘truth’. You probably know the guessing game of Twenty Questions, in which one player leaves the room and the others agree on a word, or a person, or an object. Then the questioner returns and asks questions to which the only permitted answers are ‘Yes’ and ‘No’. (You see: now you realize that this is a quantum game!) Imagine you’re the questioner. You start your questions, and receive the answers – but you find that after a few questions the answers take longer and longer to arrive. That’s odd. Still, you sense you’re closing in on the word, and finally you are sure you’ve got it: ‘A cloud!’ And everyone laughs and tells you you’re right. But then they explain what was going on. While you were out of the room, they didn’t decide on that word at all. They didn’t decide on any particular answer. The rule was simply that each person had to make sure that the answer they gave was consistent with the previous ones in fitting with something. And so the options became more and more constrained as the questions proceeded, and it took ever longer to figure out which word would still work. And everyone was forced, by the nature of the questions, to converge on the same word. If you had asked different questions, you’d...

I like to think of this informational perspective in terms of a distinction between a theory of Isness and a theory of Ifness. Quantum mechanics doesn’t tell us how a thing is, but what (with calculable probability) it could be, along with – and this is crucial – a logic of the relationships between those ‘coulds’. If this, then that.