A fine new book by Louisa Gilder called The Age of Entanglement: When Quantum Physics Was Reborn. Entanglement is that mysterious property of two or more bits of matter or light behaving, though separated, as if they were intimately and instantaneously connected -- a kind of spooky action-at-a-distance that seems to lie at the heart of the universe. Imagine human twins separated at birth and taken to different continents. Pinch one twin, and the other twin jumps, instantly. The effect is "non-local" -- that is, it happens faster than the speed of light and without the agency of any known species of causality. Physicists have been trying to "understand" entanglement for nearly a century. What it means for you and I and the evolution of the universe remains unknown. Maybe nothing. Maybe everything,
Quantum theory is astonishingly successful, predicting the results of certain subtle experiments with exquisite precision. But what -- pray -- does it "mean," and why does it seem so counterintuitive. Can a "thing" really be in two places at once? How does one particle of a pair that has once interacted "know" what is happening to its separated cousin across the universe? And where, if anywhere, is the boundary between the quantum world of atomic particles and photons and the classical world of pebbles, planets, and people?
The entanglement story is well told by Gilder. What is really interesting about her book is the story of how cutting-edge physics works, with clever like minds from around the world finding each other and pushing things forward by mutual inspiration, often as commonplace as a chat over beer.
My friend and colleague Mike Horne figures prominently in Gilder's book, as one of the authors of the famous GHZ paper of 1988 that showed when three particles are entangled, then entanglement is decisively demonstrated by a single measurement. By contrast, when two particles are entangled many thousands of measurements are required to be decisive. The paper is widely cited in all present work on entanglement.
As long as I have known Mike -- and that goes back before GHZ -- I've seen him using every minute between teaching classes and helping students with a yellow legal pad and sharp pencil figuring out the secrets of the universe. What a thing it is that one can do such things with a pad and pencil. Of course, once the theoretician has an idea what can be done, it's up to the experimentalist to do it. Every theoretical idea must be put to the test of observation. And every test of entanglement seems to demonstrate its perplexing reality.