From the Physics arXiv blog:
The only way to study the conflict between relativity and quantum mechanics is to test them over enormous distances in space. And physicists are already making plans
One of the great puzzles of modern science is that the laws that govern the universe on the largest scale are entirely different from the ones that govern on the smallest scale.
That's odd because all our intuition about the universe is that it ought to be internally consistent rather than at odds with itself. This is why physicists are inextricably wedded to the idea that relativity and quantum mechanics must be manifestations of a bigger and better idea that encompasses them both.
The differences between general relativity and quantum mechanics are so great that every attempt to reconcile them has so far failed. However, these attempts have been entirely theoretical and that gives them limited utility.
For example, physicists routinely measure the quantum phenomenon of entanglement by sending entangled pairs of photons from one location to another. In these experiments, the sender and receiver must both measure the polarisation of the photons, whether vertical or horizontal, for example. But that can only happen if both parties know which direction is up.
That's easy to specify when they are close together. But it becomes much harder if they are separated by distances over which the curvature of spacetime comes into play. The problem here is that the answer is ambiguous and depends on the path that each photon takes through spacetime.
The experimenters can work this out by tracing the path of each photon back to their common source, if this is known. But then, how does each photon 'know' the path that the other has taken? Theorists can only guess.
Another problem arises when these kinds of experiments are done with the sender and receiver travelling at relativistic speeds. This introduces the well known problem of determining the order of events, which Einstein famously showed depends on the observers' points of view....MORE
