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).

In Memory of George Sudarshan

It is an old Irish tradition to tell amusing stories about a colleague or friend who has recently died. Here is mine.

In 1998, I was attending a week-long conference at the University of Maryland on the Foundations of quantum mechanics. This conference was run every few years by Prof. Yanhua Shih throughout the 1990s. To save money, all the participants stayed in the student dormitories and ate in the student cafeteria. On this particular year we were sharing the dormitories with a large summer class of High School football cheerleaders.

I arrived at the Baltimore airport late on Sunday night, and when I went to collect my baggage, there was George Sudarshan waiting for his. I knew he was attending the conference since he was listed on the program as an invited speaker. I went up to him and introduced myself.

Me: “Prof. Sudarshan? I’m Jon Dowling. I was a student in your quantum mechanics class at the University of Texas many years ago. I see we are going to the same conference.”

Sudarshan: (Does not remember me at all.) “Indeed. When was the last time we met?”

Me: “Back in 1992 at that conference in Moscow.”

Sudarshan: “Oh that was a lovely conference. The room was dark, the chairs were comfortable, and you could snooze away in the back and nobody would notice a thing.”

Me: “Yes it was. So how are you getting to this conference?”

Sudarshan: “I had not given it a thought. What about you?”

Me: “I have a rental car. I’d be delighted to give you a ride.”

Sudarshan: “How very nice. I accept!”

So, I helped him with his luggage, we pack up the rental car, and then we sped off down the freeway towards the university. I had made this trip several times and the exit to the university is not very well marked and easy to miss, especially in the night. As I drive, Sudarshan begins to make small talk.

Sudarshan: “So what are you working on these days?”

Me: “Oh, I have been focusing on a new theory of quantum metrology.”

Sudarshan: (Seems not impressed.)

Me: “So tell me what you’re working on?”

Sudarshan: (Becomes very excited.) “Weak measurement theory!”

Me: “What is that?”

Sudarshan: (Even more excited.) “You don’t know? It is a hot new topic. Very hot. You should learn it!”

Then Sudarshan reaches into his briefcase and whips out a 14”-long, yellow, lined legal pad and begins scribbling wavefunctions on it that appear to be moving backwards and forwards in time. Periodically he shoves the pad in front of my face (while we are barreling down the freeway at 60 miles per hour) and exclaims, “See!?” I nod and say, “Oh yes!” I am trying to peer around the pad and not run off the road.

This goes on for an hour until I gasp and realize that we have missed the exit and have arrived in downtown Baltimore! As Sudarshan continues full speed on the weak measurements, I have to turn around and head back the other way. The lectures continue and suddenly we are back at the airport — I have missed the exit again! So finally say, “Prof. Sudarshan, I have to pay attention. We have missed the exit twice!”

He then puts down the pad and tells a story about how Einstein went to visit Bohr in Denmark and Bohr met him at the train station. They then took a tram to Bohr’s house but got so engrossed in the conversation that they missed their stop and went to the end station. Then they got back in the tram and went back the other way and missed the stop and made it to the other end station. I said, “Yes, it is just like that, but in our case I am driving the tram.”

We finally arrive at the university dormitory at midnight to check in. Each participant has their own private room but must share the bathroom with the person next door. Seeing that we are good friends, the clerk puts us in rooms with adjoining bedrooms.

After the long day and a hectic drive, I am tired and go to sleep. But not for long! Suddenly, at 2AM, Sudarshan comes bursting through the bathroom door in his pajamas, turns on the light, and waves the yellow pad. “I have new results!” He pulled over a chair and lectured me in my bed for another hour. When he finally left I locked the bathroom door and propped the chair under the doorknob.

And to this day, I still have no idea what weak measurements are….