Is a phenomenon called quantum entanglement really necessary to describe the physical world, or is some post-quantum theory possible without entanglement? In a new study published by phys.org, physicists have mathematically proved that any theory with a classical limit - when it can describe our observations of the classical world, referring to classical theory under certain conditions - must involve entanglement. Therefore, despite the fact that entanglement is at odds with the classical understanding, it should be an inevitable and most important property not only of quantum theory, but also of any non-classical theory, even not yet developed.
Physicists such as Jonathan Richens of Imperial College London and University College London, John Selby of Imperial College London and the University of Oxford, and Sabri Al-Safi of Nottingham Trent University have published an article stating that entanglement is an inevitable feature of any non-classical theory, in Physical Review Letters.
“Quantum theory has a lot of strange features compared to classical theory,” says Richens. “Traditionally, we study how the classical world emerges from the quantum one, but here we decided to reverse this reasoning to see how the classical world shapes the quantum one. So we showed that one of the strangest features of the latter, quantum entanglement, is an inevitable consequence of going beyond the classical theory, or perhaps even a consequence of our inability to abandon the classical theory, leave it behind."
While the complete proof is much more detailed, the basic idea is that any theory that describes reality should behave like a classical theory to some extent. This requirement seems pretty obvious, but as physicists show, it imposes serious restrictions on the structure of any non-classical theory.
Quantum theory satisfies this requirement of a classical limit in the decoherence process. When a quantum system interacts with the external environment, it loses its quantum coherence, connectedness, and everything that makes it quantum. Thus, the system becomes classical and behaves as expected in classical theory.
Physicists have shown that any non-classical theory that reconstructs a classical theory must contain entangled states. To prove this, they went from the opposite: let's say such a theory has no entanglement. And then they showed that without entanglement, any theory that reconstructs a classical theory must be classical itself - and this contradicts the original hypothesis that such a theory must be non-classical. This result means that the assumption that there is no entanglement in such a theory will be false, which means that any theory of this type must have it.
This result may only be the beginning of many other related discoveries, as it opens up the possibility that other physical features of quantum theory can be reproduced simply by requiring the theory to have a classical limit. Physicists suggest that features such as informational causation, bit symmetry, and macroscopic locality can be proven through this single requirement. These results also provide a clearer picture of what any future non-classical, post-quantum theory should look like.
“My future goals are to see if Bell's nonlocality can also be learned from the existence of the classical limit,” says Richens. "It would be interesting if all the theories replacing the classical theory would violate local realism."
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Local realism is a combination of the principle of locality with the “realistic” assumption that all objects have “objectively existing” values of their parameters and characteristics for any possible measurements that could be made on these objects before these measurements are made. Einstein, being, apparently, a supporter of local realism, liked to say in this regard that the moon does not disappear from the sky, even if no one is observing it. The data of modern quantum mechanics, based on the conducted experiments, cast doubt on the adequacy of the model of local realism to the "device" of reality.
Ilya Khel