Physicists should enjoy a variety of ways to understand quantum mechanics

The study of nature shows that disagreements about the meaning of quantum physics remain strong, even after 100 years. And that's normal.

Ask a bunch of physicists to explain the quantum experiment, and you are unlikely to get the same description twice. Everyone agrees that the mathematical basis of quantum mechanics has no analogues in predicting the results of experiments exploring the subatomic world. Moreover, it can be used to create technologies ranging from computer chips to lasers and nuclear clocks. But what theory means physically remains a mystery - and, still, a matter of mainly subjective interpretation.

Some scientists are confused 100 years after quantum theory was first formulated by this inability to agree on one narrative about the basic nature of reality, as well as the particles and forces that form it. They shouldn't be. Physicists should enjoy the variety of thoughts that this area represents and use philosophical disagreements as a way to stimulate further discoveries.

Quantum mechanics allows researchers to agree with the expected results of observations with extreme accuracy. This is strange, given that experiments seem to show that the quantum world itself is probabilistic and uncertain. Objects seem to behave like waves, interfering with each other and themselves until they are measured, and at this point they adopt a more specific behavior similar to particles. The phenomenon of entanglement can link the fate of particles together, even if they are physically separated over long distances, and this effect can be used to send secret signals. Statistical rules seem to regulate the results of experiments.

Disagreements begin when you ask physicists what exactly is meant by many of the above words. Language not only means radically different things for different people, but there are several ways to interpret what, if anything, is hidden behind such phenomena as randomness and what happens to quantum objects between dimensions.

The degree of disagreement is increasing in the news press this week. The Nature news team conducted the largest survey in history, asking quantum physicists about the basics of their discipline. The answers show that there is little or no consensus on whether a particle can really exist in two places at the same time, whether there are several universes, or whether it is reasonable to think of the mathematical quantities underlying quantum mechanics as corresponding to something real. The main tension among those who study the basics of quantum theory is between "realists" who believe that quantum physics can and should describe a visualized account of the world, and those who adhere to an "epistemic" view that interprets the theory as exclusively about knowledge and prediction of experimental results.

Such a disagreement can make it difficult to bring quantum physics to the attention of the public and avoid the ambiguity that can be used by pseudoscience pedals. But these discussions make this area even more exciting - and move science forward. Research on what physicists call quantum foundations not only helped scientists explore the boundaries of quantum mechanics, but also contributed to the development of technologies such as quantum computers and quantum cryptography. Such work may eventually point the way to an advanced theory that includes gravity - the only fundamental force of nature that quantum theory currently cannot explain - and shed light on other outstanding mysteries in physics.

In the Nature survey, about 75% of researchers believed that quantum theory would be replaced, at least partially, by a more complete theory. For experimenters such as Alain Aspect, a physicist at the University of Paris-Salet, who won the 2022 Nobel Prize in Physics for the study of quantum phenomena, the study of quantum foundations is a way to promote this field. "Thinking about the basics allows us, allows me to at least develop images, and these images are useful for my intuition," Aspect said in June at a conference on the German island of Heligoland, where scientists gathered to celebrate the 100th anniversary of quantum mechanics. Such discussions were at the forefront of the conference.

Physicists are making progress in understanding the basics of quantum physics. Thought experiments, such as those of Renato Renner and Daniela Frauchiger, theoretical physicists at the Swiss Federal Institute of Technology (ETH) in Zurich, Switzerland, have set limits on the fact that different interpretations can simultaneously assume that this is true (D. Fraushiger and R. Renner Nature Commun. 9, 3711; 2018). And experiments with dark matter using the XENONnT experiment excluded some of the proposed interpretations that would require a change in the Schrödinger equation, which controls the evolution of quantum states over time. But what is the best way to create a less fuzzy image of the quantum world? One way would be to deepen the dialogue between physicists and philosophers, suggests Alyssa Ney, a philosopher of physics from the Ludwig Maximilian University of Munich in Germany. This would improve the understanding of different languages and assumptions of quantum interpretations.

One of the most common approaches to understanding quantum theory, known as the Copenhagen interpretation, began in the merger of the views of the two founders of this field, Niels Bohr and Werner Heisenberg, who did not always agree. Their approach, which assumes that dealing with what cannot be observed is not the task of the theory, works well for experimenters. But the Nature survey shows that it finds less favor among those who study the philosophical foundations of quantum theory. And few physicists teach that other theories - even seemingly radical ones, such as the interpretation of many worlds, which assumes the existence of a huge number of universes - can also explain observations. "When you study quantum physics, somehow you never even come across the thought that there are different interpretations," says Renner.

Curiosity to study and study other interpretations can help scientists move physics forward. Physics and philosophy are now less interrelated than ever, but they need each other. Bachelor's physics teachers should accept philosophical questions, not just practical ones. Rough, though welcoming, discussions on the meaning of words and physical concepts, such as those that flourished at the Helgoland conference, should be encouraged.

A century later, quantum physicists may still disagree. But curiosity about what all this really means will ensure achievements that go beyond consensus.