Paul Sutter is an astrophysicist at The Ohio State University and the chief scientist at COSI Science Center. Sutter is also host of Ask a Spaceman, We Don’t Planet, and COSI Science Now.
Special relativity teaches us that the three dimensions of space and the solitary dimension of time are woven together like a fabric. It's impossible to think of them as separate entities, only a singular unified entity — space-time. We can't think of motion through space without being mindful of motion through time, and vice versa. Left-right, up-down, back-forth and past-future are all on equal footing.
And yet, time does seem a little different. We have complete freedom of movement within space, but we cannot avoid our future. Time seems to have an "arrow," whereas the spatial dimensions are ambidextrous. Given the unity between time and space, it leads to the obvious question: Is time travel, of any sort, possible? Under any circumstances? At all? [How Time Travel Works in Science Fiction (Infographic)]
Into the future: Sure
Oddly enough, the answer is yes! We cannot avoid moving into our futures, but we can control the rate that we move through time. This is a consequence of another lesson from relativity: Not all clocks are the same.
The speed at which you move through space determines the speed at which you move through time. In the succinct phrase: Moving clocks run slow.
IF you could build a big enough rocket (don't ask me how, that's an engineering problem) to provide a constant acceleration of 1g (9.8 meters per second per second; the same acceleration as provided by the Earth's gravity at its surface), you could reach the center of the Milky Way galaxy — a healthy 20,000 light-years away — in just a couple decades of your personal time.
You could stop for a few hours, have a picnic near Sagittarius A* (the black hole at the center of the galaxy), and then hop back in to your rocket and come back to Earth.
By the time you return you'll be eligible for retirement benefits, if the institution providing those benefits is even around, because while you only traveled for a few decades according to the clock on your ship, about 40,000 years would've passed on the Earth.
Closing the loop
Time is relative, but it still flows in the same direction for everyone. To ask if we can go into reverse is the domain of general relativity (GR) — this is the mathematical language we use to not only understand gravity, but the full connection between space-time and motion.
In GR, we ask a slightly more technical question: Is there any arrangement of matter and energy (the stuff that warps space-time) to permit the existence of closed time-like curves, or CTCs? I know this is jargon but it's a fun phrase to toss around at parties. "Curve" here means a path, "time-like" means you never go faster than the speed of light, and "closed" means it returns to its starting point — in other words, its own past.
So, Oracle of Einstein, are CTCs permitted?
Yes!
Well….
The possibilities are finite