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Alternative facts are __spread like a virus__ through society. Now, it seems they have even infected science – at least in the quantum field. This may seem counter-intuitive. The scientific method is, after all, based on reliable notions of observation, measurement and repetition. The fact, determined by measurement, should be objective so that all observers can agree.

But in the paper recently __published in Advances in Science__, we show that, in the micro-world of atoms and particles governed by the strange rules of quantum mechanics, two different observers have a right to their own facts. In other words, according to our best theory of the building blocks of nature itself, facts can actually be subjective.

Spectators are powerful players in the quantum world. In theory, particles can be in several places or states at once – this is called superposition. But strangely, this is only the case when they are not noticed. The second you invoke a quantum system, it indicates a particular location or condition – breaking down the superposition. The fact that nature behaves this way has been proven many times in the lab – for example, in the famous __double split experiment__.

**Related: ****18 largest unsolved mysteries in physics**

In 1961, a physicist __Eugene Wigner__ proposed a provocative thought experiment. He asked what would happen when applying the quantum mechanics of a self-taught observer. Imagine a friend of Wiener threw a quantum coin – which is superimposed on his head and tails – in a closed laboratory. Every time a friend throws a coin, they notice a certain outcome. We can say that Wiener's friend establishes a fact: the result of throwing coins is definitely the head or tail.

Wigner does not have access to this fact from the outside, and according to quantum mechanics he must describe the friend and the coin to be in superposition of all possible outcomes of the experiment. This is because they are "entangled" – __fearfully connected__ so that if you manipulate one, you also manipulate the other. Wigner can now in principle confirm this superposition by using the so-called.__mixing experiment__"- a kind of quantum metering that allows you to detect the superposition of the whole system, confirming that two objects are captured.

When later Wigner and a friend compare notes, the friend insists they have seen some results for each throw. Wiener, however, will never agree to see a friend and coin in superposition.

This is a puzzle. The reality perceived by a friend cannot be reconciled with reality from the outside. Wigner did not initially consider this a paradox; he argues that it would be absurd to describe a conscious spectator as a quantum object. However, he later __left in this regard__and according to formal textbooks on quantum mechanics, __the description is perfectly valid__.

## Experiment

The script has long been an interesting experiment to think about. But does it reflect reality? Scientifically, there has been little progress on this until recently, when __Chaslav Bruckner__ at the University of Vienna showed that, under certain assumptions, Wiener's idea was __can be used for formal proof__ that quantum mechanics measurements are subjective to observers.

Bruckner suggests a way to test this notion by turning the Wiener Friends script into a framework __first established__ by physicist John Bell in 1964. Bruckner counted two pairs of Vigners and friends, in two separate boxes, conducting measurements of commonality – inside and outside their respective boxes. The results can be summarized to the end can be used to estimate the so-called. __'Inequality in war'__. If this inequality is disturbed, observers could have alternative facts.

Now, for the first time, we performed this experiment experimentally at Heriot-Watt University in Edinburgh on a small quantum computer made up of three pairs of entangled photons. The first photon pair represents the coins, and the other two are used to carry the coin – measuring the polarization of the photons – in their corresponding box. Outside of the two boxes, there are two photons on each side that can also be measured.

Despite using the most advanced quantum technology, it took several weeks to gather enough data from just six photons to get enough statistics. But in the end, we were able to show that quantum mechanics may indeed be incompatible with the assumption of objective facts – we have broken the inequality.

The theory, however, is based on several assumptions. These include that the measurement results are not affected by signals traveling above the speed of light and that observers are free to choose what measurements to make. That may or may not be the case.

Another important question is whether single photons can be considered as exciters. In Bruckner's theoretical proposition, observers do not have to be aware, they must only be able to establish facts in the form of a measurement outcome. Therefore a non-live detector would be a valid exciter. And, quantum mechanics teaches us no reason to believe that a detector, which can be made as small as a few atoms, should not be described as a quantum object just like a photon. It is also possible that standard quantum mechanics may not be applied on a large scale, but testing is a particular 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 secure, but the main question arises as to how existing interpretations of quantum mechanics can fit into subjective facts.

Some physicists see these new developments as reinforcing interpretations that allow for more than one outcome of an excitation, for example __existence of parallel universes__ in which every outcome occurs. Others find it to be compelling evidence of substantive observer-dependent theories, such as __Quantum Bayesianism__, in which the agent's actions and experiences are central to the theory. But others still see this as a strong indication that quantum mechanics may fall apart over certain degrees of complexity.

It is clear that these are all deeply philosophical questions about the fundamental nature of reality. Whatever the answer, an interesting future awaits.

*This article was originally published on **The conversation.** The posting was contributed by the article Live Science *__ Expert votes: Op-Ed and Insight__.

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