Back in 1961, physicist and Nobel laureate Eugene Wigner outlined a thought experiment that demonstrated one of the least known paradoxes of quantum mechanics. The experiment shows how the strange nature of the universe allows two observers - say Wigner and Wigner's friend - to experience different realities. Since then, physicists have used the "Wigner's friend" thought experiment to investigate the nature of measurements and debate whether objective facts exist.
First, let's talk about Wigner's thought experiment:
Suppose two people open the box with Schrödinger's cat at the same time. If the result (of the collapse of the wave function) is chosen by the observer, as implied by the idealistic solution, then if the two observers make different choices, a problem arises. If we name one of the two outcomes, then only one of the observers can make a choice, and supporters of realism rightly consider this decision unsatisfactory.
The paradox has become important because scientists are conducting experiments to establish objective facts. But if they are faced with different realities, how can they agree on what these facts might be? Wigner's thought experiment was never more than a thought experiment.
But last year, physicists noticed that recent advances in quantum technology have made it possible to replicate Wigner's friend's test in a real experiment. In other words, it has become possible to create different realities and compare them in the laboratory in order to find out if they can be reconciled.
Is there an objective reality?
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And today, Massimiliano Proietti from Heriot-Watt University in Edinburgh and several of his colleagues said that they had conducted this experiment for the first time: they created different realities and compared them. And they came to the conclusion that Wigner was right: these realities can be irreconcilable to the extent that it is impossible to come to a consensus about objective facts in an experiment.
Wigner's original thought experiment began with a single polarized photon, which, when measured, can be horizontally or vertically polarized. But before the measurement, according to the laws of quantum mechanics, the photon exists in both polarization states simultaneously - the so-called superposition.
Wigner imagined a friend in another laboratory who measures the state of this photon and stores the result, while Wigner observes from afar. Wigner does not have information about his friend's measurements and therefore has to assume that the photon and its measurement are in a superposition of all possible experimental results.
Wigner may even conduct an experiment to determine if this superposition exists or not. A kind of interference experiment that will show that the photon and the measurement are indeed in superposition.
From Wigner's point of view, this is a "fact" - superposition exists. And this fact suggests that the measurement could not be carried out.
But his friend will not agree with this, since he measured the polarization of the photon and recorded it. The friend may even call Wigner and tell him that the measurement has been taken (provided the result is not disclosed).
Two realities contradict one another. “This casts doubt on the objective status of the facts established by the two observers,” says Proietti.
That's the theory, but last year, Caslav Bruckner of the University of Vienna in Australia came up with a way to recreate the Wigner friend in the laboratory using techniques that involve entangling many particles at once.
Proietti's breakthrough was that they actually did it. They have implemented an extended "friend of Wigner's" scenario in a modern six-photon experiment.
Six photons were entangled to create two alternate realities - one representing Wigner and the other representing Wigner's friend. Wigner's friend measures the polarization of the photon and stores the result. Wigner then takes an interference measurement to see if the measurement and the photon are in superposition.
The experiment gave an ambiguous result. It turns out that both realities can coexist, even if they produce irreconcilable results, as predicted by Wigner. This raises a number of interesting questions that prompt physicists to rethink the nature of reality.
The idea that observers can ultimately reconcile their measurements in some fundamental reality is based on several assumptions. First, universal facts do exist and observers can agree on them.
But there are other assumptions as well. One is that observers are free to make any observations they want. And one more thing: the choice that one observer makes does not affect the choice of other observers. This assumption of physics is called locality.
If there is an objective reality that everyone can agree with, then all of these assumptions will be correct.
But the result of Proietti and his colleagues suggests that objective reality does not exist. In other words, an experiment assumes that one or more assumptions are that there is a reality with which we agree; what is freedom of choice; or locality - should be wrong.
There is, of course, another option. The possibility that there is a loophole that the experimenters missed. In fact, physicists have tried to close the loopholes in such experiments for many years, but they admit that they may never be able to close them all.
However, work has important implications for science. The next step is to go further: to create experiments that create ever more bizarre alternative realities that are impossible to reconcile. Where this will lead us, nobody knows.
Ilya Khel