What are wormholes in astrophysics

Wormholes: Isn't physics speaking against time travel?

When Carl Sagan wrote his novel "Contact" in 1985, he had to name his main character, Dr. Quickly transport Ellie Arroway from Earth to the star Vega. He dropped it into a black hole and reappeared several light years away - but wasn't sure if that was physically possible. The astrophysicist and television star, who works at Cornell University, New York, asked his friend Kip Thorne, a black hole expert at the California Institute of Technology in Pasadena. It was immediately clear to Thorne that Arroway could not travel to Vega through a black hole, because black holes destroy everything that falls into them and nothing can escape from them again. But he had the idea that Arroway could use another type of space-time hole that is consistent with Einstein's general theory of relativity: a wormhole, a kind of tunnel that connects distant places in space-time.

It is true that the simplest theoretically imaginable wormholes collapse again immediately after their formation and thus disappear before anything can pass through them. But could an "infinitely progressive" civilization, Thorne wondered, keep wormholes open long enough to allow them to pass through? Indeed, he found, such a civilization could surround the mouths of a wormhole with "exotic matter" to counteract the collapse. This matter would have to have negative energy and thereby deflect radiation and repel the surrounding space-time. Sagan used this idea in "Contact" and ascribed the invention of exotic matter to an earlier, extinct civilization in order to avoid further details. But it was precisely these details that now captivated Thorne and his students as well as many other physicists: They studied passable wormholes and their theoretical consequences for many years. The researchers discovered that such wormholes could be used as time machines. This would lead to the well-known paradoxes of time travel - for physicists an indication that the necessary exotic matter does not exist in nature.

This article is included in Spectrum - The Week, 51/2017

Now, decades later, a new type of passable wormhole has appeared that does not require any exotic matter. And it could help physicists resolve a surprising black hole paradox. This paradox is based precisely on the problem that plagued the first version of "Contact" and that made Thorne think about passable wormholes in the first place: Everything that falls into black holes seems to disappear without a trace. However, this total destruction of information is in contradiction to the laws of quantum mechanics. Many experts have puzzled over this mystery for years. Some have even concluded from this that black holes have no "interior" at all, but that spacetime ends in a strange way on the horizon of black holes.

Passable wormholes are possible

In 2016, Ping Gao and Daniel Jafferis from Harvard University in Boston and Aron Wall from Stanford University in California showed in a technical article that wormholes that can be traversed are possible without exotic matter: the necessary repulsive negative energy at the mouths of a wormhole can come from outside created by special quantum connections between the two black holes that make up the wormhole. If the two black holes are connected in a certain way, then an object that falls into one of the black holes stumbles through the wormhole and can - through certain events in the outer universe - exit the second black hole again. Gao, Jafferis and Wall were surprised to find that this scenario is mathematically equivalent to quantum teleportation - a phenomenon that has been proven in numerous experiments and is the key to quantum cryptography.

The discovery of these new, passable wormholes came as a surprise, explains John Preskill, an expert on black holes and quantum gravity at Caltech, and has significant implications for the information paradox and the interior of black holes. "What I really like about it," he says, "is that an observer can enter a black hole and come out again - and then report what he saw in it." Because that shows that the interior of black holes actually exists and that everything that falls in must ultimately come out again.

The discovery of the new wormholes began in 2013 when Jafferis heard an engaging talk from Juan Maldacena, a professor of physics at the Institute for Advanced Studies at Princeton, during a conference on strings in South Korea. Maldacena had come to the conclusion "ER = EPR" on the basis of various ideas and arguments. This means that wormholes between distant places in space-time - the simplest variants of which are known as Einstein-Rosen bridges (ER) - are equivalent (in a rather poorly defined way) to entangled particles, which are also Einstein-Podolsky-Rosen pairs (EPR). This "ER = EPR conjecture", developed by Maldacena and Leonard Susskind of Stanford University, was an attempt to solve a modern version of the infamous information paradox of black holes (PDF). To this end, the two researchers linked the space-time geometry described by the general theory of relativity with the instantaneous quantum connections between distant particles, which Einstein had reviled as "spooky long-range effects".