The great mystery of quantum mechanics is that particles can be connected without a connector. They can coordinate their behavior in ways that are too complicated to be preprogrammed into them, even though no process is acting across the distance between them. “We can’t say that one thing led to another,” Nicolas Gisin of the University of Geneva tells me. “There’s no story in spacetime.”
Our interview focused on the meaning of correlations and Gisin’s argument that they cannot be explained within spacetime. He has demonstrated this experimentally by gauging how fast a putative spatial process would have to act in order to ensure that particles act in lockstep. His team measured particles in laboratories that were about 10 kilometers apart at the same instant, to a precision of 5 picoseconds—implying that any coordinating process would have to zap from one lab to the other 10 million times faster than light. That speed estimate was relative to the ground; later experiments monitored the correlations over a 24-hour period, during which the baseline between the laboratories rotated through all possible frames of reference. In all frames, the speed exceeded 10,000 times that of light.
When you talk about speeds faster than light, you are scratching your nails on a blackboard as far as Einstein’s special theory of relativity is concerned. According to that theory, each laboratory in Gisin’s experiment had a different notion of what constitutes the present moment and thought it performed the measurement before the other lab did. So, it makes no sense to say that an influence passed from one lab to the other, because each lab had an equal claim to be the source of that influence. To resolve the contradiction, relativity theory would have to be wrong, so that some absolute frame of reference would define the order of events.
Giving up relativity theory would be a heavy price to pay and wouldn’t even solve the problem. When you analyze groups of particles, any spatial process would have to cross the distance among them infinitely fast. And that doesn’t make any sense. It is tantamount to saying that the very category of distance, and therefore of space, dissolves. Gisin has written: “If [the velocity of quantum information transfer] is infinite, then quantum information is simultaneously everywhere, in all reference frames. In this case space-time is not really out there, but seems to be part of the quantum state of the Universe. One would have to explain why there is apparent space-time? And apparent locality?”