Only last year the world of physics celebrated the 50th anniversary of Bell's theorem, a mathematical proof that certain predictions of quantum mechanics are incompatible with local causality. Local causality is a very natural scientific assumption and it holds in all modern scientific theories, except quantum mechanics.
Local causality is underpinned by two assumptions. The first is Albert Einstein's principle of relativistic causality, that no causal influences travels faster than the speed of light. This is related to the "local" bit of local causality.
The second is a common-sense principle named after the philosopher Hans Reichenbach which says roughly that if you could know all the causes of a potential event, you would know everything that is relevant for predicting whether it will occur or not.
Although quantum mechanics is an immensely successful theory – it has been applied to describe the behaviour of systems from subatomic particles to neutron stars – it is still only a theory.
Thus, because local causality is such a natural hypothesis about the world, there have been decades of experiments looking for, and finding, the very particular predictions of quantum mechanics that John Bell discovered in 1964.
But none of these experiments definitively ruled out a locally causal explanation of the observations. They all had loopholes because they were not done quite in the way the theorem demanded.
No loopholes
Now, the long wait for a loophole-free Bell test is over. In a paper published today in Nature, a consortium of European physicists has confirmed the predictions required for Bell's theorem, with an experimental set-up without the imperfections that have marred all previous experiments.
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