What is time travel and how does time travel work?

Physicists can mathematically calculate how traveling back in time might work. But if time travel is possible, what are the consequences?

I'm stuck at home, you're stuck at home, we're all stuck at home. We may have to do without for a while when we travel to fun holiday destinations or festivals. But how about a journey through time? And not just in the boring way where we wait for the future to arrive one second at a time. How about if you could jump through time at will? Forward to the future or backward into the past, as easy as pushing buttons on the dashboard of a souped-up DeLorean, just like in the Back to the Future movie?

People have been dreaming of time travel for at least 125 years. HG Wells wrote his seminal novel The Time Machine in 1895, and physicists and philosophers have been serious treatises on the subject for nearly a century.

What really got the scientific exploration of time travel going was the notion of time as a dimension that emerged in the last few years of the 19th century. We can move through space without any problems - so why not also through time?

“You can go wherever you want in the room. So maybe you can go wherever you want in time, ”says Nikk Effingham, a philosopher at the University of Birmingham in Great Britain. "From there it's just a short leap to time machines."

Wells was a writer, not a physicist. But physics should catch up with him soon. In 1905 Albert Einstein published the first part of his theory of relativity, known as the special theory of relativity. In it, space and time are malleable; Measurements of space and time depend on the relative speed of the person measuring.

A few years later, the German mathematician Hermann Minkowski showed that, in Einstein's theory, space and time can be viewed as two aspects of a single four-dimensional unit called space-time. In 1915 Einstein developed the second part of his theory, the general theory of relativity. General relativity looks at gravity in a new light: instead of looking at it as a force, general relativity describes it as a curvature or deformation of space-time.

But the special theory of relativity is enough for now to start looking at time travel. The theory "states that time is much more like space than we previously thought," says Clifford Johnson, a physicist at the University of Southern California. "So we can possibly do everything we can do with space, with time."

Well, almost everything. Special relativity doesn't give us a way to travel back in time, but it does give us a way to travel forward - at a speed that can actually be controlled. Thanks to the special theory of relativity, it can even happen that two twins are of different ages - the famous "twin paradox".

Let us assume that you set off in a spaceship at an extremely high speed (close to the speed of light) to the neighboring binary star system Alpha Centauri, while your own twin remains on earth. When you get home you will find that you are now much younger than your twin. That seems counterintuitive, but even after more than a century there is still nothing to be shaken about the physical explanation.

"It is absolutely provable with the special theory of relativity that the astronaut who travels at almost the speed of light will be much younger than his twin when he comes back," says Janna Levin, a physicist at Barnard College in New York. Interestingly, for both twins, time seems to pass the same way it always does; only when they are reunited does the difference show.

Maybe they were both in their twenties when the trip began. When the traveler returns, they will only look a few years older than they did when they left, while the twin may already have grandchildren. “His experience of the past is completely normal for him. His clocks tick at a normal rhythm, he ages normally, films run at the right pace, ”says Levin. “He hasn't traveled further into his own future than normal. But he's traveled into the future of his twin. ”

With general relativity, things are just beginning to get interesting. According to this theory, a massive object bends space and time. You may be familiar with this from diagrams or videos in which this is illustrated with a ball that sinks into a stretchable surface.

It follows not only that traveling at high speed affects the passage of time, but also that being close to a massive object like a black hole affects one's own experience of time. (This fact was central to the plot of the 2014 film Interstellar, in which Matthew McConaughey's character spends time near a massive black hole. When he returns home, he finds that his actually young daughter has grown old is).

But black holes are just the beginning. Physicists also speculate on the effects of a much more exotic structure they call a wormhole. Wormholes could - if they exist - connect one place in space-time to another. An astronaut who flies into a wormhole in the Andromeda galaxy in 3000 could come out at the other end in our own galaxy in 2000. But there's a catch: while there is more than enough evidence of the existence of black holes - astronomers even photographed one last year - wormholes are far more speculative.

“You can imagine building a bridge from one region of spacetime to another region of spacetime,” explains Levin, “but that would require types of mass and energy that we don't know exist in reality - Things like negative energy. ”She says that while it is“ mathematically conceivable ”that structures like wormholes could exist, they may not be part of physical reality.

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