In an article published in Nature Communication, researchers described for the first time a quantum entanglement between events. The article describes how the principle of causality in combination with quantum theory allows changing the order of events or allows them to exist simultaneously
Quantum theory is famous for mysterious phenomena that contradict the natural logic of all of us. Particles can penetrate walls, exist in several places at the same time and even join forces and exist as one entity even if its parts are light years apart. Scientists call the latter phenomenon "entanglement" because the particles expressing such properties intertwine their fate when they are measured. Entanglement was the honey and the sting of quantum theory, it is considered a miraculous phenomenon but scientists saw it as the detonator for the entire theory. Among them was Einstein who presented the phenomenon of interweaving to the world for the first time and alongside it its problematic in relation to the principle of locality.
Although many think that Einstein was a complete opponent of quantum theory he was quoted as saying that quantum theory is indeed impressive. Alongside these words he added that his inner feeling suggests that this is not the end of the story. The difficulty in the interweaving phenomenon stems from the deeper idea in relation to reality - is reality determined at the time of measurement and influenced by the viewer or is it determined ahead of time but hidden from us until we open our eyes? Einstein's inner feeling is shared by almost all of us, maybe it works on children when they hide their eyes, but we all know that our existence does not depend on the gaze of a baby. To solve the problem, the physicist Bell proposed a set of basic rules to determine whether a measurement is indeed interpreted as quantum, meaning that entanglement is the only explanation for the observation or whether there are indeed hidden properties in nature that are discovered at the time of measurement and are predetermined. Thanks to Bell and countless measurements in laboratories around the world, today we know that physics obeys quantum theory. The strange phenomenon that binds the fate of particles in nature and affects them instantly exists in our universe.
Even though particles can affect each other instantaneously without distance affecting it, the laws of causality still apply. The past belongs to the past, the present is now and the future has not yet arrived. In a study conducted by researchers from Australia and the United States, the question was raised, can the phenomenon of interweaving affect natural events? Quantum theory like general relativity maintains the principle of causality. An event in the past affects an event in the future and not the other way around. In addition, the theory of relativity gives each point in space a unique clock that depends on the total energy in its environment or in the presence of massive bodies. This principle is still defined to be a classical (non-quantum) phenomenon, since causality is affected by predetermined variables. The obvious question was how does quantum theory affect the principle of causality? When a massive body can exist in several places at the same time, it should affect the clocks around it, but how? What will be the clock of a body both far and close to a massive star? What will the space of time look like in such a situation? Without knowing what the space looks like, researchers find it difficult to describe the quantum states that exist on it and in particular their evolution in time.
Previous studies have even suggested that situations that scatter massive bodies in several places at the same time are not physical and will collapse very quickly to preserve the principle of causality and the classical development of the theory of gravity. To date, even various quantum gravity theories are unable to describe the phenomenon of macroscopic interweaving with massive bodies. The latest research published in the magazine Nature presents an initial example of the phenomenon and is able to assess in a similar way to Bell whether the events are quantum entwined or whether it was the hand of chance based on hidden variables.
I won't go into the depth of the ideas, but we can conduct the following thought experiment: imagine two spaceships traveling in front of each other so that at a certain moment they are asked to shoot at each other and at another moment to stop the shooting. If one of the spacecraft fires earlier than expected it may destroy the other spacecraft. Now suppose we place a massive body between them, for example a planet so that one of the spacecraft is close enough to the star that the clocks are not clocked. In such a situation the other spaceship will try to stop the fire too late or launch a fire too early and both will be destroyed. If we add quantum mechanics to the story and build the spaceship so that it is close and far from the planet at the same time, it will interweave its state with the other spaceship so that the physical state describes a system in which the spaceships are either completely destroyed or saved together until the moment of measurement.
This is certainly the first time that researchers demonstrate the effect of gravity on quantum entanglement at the macroscopic level, describe it mathematically and provide a clear test similar to the one Bell proposed. Such an idea could form a deeper basis for advanced quantum gravity theories. The mathematical description published in the article is entirely based on quantum mechanics at low energies. To integrate the theory of gravity and quantum mechanics to the depth of details it is important to also include high energy scales. The researchers hope this will inspire what quantum gravity theory would look like at low energies.
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