What would happen if a black hole fell into a wormhole?


If wormholes exist, scientists may one day spot black holes falling into them, a new study suggests. (Image credit: Shutterstock)

Astronomers believe they may be able to detect black holes colliding with wormholes using gravitational waves, but only if wormholes exist and such a scenario has ever occurred, according to a new study.

Gravity arises from the way mass warps space and time, according to Einstein, who predicted the presence of gravitational waves in 1916. Gravitational waves are produced when two or more objects move through a gravitational field at the speed of light, stretching and squeezing space-time in the process.

Because gravitational waves are incredibly faint, they are extremely difficult to detect, and even Einstein was unsure whether they existed and if they would be discovered. In 2016, scientists released the first direct evidence of gravitational waves, which was identified using the Laser Interferometer Gravitational-Wave Observatory, after decades of research (LIGO).

Black holes vs. wormholes

More than 20 large collisions involving extremely dense and massive objects like black holes and neutron stars have been discovered by gravitational-wave observatories. More unusual objects, such as wormholes, may theoretically exist, and their collisions should emit gravitational signals that scientists can detect.

Wormholes are spacetime tubes that, in theory, can transport you anywhere in time and space, or even to another universe. The concept of wormholes is allowed by Einstein's theory of general relativity, albeit whether they exist is debatable.

In theory, all wormholes are unstable and close as soon as they open. Only an alien type of matter with so-called "negative mass" can keep them open and traversable. Exotic matter has peculiar qualities, such as flying away from a typical gravitational field rather than sinking toward it as normal matter does. No one knows if such a strange substance exists.

A wormhole is similar to a black hole in many respects. Both types of objects are extremely dense and have extremely strong gravitational forces for their size. The main difference is that after entering a black hole's event horizon — the point at which the speed required to escape the black hole's gravitational pull exceeds the speed of light — no object can theoretically reverse course, whereas any object entering a wormhole can theoretically reverse course.

Scientists explored the gravitational signals generated when a black hole orbits a wormhole for a new work that has not yet been peer-reviewed, assuming wormholes exist. The researchers also looked into what might happen if a black hole enters one mouth of a wormhole, exits out the other mouth into another point in space-time, and then falls back into the wormhole and emerges out the other side, assuming the black hole and wormhole are gravitationally bound to one another.

No escape

The interactions between a black hole five times the mass of the sun and a stable traversable wormhole 200 times the mass of the sun with a throat 60 times wider than the black hole were studied using computer models. When the black hole travelled into and out of the wormhole, the models predicted that gravitational signals unlike any seen before would arise.

The orbital speeds of two black holes rise as they spiral closer together, similar to how spinning figure skaters drag their arms closer to their bodies. As a result, the gravitational wave frequency increases. These gravitational waves would make a chirping sound, similar to when you rapidly increase the pitch on a slide whistle, because any increase in frequency equates to an increase in pitch.

If you observed a black hole spiral into a wormhole, you'd hear a chirp that sounded like two black holes colliding, but the black hole's gravitational signal would diminish quickly as it radiated most of its gravitational waves on the other side of the wormhole. (When two black holes collide, however, the result is a massive burst of gravitational waves.)

An "anti-chirp" could be heard as a black hole emerges from a wormhole. As the black hole travelled further away from the wormhole, the frequency of gravitational waves from it would diminish.

A cycle of chirps and anti-chirps would be generated as the black hole travelled in and out of each wormhole mouth. The interval between each chirp and anti-chirp would get shorter and shorter until the black hole became lodged in the wormhole's throat. The discovery of such a gravitational signal could support the existence of wormholes.

"Though wormholes are very, very hypothetical," research co-author William Gabella, a physicist at Vanderbilt University in Nashville, said, "the notion that we might be able to verify or at least give credence to their existence is quite exciting."

The black hole would finally stop falling in and out of the wormhole and settle near its throat in this scenario. The ramifications of such a conclusion are entirely dependent on the highly speculative qualities of the exotic materials discovered in the wormhole's throat. One scenario is that the wormhole's mass has been effectively enhanced by the black hole, and the wormhole no longer possesses enough exotic matter to remain stable. According to Gabella, the subsequent disruption in space-time may cause the black hole to convert its mass to energy in the form of a large number of gravitational waves.

A wormhole should remain stable as long as its mass exceeds that of any black hole it meets. If a wormhole collides with a larger black hole, the black hole may destabilise the wormhole's exotic matter, leading it to collapse and likely generate a new black hole, according to Gabella.

What would happen if a black hole just clipped the margins of a wormhole, with only a portion of the black hole entering the wormhole's mouth and the remainder remaining outside it? "I suspect some bizarre behaviour at the black hole event horizon, resulting in even more gravitational waves and energy loss," Gabella added. He said that such a collision could disturb the exotic matter in the wormhole, "leading to an unstable wormhole."

Future research might look at the interactions between the exotic matter inside a wormhole and any ordinary matter that enters it, as well as more complex scenarios like what might happen if the wormhole spins, according to Gabella. Other study avenues might include looking into how gravitational waves interact with ordinary and exotic matter in these scenarios, as well as "the variety of orbits that might occur between wormholes and you name it," he noted.


James B. Dent, William E. Gabella, Kelly Holley-Bockelmann, Thomas W. Kephart             arXiv:2007.09135

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