14 September 2019

Copenhagen versus the universe

The trouble with the Copenhagen interpretation of quantum physics.

Adam Becker’s What Is Real? is an impressive book. And infuriating. Becker traces in painstaking detail the development of the Copenhagen interpretation of quantum physics, the role that its main architect Niels Bohr (and others) had in suppressing competing theories, and the consequences not only within the field of physics, but the general intellectual atmosphere of the past hundred years.

Regarding the Copenhagen interpretation, Einstein wrote to a friend that it “reminds me a little of the system of delusions of an exceedingly intelligent paranoiac….” Philosopher Imre Lakatos went further:

…Bohr and his associates introduced a new and unprecedented lowering of critical standards for scientific theories. This led to a defeat of reason within modern physics and to an anarchist cult of incomprehensible chaos.

Despite this, the theory has served as the standard interpretation of quantum physics since the 1920s.

“There is no quantum world”

Bohr, in both speech and writing, was notoriously difficult to pin down. That is part of the reason why it is so difficult to state exactly what the Copenhagen interpretation claims. In some ways, it’s easier to explain what it doesn’t say:

Rather than telling us a story about the quantum world that atoms and subatomic particles inhabit, the Copenhagen interpretation states that quantum physics is merely a tool for calculating the probabilities of various outcomes of experiments. According to Bohr, there isn’t a story about the quantum world because “there is no quantum world. There is only an abstract quantum physical description.” That description doesn’t allow us to do more than predict probabilities for quantum events, because quantum objects don’t exist in the same way as the everyday world around us.

According to the Copenhagen interpretation, at the level of quantum reality particles have no definite position until someone or something “measures” them. They are both everywhere and nowhere until they suddenly snap into a definite position when someone comes looking for them. This aspect of the theory follows from the fact that it isn’t concerned about the underlying physical reality. It’s a theory for calculating the outcomes of experiments and isn’t bothered by what the particles are up to in between measurements.

As Tim Maudlin explains in his excellent review of Becker’s book, Einstein’s opposition to the theory was due in part to his commitment to a belief in “a real, objective, mind-independent physical world”, where the idea of things being nowhere in particular goes against common sense. He believed “the goal of physics is to describe that world. Mere prediction, no matter how precise, is not enough: explanation is the goal.”

But still, the math worked and physics moved forward. The outbreak of World War II, where applied physics would play a leading role, followed by a post-war boom in jobs within applied physics led to a premature halt to debates about the foundations of quantum physics. Physicists were now expected to “shut up and calculate”.

Renegades

Which brings me to the part of Becker’s book that is so infuriating. There have been a number of alternative theories that have not been given a proper hearing. More than that, their authors and advocates were sometimes driven out of academia for refusing to disavow their ideas and accept the orthodoxy.

One of these “renegades” was Hugh Everett:

Also rejecting Copenhagen, Hugh Everett took Schrödinger’s evolving wavefunction and removed the collapse. He argued that rather than an incomprehensible smear resulting, as Schrödinger’s neither-alive-nor-dead cat suggested, a multiplication of worlds results. Schrödinger’s cat ends up both dead and alive, as two cats in two equally real physical worlds. Today this approach is called the many-worlds interpretation.

Everett’s thesis advisor, John Wheeler, had great enthusiasm for Everett’s innovation. But he insisted that Everett get the nod of approval from Bohr. Bohr refused, and Wheeler required Everett to bowdlerize his thesis. Everett left academia and did not look back. His work lay in obscurity.

Another was David Bohm:

In Bohm’s interpretation of quantum physics, much of the mystery of the quantum world simply falls away. Objects have definite positions at all times, whether or not anyone is looking at them. Particles have a wave nature, but there’s nothing “complementary” about it—particles are just particles, and their motions are guided by pilot waves. Particles surf along these waves, guided by the waves’ motion (hence the name).

Robert Oppenheimer suggested to a roomful of physicists that “if we cannot disprove Bohm, then we must agree to ignore him.”

The third was John Stewart Bell. Maudlin writes:

Spurred by Bohm’s papers, Bell queried whether Einstein’s dreaded spooky action at a distance could be avoided. Copenhagen and the pilot wave theory had both failed this test. Bell proved that the nonlocality is unavoidable. No local theory—the type Einstein had sought—could recover the predictions of quantum mechanics. The predictions of all possible local theories must satisfy the condition called Bell’s inequality. Quantum theory predicts that Bell’s inequality can be violated. All that was left was to ask nature herself. In a series of sophisticated experiments, the answer has been established: Bell’s inequality is violated. The world is not local. No future innovation in physics can make it local again. The spookiness that Einstein spent decades deriding is here to stay.

How did the physics community react to this epochal discovery? With a shrug of incomprehension. For decades, discussion of the foundations of quantum theory had been suppressed. Physicists were unaware of the problems and unaware of the solutions.

The worst of the lot

The “anarchist cult of incomprehensible chaos” has proved to be an alluring force within both intellectual and pop culture. The paradoxical, counterintuitive ideas of the Copenhagen interpretation have served the purposes of countless New Age thinkers and sloppy philosophers.

The TV show Futurama skewered this fairly accurately, showing a physics professor in the year 3008 claiming that “as Deepak Chopra taught us, quantum physics means anything can happen at any time for no reason.” Chopra does in fact claim that consciousness arises from quantum entanglement and that “quantum healing” allows the mind to heal the body through sheer willpower.

Becker points to the role of logical positivism in propping up the Copenhagen interpretation. The positivist believes that two different theories which make the same predictions are for all purposes equivalent. The details of the underlying reality can be discarded. As Maudlin points out, “[l]ogical positivism has been killed many times over by philosophers. But no matter how many stakes are driven through its heart, it arises unbidden in the minds of scientists.”

In 1951, Einstein expressed his belief that the Copenhagen interpretation would continue to hold sway “for many more years, mainly because physicists have no understanding of logical and philosophical arguments.”

In terms of the current state of quantum physics and the on-going work to find better explanations, Becker summarizes the situation like this:

So what is real? Pilot waves? Many worlds? Spontaneous collapse? Which interpretation of quantum physics is the right one? I don’t know. Every interpretation has its critics (though the proponents of basically every non-Copenhagen interpretation are usually agreed that Copenhagen is the worst of the lot).