Travel in the past is practically possible
Time travel: tunnels into other worlds
A "politically incorrect" topic
How about: travel to ancient Egypt to see how the pyramids were built? Or take a look at the New York of the distant future? So far, time travel has only existed as science fiction, in books and films. So nothing serious scientists study? "Anyone who openly speculates about time travel either risks an outcry from the public that research funds are being wasted on something absolutely ridiculous," says physicist Stephen Hawking, "or the research is classified as secret by the military."
A "politically incorrect" topic
Time travel is a "politically incorrect" topic, says Hawking. But his guild knows what to do: "We hide our preoccupation with the subject behind complicated technical terms that are a code for time travel." For example behind the term "closed time-like curve". In fact, well-known physicists have been discussing the possibility of breaking into the past and the future in reputable scientific journals since the late 1980s. Albert Einstein opened the gate there. His special theory of relativity and the general theory of relativity created a completely new picture of the order of the universe: According to this, time elapses at different speeds, depending on the speed at which a person moves through space.
Travel at the speed of light
For an astronaut, for example, when viewed from Earth, the time on board goes by the slower the closer his speed approaches the speed of light. With the help of this effect one could very well travel to New York in the year 3005. "All you have to do is get into a spaceship, travel to a star that is almost 500 light-years away, and return," says US physicist Richard Gott, explaining the concept of time travel. The prerequisite is that the way there and back is covered at 99.995 percent of the speed of light. "When you return, the earth will be 1,000 years old, but you will only be 10 years older."
What appears theoretically possible faces enormous hurdles in practice. So far there is no rocket engine for such speeds. And above all: How would the future traveler get back to the present? According to the special theory of relativity, not at all: in it, time runs like a one-way street.
Travel into the past is not much better off. In Einstein's general theory of relativity, spacetime is "curved" under the influence of gravity - up to a so-called singularity, which the physicist John Wheeler described in 1967 as a "black hole". Even light cannot escape this extremely dense accumulation of matter due to its enormous gravity. Albert Einstein and his student Nathan Rosen tried in 1935 to prove that such singularities cannot exist. But they didn't succeed. They even discovered that - purely mathematically - two of these space-time funnels can touch each other in such a way that they form a bridge between our world and a parallel universe.
Fast connection: wormholes
Such a connection, if it represents a tunnel between two points in the same universe, was later dubbed "wormhole". The crew of the spaceship "Enterprise" let themselves be thrown into the past through such a passage in the "Star Trek" television series.
However, wormholes have a catch: they are not stable. When the American astrophysicist and author Carl Sagan wrote the novel "Contact" in the 1980s, he asked his friend Kip Thorne, a physicist at the California Institute of Technology in Pasadena, to check whether a stable wormhole might not be possible. Through this Sagan wanted to send his heroine Ellie Arroway to aliens in the Vega planetary system. Thorne plugged this into Einstein's equation and actually found a mathematical solution that could also enable a journey into the past: a time loop that leads directly through a wormhole.
The British physicist Paul Davies was the most convincing example of how a time machine could be constructed from this. As components he suggests a particle accelerator, an "imploder", an "inflator" and a "differentiator".
The difficulties start with finding or making a wormhole. At least none has been directly observed in the cosmos. Quantum physicists suspect, however, that a particularly short-lived and tiny wormhole variant is permanently formed in the fine structure of the universe, the so-called space-time foam. These "virtual" wormholes would only exist 100 trillionths of a trillionths of a trillionth of a nanosecond. Its diameter would be a so-called Planck length, i.e. it would be 20 powers of ten smaller than an atomic nucleus.
"If a permanent wormhole is to be created, sufficient energy must be artificially supplied to the space-time foam," says Paul Davies. "Surprisingly, only about ten billion joules are required for this. That corresponds to the energy output of a typical large power plant within a few seconds." The energy can be generated by a particle accelerator in which heavy atomic nuclei are shot at one another and atomized.
A time machine would have gigantic proportions
The second step is only manageable for a technically very advanced civilization: The ten billion joules have to be compressed in an imploder to the unimaginably small space of Planck's length. If you wanted to do this with magnetic fields - as is common in today's particle accelerators - a facility the size of our solar system would be required, says Davies.
But that's not all: In the next step, the wormhole has to be enlarged so that a time traveler can also fit through: for example with the help of "exotic" matter that has a negative gravity opposite to gravitation. It is so far unknown in our everyday world, but some physicists consider its existence to be possible. It could be harvested with the help of negative energy, generated in an inflator, a "pumping up machine" made of high-energy lasers with an extremely fast rotating mirror system, as Davies writes. To create a wormhole with a diameter of one meter, however, a negative amount of energy would be required, which would correspond to the mass of Jupiter, the largest planet in the solar system, as the American Matt Visser has calculated.
Neutron star as a time warper
If you had created a passable wormhole a few meters long in this way, the last step would remain: a differentiator would have to create a time difference between the two openings. That could be an extremely massive neutron star, says Davies, because according to general relativity, strong gravity slows the passage of time. To do this, one opening of the wormhole would have to be pushed close to the neutron star. While time slows down there - the more massive the neutron star, the more time is stretched - it passes quite normally at the other opening.
Then the opening would be brought back into our solar system - although Davies, of course, cannot say how this could be practically possible. The traveler would now step into the past through the wormhole, as if he were walking down a long hallway at the end of which a door opened to Berlin in the 1920s. "Instead of letting time run backwards, the time traveler embarks on a journey through space that ends in the past," says Davies, characterizing the functionality of a wormhole time machine.
Why nobody from the future has visited us yet
However, the past cannot go back indefinitely. There would be a so-called time travel horizon: the completion of the time machine construction, i.e. the moment when the differentiator - for example the neutron star - begins to slow down time. The fact that there is demonstrably no such time machine today would also explain why no traveler from the future has visited us so far. The time machine described by Paul Davies has a further consequence: time travel to ancient Egypt, the Middle Ages or even just up to the fall of the wall is therefore impossible in principle - because no such time machines have been built in the past either.
In any case, physicists are not entirely satisfied with the hypothetical possibility of time travel into the past. Causality, one of the basic principles of physics, could be violated. This is shown, for example, in the film "Back to the Future", in which a boy goes back to the time before he was born. His mother falls in love with him. But what would happen if because of that she never married his father and he wasn't even born?
Is the will free?
Some physicists believe that such paradoxes cannot arise. Whatever a time traveler undertakes, he could only confirm the course of events. Just like in the film "Terminator", when the machines in the future send the killer robot played by Arnold Schwarzenegger into our present. He is supposed to kill the woman who will give birth to the future leader of insurgent humanity. But the people of the future will succeed in sending a rescuer to protect their mother, and he will also become the father of their child, who will later become the leader. The course of history has not changed in the case. But that would fundamentally call human free will into question. A high price to pay for avoiding the paradox.
Journey in parallel universes
The Oxford physicist David Deutsch has shown a way out of this dilemma, which insists on a conceivable interpretation of quantum physics: Our universe is therefore only one of an infinite number of parallel universes in which events develop differently. "Many of the leading quantum physicists take this many-worlds interpretation pretty seriously," admits Richard Gott. A time traveler would therefore simply be the trigger for an alternative world history that does not correspond to the one from which he comes. It would take place in another universe.
Free will would be saved, but how could the time traveler ensure that when they return they will find their original world again? Stephen Hawking, who is intellectually stimulated by the mind games about time travel, ultimately hopes for the validity of the "chronology protection conjecture" that he formulated in 1992: "The laws of physics have conspired to allow macroscopic objects to travel through time prevent." But as long as this is just a guess, time travel is not completely ruled out - at least in theory.
Falko Blask and Ariane Windhorst Time machines - myth and technology of a human dream Atmospheres Verlag 2005 ISBN 3-86533-020-7#Subjects
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