http://www.bbc.co.uk/programmes/b018nn7l I did enjoy Brian Cox's program on quantum mechanics last night, but one bit left me thinking "no, that's not right!". The gist of it was that all the electrons in the universe have to be in constant communication to ensure that no two of them are ever in the same state. If he changed the energies of electrons in a diamond, by heating it in his hand, all the other electrons in the world would have to adjust their energies too. I think this may have been an attempt to show that entanglement follows from the Pauli exclusion principle, but was it a simplification too far? The Pauli principle confused me when I first heard it at school: did it mean that no two hydrogen atoms in the universe could be in their ground states simultaneously? I have always understood, since then, that it doesn't mean that, because which proton the electron is bound to is part of its state. So "in the first energy level around this proton" is a different state from "in the first energy level around that proton". The exclusion principle states that no two electrons can be in the same *state* not, as Cox seemed to be implying, that they may not have numerically the same energies. That is not forbidden as far as I know. We would not see nice spectral lines from billions of hydrogen atoms all making the same state transition at the same time, if it was. I now know there is a deeper explanation of the exclusion principle, namely that the multi-particle wave-function of a half-integral spin particle is antisymmetric, and that means the probability of finding two of them in the same place is zero. So OK, Pauli and entanglement are connected. But I always like a simple explanation if one is available. What does the panel think? Did what Cox said amount to a good explanation for a general audience, or does it risk perpetuating a misunderstanding?