Why is quantum theory so strange? The weirdness could be in our heads

PARTICLES that also act like waves; the “spooky action at a distance” of entanglement; those dead-and-alive cats. Small wonder people often trot out physicist Richard Feynman’s line that “nobody understands quantum mechanics”. With quantum theory, we have developed an exceedingly successful description of how fundamental reality works. It also amounts to a full-frontal assault on our intuitions about how reality should work.

Or does it? “It only seems strange to us because our immediate everyday experience of the world is so very limited,” says Sean Carroll at the California Institute of Technology. Intuitive-feeling classical physics is largely devoted to describing macroscopic objects – the things we see and feel directly in the world around us. “It should not be surprising that this breaks down when we push it into domains that we never experience directly,” says Carroll.

There is a big difference between seeming strange and being strange, too. “If quantum mechanics is right, it can’t truly be strange – it’s how nature works,” says Carroll. You can say something similar, after all, about other areas of physics, such as Albert Einstein’s space-and-time-warping theories of relativity. Their effects only truly kick in at close to light speed, or in humongous gravitational fields of the sort we never experience, so their picture of the world seems alien to us.

For all that, there does seem to be something peculiarly alien about quantum theory. Take the way the mathematics of the theory allows us only to know the probability, on average, of what we will find when we measure the properties of a quantum object many times over, not tell us the outcome of any one measurement. This “measurement problem” raises the question of what quantum reality is doing before we measure it, and is the origin of Erwin Schrödinger’s notorious thought experiment about cats that are both dead and alive until we determine which. Attempting to answer it leads physicists down all sorts of strange byways, from suggesting the existence of constantly branching parallel quantum worlds to suggesting an active role for consciousness in constructing reality.

Maybe we shouldn’t worry so much about quantum theory’s strangeness, says Nicolas Gisin at the University of Geneva in Switzerland. The theory works for what it is supposed to do, which is make predictions about the subatomic world. We don’t know the extent to which it can or should apply to macroscopic objects. “I am certain that quantum theory is not the ultimate theory and there is no reason to believe it applies to the entire world,” says Gisin. We should expect it to be strange, at least to the extent it is.

Others think a fundamental ambiguity is exposed by the measurement problem and other aspects of quantum weirdness such as entanglement, in which we see correlations that can’t be explained by classical physics between measurements made on objects too far away from one another for any influence to pass between them. Is quantum theory a theory of the world as it exists objectively, or of how we interact with it?

“Part of the problem is that at present quantum mechanics is both,” says Emily Adlam at the University of Western Ontario, Canada. Some bits of the mathematics seem to refer to objective physical processes, and others to our subjective inferences about them. “Whatever the underlying reality might be, it is mixed up with the inferential parts of the theory in such a complex way that it’s hard to separate the two,” says Adlam. In the words of physicist Edwin Thompson Jaynes, quantum theory is an “omelette that nobody has seen how to unscramble”.

Even if we work out how to unscramble it, we may be left with a curate’s egg. Broadly, Adlam agrees with Carroll that the problem lies with us. “Whatever that underlying reality turns out to be, it is almost certainly ‘strange’ relative to our classical experience,” she says.

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