Einstein’s unwanted phantom

Scientists from AEI are planning an unusual experiment

January 07, 2008

Physicists from Hannover and Potsdam are planning an unusual experiment: they are seeking to engage two mirrors in a quantum mechanical entanglement with one another. This would result in a connection that is so strange that Albert Einstein called it “spooky action at a distance”. Should the experiment prove to be successful, this would be the first time that such a coupling was established between large objects. Scientists of the Max Planck Institute for Gravitational Physics and the Leibniz Universität Hannover (Albert Einstein Institute) are publishing their proposal for the experiment at the beginning of January in the journal “Physical Review Letters”. Their article bears the title “Entanglement of macroscopic test masses and the standard quantum limit in laser interferometry”. The online article will appear on 7 January and the printed edition of the article on 11 January.  It is planned to carry out the experiment later this year, after the vacuum system necessary therefor has been set up. The first results are expected in 2009 or 2010.

Up to now quantum mechanical entanglements have only been encountered in the world of the very small. They arise between elementary particles, atoms, and molecules. If two particles are entangled with one another, then they behave in an extremely strange fashion; an imperceptible spell seems to unite them.


Assume that two dice were entangled quantum mechanically. This would have a very strange result: if one threw both dice it would be certain that each die would show the same number. Yet the number itself would be impossible to predict – except that it would appear on both dice, something that would be a one hundred percent certainty. The dice are not loaded. Each of them arrives at a perfectly random number - exactly as it should be. This can be demonstrated by throwing the die frequently and recording the results: clean random statistics emerge.

One might think that one die can somehow be watching the other and thereby always come up with the same number as the other. Yet that too is not true. If they are separated from each other, for example, if they are placed in two different buildings, each die supplies the same number. Does some sort of force field exist between them? The answer is still no: while you can shield the two from each other in any conceivable way, the numbers on the dice will nonetheless continue to agree each time they are thrown.

This “spooky action at a distance” is completely puzzling because it happens infinitely fast. As soon as one die “decides” in favour of a certain number, the other also displays it. This occurs instantly, without time delay - no matter how far apart the two are.

Albert Einstein remained sceptical. In the meantime, however, quantum mechanical entanglements have been proven.

In 1935, together with his colleagues Boris Podolsky and Nathan Rosen, Albert Einstein recognised the fact that two objects could be connected in such a mysterious way. Einstein was not happy with this result. It did not fit in with his vision of the world and made him feel uneasy. That's why he described the matter as “spooky”. Today researchers know that quantum mechanical entanglements exist. They have been detected in the case of light particles, atoms and molecules, as well as elsewhere. Of course, such particles are not dice that show numbers. But they do possess other random qualities. If, for example, one first measures location and then speed, this gives rise to unpredictable values - that is the Heisenberg uncertainty principle. If one undertakes such measurements regarding two entangled particles, then the results are still random, but they are clearly related. The values ​​of the two particles can be the same or clearly complementary to one another - for example, by their sum always being the same.

The purpose of the experiment is to show that quantum mechanical entanglements also occur in the case of large objects

Researchers from the Albert Einstein Institute (AEI) in Hannover and the Golm district of Potsdam are now seeking to show that quantum mechanical entanglements do not only occur in the case of small particles but also in that of large objects. Their goal is to entangle two highly reflective supermirrors, which are supposed to weigh up to several kilograms. Matters are to proceed as follows: both mirrors hang in the vacuum on pendulums, where they are protected from sound and vibrations. Then scientists direct a laser beam at each of them. The rays are then thrown back at the mirrors, after which they meet and superimpose. Two light patterns emerge, from which the researchers can extremely precisely read just how far away the mirrors are and how fast they move relative to each other.

However, the laser beams constantly give the mirrors tiny impacts that are unpredictable. They thereby begin to swing in random directions. If the mirrors were intertwined, their pendulum motions would not only be unpredictable, but would also be completely in unison (common mode) and therefore perfectly synchronous. This strange behaviour would be directly analogous to that of the entangled dice if they were thrown repeatedly. In the planned experiment, this behaviour would be seen in the two light patterns. The two light patterns would constantly change in an unpredictable manner, but surprisingly in unison. The entanglement of the two mirrors would thus be clearly visible through the observation of the respective light patterns.

“We want to carry out this experiment at the Albert Einstein Institute in Hannover;” said Roman Schnabel, Junior Professor at the AEI, “if we were indeed to find an entanglement between the mirrors, it would be a small sensation. This result would show that quantum mechanics does not only manifest itself in the case of small particles, but also in the world of large objects which are visible to us.” According to Schnabel, “there is a well-regarded theory that quantum effects cannot be detected in large objects, because the gravity of the objects destroys them. Our experiment could prove this theory to be false.”

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