Scientists at the University of Queensland in Australia have demonstrated that, in terms of quantum mechanics, two different events can precede each other simultaneously. The breakdown of the causal relationship was demonstrated using the polarization of photons in an interferometer. This is reported by Science.
In the course of the study, physicists sent photons through an interferometer - a device with which a beam of electromagnetic radiation is divided into several beams that travel through different optical paths (A and B). Eventually, the two beams reunite and overlap, resulting in interference. The setup was assembled in such a way that, with vertical polarization, the photon will choose the left path, then return back and hit the right side of the interferometer. With horizontal polarization, the particle first goes along the right path, and then along the left.
However, with diagonal polarization, the quantum wave describing the position of the photon “splits”, moving along both paths simultaneously. Vertically and horizontally polarized components first go along their own path, come back, and go to the adjacent path. Thus, both components pass along each path at once, that is, the photon seems to go along both paths simultaneously. At the end of each path, the photon splits again, with one component coming back and the other leaving the setup.
Experiment Layout / Image: Arxiv.org
In this case, it is very difficult to determine which event precedes another: either the return of the polarized components to the beginning of the paths creates the appearance of the passage of a photon along A and B simultaneously (the photon first passes along one path, and then along the other), or the splitting of a "bifurcated" photon into at the end of each of the paths causes a one-time return of the components to the beginning of each path (and then the photon actually travels along both paths simultaneously).
To solve this problem, scientists conducted a series of experiments, each time inserting additional lenses into the installation, which change the spatial distribution of the light beam. This allows you to change the polarization of the photon at the moment when the quantum waves are again superimposed on each other. If each photon in the beam first passed one path, and then another, then the final polarization of the photon must correspond to a certain value. However, the researchers found that it was impossible to experimentally determine which event actually causes the other. In other words, both processes are cause and effect of each other.
