Back in 1961, the physicist and Nobel laureate EugeneWigner laid out a mental experiment that was demonstrated by 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 been using the “Wigner friend” thought experiment to study the nature of the measurements and the debate about whether there are objective facts.
First, let us tell you about Wigner's thought experiment:
Suppose two people are simultaneouslyopen the box with the cat Schrodinger. If the result (of the collapse of the wave function) is chosen by the observer, as an idealistic solution implies, then if two observers make different choices, a problem arises. If we call one of the two outcomes, then only one of the observers can make the choice, and supporters of realism rightly consider this decision unsatisfactory.
"In the paradox of Wigner's friend, formulatedas a physicist Eugene Wigner, the following happens: suppose that instead of watching a cat yourself, Wigner asks his friend to do it. His friend opens the box, sees the cat and then reports the results of his observation to Vigner. At this stage we can say that Wigner has just actualized the reality, which includes his friend and the cat. There is a paradox here: Was the cat alive or dead when Wigner's friend watched her, but before he reported the result of the observation? To say that when Wigner's friend observed a cat, her condition did not collapse, it means to assert that his friend was unconscious until Wigner asked him - that his friend's mind could not decide whether the cat was alive or dead, without prompting from Wigner .
The paradox became important because scientists spendexperiments to establish objective facts. But if they face different realities, how can they agree on what these facts might be? Wigner's mental experiment was never more than just a thought experiment.
But last year, physicists noticed that the lastadvances in quantum technology allowed Wigner's friend to reproduce the test in a real experiment. In other words, it became possible to create different realities and compare them in the laboratory in order to find out if they could be reconciled.
Is there an objective reality?
And today Massimiliano Proietti from the UniversityHeriot-Watt in Edinburgh and several colleagues said that they 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 as irreconcilable as possible, that it is impossible to come to a common opinion about objective facts in the experiment.
Wigner's original thought experimentIt began with a single polarized photon, which, when measured, can have horizontal or vertical polarization. But before measurement, according to the laws of quantum mechanics, a photon exists in both states of polarization at the same time - the so-called superposition.
Wigner imagined a friend in anotherlaboratory, which measures the state of this photon and stores the result, while Wigner observes from afar. Wigner does not have information about the measurements of his friend and is therefore forced to assume that the photon and its measurement are in a superposition of all possible experimental results.
Wigner may even conduct an experiment todetermine whether this superposition exists or not. A kind of interference experiment, which shows that the photon and the measurement are indeed in a superposition.
From the point of view of Wigner, this “fact” - there is a superposition. And this fact suggests that the measurement could not be taken.
But his friend does not agree with this, since he measured the polarization of the photon and wrote it down. A friend may even call Wigner and say that the measurement has been made (provided that the result is not disclosed).
Two realities contradict one another. “This casts doubt on the objective status of the facts ascertained by two observers,” says Proietti.
That is the theory, but last year Kaslav Brucknerfrom the University of Vienna in Australia, he invented a way to recreate a “Wigner friend” in the laboratory using methods involving entanglement of many particles simultaneously.
Breakthrough Proyetti was that they actually did it. They implemented the extended scenario of the “friend of Wigner” in a modern six-photon experiment.
Six photons were entangled to create twoalternate realities - one representing Wigner, and the second - Wigner friend. Wigner's friend measures the photon's polarization and saves the result. Wigner then measures the interference 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 give irreconcilable results, as Wigner predicted. This raises a number of interesting questions that motivate physicists to reconsider the nature of reality.
The idea that observers may end upto reconcile their measurements in some fundamental reality, based on several assumptions. First, universal facts do exist and observers can agree on them.
But there are other assumptions. One of them 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 with which everyone can agree, all these assumptions will be true.
But the result of Proietti and his colleagues suggeststhat objective reality does not exist. In other words, the experiment suggests that one or more assumptions are that there is a reality with which we agree; what is freedom of choice; or about locality - should be wrong.
Of course, there is another option. Option that there is a loophole that experimenters missed. In fact, physicists have tried to close loopholes in such experiments for many years, but they admit that they may never be able to close them all.
However, the work has important implications forscience. The next step is to go further: to create experiments that create increasingly bizarre alternative realities that cannot be reconciled. Where it takes us, no one knows.
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