The Past and Future of Time Travel
By Elizabeth Howell / Space.com
How would you like to head back to the future in a DeLorean car? Or travel with the crew of the USS Enterprise to save the whales? These two examples (from "Back to the Future" and "Star Trek IV: The Voyage Home") show a very common trope in science fiction — time travel.
We all have things we regret in life, so the concept of turning back time (or in the case of one "Superman" movie, reversing Earth's rotation) is an inviting one.
As one scientist points out, we all constantly time travel — but it's in only one direction. We're inevitably moving 1 second at a time into the future, and we could go faster if we wanted.
"Indeed, we can jump forward into the future as much as we want. It's only a matter of going really, really fast," Paul Sutter, an astrophysicist at Ohio State University, told Space.com in an email. He began by citing evidence from Albert Einstein's theory of special relativity, which shows that time is relative depending on how fast you are moving.
"The faster you move through space, the slower you move through time. We've been able to measure this with ultra-precise atomic clocks in jet airplanes, and the precision offered by the GPS system needs to take this into account. Sci-fi always seem to require complicated contraptions to jump in time, when all you need is a very large rocket," Sutter wrote.
This means that astronauts, for example, are already time travelers of a sort. That's because they go into space and live on the International Space Station, sometimes for months at a time. At a speed of about 5 miles (8 kilometers) a second, astronauts on the space station are moving faster than we are on Earth. This means that on the station, astronauts age just a tiny bit slower than they would on the planet's surface. (And that when astronaut Scott Kelly came back from a year in space, the age gap with his slightly older identical twin, Mark, widened by just a little bit.)
Jumping into the past
Sutter pointed out that the physics of our universe appear to forbid this situation, at least as far as we can see. But surprisingly, some of Einstein's equations from the theory of general relativity may allow time travel into the past. (That theory basically discusses how huge objects distort space-time, which we feel as gravity.)
So how could Einstein's theory make time travel possible? Well, one way would be to break the cosmic speed limit and go faster than the speed of light — but that likely wouldn't work, because an object going at that speed would have infinite mass. Another possibility would be to form "wormholes" between points in space-time, although this would likely work for only small particles. There are even more exotic possibilities out there, such as using black holes, huge cylinders or cosmic strings to play with the fabric of space-time.
"When it comes to the past," Sutter said, "the mathematics of general relativity does allow a few strange scenarios where you can end up in your own past. But all of these scenarios end up violating other known physics, like requiring negative mass or infinitely long rotating cylinders. Why does general relativity allow past time travel, but other physics always jump in to spoil the fun? We honestly don't know."
But that doesn't mean that scientists are giving up. In 2015, Ali Övgün of Eastern Mediterranean University in Cyprus said wormholes might be possible in zones with dark matter. (This is a theoretical form of matter that cannot be seen or otherwise sensed with telescopes, but does show itself in its gravitational effects on other bodies.) While his equations show wormholes could occur in these regions, Övgün said he is still searching for proof. "It is only mathematical proof," he said. "I hope one day it will be possible to also find direct experimental evidence."
Even the world-renowned physicist Stephen Hawking was entranced by the idea of time travel before his death this year, when he discussed in the Daily Mail how a black hole could make it possible. "Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time," he wrote in 2010. However, physicist Amos Iron at the Technion-Israel Institute of Technology in Haifa, Israel, said a machine circling a black hole would probably disintegrate before moving that quickly.
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