A wormhole, in astrophysics and cosmology, it is a passageway connecting two points in the fabric of space-time. Just as the falling apple inspired Isaac Newton's theory of gravitation in 1687, worms that pierce apples have inspired new theories, also in the framework of gravitation..
Just as the worm manages to reach another point on the surface of the apple through a tunnel, space-time wormholes constitute theoretical shortcuts that allow it to travel to distant places in the universe in less time..
It is an idea that has captured and continues to capture the imagination of many. Meanwhile, cosmologists are busy looking for ways to prove its existence. But at the moment they are still the subject of speculation.
To get a little closer to understanding wormholes, the possibility of time travel through them and the differences that exist between wormholes and black holes, we must look at the concept of space-time.
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The concept of space-time is closely linked to that of the wormhole. That is why it is necessary to first establish what it is and what its main characteristic is..
Spacetime is where each and every event in the universe occurs. And the universe in turn is the totality of space-time, capable of housing all forms of matter-energy and more ...
When the groom meets the bride it is an event, but this event has spatial coordinates: the meeting place. And a time coordinate: year, month, day and time of the meeting.
The birth of a star or the explosion of a supernova are also events that take place in space-time..
Now, in a region of the universe free of mass and interactions, spacetime is flat. This means that two light rays that start parallel continue like this, as long as they stay in that region. By the way, for a ray of light time is eternal.
Of course, space-time is not always flat. The universe contains objects that have mass that modify space-time, causing a space-time curvature 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 is the famous equivalence principle.
And an accelerated observer bends space-time, that is, Euclidean geometry is no longer valid.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 is curved..
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 pulls us forward when the bus in which we are traveling comes to a sudden stop. It is as if suddenly something invisible, dark and massive, for a few moments comes forward and attracts us, suddenly propelling us forward..
The planets move elliptically around the Sun because its mass produces a depression in the space-time surface that causes the planets to curve their paths. A light ray also curves its path following the space-time depression produced by the Sun.
If space-time is a curved surface, in principle nothing prevents one area from connecting with another through a tunnel. Traveling through such a tunnel would imply not only changing places, 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 more recently "Deep Space 9" from the Star Trek franchise and the 2014 movie Interstellar..
The idea came from Einstein himself, who, looking for solutions to the field equations of General Relativity, found together with Nathan Rosen a theoretical solution that allowed connecting two different regions of space-time through a tunnel that functioned as a shortcut..
That solution is known as the Einstein Bridge - Rosen and appears in a work published in 1935.
However, the term "wormhole" was first used in 1957, thanks to theoretical physicists John Wheeler and Charles Misner in a publication from 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 that was later made into a movie. The protagonist named Elly discovers intelligent extraterrestrial life 25 thousand light years away. Carl Sagan wanted Elly to travel there, but in a way that was scientifically credible..
Traveling 25 thousand light-years away is not an easy task for a human, unless a shortcut is sought. A black hole cannot be a solution, since when approaching the singularity, differential gravity would tear the spacecraft and its crew apart..
In search of other possibilities, Carl Sagan consulted one of the leading experts on black holes of the time: Kip Thorne, who began to think about the matter and realized that the Einstein-Rosen bridges or the wormholes of Wheeler were the solution.
However Thorne also realized that the mathematical solution was unstable, that is, the tunnel opens, but soon after it strangles and disappears.
Is it possible to use wormholes to travel great distances in space and time??
Since they were invented, wormholes have served in numerous science fiction plots to take their protagonists to remote places and to experience the paradoxes of non-linear time..
Kip Thorne found two possible solutions to the problem of wormhole instability:
The remarkable thing about this last solution is that unlike black holes, there is no singularity or quantum phenomena, and the passage of humans through this type of tunnel would be feasible..
In this way, wormholes would not only allow distant regions in space to be connected, but also separated in time. Therefore they are machines to travel in time.
Stephen Hawking, the great reference in cosmology at the end of the 20th century, did not believe that wormholes or time machines were feasible, because of the many paradoxes and contradictions that arise from them..
That has not dampened the spirits of other researchers, who have suggested the possibility that two black holes in different areas of space-time are internally connected by a wormhole..
Although this would not be practical for space-time travel, since apart from the tribulations that entering the singularity of the black hole would bring, there would be no possibility of exiting through the other end, since it is another black hole.
When you talk about a wormhole, you also immediately think of black holes.
A black hole forms naturally, after the evolution and death of a star that has a certain critical mass.
It arises after the star exhausts its nuclear fuel and begins to contract irreversibly due to its own gravitational force. It continues relentlessly until it causes such a collapse that nothing closer than the radius of the event horizon can escape, not even light..
By comparison, a wormhole is a rare occurrence, the consequence of a hypothetical anomaly in the curvature of space-time. In theory it is possible to go through them.
However, if someone were to try to pass through a black hole, the intense gravity and extreme radiation in the near vicinity of the singularity would turn it into a thin thread of subatomic particles..
There is indirect and only very recently direct evidence for the existence of black holes. Among these evidences are 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 a super massive black hole exists at the center of large galaxies, such as our Milky Way..
The rapid rotation of the stars near the center, as well as the enormous amount of high-frequency radiation that emanates from there, are indirect evidence that there is a huge black hole that explains the presence of these phenomena.
It was just on April 10, 2019 that the world was shown the first photograph of a supermassive black hole (7 billion times the mass of the Sun), located in a very distant galaxy: Messier 87 in the constellation Virgo, at 55 million light years from Earth.
This photograph of a black hole was made possible thanks to the worldwide network of telescopes, called “Event Horizon Telescope”, with the participation of more than 200 scientists from around the world.
On the other hand, there is no evidence of wormholes to date. Scientists have been able to detect and track a black hole, however the same has not been possible with wormholes..
Therefore, they are hypothetical objects, although theoretically feasible, as black holes were once also..
Although they have not yet been detected, or perhaps precisely because of this, different possibilities for wormholes have been imagined. They are all theoretically feasible, since they satisfy Einstein's equations for general relativity. Here are some:
All are perfectly possible, since they are not contradictory with Einstein's equations of general relativity.
For a long time, black holes were theoretical solutions to Einstein's equations. Einstein himself questioned the possibility that they could ever be detected by humanity.
So for a long time, black holes remained a theoretical prediction, until they were found and located. Scientists have the same hope for wormholes.
It is very possible that they are also there, but it has not yet been learned to locate them. Although according to a very recent publication, wormholes would leave traces and shadows observable even with telescopes.
The photons are believed to travel around the wormhole, generating a luminous ring. The closest photons fall inside and leave behind a shadow that will allow them to be differentiated from black holes.
According to Rajibul Shaikh, a physicist at the Tata Institute for Fundamental Research in Mumbai in India, a type of rotating wormhole would produce a larger and warped shadow than that of a black hole..
In his work, Shaikh has studied the theoretical shadows cast by a certain class of spinning wormholes, focusing on the crucial role of the hole throat in the formation of a photon shadow that allows it to be identified and differentiated from a black hole..
Shaikh has also analyzed the dependence of the shadow on the wormhole's spin and has also compared it with the shadow cast by a spinning Kerr black hole, finding significant differences. It is a completely theoretical work.
Apart from that, for the moment, wormholes remain as mathematical abstractions, but it is possible that some will be spotted very soon. What is at the other end is still a subject of conjecture for the moment.
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