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SUNDAY SCIENCE: PHYSICISTS DISAGREE WILDLY ON WHAT QUANTUM MECHANICS
SAYS ABOUT REALITY, NATURE SURVEY SHOWS  
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Elizabeth Gibney
July 30, 2025
Nature [[link removed]]

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_ First major attempt to chart researchers’ views finds
interpretations in conflict. _

, Illustration: Olena Shmahalo/Nature

 

Quantum mechanics is one of the most successful theories in science
— and makes much of modern life possible. Technologies ranging from
computer chips to medical-imaging machines rely on the application of
equations, first sketched out a century ago
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the behaviour of objects at the microscopic scale.

But researchers still disagree widely on how best to describe the
physical reality that lies behind the mathematics, as
a _Nature_ survey reveals.

At an event to mark the 100th anniversary
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mechanics last month, lauded specialists in quantum physics argued
politely — but firmly — about the issue. “There is no quantum
world,” said physicist Anton Zeilinger, at the University of Vienna,
outlining his view that quantum states exist only in his head and that
they describe information, rather than reality. “I disagree,”
replied Alain Aspect, a physicist at the University of Paris-Saclay,
who shared the 2022 Nobel prize
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for work on quantum phenomena.

To gain a snapshot of how the wider community interprets quantum
physics in its centenary year, _Nature_ carried out the largest ever
survey on the subject. We e-mailed more than 15,000 researchers whose
recent papers involved quantum mechanics, and also invited attendees
of the centenary meeting, held on the German island of Heligoland, to
take the survey.

The responses — numbering more than 1,100, mainly from physicists
— showed how widely researchers vary in their understanding of the
most fundamental features of quantum experiments.

As did Aspect and Zeilinger, respondents differed radically on whether
the wavefunction — the mathematical description of an object’s
quantum state — represents something real (36%) or is simply a
useful tool (47%) or something that describes subjective beliefs about
experimental outcomes (8%). This suggests that there is a significant
divide between researchers who hold ‘realist’ views, which project
equations onto the real world, and those with ‘epistemic’ ones,
which say that quantum physics is concerned only with information.

The community was also split on whether there is a boundary between
the quantum and classical worlds (45% of respondents said yes, 45% no
and 10% were not sure). Some baulked at the set-up of our questions,
and more than 100 respondents gave their own interpretations (the
survey, methodology and an anonymized version of the full data are
available in supplementary information at the foot of this page).

“I find it remarkable that people who are very knowledgeable about
quantum theory can be convinced of completely opposite views,” says
Gemma De les Coves, a theoretical physicist at the Pompeu Fabra
University in Barcelona, Spain.

_Nature_ asked researchers what they thought was the best
interpretation of quantum phenomena and interactions — that is,
their favourite of the various attempts scientists have made to relate
the mathematics of the theory to the real world. The largest chunk of
responses, 36%, favoured the Copenhagen interpretation — a practical
and often-taught approach. But the survey also showed that several,
more radical, viewpoints have a healthy following.

Asked about their confidence in their answer, only 24% of respondents
thought their favoured interpretation was correct; others considered
it merely adequate or a useful tool in some circumstances. What’s
more, some scientists who seemed to be in the same camp didn’t give
the same answers to follow-up questions, suggesting inconsistent or
disparate understandings of the interpretation they chose.

“That was a big surprise to me,” says Renato Renner, a theoretical
physicist at the Swiss Federal Institute of Technology (ETH) in
Zurich. The implication is that many quantum researchers simply use
quantum theory without engaging deeply with what it means — the
‘shut up and calculate’ approach, he says, using a phrase coined
by US physicist David Mermin. But Renner, who works on the foundations
of quantum mechanics, is quick to stress that there is nothing wrong
with just doing calculations. “We wouldn’t have a quantum computer
if everyone was like me,” he says.

COPENHAGEN STILL REIGNS SUPREME

Over the past century, researchers have proposed many ways to
interpret the reality behind the mathematics of quantum mechanics,
which seems to throw up jarring paradoxes. In quantum theory, an
object’s behaviour is characterized by its wavefunction: a
mathematical expression calculated using an equation devised by German
physicist Erwin Schrödinger in 1926. The wavefunction describes a
quantum state and how it evolves as a cloud of probabilities. As long
as it remains unobserved, a particle seems to spread out like a wave;
interfering with itself and other particles to be in a
‘superposition’ of states, as though in many places or having
multiple values of an attribute at once. But an observation of a
particle’s properties — a measurement — shocks this hazy
existence into a single state with definite values. This is sometimes
referred to as the ‘collapse’ of the wavefunction.

It gets stranger: putting two particles into a state of joint
superposition can lead to entanglement, which means that their quantum
states remain intertwined even when the particles are far apart.

The German physicist Werner Heisenberg, who helped to craft the
mathematics behind quantum mechanics in 1925, and his mentor, Danish
physicist Niels Bohr, got around the alien wave–particle duality
largely by accepting that classical ways of understanding the world
were limited, and that people could only know what observation told
them. For Bohr, it was OK that an object varied between acting like a
particle and like a wave, because these were concepts borrowed from
classical physics that could be revealed only one at a time, by
experiment. The experimenter lived in the world of classical physics
and was separate from the quantum system they were measuring.

Heisenberg and Bohr not only took the view that it was impossible to
talk about an object’s location until it had been observed by
experiment, but also argued that an unobserved particle’s properties
really were fundamentally unfixed until measurement — rather than
being defined, but not known to experimenters. This picture famously
troubled Einstein
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the view that there was a pre-existing reality that it was science’s
job to measure.

Decades later, an amalgamation of Heisenberg’s and Bohr’s
not-always-unified views became known as the Copenhagen
interpretation, after the university at which the duo did their
seminal work. Those views remain the most popular vision of quantum
mechanics today, according to _Nature_’s survey. For Časlav
Brukner, a quantum physicist at the University of Vienna, this
interpretation’s strong showing “reflects its continued utility in
guiding everyday quantum practice”. Almost half of the experimental
physicists who responded to the survey favoured this interpretation,
compared with 33% of the theorists. “It is the simplest we have,”
says Décio Krause, a philosopher at the Federal University of Rio de
Janeiro, Brazil, who studies the foundations of physics, and who
responded to the survey. Despite its issues, the alternatives
“present other problems which, to me, are worse”, he says.

But others argue that Copenhagen’s emergence as the default comes
from historical accident, rather than its strengths. Critics say it
allows physicists to sidestep deeper questions.

One concerns the ‘measurement problem’, asking how a measurement
can trigger objects to switch from existing in quantum states that
describe probabilities, to having the defined properties of the
classical world.

Another unclear feature is whether the wavefunction represents
something real (an answer selected by 29% of those who favoured the
Copenhagen interpretation) or just information about the probabilities
of finding various values when measured (picked by 63% of this group).
“I’m disappointed but not surprised at the popularity of
Copenhagen,” says Elise Crull, a philosopher of physics at the City
University of New York. “My feeling is that physicists haven’t
reflected.”

The Copenhagen interpretation’s philosophical underpinnings have
become so normalized as to seem like no interpretation at all, adds
Robert Spekkens, who studies quantum foundations at the Perimeter
Institute for Theoretical Physics in Waterloo, Canada. Many advocates
are “just drinking the Kool-Aid of the Copenhagen philosophy without
examining it”, he says.

Survey respondents who have carried out research in philosophy or
quantum foundations, studying the assumptions and principles behind
quantum physics, were the least likely to favour the Copenhagen
interpretation, with just 20% selecting it. “If I use quantum
mechanics in my lab every day, I don’t need to go past
Copenhagen,” says Carlo Rovelli, a theoretical physicist at
Aix-Marseille University in France. But as soon as researchers apply
thought experiments that probe more deeply, “Copenhagen is not
enough”, he says.

WHAT ELSE IS ON THE MENU?

In the years after the Second World War and the development of the
atomic bomb, physicists began to exploit the uses of quantum
mechanics, and the US government poured cash into the field.
Philosophical investigation was put on the back burner. The Copenhagen
interpretation came to dominate mainstream physics
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physicists found it unsatisfying and came up with alternatives (see
‘Quantum mechanics: five interpretations’).

Quantum mechanics: Five interpretations

Here are five broad approaches to interpreting quantum mechanics —
and how they address the quantum measurement problem.

In quantum theory, an unobserved system can be described as being in a
superposition of multiple possible states at once, for example in
different locations. Its quantum state is given by a wavefunction,
which evolves according to Schrödinger’s equation in a smooth,
predictable way. But when interacting with measuring equipment, the
system acquires a well-defined state, unknowable in advance. Its
wavefunction ‘collapses’, as some say. How to make sense of this?

The ‘Schrödinger’s cat’ thought experiment showcases the
conundrum. Here, whether poison is released — potentially killing a
cat in a box — depends on radiation being emitted, a random quantum
event. Until the box is opened, the cat can be described as a
superposition of alive and dead; on looking inside the box, it is in
only one of the two states.

Illustrations: Nik Spencer/_Nature_

In 1952, US physicist David Bohm resurfaced an idea first touted in
1927 by French physicist Louis de Broglie, namely that the strange
dual nature of quantum objects made sense if they were point-like
particles with paths determined by ‘pilot’ waves. ‘Bohmian’
mechanics had the advantage of explaining interference effects while
restoring determinism, the idea that the properties of particles do
have set values before being measured. _Nature_’s survey found that
7% of respondents considered this interpretation the most convincing.

Then, in 1957, US physicist Hugh Everett came up with a wilder
alternative, one that 15% of survey respondents favoured. Everett’s
interpretation, later dubbed ‘many worlds’, says that the
wavefunction corresponds to something real. That is, a particle really
is, in a sense, in multiple places at once. From their vantage point
in one world, an observer measuring the particle would see only one
outcome, but the wavefunction never really collapses. Instead it
branches into many universes, one for each different outcome. “It
requires a dramatic readjustment of our intuitions about the world,
but to me that’s just what we should expect from a fundamental
theory of reality,” says Sean Carroll, a physicist and philosopher
at Johns Hopkins University in Baltimore, Maryland, who responded to
the survey.

In the late 1980s, ‘spontaneous collapse’ theories attempted to
resolve issues such as the quantum measurement problem. Versions of
these tweak the Schrödinger equation, so that, rather than requiring
an observer or measurement to collapse, the wavefunction occasionally
does so by itself. In some of these models, putting quantum objects
together amplifies the likelihood of collapse, meaning that bringing a
particle into a superposition with measuring equipment makes the loss
of the combined quantum state inevitable. Around 4% of respondents
chose these sorts of theories.

_Nature_’s survey suggests that ‘epistemic’ descriptions, which
say that quantum mechanics reveals only knowledge about the world,
rather than representing its physical reality, might have gained in
popularity. A 2016 survey1
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physicists found that only around 7% picked epistemic-related
interpretations, compared with 17% in our survey (although the precise
categories and methodology of the surveys differed). Some of these
theories, which build on the original Copenhagen interpretation,
emerged in the early 2000s, when applications such as quantum
computing and communication began to frame experiments in terms of
information. Adherents, such as Zeilinger, view the wavefunction as
merely a tool to predict measurement outcomes, with no correspondence
to the real world.

The epistemic view is appealing because it is the most cautious, says
Ladina Hausmann, a theoretical physicist at the ETH who responded to
the survey. “It doesn’t require me to assume anything beyond how
we use the quantum state in practice,” she says.

One epistemic interpretation, known as QBism (which a handful of
respondents who selected ‘other’ wrote down as their preferred
interpretation), takes this to the extreme, stating that observations
made by a specific ‘agent’ are entirely personal and valid only
for them. The similar ‘relational quantum mechanics’, first
outlined by Rovelli in 1996 (and selected by 4% of respondents), says
that quantum states always describe only relationships between
systems, not the systems themselves.

_Nature_ 643, 1175-1179 (2025)

_doi: [link removed]

Additional survey analysis by Richard Van Noorden and Jeffrey M.
Perkel.

References

*
Sivasundaram, S. & Nielsen, K. H. Preprint at
arXiv [link removed] (2016).

*
Hallas, A. M. _Nature Phys._ 21, 491–493 (2025).

Article [[link removed]] Google Scholar
[[link removed].] 

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Sharoglazova, V., Puplauskis, M., Mattschas, C., Toebes, C. & Klaers,
J. _Nature_ 643, 67–72 (2025).

Article [[link removed]] PubMed
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Scholar
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Supplementary Information

* Survey methodology
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* Survey questions
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* Full survey results data
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ELIZABETH GIBNEY is a senior physics reporter at _Nature
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[[link removed]] She
has written for _Scientific American
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_Nature_ is a weekly international journal publishing the finest
peer-reviewed research in all fields of science and technology on the
basis of its originality, importance, interdisciplinary interest,
timeliness, accessibility, elegance and surprising
conclusions. Nature also provides rapid, authoritative, insightful
and arresting news and interpretation of topical and coming trends
affecting science, scientists and the wider public.

_Nature_'s mission statement

First, to serve scientists through prompt publication of significant
advances in any branch of science, and to provide a forum for the
reporting and discussion of news and issues concerning science.
Second, to ensure that the results of science are rapidly disseminated
to the public throughout the world, in a fashion that conveys their
significance for knowledge, culture and daily life.

Nature's original mission statement
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published for the first time on 11 November 1869.

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