But eventually, we succeeded in showing that quantum mechanics might indeed be incompatible with the assumption of objective facts-we violated the inequality!
Outside the two boxes, two photons remain on each side that can also be measured.ĭespite using state-of-the-art quantum technology, it took weeks to collect sufficient data from just six photons to generate enough statistics. The first photon pair represents the coins, and the other two are used to perform the coin toss-measuring the polarization of the photons-inside their respective box. We have now for the first time performed this test experimentally at Heriot-Watt University in Edinburgh on a small-scale quantum computer made up of three pairs of entangled photons. If this inequality is violated, observers could have alternative facts. The results can be summed up to ultimately be used to evaluate a so called "Bell inequality". Brukner considered two pairs of Wigners and friends, in two separate boxes, conducting measurements on a shared state-inside and outside their respective box. But does it reflect reality? Scientifically, there has been little progress on this until very recently, when Časlav Brukner at the University of Vienna showed that, under certain assumptions, Wigner's idea can be used to formally prove that measurements in quantum mechanics are subjective to observers.īrukner proposed a way of testing this notion by translating the Wigner's friend scenario into a framework first established by the physicist John Bell in 1964. The scenario has long remained an interesting thought experiment. Wigner can now in principle verify this superposition using a so-called " interference experiment"-a type of quantum measurement that allows you to unravel the superposition of an entire system, confirming that two objects are entangled. That's because they are "entangled"-spookily connected so that if you manipulate one you also manipulate the other. Wigner doesn't have access to this fact from the outside, and according to quantum mechanics, must describe the friend and the coin to be in a superposition of all possible outcomes of the experiment. We can say that Wigner's friend establishes a fact: the result of the coin toss is definitely head or tail. Every time the friend tosses the coin, they observe a definite outcome. Imagine that a friend of Wigner tosses a quantum coin-which is in a superposition of both heads and tails-inside a closed laboratory. He questioned what would happen when applying quantum mechanics to an observer that is themselves being observed. In 1961, physicist Eugene Wigner proposed a provocative thought experiment. The fact that nature behaves this way has been proven multiple times in the lab-for example, in the famous double slit experiment (see video).
The second you observe a quantum system, it picks a specific location or state-breaking the superposition. But oddly, this is only the case when they aren't observed. According to the theory, particles can be in several places or states at once-this is called a superposition. Observers are powerful players in the quantum world. In other words, according to our best theory of the building blocks of nature itself, facts can actually be subjective. But in a paper recently published in Science Advances, we show that in the micro-world of atoms and particles that is governed by the strange rules of quantum mechanics, two different observers are entitled to their own facts.