How to Find a Wormhole

Wormholes, passageways that connect one area of spacetime to another, are still only theoretical — but that doesn't mean we shouldn't be looking for them.

Einstein's theory of general relativity predicted the warping of spacetime due to gravitational forces. When this bending becomes so strong that a region of space folds like a piece of paper, the hypothetical tunnels known as wormholes can be formed. The extreme gravitational conditions necessary for this to happen may occur around supermassive black holes. But do wormholes really exist?

When taken to the extreme, gravity can create some intriguing visual effects known as gravitational lensing, that can be observed through telescopes. From the outside, wormholes can appear similar to black holes, but while moving in front of a distant star, a wormhole big enough to fit a spaceship would create a very specific type of this weird laughing mirror effect. Therefore, when it comes to hunting wormholes, looking for the distant signatures of smaller gravity lenses known as micro lenses could be the way to go.

Just as optical telescopes collect visible light, so do radio telescopes collect the radio waves emitted by stars, galaxies, black holes and perhaps wormholes. Investigating ways to distinguish a wormhole from a black hole, researchers have focused on supermassive black holes with masses millions to billions of times that of the sun, which are thought to dwell at the hearts of most, if not all, galaxies. For example, at the center of our Milky Way galaxy lies Sagittarius A, a monster black hole that is about 4.5 million solar masses in size. What if such an “active galactic nucleus” (AGN) were a wormhole mouth rather than a supermassive black hole?

Any matter falling into the mouth of a supermassive wormhole would likely travel at extraordinarily high speeds due to its powerful gravitational fields. The collision in the wormhole's throat of matter flowing through both mouths would result in spheres of plasma expanding from the wormhole at nearly the speed of light. Those spheres can reach temperatures of about 10 trillion degrees Celsius and would produce gamma rays with extremely high energies. In contrast, accretion disks of AGNs are too cold to emit gamma radiation.

Moreover, although jets from AGNs can emit gamma rays, they would mostly travel in the same direction as the jets — any traveling out in a sphere might suggest they came from a wormhole. All in all, there are several ways in which the gamma radiation emitted by wormholes has a distinctive spectrum much different from those of supermassive black holes. An observation of such radiation would serve as convincing evidence of their existence.

Researchers have also considered the possibility that wormholes can act as natural particle accelerators. The high-energy cosmic rays produced by such a scheme could be detected by sensitive radio telescopes as well. It is thought that this phenomenon may occur when a wormhole generates and maintains a magnetic field, which might also explain the observation of such fields in cosmic voids.

In addition to telescopes, gravitational wave detectors could be used to detect wormholes in deep space. The United States–based Advanced LIGO, or Laser Interferometer Gravitational-Wave Observatory, as well as the Advanced Virgo detector near Pisa, Italy, have already detected ripples in spacetime caused by the merging of two black holes or dense stellar corpses called neutron stars.

But scientists have also considered a black hole with a mass five times the sun’s, orbiting a wormhole billions of light years from Earth. As the black hole swings around the wormhole, the researchers calculated, it would begin to spiral inward as if it were orbiting another black hole. Initially, the resulting gravitational waves would look like a standard signature for two black holes, a pattern of waves that increase in frequency over time called a chirp.

But when it reached the wormhole’s center, known as the “throat,” the black hole would pass through. It would then emerge in a totally different spacetime and the observed gravitational waves would abruptly die off. But eventually the black hole could fall back into the wormhole and as it returns, it would initially spiral outward, perhaps producing an “anti-chirp.” This pattern of gravitational waves is opposite to a chirp’s, but would be followed by a chirp when plunging back in again.

The black hole would continue bouncing between the two regions of spacetime, causing repeated bursts of gravitational waves punctuated by silence, until it settled down in the wormhole’s throat. The signals that would be produced in this way could not be explained by the merging of two black holes and would be clear evidence for the presence of a wormhole.

Last but not least, researchers have come up with a method to search for wormholes around the Milky Way's central, supermassive black hole. If a wormhole were hiding there, the stars on one side of the passage would be influenced by the gravity of stars on the other side. If physicists can detect small changes in the expected orbits of those stars, then that may indicate that there is a wormhole nearby. While current methods aren't sensitive enough to make those measurements, new techniques and longer observations might render it possible within the near future.

To be sure, none of the methods described here has so far given any indication of the existence of wormholes in our universe or beyond. On the other hand, we've only recently found hard evidence for the existence of black holes, although the theoretical possibility has been known for a long time. Will wormholes eventually follow the same route from theoretical possibility to actual natural phenomena?

(November 2020)

Parkes radiotelescope