Its equations cannot predict the exact outcome of a measurement — for example, of the position of an electron — only the probabilities that it can yield particular values. The most common way of understanding this was formulated in the s by quantum-theory pioneers Niels Bohr and Werner Heisenberg, and is called the Copenhagen interpretation, after the city where Bohr lived. The Copenhagen interpretation left open the question of why different rules should apply to the quantum world of the atom and the classical world of laboratory measurements and of everyday experience.

But it was also reassuring: although quantum objects live in uncertain states, experimental observation happens in the classical realm and gives unambiguous results. Now, Frauchiger and Renner are shaking physicists out of this comforting position. Their theoretical reasoning says that the basic Copenhagen picture — as well as other interpretations that share some of its basic assumptions — is not internally consistent.

In that case, the state of the cat was uncertain until the experimenter opened the box and checked it. In , the Hungarian physicist Eugene Wigner proposed a version of the paradox in which he replaced the cat and the poison with a physicist friend who lived inside a box with a measuring device that could return one of two results, such as a coin showing heads or tails. One school of thought says that it does, suggesting that consciousness is outside the quantum realm. But if quantum mechanics applies to the physicist, then she should be in an uncertain state that combines both outcomes until Wigner opens the box.

They have two Wigners, each doing an experiment on a physicist friend whom they keep in a box. One of the two friends call her Alice can toss a coin and — using her knowledge of quantum physics — prepare a quantum message to send to the other friend call him Bob.

## Schrödinger's Cat Paradox: Who Killed the Cat?

When the two Wigners open their boxes, in some situations they can conclude with certainty which side the coin landed on, Renner says — but occasionally their conclusions are inconsistent. Yet it might be feasible to make two quantum computers play the parts of Alice and Bob: the logic of the argument requires only that they know the rules of physics and make decisions based on them, and in principle one can detect the complete quantum state of a quantum computer.

Quantum computers sophisticated enough to do this do not yet exist, Renner points out. Physicists are still coming to terms with the implications of the result.

### File Extensions and File Formats

It has triggered heated responses from experts in the foundations of quantum theory, many of whom tend to be protective of their pet interpretation. And different researchers tend to draw different conclusions. For Leifer, producing inconsistent results should not necessarily be a deal breaker. Some interpretations of quantum mechanics already allow for views of reality that depend on perspective. That could be less unsavoury than having to admit that quantum theory does not apply to complex things such as people, he says. Robert Spekkens, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada, says that the way out of the paradox could hide in some subtle assumptions in the argument, in particular in the communication between Alice and Bob.

But he admits that he has not found a solution yet. For now, physicists are likely to continue debating. In fact, the wave function seems to describe bizarre situations, like physical systems existing simultaneously in multiple states, such as different positions or velocities. It gives very precise probabilities for the possible outcomes of laboratory experiments, but it defies an intuitive interpretation. Other fragments of quantum theory were later shown to fit similar models, but the question was open whether or not this was possible for all of quantum theory.

To understand what this kind of model is, imagine I hold two decks of cards: one contains only red cards, the other only aces, and I ask you to pick a card from one, without knowing which is which. In an epistemic interpretation, the wave function would play the role of the deck you pick the card from. In fact, it is possible that you have picked an ace of hearts, which is compatible with both decks. A wake-up call came in , when Matthew Pusey, Jonathan Barrett and Terry Rudolph showed that in any objective model of quantum theory, the wave function must be a real property of individual systems, unlike the deck of cards.

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But, their theorem had an extra assumption that has called the implications of the theorem into question. However, a series of theorems published within the last year, starting with work from myself and colleagues , puts strong bounds on the viability of epistemic models, even without those extra assumptions.

## The Physics Behind Schrödinger's Cat Paradox

These theorems consider the fact that some pairs of quantum states cannot be distinguished on a single experiment. This is analogous to not always being able to tell whether a randomly picked card came from the red deck or the ace deck. If you pick a non-ace card, you can be sure it came from the red deck.

If you pick a black ace, you can be sure it came from the ace deck. Counting the cards in the decks, we can determine how often this is supposed to happen.

But the theorems show that this explanation simply cannot work. These predictions were partially confirmed by the experiment I was involved with, performed by Martin Ringbauer and the Brisbane team led by Alessandro Fedrizzi. They followed an improved version of our theorem due to Cyril Branciard, a co-author in the study.

## Reimagining of Schrödinger’s cat breaks quantum mechanics — and stumps physicists

The experiment involved preparing single photons particles of light in those specially designed states and subjecting them to a number of alternative measurements. The results give bounds on how well a model like the one outlined above can describe the statistics they observe.

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If further experiments confirm the implications of the theorems, viable epistemic models of quantum mechanics will be essentially ruled out. But there are alternatives. One could be to reconsider assumptions of the framework used to derive the theorems, perhaps by introducing backwards-in-time causality or parallel universes.