Alive or dead? Schrödinger's cat is a paradox that explains quantum physics

The secrets of quantum physics

The science in Philip Ball's new book takes us to a place beyond weird
May 15, 2018

There are two prerequisites to tackling this ambitious work: a passing familiarity with the basics of quantum physics, and the willingness to suspend your natural intuition. After all, we are talking about a realm in which a single particle can be both here and there, its location happily indistinct until it’s measured.

Philip Ball, a gifted and prolific science writer familiar to Prospect readers, is a demanding but engaging guide to this daunting terrain. One canonical example of quantum weirdness that Ball uses is that of Schrödinger’s cat. This thought experiment holds that, in the case of a feline trapped in a box, the cat is both dead and alive until its physical state is measured (by, say, opening the box and looking at it). In the parlance, opening the box “collapses the wavefunction,” or imposes a defined state on the cat.

This example illuminates the argument that has been rumbling since the 1900s, when quantum theory was first posited: what does the associated mathematics tell us, if anything, about the nature of reality? Does the cat even exist before we look at it?

As the great physicist Richard Feynman said: “I think I can safely say that nobody understands quantum mechanics.” This is reassuring, coming from someone who won a Nobel Prize for his contribution to the subject. Albert Einstein and Niels Bohr, two early protagonists in a discipline peopled by geniuses, were similarly occupied by the metaphysical issues emerging from their calculations.

Today, Ball says, quantum theory is best interpreted as a theory of information: about what can be known about any system, and how this knowledge can be deduced. Scientists are busy developing quantum computers—though, again, nobody knows how they are supposed to work. Amid all this uncertainty one thing is certain: if you grasp everything in Beyond Weird, you’re beyond clever.

Beyond Weird: Why Everything You Thought You Knew About Quantum Physics is Different by Philip Ball (Bodley Head, £17.99)