Astronomers think they might be able to detect black holes falling into wormholes using ripples in spacetime known as gravitational waves, but only if wormholes actually exist and such a scenario ever happened, a new study finds.
According to Einstein, who first predicted the existence of gravitational waves in 1916, gravity results from the way in which mass warps space and time. When two or more objects move within a gravitational field, they produce gravitational waves that travel at the speed of light, stretching and squeezing space-time along the way.
Gravitational waves are extraordinarily difficult to detect because they are extremely weak, and even Einstein was uncertain whether they really existed and if they would get discovered. After decades of work, scientists reported the first direct evidence of gravitational waves in 2016, detected using the Laser Interferometer Gravitational-Wave Observatory (LIGO).
Black holes vs. wormholes
Gravitational-wave observatories have detected more than 20 giant collisions between extraordinarily dense and massive objects such as black holes and neutron stars. However, more exotic objects may theoretically exist, such as wormholes, the collisions of which should also produce gravitational signals that scientists could detect.
Wormholes are tunnels in spacetime that, in theory, can allow travel anywhere in space and time, or even into another universe. Einstein's theory of general relativity allows for the possibility of wormholes, although whether they really exist is another matter.
In principle, all wormholes are unstable, closing the instant they open. The only way to keep them open and traversable is with an exotic form of matter with so-called "negative mass." Such exotic matter has bizarre properties, including flying away from a standard gravitational field instead of falling toward it like normal matter. No one knows if such exotic matter actually exists.
In many ways, a wormhole resembles a black hole. Both types of objects are extraordinarily dense and have powerful gravitational pulls for objects their size. The main difference is that no object can theoretically get back out after entering a black hole's event horizon — the threshold where the speed needed to escape the black hole's gravitational pull exceeds the speed of light — whereas any object entering a wormhole could theoretically reverse course.
Assuming wormholes might exist, scientists investigated the gravitational signals generated when a black hole orbits a wormhole for a new paper, which has not yet been peer-reviewed. The researchers also explored what might happen when the black hole enters one mouth of the wormhole, exits out the wormhole's other mouth into another point in space-time, and then — assuming the black hole and wormhole are gravitationally bound to one another — falls back into the wormhole and emerges out the other side.
In computer models, the researchers analyzed 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. The models suggested that gravitational signals unlike any seen up to now would occur when the black hole journeyed into and out of the wormhole.
When two black holes spiral closer to one another, their orbital speeds increase, much like spinning figure skaters who draw their arms closer to their bodies. In turn, the frequency of the gravitational waves rises. The sound these gravitational waves would produce is a chirp, much like when one increases the pitch rapidly on a slide whistle, since any increase in frequency corresponds to an increase in pitch.
If one watched a black hole spiral into a wormhole, one would see a chirp much like two black holes meeting, but the gravitational signal from the black hole would quickly fade as it radiated most of its gravitational waves on the other side of the wormhole. (In contrast, when two black holes collide, the result is a giant burst of gravitational waves.)
If one watched a black hole emerge from a wormhole, one would see an "anti-chirp." Specifically, the frequency of gravitational waves from the black hole would decrease as it moved farther away from the wormhole.
As the black hole keeps journeying in and out each mouth of the wormhole, it would generate a cycle of chirps and anti-chirps. The length of time between each chirp and anti-chirp would shrink over time until the black hole got stuck in the throat of the wormhole. Detecting this kind of gravitational signal might support the existence of wormholes.
"Though wormholes are very, very speculative, the fact that we might have the ability to prove or at least give credibility to their existence is pretty cool," study co-author William Gabella, a physicist at Vanderbilt University in Nashville, told Space.com.
In this scenario, eventually the black hole would stop falling in and out of the wormhole and settle near its throat. The consequences of such a finale depend on the completely speculative properties of the exotic matter found in the wormhole's throat. One possibility is that the black hole has effectively increased the mass of the wormhole and the wormhole may not possess enough exotic matter to keep stable. Maybe the resulting disruption in space-time causes the black hole to convert its mass to energy in the form of an extraordinary amount of gravitational waves, Gabella said.
As long as a wormhole has a greater mass than any black hole it encounters, it should remain stable. If a wormhole encounters a larger black hole, the black hole may disrupt the wormhole's exotic matter enough to destabilize the wormhole, causing it to collapse and likely form a new black hole, Gabella said.
It remains uncertain what might happen if a black hole only clipped the edges of a wormhole, with part of the black hole entering a wormhole's mouth with the rest staying outside it. "I suspect that there would be some crazy behavior at the black hole event horizon giving rise to even more gravitational waves and more energy loss," Gabella said. Such a collision may also disrupt the wormhole's exotic matter, "leading to an unstable wormhole," he added.
Future research can explore the interactions between a wormhole's exotic matter and any normal matter entering the wormhole, as well as more complex scenarios, such as what might happen if the wormhole is spinning, Gabella said. Other research directions could investigate how gravitational waves interact with both the normal and exotic matter in these scenarios, as well as "the variety of orbits that might occur between wormholes and you name it," he added.
The scientists detailed their findings online July 17 in a study they plan to submit to the journal Physical Review Letters. The research was detailed on the preprint site arXiv.org.
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