Alternative facts are spreading like a virus across society. Now, it seems they have even infected science — at least the quantum realm. This may seem counter intuitive. The scientific method is after all founded on the reliable notions of observation, measurement and repeatability. A fact, as established by a measurement, should be objective, such that all observers can agree with it.
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. In other words, according to our best theory of the building blocks of nature itself, facts can actually be subjective.
Observers are powerful players in the quantum world. According to the theory, particles can be in several places or states at once — this is called a superposition. But oddly, this is only the case when they aren't observed. The second you observe a quantum system, it picks a specific location or state — breaking the superposition. The fact that nature behaves this way has been proven multiple times in the lab — for example, in the famous double slit experiment. In 1961, physicist Eugene Wigner proposed a provocative thought experiment. He questioned what would happen when applying quantum mechanics to an observer that is themselves being observed. Imagine that a friend of Wigner tosses a quantum coin — which is in a superposition of both heads and tails — inside a closed laboratory. Every time the friend tosses the coin, they observe a definite outcome. We can say that Wigner's friend establishes a fact: the result of the coin toss is definitely head or tail.
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. That's because they are "entangled" — spookily connected so that if you manipulate one you also manipulate the other. 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.
When Wigner and the friend compare notes later on, the friend will insist they saw definite outcomes for each coin toss. Wigner, however, will disagree whenever he observed friend and coin in a superposition.
This presents a conundrum. The reality perceived by the friend cannot be reconciled with the reality on the outside. Wigner originally didn't consider this much of a paradox, he argued it would be absurd to describe a conscious observer as a quantum object. However, he later departed from this view, and according to formal textbooks on quantum mechanics, the description is perfectly valid.
