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10 | NewScientist | 27 July 2013 THIS WEEK SPACE-TIME, the very fabric of our universe, may be a tangled place. Entanglement, a feature of quantum mechanics that links objects over great distances, could be responsible for its structure. What’s more, entanglement may fill the universe with a thicket of cosmic tunnels called wormholes, All these ideas fall out of a new theory that is making inroads into unifying gravity, which operates on large scales, with quantum mechanics, the science of the very small. A successful theory of quantum gravity is one of the biggest goals of modern physics. The theory also raises the bizarre possibility of using wormholes to enable a futuristic version of a lover’s leap. Two people separated by hundreds of light years could in principle meet inside an “entanglement wormhole”. But their love had better be strong: there is no escape from a quantum wormhole, forcing the pair to stay together until they die (see “Thoroughly modern Romeo and Juliet”, below). It all sounds a little wild, but the ideas are gaining traction and sparked lively discussion at a recent meeting of the top minds in theoretical physics and philosophy at the University of California, Santa Cruz. “This is all crazy,” Leonard Susskind of Stanford University in California, one of the brains behind the new kind of wormhole, told the conference on 5 July. “It’s also believed to be correct.” The connection between entanglement and wormholes was first proposed a month ago. Its backstory, however, began in 2009, when Mark Van Raamsdonk at the University of British Columbia in Vancouver, Canada, proposed a way to connect the bendy space-time of general relativity, Einstein’s theory of gravity, with quantum mechanics, which usually assumes an unrealistic, rigid space-time. Van Raamsdonk’s innovation involved the fields that appear in a quantum view of the universe, such as the electromagnetic field and the more exotic Higgs field. Neighbouring fields are generally more entangled with each other than with regions farther away in space or time. This led Van Raamsdonk to wonder if entanglement has a role in space- time’s geometry. Sure enough, he was able to show that altering the entanglement of quantum fields can alter the shape of space-time, making it flexible. “If you change the pattern of entanglement, you also change the geometry of space-time,” says Juan Maldacena of the Institute for Advanced Study in Princeton, New Jersey. Last month, Maldacena and Susskind enriched Van Raamsdonk’s idea by adding wormholes to the picture. They suggested that wormholes, previously only described by Adam Becker, Santa Cruz The great quantum space-time tangle “Hey, I just met you, and this is crazy, but here’s some entangled photons, so jump into a wormhole with me maybe?” It’s Alice’s last desperate plea to her sweetheart, Bob. The lovebirds, rebellious teenagers with a firm grasp of advanced physics, have just been sent light-years apart by their warring families. These star-crossed lovers don’t need to cross the stars to meet again though, as long as they are willing to pay the ultimate price. Welcome to the first romance enabled by quantum physics. The pair can be reunited thanks to a new theory of wormholes that says these tunnels in space-time emerge via quantum entanglement (see main story). Wormholes are a sci-fi staple, but until now there was no sure-fire way to make them using known materials. The theory offers a recipe. The resulting wormholes are not ideal — you could not use them to go time-travelling, say — but there’s at least one instance where they might be useful. Here’s what Alice and Bob need to do to meet again: THOROUGHLY MODERN ROMEO AND JULIET ♥ Alice may be locked in her room, but she has still got access to her physics kit. Burning out several stars to keep the electricity running, she prepares an enormous batch of entangled photon pairs. She sends one half of each pair off to Bob at the speed of light, keeping the other for herself. ♥ Photons are massless, but energy and mass are equivalent — so pack them together densely enough and Alice and Bob can create two entangled black holes. ♥ At a pre-arranged moment, Alice and Bob each jump into their black holes. ♥ Alice and Bob can meet up in the common interior of the entangled black holes — the connecting wormhole that arises from the entanglement. ♥ As they speed toward the singularity inside the wormhole, which will end their lives, Alice and Bob spend their last moments together. Depending on the mass of the black holes, they could have anywhere from a fraction of a second together (for a black hole the mass of a planet or star) to days or even weeks (for a black hole with the mass of a small galaxy or more). “A wormhole arising from quantum entanglement is crazy – but it’s also believed to be correct”
