Physicists categorically disagree with the fact that quantum mechanics speaks of reality, the Nature study shows

The first major attempt to map the views of researchers finds the interpretation in contradiction.

Quantum mechanics is one of the most successful theories in science - and makes most of modern life possible. Technologies, ranging from computer chips to medical machines, are based on the use of equations first sketched a century ago, which describe the behavior of objects on a microscopic scale.

But researchers still widely disagree on how best to describe the physical reality behind mathematics, as shown by the Nature study.

Last month, at an event dedicated to the 100th anniversary of quantum mechanics, recognized experts in the field of quantum physics politely, but firmly, argued on this issue. "There is no quantum world," said physicist Anton Seilinger, at the University of Vienna, stating his opinion that quantum states exist only in his head and that they describe information, not reality. "I don't agree," replied Alain Aspect, a physicist at the University of Paris-Saclay, who shared the 2022 Nobel Prize with Zeilinger for his work on quantum phenomena.

To get a picture of how the wider community interprets quantum physics in its centenary, Nature conducted the largest ever research on this topic. We sent an email to more than 15,000 researchers whose recent articles were related to quantum mechanics, and also invited participants of the centenary of the meeting held on the German island of Helgoland to take part in the survey.

Answers of more than 1100, mainly from physicists, showed how much researchers differ in their understanding of the most fundamental features of quantum experiments.

Like Aspect and Zeilinger, respondents radically disagreed on whether the wave function - a mathematical description of the quantum state of an object - represents something real (36%) or just a useful tool (47%) or something that describes subjective beliefs about experimental results (8%). This suggests that there is a significant gap between researchers who adhere to "realistic" views that project equations on the real world, and those who have "epistemic" views that say that quantum physics is related only to information.

The community was also divided on whether there was a boundary between the quantum and classical worlds (45% of respondents said "yes", 45% - no" and 10% were not sure). Some objected to customizing our questions, and more than 100 respondents gave their own interpretations (the survey, methodology and anonymous version of the full data are available in additional information at the bottom of this page).

"I find it remarkable that people who are well versed in quantum theory can be convinced of completely opposite views," says Gemma De Les Coves, a theoretical physicist at Pompeu Fabra University in Barcelona, Spain.

Nature asked researchers what, in their opinion, is the best interpretation of quantum phenomena and interactions, that is, their favorite of the various attempts that scientists have made to link the mathematics of the theory with the real world. Most of the answers, 36%, were in favor of interpreting Copenhagen - a practical and often taught approach. But the survey also showed that several more radical points of view have healthy followers.

Answering the question about their confidence in their answer, only 24% of respondents believed that their preferred interpretation was correct; others considered it simply adequate or useful in some cases. Moreover, some scientists who seemed to be in the same camp did not give the same answers to subsequent questions, assuming an inconsistent or scattered understanding of their chosen interpretation.

"It was a big surprise for me," says Renato Renner, a theoretical physicist at the Swiss Federal Institute of Technology (ETH) in Zurich. It is implied that many quantum researchers simply use quantum theory without giving in to a deep attitude to what it means - the "shut up and count" approach, he says, using a phrase invented by American physicist David Mermin. But Renner, who is working on the basics of quantum mechanics, quickly emphasizes that there is nothing wrong with just performing calculations. "We wouldn't have a quantum computer if everyone was like me," he says.

Copenhagen still reigns

Over the last century, researchers have proposed many ways to interpret the reality behind the mathematics of quantum mechanics, which seems to throw out annoying paradoxes. In quantum theory, the behavior of an object is characterized by its wave function: a mathematical expression calculated using an equation developed by the German physicist Erwin Schrödinger in 1926. The wave function describes the quantum state and how it develops in the form of a probability cloud. As long as it remains unobservable, the particle seems to spread like a wave; preventing itself and other particles from being in the "superposition" of states, as if in many places or having several attribute values at once. But the observation of the properties of the particle - measurement - shocks this foggy existence into a single state with certain values. This is sometimes called the "folding" of the wave function.

It gets weirder: placing two particles in a state of joint superposition can lead to entanglement, which means that their quantum states remain intertwined even when the particles are far from each other.

German physicist Werner Heisenberg, who helped create the mathematics underlying quantum mechanics in 1925, and his mentor, the Danish physicist Niels Bohr, bypassed the duality of the alien wave and particles to a large extent, accepting that the classical ways of understanding the world are limited, and that people can only know what observation tells them. For Bohr, it was normal for an object to vary between action as a particle and as a wave, because these were concepts borrowed from classical physics that could be revealed only one at a time, through an experiment. The experimenter lived in the world of classical physics and was separated from the quantum system they measured.

Heisenberg and Bohr not only came to the conclusion that it was impossible to talk about the location of the object until it was noticed by the experiment, but also argued that the properties of the unobservable particle were indeed fundamentally unfixed before the measurement, and were not determined, but not known to the experimenters. This picture, as is known, worried Einstein, who insisted that there was a pre-existing reality, that the work of science was to measure.

Decades later, the merger of not always united views of Heisenberg and Bohr became known as an interpretation of Copenhagen after the university in which the duo did their fundamental work. These views remain the most popular vision of quantum mechanics to date, according to the Nature survey. For Chaslav Bruckner, a quantum physicist from the University of Vienna, a strong demonstration of this interpretation "reflects its constant usefulness in guiding everyday quantum practice". Almost half of the experimental physicists who answered the survey supported this interpretation, compared to 33% of theorists. "This is the simplest thing we have," says Desio Krause, a philosopher at the Federal University of Rio de Janeiro, Brazil, who studies the basics of physics and answered the survey. Despite their problems, the alternatives "present other problems, which, in my opinion, are worse," he says.

But others argue that the appearance of Copenhagen as a default is due to historical accident, not because of its strengths. Critics say that this allows physicists to bypass deeper questions.

One concerns the "problem of measurement", asking how measurement can initiate objects to switch from existing in quantum states that describe probabilities to having certain properties of the classical World.

Another unclear feature is whether the wave function represents something real (the answer chosen by 29% of those who preferred the interpretation of Copenhagen) or simply information about the probabilities of finding different values during measurement (chosen by 63% of this group). "I'm disappointed, but not surprised by the popularity of Copenhagen," says Elisa Krall, a philosopher of physics from the City University of New York. "I feel that physicists didn't think."

The philosophical foundations of the Copenhagen interpretation have become so normalized that they do not seem to be an interpretation at all, adds Robert Speckens, who studies quantum foundations at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. According to him, many supporters "just drink Kool-Aid of the Copenhagen philosophy without studying it".

Respondents to the survey who conducted research in the field of philosophy or quantum foundations, studying the assumptions and principles of quantum physics, were least likely to prefer Copenhagen's interpretation, and only 20% chose it. "If I use quantum mechanics in my laboratory every day, I don't have to pass by Copenhagen," says Carlo Rovelli, a theoretical physicist from the University of Aix-Marseille in France. But as soon as researchers apply thought experiments that explore more deeply, "Copenhagen is not enough," he says.

What else is on the menu?

In the years after World War II and the development of the atomic bomb, physicists began to use quantum mechanics, and the U.S. government invested money in the field. Philosophical research was put in the background. The Copenhagen interpretation began to dominate mainstream physics, but nevertheless some physicists found it unsatisfactory and came up with alternatives.

Quantum mechanics: five interpretations

Here are five broad approaches to the interpretation of quantum mechanics - and how they solve the problem of quantum measurement.

In quantum theory, an unobservable system can be described as being in the superposition of several possible states at the same time, for example, in different places. Its quantum state is given by a wave function, which develops in accordance with the Schrödinger equation in a smooth, predictable way. But when interacting with the measuring equipment, the system acquires a clearly defined state, unknown in advance. Its wave function is "folded", as some say. How to understand it?

The thought experiment "Schrödinger's cat" demonstrates a mystery. Here, whether the poison is released - potentially killing the cat in the box - depends on the radiation, a random quantum event. Until the box is opened, the cat can be described as super-positioned alive and dead; when looking inside the box, it is in only one of two states.

In 1952, the American physicist David Bohm reappeared the idea, first projected in 1927 by the French physicist Louis de Broglie, namely that the strange dual nature of quantum objects makes sense if they are point particles with paths defined by "pilot" waves. The mechanics of "Bohm" had an advantage in explaining the interference effects in the restoration of determinism, the idea that the properties of particles have established values before measurement. The Nature survey showed that 7% of respondents consider this interpretation to be the most convincing.

Then, in 1957, American physicist Hugh Everett came up with a wilder alternative, which was preferred by 15% of respondents. Everett's interpretation, later called "many worlds", states that the wave function corresponds to something real. That is, the particle is really located, in a sense, in several places at the same time. From their point of view, in one world, an observer who measured a particle will see only one result, but the wave function is never really destroyed. Instead, it branches into many universes, one for each individual result. "This requires a radical adjustment of our intuition about the world, but for me this is exactly what we should expect from the fundamental theory of reality," says Sean Carroll, a physicist and philosopher at Johns Hopkins University in Baltimore, Maryland, who answered the survey.

In the late 1980s, theories of "spontaneous collapse" tried to solve problems such as the problem of quantum measurement. These versions adjust the Schrödinger equation, so instead of requiring an observer collapse or measurement, the wave function sometimes does it by itself. In some of these models, the unification of quantum objects increases the probability of collapse, which means that bringing the particle to a superpositioned position using measuring equipment makes the loss of the combined quantum state inevitable. About 4% of respondents chose such theories.

The study of nature suggests that "epistemic" descriptions, which say that quantum mechanics reveals only knowledge about the world, and do not represent its physical reality, could become popular. The 2016 survey Physicists found that only about 7% chose interpretations related to epistemics, compared to 17% in our survey (although the exact categories and survey methodology differed). Some of these theories, based on Copenhagen's original interpretation, appeared in the early 2000s, when applications such as quantum computing and communication began to formulate experiments in terms of information. Binders such as Zeilinger consider the wave function simply as a tool for predicting measurement results, not corresponding to the real world.

The epistemic point of view is attractive because it is the most cautious, says Ladina Hausmann, a theoretical physicist in ETH, who answered the survey. "It doesn't require me to assume anything other than how we use the quantum state in practice," she says.

One epistemic interpretation known as QBism (which several respondents who chose "other" recorded as the preferred interpretation) takes this to the extreme, stating that the observations made by a particular "agent" are completely personal and valid only for them. A similar "relational quantum mechanics", first described by Rovelli in 1996 (and chosen by 4% of respondents), suggests that quantum states always describe only the relationship between systems, not the systems themselves.