Book Review: “Through Two Doors at Once” by Anil Ananthaswamy

Through Two Doors at Once: The Elegant Experiment That Captures the Enigma of Our Quantum Reality Hardcover 
By Anil Ananthaswamy
Dutton (August 7, 2018)

The first time I saw the double-slit experiment, I thought it was a trick, an elaborate construction with mirrors, cooked up by malicious physics teachers. But no, it was not, as I was soon to learn. A laser beam pointed at a plate with two parallel slits will make 5 or 7 or any odd number of dots aligned on the screen, their intensity fading the farther away they are from the middle. Light is a wave, this experiment shows, it can interfere with itself.

But light is also a particle, and indeed the double-slit experiment can, and has been, done with single photons. Perplexingly, these photons will create the same interference pattern; it will gradually build up from single dots. Strange as it sounds, the particles seem to interfere with themselves. The most common way to explain the pattern is that a single particle can go through two slits at once, a finding so unintuitive that physicists still debate just what the results tell us about reality.

The double-slit experiment is without doubt one of the most fascinating physics experiments ever. In his new book “Through Two Doors at Once,” Anil Anathaswamy lays out both the history and the legacy of the experiment.

I previously read Anil’s 2013 book “The Edge of Physics” which got him a top rank on my list of favorite science writers. I like Anil’s writing because he doesn’t waste your time. He says what he has to say, doesn’t make excuses when it gets technical, and doesn’t wrap the science into layers of flowery cushions. He also has a good taste in deciding what the reader should know.

A book about an experiment and its variants might sound like a washing list of technical detail with increasing sophistication, but Anil has picked only the best of the best. Besides the first double-slit experiment, and the first experiment with single particles, there’s also the delayed choice, the quantum eraser, weak measurement, and interference of large molecules (“Schrödinger’s cat”). The reader of course also learns how to detect a live bomb without detonating it, what Anton Zeilinger did on the Canary Islands, and what Yves Couder’s oil droplets may or may not have to do with any of that.

Along with the experiments, Anil explains the major interpretations of quantum mechanics, Copenhagen, Pilot-Wave, Many Worlds, and QBism, and what various people have to say about this. He also mentions spontaneous collapse models, and Penrose’s gravitationally induced collapse in particular.

The book contains a few equations and Anil expects the reader to cope with sometimes rather convoluted setups of mirrors and beam splitters and detectors, but the heavier passages are balanced with stories about the people who made the experiments or who worked on the theories. The result is a very readable account of the past and current status of quantum mechanics. It’s a book with substance and I can recommend it to anyone who has an interest in the foundation of quantum mechanics.

[Disclaimer: free review copy]