Is entanglement really necessary for describing the physical world, or is it possible to have some post-quantum theory without entanglement?
In a new study, physicists have mathematically proved that any theory that has a classical limit-meaning that it can describe our observations of the classical world by recovering classical theory under certain conditions-must contain entanglement.
So despite the fact that entanglement goes against classical intuition, entanglement must be an inevitable feature of not only quantum theory but also any non-classical theory, even those that are yet to be developed.
The physicists, Jonathan G. Richens at Imperial College London and University College London, John H. Selby at Imperial College London and the University of Oxford, and Sabri W. Al-Safi at Nottingham Trent University, have published a paper establishing entanglement as a necessary feature of any non-classical theory in a recent issue of Physical Review Letters.
“Traditionally we study how the classical world emerges from the quantum, but we set out to reverse this reasoning to see how the classical world shapes the quantum. In doing so we show that one of its strangest features, entanglement, is totally unsurprising. This hints that much of the apparent strangeness of quantum theory is an inevitable consequence of going beyond classical theory, or perhaps even a consequence of our inability to leave classical theory behind.”
Although the full proof is very detailed, the main idea behind it is simply that any theory that describes reality must behave like classical theory in some limit.
This result implies that the assumption that such a theory does not have entanglement is false, which means that any theory of this kind must have entanglement.