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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.    

That’s in stark contrast to the prediction of quantum mechanics. Here the measurement of one entangled photon instantly determines the result of a future measurement on the other, regardless of the distance between them. 

If special relativity ensures that the order of events is ambiguous, what gives?  Once again, theorists are at a loss.

Of course, the way to answer these questions is to test them and see. 

Today, David Rideout at the University of California, San Diego and a few friends outline various ways to crack these nuts and they say these kinds of experiments ought to be possible in the near future.  

That’s largely because the required experimental gear is standard in many optics laboratories, so qualifying it for use in space should be straightforward. 

Two groups have already proposed to do these kinds of experiments in space. One group wants to put a package capable of producing entangled photons on the International Space Station, for beaming back to Earth. Another wants to keep the quantum equipment on the ground and bounce photons off a simple microsatellite in low Earth orbit, an option they say will be cheaper, easier and better. 

Neither group has a launch date in mind or even the guaranteed funds to built their gear. But that could change, given the increasing level of interest in this area and the possibility that Chinese work could leapfrog western efforts.  

Beyond this, there are longer term options to beam photons from further afield–from the Moon or interplanetary spacecraft, for example.

The bigger picture is that to find new physics, scientists need to push experiments to new limits. Physicists have not been able to test general relativity on the quantum scale (ie the Planck scale of 10^-34m). However, efforts are now afoot to explore this scale using atom interferometers.

And until now, physicists have not been able to test quantum mechanics on the scale of general relativity, because the distances over which the curvature of space time become significant are so large. We saw just a few weeks ago that the record for teleporting quantum objects is only 150km, which is too little for general relativity to work its magic.   

Rideout and co say that’s bound to change over the coming years. The paradoxes of quantum mechanics were first debated by Einstein, Bohr and others in the 1920s and 30s. But for various reasons, not least of which was blind prejudice against this type of work, it wasn’t until the 1970s and 80s that physicists began to test them experimentally. 

The paradoxes raised by the meeting of quantum mechanics and relativity are just as old and arguably more profound. And yet, physicists have yet to begin a concerted effort to explore them experimentally.

High time to grip this nettle.

Ref: arxiv.org/abs/1206.4949: Fundamental Quantum Optics Experiments Conceivable With Satellites Reaching Relativistic Distances And Velocities

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