Wormholes and entanglement—two of science fiction's favorite concepts from modern physics—may in reality be two sides of the same coin, physicists say. The findings may offer a way to solve puzzling mysteries about black holes and perhaps help reconcile theories of gravity and quantum physics, which has been the dream of physicists since the mid–20th century.
Wormholes are hypothetical shortcuts through spacetime, also known as Einstein–Rosen bridges, after Albert Einstein and Nathan Rosen, who predicted them in 1935. Entanglement is another way of connecting two distant objects: When two particles are entangled, they retain a connection even when separated over great distances, so that actions performed on one affect the other. Entanglement has been demonstrated in quantum physics experiments with particles, but wormholes, which arise from general relativity, are purely theoretical. The two phenomena were long thought to be unrelated.
Then physicists Leonard Susskind of Stanford University and Juan Maldacena of the Institute for Advanced Study in Princeton, N.J. began to think about entangling two black holes to one another. Entanglement is generally thought to occur between tiny particles, not giant cosmic objects. But if two black holes were entangled, and then separated from one another, the result, the physicists reasoned, would be a wormhole connecting them. Susskind and Maldacena postulated such a link between wormholes and entanglement earlier this year.
Two independent teams have since found support for the idea. They show theoretically that entangled quarks are indeed connected by a wormhole within a stripped-down version of reality. In this model it's as if the wormhole exists in our real 4-D world (three dimensions of space and one of time), but the quarks are entangled only in a flattened 3-D simulacrum of reality. (This kind of modeling is akin to using a two-dimensional hologram to represent a 3-D object.) "What Maldacena and Susskind want to say is that literally whenever you have entanglement, you have wormholes," says Andreas Karch of the University of Washington, co-author of one of the two new papers.
Maldacena and Susskind go so far as to equate the two in the relation ER = EPR, where ER refers to an Einstein–Rosen bridge (or wormhole) and EPR, short for Einstein–Podolsky–Rosen, is another term for entanglement. "Ours is a weaker statement, but it's easier to demonstrate to be true," Karch says. In his version wormholes aren't equivalent to entanglement; rather, 4-D wormholes are mathematically analogous to 3-D entanglement. Karch and his collaborator, Kristan Jensen of the University of Victoria in British Columbia, published their findings November 20 in Physical Review Letters. Building on their work, Julian Sonner of the Massachusetts Institute of Technology strengthened the argument in a paper published in the same journal issue.
"These papers are interesting from the point of view of studying the connection between entanglement and geometry in the context of the gauge/gravity duality," or the holographic principle the researchers used to simplify their calculations, says Maldacena, who is also the original architect of that holographic principle. He agrees, however, with Susskind that the entangled quarks in the new papers are too different a system to be compared with ER = EPR, in part because the studies ignore the effects of gravity. "ER = EPR is something that only makes sense in a theory with gravity," Susskind says. "At best they are proposing some kind of analogy."
If wormholes are shortcuts that nothing can travel through, what are they good for? "Even though you and I can't travel through the wormhole to get from point A to point B, quantum mechanics knows about it," Jensen says. "It allows different regions in spacetime to talk to each other quantum mechanically."
The ER=EPR conjecture might be a step toward formulating a quantum theory of gravity that can fully describe black holes and other sticking points of quantum mechanics and general relativity. "People have been proposing various ways in which entanglement could actually be used to have the geometry of spacetime predicted by Einstein emerge," Sonner says, "so that the curvature has to do with the entanglement that really underlies it all."
The correlation between entanglement and wormholes might also be a way of solving a puzzling paradox discovered last year about black holes. Scientists realized that for several rules of physics to hold true, the boundaries of black holes would have to be enclosed by firewalls—walls of energy that would immediately destroy anything that hit them. But firewalls themselves conflict with tenets of physics, such as the idea that a person falling into a black hole should not immediately notice anything amiss when they cross its boundary, or event horizon. Susskind and Maldacena came up with the ER = EPR relation in response to the paradox. "If it's correct, it may be part of the reason why firewalls don't exist," Jensen says.