Quantum’s Leaping Lizards

Let me say before I go any further that I forgive nobody.

—Samuel Beckett

Quantum mechanics is the most accurate and most general scheme for making predictions about the behavior of physical systems in the history of natural science. Its arrival in the 1920s reduced the entire field of chemistry, more or less overnight, to a special application of the laws of physics, and suddenly made it possible to understand the shining of stars, the conduction of electricity, and the very existence and stability of familiar material things. It has been indispensable to the development of many of the twentieth-century technologies that have so radically transformed modern life and, most importantly for our purposes here, it is the logical foundation upon which the entirety of physics is carried out.

But controversies about what quantum mechanics implies, about both the nature of the world and the nature of science, have been going on for almost a hundred years, and have been the focus of enormous interest not only in theoretical physics but in philosophy, popular media, and other numerous and far-flung corners of contemporary culture as well. The history of these controversies is the subject of Adam Becker’s new book, What Is Real?

The story that Becker tells (and this is perhaps the place to say that he interviewed me in the course of his research, and that I am both mentioned and thanked in his book) runs, in a nutshell, like this: Once upon a time, physics aspired to offer us an objective, literal, realistic, comprehensive, and mechanical account of what the world is actually like. But that aspiration suddenly began to look quaint, naive, and out of date in the early decades of the twentieth century, under the assault of new and unsettling discoveries about the behaviors of subatomic particles.

One famous example is the “double-slit” experiment: on the left (as shown in the figure below) there is a source of electrons, and in the middle there is a screen through which electrons cannot pass. The screen has two slits in it, both of which can be covered with shutters. On the right is a second screen, which is coated with a fluorescent material that lights up for a moment at the point where an electron strikes it.

Mike King
The ‘double-slit’ experiment

Let’s consider three different cases. (1) Suppose that we cover one of the slits and allow a fairly large number of electrons to stream out of the source. Some of those electrons will make it to the fluorescent screen and reveal themselves there as little flashes of light—and if you make a graph of the number that land in each particular location on the fluorescent screen, it will form a bell-shaped curve centered on x1, if you have covered the top slit, or on x3, if you have covered the bottom…



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