A black hole is an object with such a powerful gravitational field that nothing, not even light, can escape it if it strays within a boundary known as the event horizon. Einstein's theory of general relativity says black holes should form whenever matter is squeezed into a small enough space.

Though black holes are not seen directly, astronomers have identified many objects that appear to be black holes based on observations of how matter swirls around them. But physicists Thibault Damour of the Institut des Hautes Etudes Scientifiques in Bures-sur-Yvette, France, and Sergey Solodukhin of International University Bremen in Germany now say that these objects could be structures called wormholes instead.

Wormholes are warps in the fabric of space-time that connect one place to another. If you imagine the universe as a two-dimensional sheet, you can picture a wormhole as a "throat" connecting our sheet to another one. In this scenario, the other sheet could be a universe of its own, with its own stars, galaxies and planets.

Damour and Solodukhin studied what such a wormhole might look like, and were surprised to discover that it would mimic a black hole so well that it would be virtually impossible to tell the difference.

Hawking Radiation

Matter would swirl around a wormhole in the same way as for a black hole, since both objects distort the space around them in the same way. One might hope to distinguish the two by something called Hawking radiation, an emission of particles and light which should only come from black holes and would have a characteristic energy spectrum. But this radiation is so weak that it would be completely swamped by other sources, such as the background glow of microwaves left over from the big bang, making it unobservable in practice.

Another difference one might hope to exploit is that unlike black holes, wormholes have no event horizon. This means that things could go in a wormhole and come back out again. In fact, theorists say one variety of wormhole wraps back onto itself, so that it leads not to another universe, but back to its own entrance.

Daring Plunge

But this does not provide a foolproof test either. Depending on the detailed shape of the wormhole, it could take billions of years or more for things to pop back out after falling in. With the right shape, even the oldest wormholes in our universe would not have had time to spit anything back out yet.

It seems the only way to decide the issue for sure with astronomical black holes is to make a daring plunge inside. That would be a dangerous gamble, because if it is a black hole, the incredibly strong gravitational field inside would tear apart every atom in your body. Even if it turns out to be a wormhole, the forces inside could still be deadly. Assuming you could survive, and the wormhole was not symmetric, you might find yourself in another universe on the other side. Without further intervention, the wormhole would tend to suck you back in and carry you back to the opening in your universe.

Yo-yo Motion

"The spaceship would do this yo-yo motion," Damour told New Scientist. "[But] if you use your fuel, then you can escape from the attracting power of the wormhole and explore" the space on the other side, he says. But a friend in either universe might have to wait billions of years to hear back from you, since the transit time could be excruciatingly long.

Such a delay would make meaningful communication with anyone on the other side impossible. But the delay gets smaller with smaller wormholes. If a microscopic wormhole could be found or constructed, the delay across it could be as short as a few seconds, Solodukhin says, potentially making two-way communication possible.

Stephen Hsu of the University of Oregon in Eugene, US, who has studied the formation of black holes and the properties of wormholes, says he agrees that distinguishing between the two types of object with observations is practically impossible, at least with current technology.

Exotic Matter

"The most important property of a black hole – that there is a 'point of no return' for an object falling in – is not something we can test at the moment," he told New Scientist.

Still, he says the objects out there suspected to be black holes probably really are black holes rather than wormholes. There are plausible scenarios for forming black holes, he says, such as the collapse of a massive star, but it is not clear how you would form a wormhole.

"Wormholes that might be confused with a macroscopic black hole require some kind of exotic matter to stabilise them, and it is not known whether such exotic matter exists," he says. Solodukhin says that a wormhole might form in much the same way that black holes form, such as from a collapsing star. Physicists normally expect in these situations that a black hole would be produced, but Solodukhin says that quantum effects may stop the collapse just short of producing a black hole, leading to a wormhole instead.

Microscopic Black Holes

He says this mechanism might be inevitable in a more complete picture of physics that unites gravity and quantum mechanics – a longstanding goal of physics. If he is right, then wherever we used to expect black holes to form, wormholes would form instead. And there might be a way to test the conjecture. Some physicists say that future particle accelerator experiments could produce microscopic black holes.

Such tiny black holes would emit measurable amounts of Hawking radiation, proving that they are black holes rather than wormholes. But if Solodukhin is right, and microscopic wormholes are formed instead, no such radiation would be expected. "In that case, you would actually see if it is a black hole or a wormhole," he says.

An added benefit of wormholes is that they could resolve the so-called black hole information paradox. The only way anything can exit a black hole is in the form of Hawking radiation, but it is not clear how the radiation carries information about the original object that was swallowed. This scrambling effect conflicts with quantum mechanics, which forbids such erasing of information.

"Theoretically, wormholes are much better than black holes because all these problems with information loss don't exist in this case," Solodukhin says. Since wormholes have no event horizons, things are free to leave without first being converted into Hawking radiation, so there is no problem with lost information.

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