One of the most controversial parts of modern physics is what it tells us about philosophy. Both quantum mechanics and special relativity have much to say on the limits of what can be casually related, the degree to which reality is deterministic, how local (or non-local) the universe is, and even whether some forms of realism are scientifically valid. Many people find the conclusions of quantum mechanics unpalatable and have proposed alternatives, such as Bohm's pilot wave quantum mechanics, to match their personal preference for reality. However, most of these proposals require that there is some privileged view of the universe that keeps both special relativity and their particular ideas in agreement. Researchers have now tested the mechanism that would have to be behind this agreement, and found it to be extremely unlikely.

If you are experiencing déjà vu about now, you are not alone—we have been here before. In the late 19th and early 20th century, there was considerable debate about the speed of light. Maxwell's then-new theory of electromagnetism had predicted the speed of light. However, speed is always measured between two objects, so the natural question was what the predicted speed of light measured relative to? The question seemed to imply that there was some absolute reference from which everything could be measured. Experiments showed that this frame of reference was extremely unlikely, and it took Einstein to suggest that perhaps our ideas about relativity should be re-thought.

Now, here we are just over 100 years later, and the same question has returned, albeit in a slightly different form. Quantum mechanics has a property called entanglement that tells us that the states of two particles can be correlated. Furthermore, if we separate the correlated particles by an arbitrary distance and then do something to one particle, our actions will be instantaneously reflected in changes to the measured properties of the other particle. Although this sounds like it can be used for information transfer, it cannot, so special relativity is safe.

If, like Bohm, you happen to dislike the nondeterminism inherent in quantum mechanics, this is a problem because the two particles must communicate. He rewrote quantum mechanics so that a pilot wave kept the two particles entangled and everything remained deterministic. However, to do this and not violate special relativity, a privileged way to observe the universe, called a frame, is required. These frames should always be detectable, because the Earth is in motion. Unless the special frame is both centered on, and rotating with, the Earth, the frame could always be detected by determining how the movement of the Earth changes experimental results.

This is exactly what a group of Swiss scientists have now done. From their Geneva location, they created entangled pairs of photons. These photons were sent down optical fibers to two villages separated by 18km in an approximately east-west direction. At each end, the single photon was offered two choices—a long path and a short path to a photodetector. If both photons took the same choice, then the detectors in each village would click at the same time.

To observe the entangled nature of the photons, the lengths of the paths in one village were changed slightly over time, so the timing of photon arrivals fluctuated periodically as the path lengths oscillated back and forth, creating interference fringes. The key to the experiment was measuring how deep the fringes are. If the correlations are not due to entanglement, there will be no, or very shallow, fringes. Fringes deeper than a certain threshold can only be due to entanglement.

The researchers reasoned that if there is a special frame through which the entangled particles keep track of each other, then at certain times of the day the fringes will vanish. This will occur when the surface of the Earth is at right angles to the special frame. So, they looked for periodic changes to the fringe visibility, but found that the fringe visibility is independent of the Earth's rotation. No matter what time of the day, the fringes were always too deep to be due to anything but entanglement.

One can always explain away these results by postulating that the speed of the proposed pilot wave is faster than light. Well, the researchers considered that as well. Their analysis shows that the pilot wave must travel at least 10,000 times faster than the speed of light to explain their results, a possibility they consider extremely unlikely.

It seems that there are always people who will argue for specialness. First there was the luminiferous aether, then the anthropic principle, and now an entanglement frame. With the possible exception of the anthropic principle (and I wouldn't put money on that surviving the next 50 years), none of these ideas survive for very long.

Nature, 2008, DOI: 10.1038/nature07121