Alternative facts have been spread as a virus in society. It seems that they have even infected science – at least the quantum sphere. This may seem counter-intuitive. The scientific method is ultimately based on reliable ideas for observation, measurement and repeatability. The fact established through measurement must be objective so that all observers can agree with it.
But in an article recently published in Science Advances we show that in a micro-world of atoms and particles 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 about the building blocks of nature itself, facts can actually be subjective.
Observers are powerful players in the quantum world. According to theory, particles can be in several places or states at once ̵
Related: The 18 Greatest Solved Mysteries in Physics
In 1961, the physicist Eugene Wigner proposed an experimental thought experiment. He questioned what would happen when applying quantum mechanics to an observer who he observes. Imagine being a friend of Wigner throwing a quantum coin – which is in superposition on both head and tail – in a closed lab. Each time a friend throws a coin, they observe a certain result. We can say that Wigner's friend makes a point: the result of a coin toss is definitely a head or a tail.
Wigner has no access to this fact from the outside, and according to quantum mechanics, he must describe his friend and coin to be in superposition of all possible results of the experiment. This is because they are "entangled" – ghostly connected so if you manipulate one, you manipulate the other. Now Wigner can generally verify this superposition using the so-called interference experiment a type of quantum measurement that allows you to extend the superposition of an entire system, confirming that two objects are entangled.  When Wigner and the friend later compare the notes, the friend will insist that they have seen certain results for each coin toss. However, Wigner will not agree every time he sees a friend and a coin in superposition.
This is a puzzle game. The reality perceived by a friend cannot be reconciled with the reality outside. Initially, Wigner did not consider this a paradox; he argued that it would be absurd to describe a conscious observer as a quantum object. However, later deviated from this view and according to the official textbooks on quantum mechanics, the description is completely valid .
The script has long remained an interesting thought experiment. But does it reflect reality? There has been little progress on the subject until very recently, when Chaslav Bruckner at the University of Vienna showed that, under certain assumptions, the idea of Wigner could be used to formally prove that measurements in quantum mechanics are subjective to observers.
Bruckner offers a way to test this notion by translating the script of Wigner's friend into a frame first created by physicist John Bell in 1964. Bruckner considers two pairs of Wigners and friends in two separate boxes. sharing measurements – inside and outside the box. The results can be summarized and ultimately used to evaluate the so-called Bell inequality . If this inequality is breached, observers may have alternative facts.
For the first time, we conducted this test experimentally at the Heriot-Wat University in Edinburgh on a small scale quantum computer composed of three pairs of interlaced photons. The first photonic pair represents the coins and the other two are used to throw coins – measuring the polarization of the photons – into their respective box. Outside the two boxes are two photons on each side that can also be measured
Despite the use of state-of-the-art quantum technology, it took weeks to collect enough data from just six photons to generate enough statistics. But in the end, we were able to show that quantum mechanics may indeed be incompatible with the admission of objective facts – we have broken the inequality.
However, the theory is based on several assumptions. These include that the measurement results are not affected by signals traveling above light speed and that observers are free to choose what measurements to make.
Another important question is whether single photons can be considered as observers. According to Bruckner's theory, observers need not be conscious but simply have to be able to establish facts in the form of a measurement result. An inanimate detector would therefore be a valid observer. And the quantum mechanics of the textbook gives us no reason to believe that a detector that can be made as small as several atoms should not be described as a quantum object just like a photon. It is also possible that standard quantum mechanics may not apply to large length scales, but testing is a separate problem.
Therefore, this experiment shows that, at least for local models of quantum mechanics, we need to rethink our notion of objectivity. The facts we experience in our macroscopic world seem to remain safe, but the fundamental question arises as to how existing interpretations of quantum mechanics can accommodate subjective facts.
Some physicists view these new developments as affirmative interpretations that allow more than one observation result to emerge, for example the existence of parallel universes in which each result occurs. Others see this as convincing evidence of the theories of the observer theory such as quantum Bayesianism in which the agent's actions and experience are major problems of the theory. But others take it as a strong indication that quantum mechanics might fall apart over certain rocks of complexity.
Obviously these are deeply philosophical questions about the fundamental nature of reality. Whatever the answer, there is an interesting future ahead.
This article was originally published in Conversation. Publication contributed to the article on Living Research Expert Voices: Op-Ed & Insights .