After 100 Years of Quantum Mechanics, Physicists Still Can’t Agree on Anything

In July 1925—exactly a century ago—famed physicist Werner Heisenberg wrote a letter to his equally famous colleague, Wolfgang Pauli. In it, Heisenberg confesses that his “views on mechanics have become more radical with each passing day,” requesting Pauli’s prompt feedback on an attached manuscript he’s considering whether to “complete…or to burn.”

That was the Umdeutung (reinterpretation) paper, which set the foundation for a more empirically verifiable version of quantum mechanics. For that reason, scientists consider Umdeutung’s publication date as quantum mechanics’s official birthday. To commemorate this 100th anniversary, Nature asked 1,101 physicists for their take on the field’s most fiercely debated questions, revealing that, as in the past, the field of quantum physics remains a hot mess.

Published today, the survey shows that physicists rarely converge on their interpretations of quantum mechanics and are often unsure about their answers. They tend to see eye-to-eye on two points: that a more intuitive, physical interpretation of math in quantum mechanics is valuable (86%), and that, perhaps ironically, quantum theory itself will eventually be replaced by a more complete theory (75%). A total of 15,582 physicists were contacted, of which 1,101 responded, giving the survey a 7% response rate. Of the 1,101, more than 100 respondents sent additional written answers with their takes on the survey’s questions. 

‘Textbook’ approach still tops, with a caveat

Participants were asked to name their favored interpretation of the measurement problem, a long-standing conundrum in quantum theory regarding the uncertainty of quantum states in superposition. No clear majority emerged from the options given. The frontrunner, with 36%, was the Copenhagen interpretation, in which (very simply) quantum worlds are distinct from classical ones, and particles in quantum states only gain properties when they’re measured by an observer in the classical realm.

© Nature

It’s worth noting that detractors of the Copenhagen interpretation scathingly refer to it as the “shut up and calculate” approach. That’s because it often glosses over weedy details for more practical pursuits, which, to be fair, is really powerful for things like quantum computing. However, more than half of physicists who chose the Copenhagen interpretation admitted they weren’t too confident in their answers, evading follow-up questions asking them to elaborate. 

Still, more than half of the respondents, 64%, demonstrated a “healthy following” of several other, more radical viewpoints. These included information-based approaches (17%), many worlds (15%), and the Bohm-de Broglie pilot wave theory (7%). Meanwhile, 16% of respondents submitted written answers that either rejected all options, claimed we don’t need any interpretations, or offered their personal takes on the best interpretation of quantum mechanics. 

So, much like many other endeavors in quantum mechanics, we’ll just have to see what sticks (or more likely, what doesn’t).

Divided results, equivocal reviews

Physicists who discussed the results with Nature had mixed feelings about whether the lack of consensus is concerning. Elise Crull at the City University of New York, for instance, told Nature that the ambiguity suggests “people are taking the question of interpretations seriously.”

Experts at the cross-section of philosophy and physics were more critical. Tim Maudlin, a philosopher of physics at New York University, told Gizmodo that the survey’s categorization of certain concepts is misleading and conducive to contradictory answers—a discrepancy that the respondents don’t seem to have realized, he said. “I think the main takeaway from this is that physicists do not think clearly—and have not formed strongly held views—about foundational issues in quantum theory,” commented Maudlin, my professor in graduate school.

In an email to Gizmodo, Sean Carroll, a theoretical physicist at Johns Hopkins who responded to the survey, expressed similar concerns. Several factors may be behind this lack of consensus, he said, but there’s a prevalent view that it “doesn’t matter as long as we can calculate experimental predictions,” which he said is “obviously wrong.”

“It would be reasonable if we thought we otherwise knew the final theory of physics and had no outstanding puzzles,” added Carroll, who was part of an expert group consulted for the survey. “But nobody thinks that.”

“It’s just embarrassing that we don’t have a story to tell people about what reality is,” admitted Carlton Caves, a theoretical physicist at the University of New Mexico in Albuquerque who participated in the survey, in Nature’s report. 

However, the survey’s results do seem to hint at a general belief in the importance of a solid theoretical groundwork, with almost half of the participants agreeing that physics departments don’t give sufficient attention to quantum foundations. On the other hand, 58% of participants answered that experimental results will help inform which theory ends up being “the one.”

Schrödinger’s consensus, kind of

For better or worse, the survey represents the lively, fast-developing field of quantum science—which, if you’ve been following our coverage, can get really, really weird. A lack of explanation or consensus isn’t necessarily bad science—it’s just future science. After all, quantum mechanics, for all its complexity, remains one of the most experimentally verified theories in the history of science.

It’s fascinating to see how these experts can disagree so wildly about quantum mechanics, yet still offer solid evidence to support their views. Sometimes, there’s no right or bad answer—just different ones.

Wolfgang Pauli, Werner Heisenberg, and Enrico Fermi during the 1927 International Congress of Physicists, where the new quantum mechanics was discussed in depth. To the left are the first lines of Heisenberg’s letter to Fermi on July 9, 1925. Credit: Heisenberg Society/CERN, Wolfgang Pauli Archive

For you fellow quantum enthusiasts, I highly recommend that you check out the full report for the entire account of how and where physicists were split. You can also find the original survey, the methodology, and an anonymized version of all the answers at the end of the report. 

And if you do take the survey, or at least part of it, feel free to share your answers. Oh, and let me know whether you believe Heisenberg should have burned Umdeutung after all.