The audience at the Skirball Center was treated to a quadruple-header of competing physics theories on Thursday night at “Measure for Measure: Quantum Physics and Reality,” part of the Big Ideas Series at the 2014 World Science Festival. A lineup of renowned physicists met to weigh various competing interpretations of quantum mechanics, and proclaim which ones they found wanting.

The lingering debate is intriguing, given that, as panel moderator and Festival co-founder Brian Greene noted, quantum mechanics is hardly a new arrival; the basic framework was set in place by the 1930s. “Yet 80 years later, there are still questions that are ripe for discussion at the foundation of the subject—questions that are still controversial,” Greene said.

To panelist and Caltech physicist Sean Carroll, the quick and near-universal acceptance of quantum mechanics is an intellectual marvel. “Every part of it seemed to violate some cherished notion that we had clung to,” he said. “Quantum mechanics comes along and just changes all of the rules.”

But there is still the “measurement problem”; the fact that, despite the fact that the elegant equations of quantum mechanics give rise to probability waves describing the likelihood of a particle’s position, we still experience reality, and the particles in it, as defined points. Does the wave function of a particle really collapse when we look at it? Or is there some other kind of relationship between wave and particle?

The four panelists and Greene sifted through four proposed solutions to the measurement problem: the De Broglie-Bohm theory, the many worlds interpretation (also known as the Everett formulation), dynamical collapse theories, and a new arrival on the scene called Quantum Bayesianism, nicknamed QBism.

Under the Bohmian interpretation of quantum mechanics, it’s not that the wave function collapses into a position; both wave and particle exist, with the former is a guiding force for the latter.

“You’ve always got waves, you’ve always got particles,” says Rutgers University physicist and mathematician Sheldon Goldstein.

But Carroll wasn’t a fan.

“Faced with the measurement problem, the philosophers instantly want to add new physics; the physicists are like, we just need better philosophy,” Caroll said (borrowing the words of University of Oxford philosopher of physics David Wallace). For Carroll, the fact that Bohmian theory has to add more equations and concepts into the mix is a sign of weakness. “I believe in a much leaner and meaner version of quantum mechanics, so I just think that none of this is necessary,” Carroll said.

His preferred interpretation is the many worlds theory, in which there are various branching realities that account for all predictions on the probability wave. This approach, Carroll says, basically just trusts the current equations that we have are accurate.

But Ruediger Shack, a Royal Holloway, University of London mathematician, was skeptical: “My worry is that in a theory in which anything happens, we are not saying anything about the world we are in.”

The third possible solution to the measurement problem, dynamical collapse, posits that wave functions really do collapse unpredictably. On the subatomic level, this collapse happens rarely enough to be negligible, but when you get to the macroscopic level (things we see with our naked eyes), the sum of all those wave function collapses adds up to our superficially stable reality.

“This approach is more or less the most pedestrian, straight-forward” approach, Columbia University philosopher and physicist David Albert said. When solving equations yields the puzzling problems that don’t quite line up with observations, “the obvious thing to do is to think about modifying these equations.”

Carroll said spontaneous collapse came along with too many weird and unnecessary consequences for his taste: “If it turned out to be true, I would probably retire.”

In QBism, a lot of the mess in quantum mechanics can be solved by accounting for the presence of the observer him or herself. As mentioned before, the ‘B’ in QBism stands for Bayesian, referring to a branch of statistics that, although it’s come into greater use in recent years, remains a little controversial. This is because it adds a somewhat subjective element to science; Bayesian analysis factors in “prior probability,” or a guess at what the outcome of an experiment’s result will be, based on previous data.

When you apply Bayesian principles to quantum mechanics, interesting principles arise. In this interpretation, the wave function is not independently, objectively real; it is the mathematical tool a scientist uses to bet on where the particle is located. A single particle may have several different wave functions if it is observed by several different observers.

Once a scientist measures a particle’s position, “there is no collapse problem,” Shack said. “The probability becomes useless; it has done its job, and we don’t need it any longer.” It’s not hard to see why this idea spooks some scientists.

Just how long will physicists refuse to agree? Will one of the four solutions to the measurement problem presented here eventually reign supreme? The outlook is far from certain: Goldstein thought that, in another century or so, quantum mechanics might be replaced by something entirely different. Carroll predicted that 50 years from now, 97 percent of working philosophers/physicists will have settled on an interpretation of QM.

“Come back to the World Science Festival 2100 to see if the predictions are borne out!” Greene joked.

*Photo Credit: Greg Kessler*

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