You have probably heard that “the world is not only stranger than we understand, but stranger than we can understand”. That such an irrational, defeatist maxim (attributed to several great 20th-century physicists) has become conventional wisdom is appalling. But it is a measure of the shock experienced by everyone who gets to grips with quantum theory.
Quantum theory gives us our deepest knowledge of the physical world, unrivalled in its predictions of how subatomic particles behave, and integral to our understanding of everything providing a framework for most of our physical theories. One day computers using quantum effects will revolutionise cryptography.
But what is shocking about any of that? Why should we care whether subatomic particles behave in one weird way rather than another?
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Precisely because, when we imagine how particles might behave, the full range of common-sense possibilities does not include the truth.
Therefore, philosophically minded people want to know that truth for its own sake. Pragmatists may point out that it cannot resolve today’s burning controversies about war and peace, crime and punishment, democracy, human rights…yet the astonishing thing is, discussing such issues in ignorance of the quantum-mechanical nature of the world is like worrying about overpopulation without knowing what causes pregnancy. In conversation about any controversial issue, questions about determinism and free will, identity and distinguishability, probability and “what would have happened”, will come up. For many of those fundamental questions, quantum theory implies that none of the common sense answers is true. For instance, common sense defines a “person” partly by continuity: you are “you” today because you consist mostly of the same atoms as you did yesterday; an identical copy made of different atoms would not be you.
But quantum theory says that in certain situations the notion of which atoms are the original ones and which the new doesn’t arise. Yet that conception of continuity of personhood is essential to many people’s reasoning about, say, abortion.
Controversy rages about what quantum reality is, but everyone agrees that, as the philosopher Michael Lockwood put it, “there are no conservative options”. Unfortunately, with physicists disagreeing among themselves and many explicitly renouncing all reference to reality or even truth, every kind of mysticism and nonsense has rushed in to fill the explanatory vacuum. Silly doctrines based on misunderstandings of quantum theory abound. So, while you should think hard about anything advocated under the heading “quantum”, and insist that it make sense before giving it credence, you ignore quantum theory at your philosophical peril.
