Time travel is true

: Can time travel like in the new Netflix series Dark be possible?

"The difference between past, present and future is an illusion, albeit a very persistent one," Albert Einstein once wrote. This bon mot became the motto of the first German Netflix series "Dark". She tells how young people in the small town of Winden disappear inexplicably. The viewer soon learns that the missing found their way into the past through a “wormhole” and became time travelers.

For decades, even great minds like Stephen Hawking or the co-discoverer of gravitational waves and Nobel laureate in physics, Kip Thorne, have been wondering whether something like this could be physically possible. They have not found a definitive answer. But there are many indications that time travel is impossible. We are caught in the here and now.

In “Dark” the gate to the time tunnel is hidden in a cave, the passages of which run through the rock below a nuclear power plant. The path into the past begins behind a heavy iron door. If you cross the corridor, you come out again at the entrance, but decades in the past. This is where the time travelers encounter their ancestors and, in the end, even themselves. A tricky story unfolds in this spacetime web that gives a lot to think about and, above all, raises serious causal paradoxes: cause and effect lose their well-known connection.

Einstein never had the possibility of time travel in mind

The history of the theory of spacetime tunnels is almost as tricky as the story from Winden. It is based on Einstein's general theory of relativity. One of their basic statements is: Celestial bodies curve the space around them, and all other bodies and also the light must follow these dimples in space. To illustrate this, our three-dimensional space is reduced to two dimensions. Far away from all matter, the space is not curved, correspondingly the two-dimensional simplification is flat like a cloth. If you place a ball, which represents a celestial body, on this cloth, a hollow is created around it. This is how you can imagine the curved space.

Shortly after Einstein's publication of the theory of relativity, the unknown physicist Ludwig Flamm from the University of Vienna came across the possibility that two curved areas of space could be connected by a tunnel. However, the work received no attention and was also unknown to Einstein when in 1935 he and his colleague Nathan Rosen also came across the possibility of a “bridge” between two areas of the room. The connection could be created by particles or energy. Such an Einstein-Rosen bridge would be a shortcut through a hypothetical, four-dimensional hyperspace. Einstein and Rosen never had the possibility of time travel in mind.

In the 1950s, the American pioneer of the theory of relativity, John Archibald Wheeler, paved the way from science to the world of science fiction. He found out that an Einstein-Rosen Bridge might really be possible. He created the term wormhole for this. To understand this, imagine the space reduced to two dimensions as the surface of an apple. For an ant, the shell is the habitat that it cannot leave, just as we cannot leave our universe. Suddenly she comes across a hole. She slips in and finds herself in a worm channel. She crosses it and comes out on the other side of the apple. She has found a shortcut.

Time stands still at the edge of a black hole

This is how humans could do it: Instead of flying thousands of years to the next star, you take a shortcut through a wormhole and you'll be there in no time. This idea was reflected in the 1997 film "Contact".

Under certain circumstances, a wormhole could even make it possible to travel through time, as Wheeler and later his student Kip Thorne found out. The entrance to the wormhole would have to be close to a celestial body because time passes a little slower here than it does far away. The effect is strongest close to a black hole, because time is extremely slow there.

It even stops on its edge. Christopher Nolan implemented this idea in his box office hit "Interstellar", in which Thorne worked as a scientific advisor. In "Dark" - copied from Nolan - someone explains the journey through time through a wormhole by rolling up a sheet of paper so that its beginning and end come together.

"I doubt that the laws of physics allow traversable wormholes"

There are just a few catches. On the one hand, you need a special kind of negative matter, or better negative energy, to open the hole. There is no idea how one could create negative energy in such great density. In the film "Dark" it comes from an accident in the nuclear power plant. A time traveler also whispers about a Higgs field and the amplification of a radioactive substance. It would certainly not work that way.

In addition, wormholes would be unstable. They dissolve again in a fraction of a second. How to keep such a tunnel open has been investigated by many theoretical physicists without any tangible result. "I doubt the laws of physics allow traversable wormholes," says Thorne. Without a far-reaching theory that brings together the laws of relativity and quantum physics, which also play a role, this topic is likely to remain speculation.

Apart from the physical problems, time machines also generate paradoxes. The problem is obvious: what if I go back in time and kill my parents? Am I dissolving myself in this moment because I was never conceived? Or does a new reality arise at this moment, which is developing in parallel to the one that has already taken place in a different universe, so to speak? "Dark" plays with this very possibility, with a father going back in time to kill a boy who, years later, is believed to have killed his son.

Where are the tourists from the future?

Some physicists have suggested that time travel is only possible if it does not change the future. How does that work? Even a minimal change can change the further course of history - according to the creed of chaos research “small cause, big effect”.

In order to avoid time paradoxes in general, Stephen Hawking suggested that there is a law of nature to “protect the order of time”, which prevents the creation of closed, time-like curves. He also points out: "Where are the tourists from the future when time travel is possible?"

There could, however, be an answer to this: In principle, one could not travel back to a time before the wormhole opened. Incidentally, this is a mistake in "Dark", where a person travels back to 1953. That year the nuclear power plant had not yet been built and therefore the wormhole had not yet been created.

Time travel gives you a lot to think about; physicists have already published hundreds of papers on the subject. Einstein would probably have banished them completely to the realm of fantasy, because he firmly believed in the irreversible order of cause and effect.