Some of the world’s leading physicists gathered on German the island of Heligoland and had an unusual disagreement. “There is no quantum world,” declared Anton Zeilinger, the Nobel Prize-winning physicist from the University of Vienna. “Quantum states exist only in our heads.” Alain Aspect, who shared that same Nobel in 2022, gently pushed back. “I disagree,” he replied.

It’s not uncommon for scientists (including leading scientists) to have a disagreement. But it is a bit unusual that people from the same field disagree on the fundamental interpretation involved in their work.

It’s the weirdest thing: quantum theory works incredibly well in practice. It powers devices ranging from MRI machines to lasers and the very computer you’re reading this on. But even after 100 years, physicists still can’t agree on what it means.

The Physics Works. The Philosophy Doesn’t

Quantum mechanics is the branch of physics that describes the behavior of nature at the smallest scales (from atoms to electrons, photons, and other subatomic particles). Unlike classical physics, which governs the motion of planets or falling apples, quantum mechanics reveals a world governed by probabilities, uncertainty, and phenomena that defy everyday intuition.

In 1900, Max Planck proposed that energy comes in discrete packets, or “quanta.” Albert Einstein extended this idea in 1905, suggesting that light itself is made of particles — photons. Over the next two decades, experiments continued to build on this idea. By 1925, Werner Heisenberg developed a version of quantum theory based on observable quantities, followed by Erwin Schrödinger’s wave mechanics a year later, a thorough mathematical description of how quantum systems evolve over time.

To mark the theory’s centenary, Nature conducted the largest-ever survey of quantum physicists, sending questions to over 15,000 researchers and receiving more than 1,100 responses. The results show researchers confident in the field but disagreeing on interpretation.

At the heart of these disagreements lies a fundamental problem: what happens when you observe a quantum system?

The classic example (Schrödinger’s cat) imagines a cat trapped in a box. Inside the sealed box, a quantum event with 50-50 odds determines whether a vial of poison is released and whether the cat is killed or not. You don’t know what happened, and the cat is both alive and dead until someone opens the box and observes the outcome.

This is a simple thought experiment designed by Austrian physicist Erwin Schrödinger to highlight the peculiar consequences of quantum superposition. According to quantum mechanics, until the box is opened and the system is observed, the cat exists in a superposition of both life and death. Only when someone looks does one definite outcome emerge.

But what are you actually doing when you open the box and look at the outcome? Surprisingly, there are not two or three, but five competing theories.

Five Ways To Understand Reality

In the Copenhagen view, opening the box creates reality. The large, macroscopic systems and the quantum ones are fundamentally different. The particle has properties only when measured by an observer; they are not intrinsic. This approach basically says that quantum mechanics doesn’t describe a physical reality that exists independently of measurement. Instead, it describes what we can know through measurement. But critics argue that it sidesteps deeper questions: What is a measurement and when exactly is the outcome decided?

The many-worlds interpretation addresses those questions by taking a radically different path. Instead of a range of possibilities collapsing into a single outcome, it claims that all possible outcomes actually occur, but each in a separate, branching universe. So, in Schrödinger’s thought experiment, the cat is both alive and dead but in two distinct, non-communicating worlds. The universe simply keeps branching, endlessly, into parallel realities. This explains pretty much everything, on the (rather tough) condition that you believe in an endless stream of universes branching out infinitely.

Bohmian mechanics sidesteps the problem by introducing a “pilot wave” that makes all quantum phenomena deterministic and not random, but it has the “small” problem of instantaneous effects happening faster than the speed of light, which is fundamentally incompatible with Einstein’s theory of relativity.

Spontaneous collapse takes an even weirder approach, assuming that quantum mechanics is only an approximation of a different theory. This approach is mathematically very sound but it’s not confirmed by any experiments.

Finally, epistemic interpretations suggest that quantum states represent only information detectable by an external agent. This also poses an unpleasant problem that reality can only exist subjectively and there is no objective reality.

If this all has your head buzzing, well, you’re not alone. Carlo Rovelli, a theoretical physicist at Aix-Marseille University in France, told Nature that for all practical purposes, he uses the Copenhagen view. But if you try to dive deeper into thought experiments, you run into problems.

All these interpretations have massive implications for how we understand reality. Yet, no most people don’t seem to agree.

So What Do Quantum Physicists Say?

Copenhagen interpretation was the most popular, selected by 36% of respondents. But among them, only a small fraction felt confident it was correct; most saw it as simply adequate or useful. This was the most popular option. Epistemic approaches (17%) and Many Worlds (15%) followed.

About 10% of respondents selected “other” and provided free-text answers, while a small number said none of the interpretations seemed adequate or that no interpretation was needed at all. Notably, in total, only 24% of all respondents felt confident that their preferred interpretation was correct.

Despite their differences, most respondents agreed that interpretation is worth pursuing. A full 86% said it’s a valuable endeavor. Three-quarters believed quantum theory will eventually be replaced by a more complete framework. Some hope experiments might one day help settle the score. Others believe quantum computing will help.

But perhaps, the fact that we don’t agree is exactly the point.

Quantum mechanics has always been less about certainties and more about confronting the limits of what we can know, taking the oddity of the universe head-on. That scientists can use the same equations to build quantum computers, probe the nature of black holes, and still disagree on what this actually means is not a failure. It’s a reminder that there’s still something out there waiting to be discovered. It’s also a reminder that physics describes what happens in the universe, and doesn’t necessarily seek to explain why it happens.

It’s possible that one day, a new framework will emerge to unify these competing views, or perhaps nature will prove stranger than any of them. Until then, quantum physicists will continue to calculate and create devices that improve our lives—all while not agreeing on what it actually means.