While William Orem has been celebrating Halloween by quantum ghost hunting, quantum physicist Vlatko Vedral is fearlessly tackling the controversy over the speed of Einstein's spooky-action-at-a-distance. Just how speedy is spookiness?
From Vlatko Vedral:
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Entanglement, as Schrödinger famously said, was the characteristic trait of quantum mechanics. One of its key experimental manifestations is in the way two entangled particles are correlated, so that changes made to one immediately affect the properties of its partner--even when they are separated by vast distances. This is what Einstein termed "spooky action at a distance" because the correlations exceed anything that classical physics allows. But just how fast are entangled particles communicating? What is the speed of spookiness? An answer was offered up last May, but it's proving to be pretty controversial.
More than forty years ago, John Bell came up with way to quantify this excess of correlations. He made two assumptions that most us would happily have agreed with, that of "locality" and that of "reality." The locality assumption is meant to comply with the principles of special relativity, namely that no signal can travel faster than light. The second, reality assumption, says that measurement outcomes of an experiment should be determined independently of the measurement settings--that is, there is a reality out there independent of us choosing how and what to measure. Bell came up with a set of conditions (known as "Bell's inequalities") that could be tested in experiments and should be upheld if those two assumptions are correct.
But Bell's inequalities have been violated many times and therefore we face the following stark choice, the Bell dilemma as it were: Either the world is not real, or special relativity is wrong (or both!).
A number of physicists would like the world to be real. After all, we know that the universe, the earth, other forms of life etc, all existed before we humans came along. So it should not matter whether we observe the universe or not, surely it's got to have an independent existence from measurements. Nicolas Gisin at the University of Geneva is one such experimental physicist. The conclusion that you then have to draw is that within these entanglement experiments there are signals travelling faster than light (in some frame of reference), allowing entangled particles to communicate seemingly instantaneously, and adjust their properties in unison.
Gisin and his group in Geneva recently decided to test how quickly this signal would have to propagate. They generated entangled photons between two villages is Switzerland, separated by 18 kilometers. Their results have been communicated in a recent letter to Nature and the conclusion is that the speed would have to be at least ten thousand times higher than that of light! "Physicists spooked by faster-than-light information transfer" ran the headlines! Special relativity is therefore in serious trouble. Or is it?
Well, maybe not. The argument over the interpretation of the results that were announced last May is raging on. There are some technical objections to the conclusiveness of Gisin's results, most notably by Anton Zeilinger's group in Vienna. (You can read about Zeilinger attempts to create entanglement on the largest ever scale in Grace Stemp-Morlock's article "Quantum Upsizing".) Zeilinger, by the way, when faced with the Bell dilemma, would rather keep the locality assumption and admit that the world is in some sense unreal. I know this from our numerous conversations. The latter hinges on the fact that the Gisin experiments did not exhaust all the measurements necessary to demonstrate a violation of Bell's inequalities.
I don't want to get bogged down in all the unnecessary detail, but you can read Zeilinger's latest arguments here, and Gisin's response here.
Instead, there is a much more general point that I would like to make and this one is frequently overlooked. Gisin's exposition of his group's results may give us the misleading impression that quantum physics and special relativity sit uneasily with each other. That they are in some sense incompatible: You have to either side with quantum mechanics or with special relativity, but you can't have them both, you can't have your cake and eat it.
This, however, is not true at all. Quantum physics and relativity were combined into what is called quantum field theory almost immediately after the advent of quantum mechanics, in the late 1920s. And quantum field theory is a spectacularly successful theory, the most successful description of nature known to humans. Its span is simply breathtaking, ranging as it does from the minute subatomic particles, via macroscopic phenomena in solid state physics to many problems at the grand cosmological scales.
My personal view is this. All the experiments on entanglement, including Gisin's, are really telling us one thing. It is high time we grew up and got used to quantum mechanics as it is and all its basic tenets. As grown-ups we know there are no witches or ghosts (even on Halloween) and so we should stop using archaic language, such as "reality" and "causality." This can (and does) lead to confusion. Thinking of quantum entanglement as something still spooky (despite so many experimental verifications) is, to borrow Bertrand Russell's phrase, "a relic of a bygone era, surviving, like the monarchy, only because it is erroneously supposed to do no harm."
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Vlatko Vedral is a professor of quantum information science at the University of Leeds, UK, and a professor of physics at NUS, Singapore.