A wormhole , in astrophysics and cosmology, is a passageway that connects two points in the space-time fabric. Just as the falling apple inspired Isaac Newton’s theory of gravitation in 1687, the worms that pierce apples inspired new theories, also in the framework of gravitation.
Just as the worm manages to reach another point on the surface of the block through a tunnel, the holes in spacetime are theoretical shortcuts that allow it to travel to distant places in the universe in less time.
Space-time wormhole: artistic vision. Source: Pixabay
It’s an idea that has captured and continues to capture the imagination of many. Meanwhile, cosmologists are looking for ways to prove its existence. But at the moment they are still subject to speculation.
To get a little closer to understanding wormholes, the possibility of time traveling through them, and the differences that exist between wormholes and black holes, you must place yourself in the concept of spacetime.
What is spacetime?
The concept of spacetime is closely linked to the concept of a wormhole. Therefore, it is necessary to establish first what it is and what its main characteristic is.
Spacetime is the place where each and every event in the universe takes place. And the universe, in turn, is the totality of space-time, capable of housing all forms of matter-energy and much more…
When the groom meets the bride, it is an event, but this event has spatial coordinates: the meeting place. And a temporary coordinate: year, month, day and time of the meeting.
The birth of a star or the explosion of a supernova are also events that develop in spacetime.
Now, in a region of the universe free of mass and interactions, spacetime is flat. This means that two beams of light that start out parallel remain so as long as they remain in that region. By the way, for a beam of light, time is eternal.
Obviously, spacetime is not always flat. The universe contains objects that have mass that modify spacetime, creating a curvature of spacetime on a universal scale.
It was Albert Einstein himself who realized, in a moment of inspiration, that he called “the happiest idea of my life” , that an accelerated observer is locally indistinguishable from one that is close to a massive object. It’s the famous equivalence principle.
And an accelerated observer bends spacetime, that is, Euclidean geometry is no longer valid. Therefore, in the environment of a massive object, such as a star, a planet, a galaxy, a black hole, or the universe itself, space-time curves.
This curvature is perceived by human beings as a force called gravity, everyday, but mysterious at the same time.
Gravity is as enigmatic as the force that pushes us forward when the bus we’re traveling on brakes sharply. It’s as if suddenly something invisible, dark and massive, for a few moments, was placed in front of us and attracted us, pushing us suddenly.
Planets move elliptically around the Sun because their mass produces a depression in the space-time surface that causes the planets to curve their paths. A ray of light also curves its path after the space-time depression produced by the Sun.
Tunnels through space – time
If spacetime is a curved surface, in principle nothing prevents one area from connecting to another through a tunnel. Traveling through a tunnel like this would imply not only moving from one place to another, but also offering the possibility of going to another time.
This idea has inspired many science fiction books, series and movies, including the famous 1960s American series “The Time Tunnel” and, most recently, “Deep Space 9” from the Star Trek franchise and the interstellar movie of 2014.
The idea came from Einstein himself, who was looking for solutions to the field equations of General Relativity, and Nathan Rosen found a theoretical solution that allowed two different regions of space-time to be connected through a tunnel that worked as a shortcut.
This solution is known as the Einstein-Rosen bridge and appears in an article published in 1935.
However, the term “wormhole” was first used in 1957, thanks to theoretical physicists John Wheeler and Charles Misner in a publication that year. Previously, “one-dimensional tubes” had been spoken of to refer to the same idea.
Later, in 1980, Carl Sagan was writing the science fiction novel “Contact”, a book of which a movie was later made. The protagonist Elly discovers an intelligent extraterrestrial life 25,000 light-years away. Carl Sagan wanted Elly to travel there, but in a way that was scientifically credible.
Walking 25,000 light years away is not an easy task for a human being unless a shortcut is sought. A black hole cannot be a solution, as when approaching the singularity, differential gravity would tear the ship and her crew apart.
Looking for other possibilities, Carl Sagan consulted one of the leading black hole experts at the time: Kip Thorne, who started thinking about the matter and realized that the Einstein-Rosen bridges or the Wheeler wormholes were the solution.
However, Thorne also noticed that the mathematical solution was unstable, meaning the tunnel opens but soon strangles and disappears.
The instability of wormholes
Is it possible to use wormholes to travel great distances in space and time?
Since they were conceived, wormholes have served in countless installments of science fiction to take their protagonists to remote places and experience non-linear paradoxes of time.
Kip Thorne found two possible solutions to the wormhole instability problem:
- Through the so-called quantum foam . On a Planck scale (10 to 35 m), there are quantum fluctuations capable of connecting two regions of spacetime through microtunnels. A very advanced hypothetical civilization could find a way to extend the passes and hold them long enough for a human to pass.
- Mass negative matter. According to calculations published in 1990 by Thorne himself, large amounts of this foreign material would be needed to keep the ends of the wormhole open.
What is remarkable about this last solution is that, unlike black holes, there is no singularity or quantum phenomenon, and the passage of humans through this type of tunnel would be feasible.
In this way, wormholes not only allowed remote regions to be connected in space, but also separated in time. Therefore, they are time travel machines.
Stephen Hawking, the great referent of cosmology in the late 20th century, did not believe that wormholes or time machines were viable because of the many paradoxes and contradictions that arose from them.
This has not dampened the spirits of other researchers, who have suggested the possibility that two black holes in different areas of spacetime are internally connected by a wormhole.
Although this was not practical for space travel, since, in addition to the tribulations that would bring the singularity of the black hole, there would be no possibility of exiting on the other side, as it is another black hole.
Differences between black holes and wormholes
When talking about a wormhole, black holes are also thought of immediately.
A black hole forms naturally after the evolution and death of a star that has a certain critical mass.
It appears after the star runs out of nuclear fuel and starts to contract irreversibly due to its own gravitational pull. It continues relentlessly until it causes such a collapse that nothing short of the ray of the event horizon can escape, not even light.
In comparison, a wormhole is an exceptional occurrence, a consequence of a hypothetical anomaly in the curvature of spacetime. In theory, it is possible to get past them.
However, if one were to try to pass through a black hole, the intense gravity and extreme radiation in the environment near the singularity would make it a thin strand of subatomic particles.
There is indirect and only very recently direct evidence for the existence of black holes. Among this evidence is the emission and detection of gravitational waves by the attraction and rotation of two colossal black holes, detected by the LIGO gravitational wave observatory.
There is evidence that at the center of large galaxies such as the Milky Way, there is a super massive black hole.
The rapid rotation of stars near the center, as well as the huge amount of high-frequency radiation emanating from there, are indirect evidence that there is a huge black hole that explains the presence of these phenomena.
It wasn’t until April 10, 2019 that the world’s first photograph of a supermassive black hole (7,000 million times the mass of the Sun) was shown to the world, located in a distant galaxy: Messier 87 in the constellation Virgo, 55 million years ago. Earth light.
This photograph of a black hole was made possible by the worldwide network of telescopes, called the “Event Horizon Telescope”, with the participation of more than 200 scientists from all over the world.
Of the wormholes, there is no evidence so far. Scientists were able to detect and track a black hole, but wormholes could not.
Therefore, they are hypothetical objects, although theoretically viable, just as black holes were before.
Variety / types of wormholes
Although they have not yet been detected, or perhaps precisely because of that, different possibilities have been devised for wormholes. All are theoretically viable, as they satisfy Einstein’s equations for general relativity. Here are a few:
- Wormholes connecting two spatiotemporal regions of the same universe.
- Wormholes capable of connecting one universe to another universe.
- Einstein-Rosen bridges, in which matter could pass from one opening to another. Although this passage of matter causes instability, the tunnel collapses in on itself.
- Kip Thorne’s wormhole, with a spherical shell of negative mass. It is stable and transversal in both directions.
- The so-called Schwarzschild wormhole, consisting of two connected static black holes. They are not transversal, as matter and light are trapped between the two ends.
- Charged and/or rotating or Kerr wormholes, consisting of two internally connected dynamic black holes traversable in a single direction.
- Quantum spatiotemporal foam, whose existence is theorized at the subatomic level. The foam is made up of highly unstable subatomic tunnels that connect different areas. To stabilize and expand them, it would be necessary to create a plasma of quarks and gluons, which would require an almost infinite amount of energy to generate.
- More recently, thanks to string theory, it has been theorized about wormholes maintained by cosmic strings.
- Intertwined and then separated black holes, from which emerges a hole in spacetime, or Einstein-Rosen bridge that is held together by gravity. This is a theoretical solution proposed in September 2013 by physicists Juan Maldacena and Leonard Susskind.
All are perfectly possible since they are not contradictory to Einstein’s equations of general relativity.
Is it possible to see wormholes someday?
For a long time, black holes were theoretical solutions to Einstein’s equations. Einstein himself questioned the possibility that they could be detected by humanity.
Albert Einstein (1879-1955), author of the theory of relativity. Source: Pixabay
For a long time, black holes remained a theoretical prediction until they were found and located. Scientists harbor the same hope for wormholes.
It is very possible that they are there too, but it has not yet been learned how to locate them. Although, according to a very recent publication, wormholes left traces and shadows observable even with telescopes.
Photons are believed to move through the wormhole generating a ring of light. The closest photons fall in and leave behind a shadow that differentiates them from black holes.
According to Rajibul Shaikh, a physicist at the Tata Institute for Fundamental Research in Mumbai, India, a type of rotating wormhole would cast a larger, more warped shadow than a black hole.
In his work, Shaikh studied the theoretical shadows cast by a certain type of rotating wormhole, focusing on the crucial role of the hole’s throat in forming a photon shadow that allows it to be identified and differentiated from a black hole.
Shaikh also analyzed the dependence of the shadow on the wormhole’s rotation and also compared it to the shadow cast by a Kerr rotating black hole, finding significant differences. It is completely theoretical work.
Also, at the moment, wormholes remain mathematical abstractions, but it is possible that some will be seen soon. Whatever is on the other side is at the moment still conjectured.
