Quantum Links in Time and Space May Form the Universe’s Foundation
Quantum physics offers a way. In 2012, Jay Olson and Timothy Ralph, both physicists at the University of Queensland in Australia, laid out a procedure to encrypt data so that it can be decrypted only at a specific moment in the future. Their scheme exploits quantum entanglement, a phenomenon in which particles or points in a field, such as the electromagnetic field, shed their separate identities and assume a shared existence, their properties becoming correlated with one another’s. Normally physicists think of these correlations as spanning space, linking far-flung locations in a phenomenon that Albert Einstein famously described as “spooky action at a distance.” But a growing body of research is investigating how these correlations can span time as well. What happens now can be correlated with what happens later, in ways that elude a simple mechanistic explanation. In effect, you can have spooky action at a delay.
These correlations seriously mess with our intuitions about time and space. Not only can two events be correlated, linking the earlier one to the later one, but two events can become correlated such that it becomes impossible to say which is earlier and which is later. Each of these events is the cause of the other, as if each were the first to occur. (Even a single observer can encounter this causal ambiguity, so it’s distinct from the temporal reversals that can happen when two observers move at different velocities, as described in Einstein’s special theory of relativity.)
The time-capsule idea is only one demonstration of the potential power of these temporal correlations. They might also boost the speed of quantum computers and strengthen quantum cryptography.
But perhaps most important, researchers hope that the work will open up a new way to unify quantum theory with Einstein’s general theory of relativity, which describes the structure of space-time. The world we experience in daily life, in which events occur in an order determined by their locations in space and time, is just a subset of the possibilities that quantum physics allows. “If you have space-time, you have a well-defined causal order,” said Časlav Brukner, a physicist at the University of Vienna who studies quantum information. But “if you don’t have a well-defined causal order,” he said—as is the case in experiments he has proposed—then “you don’t have space-time.” Some physicists take this as evidence for a profoundly nonintuitive worldview, in which quantum correlations are more fundamental than space-time, and space-time itself is somehow built up from correlations among events, in what might be called quantum relationalism. The argument updates Gottfried Leibniz and Ernst Mach’s idea that space-time might not be a God-given backdrop to the world, but instead might derive from the material contents of the universe.