Ruh-Roh! The World Just Got Spookier
By Jameson O’Reilly, Applied Physics and Electrical Engineering 2019
During the weeks leading up to Halloween, everything seems to get spookier. This year was no exception, as a team of scientists led by Ronald Hanson of Delft University proved “spooky action at a distance” with much greater certainty than any previous experiment had been able to achieve. True to its name, this quantum phenomenon can seem very mysterious.
The term was coined by Albert Einstein to reflect his uneasiness with one of the main tenets of quantum mechanics: entanglement. When two particles are generated or interact in certain ways, it can become impossible to describe the state of just one or the other. We can only know the state of the system as a whole. However, once entangled, the two particles can be moved farther away from each other and then one can be measured. Because the overall state of the system is also known, the state of the other particle is instantly determined. As an example, suppose that for some property a particle can only have one of two values, either 1 or -1. If the two entangled particles have a system value of 0, one particle can be measured as having a value of 1 and the other must then have a value of -1.
Some physicists, Einstein included, viewed this as problematic because theoretically the two particles could be infinitely far apart, so the information from measuring one would have to travel faster than the speed of light to reach the other one before it could be measured to confirm that it has the value we expect. According to Einstein’s own Theory of Special Relativity, traveling that fast is not physically possible. There are not yet any compelling scientific reasons to doubt the truth of this aspect of Special Relativity, but this restriction is only a problem for entanglement in a world that is local realist. Local realism is the belief that physical contact is required for something to make a change to something else and that quantum observables can have definite values. Before entanglement was predicted, this assumption was never questioned and Einstein and many others were not willing to give it up. It is this assumption that requires information to physically travel from one particle to the other because something would have to carry the information from the first particle to the second to collapse the second from its state of uncertainty to a definite value. Other quantum theorists believed that nothing had to physically travel from one to the other because of the entanglement and so “spooky action at a distance” was not really all that spooky.
In order to preserve locality, Einstein and his colleagues proposed hidden variables that were unknown and therefore could not be measured that would allow the particles to communicate with each other before being separated far enough apart to demand faster-than-light travel. Given the “hidden” nature of these variables, it is very hard to test this hypothesis. It took until 1964 for English physicist John Bell to derive a limit on how much these hidden variables could contribute to the correlation between the measurements of each particle. If we could predict the measurement of the second particle based on the first more accurately than what became known as Bell’s Inequality, it would prove that something else was going on, something spooky. We would be forced to reject the assumptions of local realism and accept entanglement as described by quantum mechanics.
Immediately, experiments were devised to test this inequality by entangling particles and seeing how correlated the measurements of the entangled values were. Many were able to violate Bell’s Inequality, implying that local realism is false, but die-hard local realists pointed to imperfections in the experiments that created loopholes that meant that it was still possible for a local realist perspective to explain their results. One of the loopholes is known as the locality loophole. This refers to the fact that measurement does not happen instantaneously. Because the particles are not infinitely far apart, there is time for the first particle to send information to the second before it is done being measured. In this way, the two particles can coordinate their values with experimentalists being none the wiser. The other major loophole, the detection loophole, comes about because individual particles can be very hard to detect, so it is possible for information to slip through with undetected particles.
Many experiments were able to close one loophole or the other, but never both at the same time. As a result, some physicists held on to the possibility of local realism until Dr. Hanson and his team were able to close both at once by measuring electrons, which are relatively easy to measure, that were over a kilometer apart. That way, there were not enough particles that bypassed detection to explain unseen communication and the measurements could be done quickly enough to rule out any hidden variables traveling between the two particles. This forces the rejection of local realism, an important issue in the interpretation of quantum mechanics. By closing both loopholes at once and measuring a violation of the Bell Inequality, they were able to prove once and for all just how spooky our world really is.
Nature (2015). DOI: 10.1038/nature15759