Time Travel. A History

Yet we have learned that in the real world things are always a little messy. Measurements are approximate. Knowledge is imperfect. “The parts have a certain loose play upon one another,” said William James, “so that the laying down of one of them does not necessarily determine what the others shall be.” James might have been pleasantly surprised by the revelations of quantum physics: the exact states of particles can never be perfectly known; uncertainty reigns; probability distributions replace the perfect clockwork dreamt of by Laplace. “It admits that possibilities may be in excess of actualities,” James might have said—that is, he did say it, but in advance of the actual science—“and that things not yet revealed to our knowledge may really in themselves be ambiguous.” Just so. A physicist with a Geiger counter can never guess when the next click will come. You might think that our modern quantum theorists would join James in cheering indeterminism.

The computers in our thought experiments, if not always the computers we own, are deterministic because people have designed them that way. Likewise, the laws of science are deterministic because people have written them that way. They have an ideal perfection that can be attained in the mind or in the Platonic realm but not in the real world. The Schrödinger equation, the screwdriver of modern physics, manages the uncertainties by bundling up the probabilities into a unit, a wave function. It’s a ghostly abstract object, this wave function. A physicist can write it as ψ and not worry too much about the contents. “Where did we get that from?” said Richard Feynman. “Nowhere. It’s not possible to derive it from anything you know. It came out of the mind of Schrödinger.” It just was, and is, astoundingly effective. And once you have it, the Schrödinger equation returns determinism to the process. Calculations are deterministic. Given proper input, good quantum physicists can compute the output with certainty and keep on computing. The only trouble comes in the act of returning from the idealized equations to the real world they are meant to describe. Finally we have to parachute in from the Platonic abstract mathematics to the sublunary stuff on laboratory benches. At that point, when an act of measurement is required, the wave function “collapses,” as physicists say. Schrödinger’s cat is either alive or dead. According to a limerick:

It comes as a total surprise

That what we learn from the ψ’s

Not the fate of the cat

But related to that:

The best we can ever surmise.

This collapse of the wave function is the trigger for a special kind of argumentation in quantum physics, not about the mathematics but about the philosophical underpinnings. What can this possibly mean? is the basic problem, and the various approaches are called interpretations. There is the Copenhagen interpretation, first among many. The Copenhagen approach is to treat the collapse of the wave function as an awkward necessity—just a kludge to live with.^*10 The slogan for this interpretation is “Shut up and calculate.” There are the Bohmian interpretation, the quantum Bayesian, the objective collapse, and—last but definitely not least—the many worlds. “Go to any meeting, and it is like being in a holy city in great tumult,” says the physicist Christopher Fuchs. “You will find all the religions with all their priests pitted in holy war.”

The many-worlds interpretation—MWI, to those in the know—is a fantastic piece of make-believe championed by some of the smartest physicists of our time. They are the intellectual heirs of Hugh Everett, if not Borges. “The MWI is the one with all the glamour and publicity,” wrote Philip Ball, the English science writer (ex-physicist), in 2015. “It tells us that we have multiple selves, living other lives in other universes, quite possibly doing all the things that we dream of but will never achieve (or never dare). Who could resist such an idea?” (He can, for one.) The many-worlds champions are like hoarders, unable to throw anything away. There is no such thing as a path not taken. Everything that can happen does happen. All possibilities are realized, if not here, then in another universe. In cosmology universes also abound. Brian Greene has named nine different types of parallel universes: “quilted,” “inflationary,” “brane,” “cyclic,” “landscape,” “quantum,” “holographic,” “simulated,” and “ultimate.” The MWI cannot be demolished by means of logic. It’s too appealing: any argument you can make against it has already been considered and (in their minds) refuted by its distinguished advocates.