Beijing time on November 22nd, “alternative facts” (generally referred to as alternative, untrue lies) spread like viruses throughout society, and now seem to have infected the scientific community, at least in the quantum field. This sounds a bit counterintuitive. After all, scientific methods are based on the concept of reliable observation, measurement and repetition. Facts determined by measurement should be objective so that all observers can reach a consensus.
However, in a recent paper published in the journal Science Advances, researchers point out that in the microcosm of microparticle spotters dominated by quantum mechanics, two different observers have the right to obtain their own facts. In other words, according to current quantum theory, the facts are actually subjective.
Observers play an important role in the quantum world. According to quantum theory, particles can be in multiple places or states at the same time, which is called “overlay states”. But strangely, this is only in the case that they have not been observed. When we look at a quantum system, the particles choose a specific position, which is to break the superposition state. In fact, this behavior in nature has been proven many times in the laboratory, such as the famous double-stitch experiment.
In 1961, physicist Eugene Wigner proposed a provocative experiment in thinking. What would happen if quantum mechanics were applied to an observer who was also observed? Imagine a friend of Wigner’s throwing a quantum coin in a closed lab – on the front and back. Every time a friend tossed a coin, they observed a definite result. We can say that Wigner’s friends have established the fact that the result of the coin toss must be positive or negative.
Wigner could not be heard outside, and according to quantum mechanics, he had to describe friends and coins as superpositions of all possible results in experiments. Because the two are “entangled”, they are strangely linked to each other. So if you manipulate one, you also manipulate the other. In principle, Wigner can verify this overlay through so-called “interference experiments”. Interference experiments are quantum measurements that allow you to unzip the entire system to confirm entanglement between two objects.
At the end of the observation, when Wigner and his friends exchange views, they insist that they see clear results every time they toss a coin. However, Wigner would not agree with this view, as he observed that “friends” and “coins” were superimposed.
This becomes a problem. The reality in the eyes of friends is at odds with what the outside world sees. Wigner initially did not consider this a paradox, arguing that it was absurd to describe a conscious observer as a quantum object. However, he later deviated from this view, which, according to the formal textbooks of quantum mechanics, was entirely correct.
The scenario siain Wigner imagines has long been an interesting thought experiment, but does it really reflect reality? In science, little progress has been made on this issue. Until recently, research by the University of Vienna’s aslaw Brukner showed that, under certain assumptions, Wigner’s thought experiments could be used to prove that measurements in quantum mechanics were subjective to observers.
Brukner came up with a test method that transformed Wigner’s friend scene into a framework first established by physicist John Bell in 1964. The final results, summarized, can be used to estimate the so-called “Bell inequality”. If this inequality is broken, the observer may get alternative facts.
For the first time, researchers conducted an experimental test at Heriot-Watt University in Edinburgh. They used a small quantum computer consisting of three pairs of entangled photons. The first pair of photons represent coins, while the other two pairs represent toss coins in their respective boxes (measuring the polarization of photons). In addition to the two boxes, there are two photons on each side, which can also be measured.
Despite the use of state-of-the-art quantum technology, it can take weeks to collect enough statistics from just six photons. In the end, the researchers succeeded in confirming that quantum mechanics may indeed be incompatible with the assumption of objective facts – inequality is broken. However, this theory is based on assumptions, including that the measurement results are not affected by signals above the speed of light, and that the observer is free to choose what to measure. Therefore, this is not a conclusive conclusion.
Another important question is whether single photons can be treated as observers. In Brukner’s theory, the observer does not need to be conscious, only the fact that the facts can be established in the form of measurements. Therefore, an inanimate detector can also be an effective observer. According to the textbook quantum theory, there is reason to believe that if a detector can be as small as an atom, it should also be described as a quantum object like a photon. It is also possible that standard quantum mechanics does not apply to large scales, but if you want to test it, it’s another problem.
Therefore, this experiment shows that we need to rethink the concept of objectivity, at least in the case of local models of quantum mechanics. In the macro world, the facts we experience still seem safe, but the main problem is how existing quantum mechanics interpretations can accommodate subjective facts.
Some physicists see these new developments as a theoretical basis for supporting multiple outcomes in a single observation, such as the existence of parallel universes, each of which can occur. Others argue that these findings are strong evidence that they rely essentially on the observer’s theory, such as the quantum Bayesian theory. In quantum Bayesian theory, the behavior and experience of subjects are the core concerns of the theory. Others argue that this is a powerful hint that it may mean that quantum mechanics will disintegrate on some complex scales.
Obviously, these are philosophical questions about the nature of reality. Whatever the answer, an interesting future awaits us. (Any day)