Quantum experiment suggests there really are ‘alternative facts’

Physics 22 February 2019

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It’s time to rethink reality


There are no objective facts in the world. This isn’t a statement about fake news. Rather, it is the implication of an experiment that suggests the nature of reality depends on who is looking.

The work is rooted in thought experiments about the nature of quantum mechanics, but this is the first time one has been done in the lab, with potentially profound implications. “I am very excited about it,” says theorist Carlo Rovelli at Aix-Marseille University in France.

The experiment, carried out by Alessandro Fedrizzi at Heriot-Watt University, UK, and his team, involved four observers: Alice, her friend Amy, Bob and his friend Brian. It begins with Amy and Brian inside their respective labs. A central source outside both labs creates a pair of photons linked by quantum entanglement, sending one each to Amy and Brian.


Amy’s lab

Amy creates another pair of entangled photons: a system photon and a test photon. Amy uses the test photon to measure the state of the original photon from outside the lab, and the result is imprinted on the system photon through quantum entanglement. In thought experiments, Amy’s measurement is stored in her memory. In the real experiment, it is stored in the system photon, making it the “observer”.

Once Amy has made her measurements, she sends the original and system photons out of the lab to Alice. She can either directly measure the system photon, which is akin to just asking Amy what she measured (the result A0), or Alice can let the two photons interfere quantum mechanically, establishing her own fact without asking Amy (the result A1).

Meanwhile, Brian does exactly the same as Amy with the other original photon, and Bob, who is outside Brian’s lab, makes similar choices to Alice to get Brian’s result, B0, or his own, B1.

If this is confusing, here is the real mind-bender: quantum mechanics says that the results A1 and B1 (the facts as established by Alice and Bob) can disagree with A0 and B0 (the facts as established by their respective friends).

This can be verified by running the experiment over and over, with Alice and Bob making their choices at random, then tallying the average probabilities of the outcomes.

Three assumptions

The process involves making three assumptions. One, Alice and Bob have the freedom to choose their measurements. Two, Alice’s choice doesn’t influence Bob’s outcomes and vice versa. Finally, there are observer-independent facts in the world. “A piece of information that’s obtained in a measurement should be a fact of the world – a fact that all observers can agree on,” says Fedrizzi.

If these assumptions are correct, the tally of probabilities should be no more than 2. The real experiment gave a value of 2.47.

While this is predicted by quantum theory, it also implies the assumptions are wrong. Prior theoretical work suggests that even if you deal with the first two assumptions, the contradicting facts can persist. “One natural way to resolve this is to say there aren’t any objective facts,” says Fedrizzi.

The experiment could have immense implications for our understanding of the nature of quantum reality, which depends on how we interpret quantum theory. According to Fedrizzi and his colleagues, their work favours interpretations saying that the outcomes of experiments are subjective, such as quantum Bayesianism and Rovelli’s relational quantum mechanics.

In turn, it questions more mainstream views, such as the Copenhagen interpretation, which says that the properties of quantum systems don’t exist until observed, but then become objective facts, and the many worlds interpretation, which says that all possible measurement outcomes are real and objective, but each in a different world.

Renato Renner at the Swiss Federal Institute of Technology in Zurich, who last year published a thought experiment along similar lines, thinks photons might not count as observers. “The validity of the conclusions depends on whether one can reasonably claim that their experiment mimics ‘observers’,” he says.

But Rovelli is thrilled. “I do take it as a great piece of evidence directly supporting the relational interpretation. I agree in full with the way they interpret it,” he says. “It is fantastic that ‘ideal experiments’ of the past become real experiments of today.”


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