The sun spends most of this month in the constellation Gemini. To the ancient Greeks, Gemini's mirror-image pattern of stars recalled the mythological brothers Castor and Pollux. These days, those stars bring to mind a more scientific symbolism: the paired phenomena that are ubiquitous throughout the universe.
Everywhere they look, astronomers see doubles. In the centers of quasars and active galaxies, massive black holes blow out vast jets in matching, perpendicular streams. Twin jets emerge from newborn stars as well, probably because of the same fundamental rules. When clouds of gas fall inward, they tend to organize into a rotating disk; when that disk fills up, some of the gas squirts out along both poles. The orbiting Chandra X-ray Observatory has spent the past year documenting this double trouble in quasars, while last year the Hubble Space Telescope captured the best views yet of outflows from young stars.
Twins seem built into the most basic laws of physics. Accelerator experiments routinely generate subatomic particles accompanied by their antimatter counterparts— precise clones with opposite electrical charges. The energetic interactions around black holes that create jets also give rise to particle-antiparticle pairs. Matter is so inextricably bound up with antimatter that cosmologists find it baffling our universe consists almost entirely of one without the other. What happened to that other half of creation?
The closer one looks at these subatomic twins, the stranger they seem. Quantum theory indicates that one particle in a newly created pair appears to respond instantly to what the other is doing, even if the two are far apart. Albert Einstein dismissed that connection as "spooky action at a distance" and declared the theory mistaken. But in 1997, physicist Nicholas Gisin of the University of Geneva conducted a startling refutation of Einstein's doubts.
Gisin and his team created two "entangled" photons— particles of light born together, so that their initial properties are linked— and sent them flying in opposite directions, 7 miles apart, along optical fibers. One photon encountered a junction where it could randomly take one of two paths. Whichever option a photon took, Gisin found, its distant twin always made the same choice. The second photon's reaction occurred less than four ten-billionths of a second later, essentially instantly. By implication, an entangled photon would immediately echo the action of its twin even if the two were on opposite ends of the universe.
This faster-than-light influence seemed so outrageous that some researchers sought an escape clause. Perhaps, they argued, the experiments revealed only those twins that behaved in sync, missing many others that did not. A paper published earlier this year by a team led by quantum expert David Wineland at the National Institute of Standards and Technology closed that loophole. The scientists created pairs of entangled beryllium ions, which are much easier to follow than photons, and studied them with a detector. The experiment proved that, yes, every single particle really does simultaneously echo the actions of its twin. "There is some spooky action at a distance," Wineland says. As for why there is, he has no answer.
Most physicists argue that entanglement doesn't violate the cosmic light-speed limit because it carries no information: The behavior of the "sending" particle is random, so it cannot convey a message. Nevertheless, tangled subatomic twins may have practical uses. Wineland and others are trying to harness them to link together bits of data in an ultrapowerful atomic-scale computer that would, as he puts it, "carry all the weird baggage that comes with quantum mechanics."
Call it a Gemini dream.