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

Quantum theory is peerless at explaining reality, but assaults our intuitions of how reality should be. It seems likely the fault lies with our intuitions

By Daniel Cossins

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 …