Interpreting the interpretations

I was weaned on the foundations of quantum mechanics. My PhD thesis was on an alternative interpretation of quantum electrodynamics. I leap from the Copenhagen interpretation to the many-worlds interpretation in the blink of a photon. I have attended many workshops on the foundations of quantum mechanics. Not so long ago these workshops were held with minimal funding in seedy places such as the Super 8 Motel at Newark Airport in New Jersey, attended by a mixed smattering of deep thinkers and dangerous cranks.

However, since the advent of quantum computing and Peter Shor's algorithm for breaking secret codes — all spin­offs from fundamental physics - the foundations of quan­tum mechanics has taken on a new aura of respectability and an infusion of cash from various top-secret agencies. Instead of dodgy motels in Newark, we now meet at up-market resorts on the island of Capri in the Adriatic Sea. Attendance is way up, but the ratio of deep thinkers to dangerous cranks remains constant.

While quantum mechanics is still one of the most suc­cessful theories around, the interpretation of quantum mechanics has always been something of a controversial subject. The conflagration ignited by the debates between Bohr and Einstein continues to rage across the metaphysical landscape. As I think Will Rodgers once said: "I never met a physical landscape I didn't like."

At several recent workshops, a physics-by-ballot phe­nomenon has arisen whereby a call for a vote on various interpretations of quantum mechanics is made. Interestingly, the Copenhagen interpretation no longer garnet's even a simple majority! (I'm sure the Copenhagen interpretation must now know how President George W Bush feels.) Typically, the Copenhageners get the biggest share of the votes, followed by the many-worlders and the Bohmians. Then a smattering of votes pop up here and there for Cramer's transactional interpretation, the consistent-histories interpretation, or some other interpret­ation cooked up at a bar the night before.

A colleague and I once invented the "many-beers" interpretation at a bar near Pisa. With zero beers, quantum mechanics makes no sense. With one, you get an inkling. With two, things seem clear. With three, all mysteries are revealed. With four, things get foggy. And with five, quan­tum mechanics makes no sense once again.

In an effort to explain this perplexing trend, I've de­veloped — in the bar the night before — an interpretation of interpretations of quantum mechanics that is best explained in the form of an allegory. In my flat in Los Angeles, I have wall-to-wall carpeting. When I first moved in, the cable TV guy showed up to install some cable, which required pulling up the carpet. My Shetland sheepdog, Charla, seeing this fellow ambling about on his hands and knees, assumed it was time to play fetch with her rubber ball. The ball was dropped on the floor and accidentally ended up stapled under the carpet. So now I have this ball under my carpet that I can move freely about the flat, but which I cannot remove unless I pull up the carpet again.

In this metaphor, the carpet is the fabric of classical reality, while the ball is the indestructible nugget of quan­tum weirdness. There is a one-to-one mapping of interpretations to rooms. So, if you imagine a plan of my flat, you'll find the Bohmian interpretation in the bathroom, the many-worlds in the kitchen and the Copenhagen in the living room. (The one exception is the bedroom: I can't help it if you don't know any better than to discuss interpretations of quantum mechanics in the bedroom.) With this idea of the ball being in different rooms, we can now answer some questions. First, is there one inter­pretation that everybody will eventually agree on? This translates to: "Is there one placement of the ball that everybody will agree on?" Clearly, there is not. Some days I might like the ball in the living room (Copenhagen) and some days in the kitchen (many-worlds). Some days the ball may be best off in the bathroom (Bohmian). The posi­tion of the ball may change from day to day depending on my tastes or on those of the future occupants. There is no one placement of the ball that everybody will agree on.

The upshot is that even die more difficult question ­"What is the one true interpretation of quantum me­chanics?" — has no meaning, just as "What is the one true placement of the ball?" has no meaning. This is a remark­ably freeing interpretation. There is, to use the jargon, an "equivalence class" of valid interpretations of quantum mechanics. Interpretations that disagree with experiment are abandoned, forthwith.

Hence, with apologies to William Bragg, I may be a Copenhagener on Monday, Wednesday and Friday, and a many-worlder on Tuesday, Thursday and Saturday. On Sunday I rest and play with Charla — with no fear of eternal damnation. Pick the interpretation that suits you and gives you insight to the problem at hand. There is — and never shall be — one true interpretation of quantum me­chanics. Isn't it time we moved on with our lives?

Jonathan Dowling

Physics World, Volume 14, Number 11 (2001).