Could we pierce spacetime with the wormhole?

They can make the characters travel without worrying about the gigantic distances between the stars.

For physicists, however, these tunnels are another headache. Because so far, no observation has yet been made in favor of wormholes … or against them. There is no evidence that nature can allow such shortcuts in space-time, which does not prevent most specialists from considering it. Thus, Sagittarius A*, the supermassive black hole located at the center of the Milky Way whose snapshot was taken recently, could even be one! And if so, could an infinitely advanced civilization cross it? Or even – we’re projecting a bit – make one?

What is certain is that the concept is not new: it comes from General Relativity, the theory of gravitation published by Einstein in 1915. In 1935, the physicist, together with his colleague Nathan Rosen, realized that an astonishing prediction emanated from his equations: two black holes separated by very large distances could theoretically share the same interior. These curiosities were first dubbed the “Einstein-Rosen Bridge” (ER) before being renamed “wormhole” by physicist John Wheeler in the 1950s.

The theoretical study of these cosmic monsters was going well when, in the 1980s, experts calculated that these gaps in space-time should naturally close after… 10-43 s! Patatras: nothing, no one, or any signal of any kind, could then pass through a wormhole. Unless we find a way to keep it open… “For this, the physicists of the time invoked the existence of a substance with negative energy, repulsive under the influence of gravitation” , traces Dejan Stojkovic, a physicist at New York University, Buffalo in the USA. Thus, by lining the wormhole with this material, an infinitely advanced civilization could prevent the tunnel from collapsing in on itself.

Problem: in 2022, no one has yet found the slightest trace of such an exotic material. “It is very likely that it does not exist, claims Juan Maldacena, world specialist in wormholes at the Institute for Advanced Study in Princeton, USA.

And even if it did exist, more and more clues tell us that wormholes formed with negative energy matter would be incompatible with the general principles of relativistic quantum mechanics. “End of the story?” Nix continues the researcher. There could be another way: the quantum entanglement of two black holes.”

Particle entanglement was born in the same year as wormholes: in 1935, Einstein, Rosen, and a third sidekick, Boris Podolsky, discovered a “strange remote action” between quantum entangled particles. A curiosity about quantum physics, which was dubbed the “EPR paradox”, after the name of its authors. “In the 2010s, discussions arose about a possible similarity between quantum entanglement and space-time connections beyond the horizon of black holes,” says Juan Maldacena. A field of research then opened up, summarized by the formula “ER=EPR”, which today enjoys great popularity among theorists. Because by quantum filtering two black holes, we would actually achieve a wormhole through which information could be sent. Recently, Juan Maldacena even took the plunge and suggested that quantum entanglements and wormholes were ultimately strictly equivalent! What Dejan Stojkovic doubts: “The idea is fascinating, but if entangled particles really were bound by mini-wormholes, we would have already measured it in the lab,” he points. “In any case, with this theory, wormholes formed by two entangled black holes would be microscopic and extremely difficult for nature to produce.” supports Juan Maldacena. It is therefore impossible for a human being to cross a… Not to mention a vessel!

Also read: What if our black hole was a wormhole? Sagittarius A*, the supermassive black hole that sits at the center of the Milky Way, could be one of those space-time passages towards another universe predicted by Einstein’s general theory of relativity. At least according to the calculations of an American astrophysicist, there is nothing to prevent it…

If entangled particles really were bound by mini-wormholes, it would have already been measured in the lab— ALREADY STOJKOVIC, Physicist at New York University. UNIVERSITY AT BUFFALO/DOUGLAS LEVERE


But there is one clue left: String Theory. According to this, our reality in three dimensions of space would be only a “brane”, a subpart of a “bulk” in 10 dimensions. And if two black holes – or two neutron stars – approached a little too close to an extra dimension, a different dimension than the three of space, they could well merge and form a bridge: either between two distant regions of our universe, or between two parallel universes , depending on whether they come from the same branch or from two different ones.

“This fusion would put the brane under tension, but it is precisely the latter that would compensate for the gravity of the black holes and would play the role of negative energy substance by keeping the wormhole open” explains Dejan Stojkovic.

According to the researcher’s calculations, two black holes with a solar mass could thus form a wormhole, the entrance and exit of which would be spheres with a radius of 3 km, connected by a tunnel of 1 million km. “The only way to get a wormhole big enough to actually pass through is to postulate the existence of extra dimensions”, Juan Maldacena nods. Which still points to a pitfall: the tidal forces generated by the wormhole would be so gigantic that a human traveler would be distended, compressed, shredded, and then “spaghettified” before even reaching the tunnel entrance!

And that’s not the only difficulty: even if a traveler found a way to preserve his physical integrity and slip through the tunnel, he would come out the other end at a late stage. Despite the apparent speed of the journey, the duration of the journey would in reality be equivalent to the journey light traveled by “normal” means. “Let’s imagine that it took the traveler one second to cross the wormhole, an observer on the outside would discover that he actually came out of it tens of thousands of years later”, says Juan Maldacena. This wormhole would therefore in no way be a shortcut through space-time, at most a machine to run through space without aging… “This may be the case in the particular setting of the Randall-Sundrum models that he studied, but not necessarily in others, objects Dejan Stojkovic. And yet some people would surely find it useful to travel fast through the cosmos, even if it means leaving their surroundings forever”. he’s joking.

We’re not there yet.

“With current knowledge, traversable black holes seem to me extremely difficult to form, either artificially or naturally. concludes Juan Maldacena. Nevertheless, this field of research, although still only theoretical, has a great future ahead of it.” And before you cross a wormhole, you should already start by observing one. Several roads are under investigation (see box) : Dejan Stojkovic, for example, has developed an ingenious method to flush out black holes that are actually wormholes. Another promising lead is the observation of stellar black hole mergers by the gravitational wave detectors Ligo and Virgo since 2015. In a few years, astronomers could thus provide observational constraints on the existence of these famous tunnels through space-time. Whether they exist or not, microscopic or Dantesque, open to a corner of our universe or another branch, turning us into spaghetti or not…

Traditional wave detectors can pick up a signal from the merger of two black holes (yellow curve) . But if it is in fact two wormholes, we would see an echo appear (blue curve). Because, unlike black holes, wormholes do not have an event horizon, the limit at which nothing can escape their gravitational field.

Soon detectors to identify wormholes

If the black hole in our galaxy Sagittarius A* (SgrA*) was a wormhole, stars could be orbiting on the other side, in the other universe. Their gravitational field would then leak through the tunnel, where it would disrupt the motion of some stars in our reality that could be detected – unfortunately, current telescopes are not fine enough. Another lead: since 2015, the gravitational wave detectors Ligo and Virgo have observed black hole mergers (see graph above). But “if they are wormholes, we will detect a characteristic echo,” explains Pablo Antonio Cano, a physicist at the Institute of Theoretical Physics in Louvain, Belgium. Ligo and Virgo are not yet sensitive enough, but everyone agrees that future detectors, like Lisa (2032) or Einstein (2035), will be able to do it!”

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