Those are the uninteresting facts as we know them today. But science fiction stirs some fun into the facts by asking, “what if?” What if we could visit the future and see what will become of ourselves and the things we know? Better yet, what if we could travel into the past, view history with our own eyes, and maybe even use the gift of hindsight to adjust events to our advantage?
Those questions have provided fertile ground for writers of short stories, books, and movies, going back at least as far as Mark Twain. An incomplete list of written works on time travel might feature H.G. Wells’s The Time Machine, Robert A. Heinlein’s The Door into Summer, Ray Bradbury’s A Sound of Thunder, Fritz Leiber’s The Big Time, Lester Del Rey’s Tunnel Through Time, Kurt Vonnegut’s Slaughterhouse-Five, Clifford D. Simak’s Mastodonia, Anne McCaffrey’s Dragon flight, Frederik Pohl’s Gateway, Julian May’s Pliocene Exile series, and the literally unsurpassable The Restaurant at the End of the Universe by Douglas Adams. Time travel movies and TV shows include Time Bandits, The Terminator, Back to the Future, Groundhog Day, 12 Monkeys, Meet the Robinsons, The Girl Who Leapt Through Time, a double handful of Star Trek episodes and movies, and decades of Doctor Who.
Plenty of exciting, inspiring, and even funny reading and viewing. But is it all just fantasy? Is our steady journey together into the future really all there is? Might science somehow, someday imitate art and make time travel – real time travel, with the ability to make big changes in speed and direction – possible?
Let’s take a look.
Fast Forward
Traveling forward in time is allowed by physics. And simulating it is downright easy. You can get all the main effects without the need for any special equipment. Just go on a long trip. You’ll return to find a lot of undone work and an overflowing mailbox. Stay away long enough (maybe for an overseas deployment or a prison sentence) and when you return you’ll be disoriented about current events. Technology will have advanced. Everyone you know will have changed, and they may not recognize you.
But time travel the old-fashioned way is too slow to satisfy the purist. Part of the allure of traveling to the future is being able to see it before our friends do, and to get there without ageing. What we want is a shortcut. Fortunately, Albert Einstein showed us not just one but two shortcuts. According to Dr. Einstein, there is a Special way to move quickly into the future, and a General way.
Special Relativity
The classroom clock crawls
As we speed through our studies.
Relativity.
Einstein’s Theory of Special Relativity predicts that things moving at almost the speed of light experience an array of strange effects. Lengths contract. Masses increase. Things that happen simultaneously as seen by one observer happen at different times as seen by another. And, crucially, time slows down.
The Twin Paradox is a famous “thought experiment” that has been used since the early 1900s to illustrate the time-distorting effects of travel at relativistic speeds. In the experiment, which has to be done in thought because we can’t do it for real yet, one twin travels out into space and then back to Earth on a ship moving at relativistic speed. Because of the immense speed she’s traveling at, time runs slow for her. If her brother (they don’t have to be identical twins) watches her through a powerful telescope, he will see her moving in slow motion, the hands of her wall clock turning at a reduced rate, and the light from her reading lamp shifted to longer, redder wavelengths. When she comes back to Earth after her voyage, she will have experienced a fraction of the time that her brother has. She will have aged less than he. She will have effectively traveled into the future.
If you are wondering why that story represents a paradox, bravo for you. It doesn’t. The paradox arises when you consider what the sister sees through her telescope when she looks back at her brother. To her, he is the one who is moving at high speed and whose clock should be running slow. Formally resolving the paradox takes a lot of math: seven pages in my college relativity textbook, the one with the rhinoceroses on the cover. Leaving the calculations as an exercise for the abnormally interested reader, the paradox really does resolve, and the far-traveling twin really does age less than her brother. Robert A. Heinlein’s classic Time for the Stars explores the Twin Paradox in detail, using plenty of actual twins. The story may be getting a little creaky in the joints, but Mr. Heinlein did his physics homework correctly.
The great thing about special relativity is that it is totally fair and square. The theory has been verified by experiment over and over again. Even lettered physicists who love deflating the balloons of science-fiction lovers can’t declare that moving quickly into the future is impossible.
So it’s not impossible … but it is very hard. To dramatically slow down your clock, you must dramatically speed up your self. The speed of light is about 300,000 kilometers per second. To get a meaningful slowing of the clock, you need to go almost as fast as that: say, 240,000 km/s for a time-dilation factor of 60%. The fastest speed any human being has ever achieved is about 11 km/s, experienced by the Apollo astronauts whose capsules fell all the way from the Moon. Gaining even that pokey velocity was so difficult and expensive that humanity managed it only a handful of times, back when NASA was enjoying a ten-times-larger share of Federal discretionary spending than it gets today. The fastest we’ve ever made an unpiloted spacecraft go is about 70 km/s, for the Helios solar mission and the Galileo Jupiter entry probe. Neither value is close to 300,000 km/s. In another branch of science, we are able to accelerate things up to within a time-dilated gnat’s eyelash of light speed, but none of those things are bigger than a single atom, and it takes a particle accelerator with the length and power consumption of a small town to do it. If we want to use special relativity for time travel, we’ve got a long way to go in the propulsion department. When we get there, though, we’ll get the stars as a side benefit.
General Relativity
The Theory of General Relativity
Attributes to mass this proclivity:
Sufficient self-gravity
Creates a dark cavity
That holds even light in captivity.
General relativity offers another way to move into the future. Get close to an object with an escape velocity near or equal to the speed of light, and your clock will run slow as seen by an observer out in free space. Again, totally kosher, and confirmed by every experiment that has investigated it, including the recent and exquisitely sensitive Gravity Probe B space mission. Even the gravitational time dilation effect of the Earth, whose 11.2 km/s escape velocity is nowhere near the speed of light, is measurable and known. The GPS unit in your phone has to compensate for general relativity or it wouldn’t work.
But the Earth doesn’t slow time very much. Neither does the Sun, which has a surface escape velocity of about 600 km/s; close flybys yield way more scorched paint than temporal displacement. For a meaningful effect on the flow of time, you need a neutron star or a black hole. These are very unneighborly objects. They raise tides strong enough to rip apart any known material, including specifically your soft pink body. They may feature intense high-energy radiation and magnetic fields strong enough to short-circuit your nervous system. You must approach a black hole or neutron star very closely indeed to make time slow down, and somehow hang out there for a while to let the days add up. Landing is not a survivable option, but perhaps you could enter a low orbit, whipping around the monster hundreds of times a second. You will somehow have to endure the tides and radiation. Then, to enjoy your trip to the future, you must get away again, which takes a vehicle that can overcome that near-light-speed escape velocity! Compared to special relativity, this approach is messy and risky – but both earn a solid endorsement from physics.