摘要：Physicists have simulated strange objects from string theory to determine what they look like – if they exist, they could be mistaken for a black hole when imaged from very far away

A strange cosmic object described by string theory could be mistaken for an ordinary black hole from far away. If these objects do exist, they could solve a long-standing paradox about black holes.

The object is known as a topological soliton – a region where space-time itself warps and forms a hole that light can fall into, like sliding down a space-time hill. In a two-dimensional space-time, this object would resemble a donut that traps light in its hole. However, string theory posits that there are many invisible dimensions, so the topological soliton’s true shape can’t be fully perceived in the three spatial dimensions that we experience.

Though light would fall into one of these structures similar to how it falls into a black hole, topological solitons aren’t fully dark in the middle. If they are real, looking at them up close would show light swirling about their centre.

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Black holes are difficult to directly image because they trap all the light that enters them, but in 2019 the Event Horizon Telescope (EHT) produced a striking image of one black hole by detecting light and matter circling around it. Ibrahima Bah at Johns Hopkins University in Maryland and his colleagues wondered whether other space objects could produce similar images, and they focused on the topological soliton. “We are not making predictions about whether these objects are in the sky, [we are] just getting at the question of if there was something that mimics a black hole, could we even see the difference,” says Bah.

He and his colleagues modelled the trajectories of light around a topological soliton and used visualisation techniques similar to those employed to create the black hole in the movie Interstellar. This gave them images of topological solitons as they would look if they were taken with an instrument like EHT.

Emanuele Berti, also at Johns Hopkins University, says that these images revealed that light that fell into the topological soliton’s hole kept bouncing inside of its edges, so the centre wasn’t as black as that of a conventional black hole. Some faint light could even escape, which is impossible for black holes.

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Nicholas Warner at the University of Southern California says that understanding objects like topological solitons could help solve the so-called black hole information paradox, in which these cosmic behemoths seem to violate the laws of quantum physics by destroying information about objects that fall into them. If signals from space that were previously interpreted as black holes are actually coming from a topological soliton that can let light or information escape, the paradox would become irrelevant. This would give physicists insight into which type of theory of quantum gravity may be most accurate, says Warner.

In general, if observations from EHT or other instruments show any structure inside of black holes, it could indicate that the phenomena are more similar to objects like topological solitons, if not exactly the same, and have profound consequences, says Steven Giddings at the University of California, Santa Barbara. “It would open completely new avenues of thought in fundamental physics,” he says.

Reference: Physical Review D, forthcoming