Time travel to the future is not only possible, it’s practically unstoppable. You can try traveling at near the speed of light or parking yourself near a giant gravity well – but no matter what you try, your clock will keep ticking forward, no matter how slowly someone on the outside is watching it.
Time travel to the past is a whole other theoretical game. Science fiction writers and scientists alike have tried to figure out ways to make this possible within the laws of physics as we currently understand them.
Several possibilities have been proposed based on Einstein’s field equations describing the curvature of spacetime by matter and energy. One idea that arose from Einstein’s work is that “closed time-like curves” might be possible, where space-time is so twisted (intentionally or by nature) that an object or observer passing through it would return to your starting point.
The natural time curve (if it existed) would be a wormhole. These are structures made possible by the mathematics of general relativity connecting two separate regions of space and/or time. Physicists have suggested that a black hole could be connected to a mirror white hole, with a wormhole forming the connection. But we’ve never observed a wormhole, and physicists have been tying themselves in knots trying to figure out if they’ll be stable enough to walk through. After all, if you say “I found a time machine”, people will expect a bit more from the demonstration than you being crushed to a fine pulp as soon as you step into it.
Another idea for a time machine using closed time-like curves is the Tipler cylinder or Tipler time machine. It was first proposed in 1923, but gained popularity after the work of physicist Frank Tipler in 1974.
The basic idea of a Tipler roller is to take a roller and spin it incredibly fast. That doesn’t sound too complicated, considering the result would be a working time machine — but before you go hunting for used toilet paper, there are a few caveats.
The cylinder itself must be incredibly long and incredibly dense, which probably requires a mass of at least ten times the mass of our Sun to function. Then you have to spin it to absurd speeds to make it spin a few billion times per second. On another practical level, you would then need to be able to approach the tube – with its incredible gravity – and pass over it, hopefully not being torn apart by the force of its own rotation.
But if you were able to overcome these problems, entering the war and accelerating along the correct trajectory inside the warped space-time war should (according to some highly speculative math) allow you to emerge thousands or billions of years away, and perhaps even several galaxies away from where you started.
“Your trajectory, which normally moves you inextricably forward, changes because moving around the cylinder in the direction of rotation moves you back in time,” explains maths teacher Steve Humble in The Conversation. “The machine causes the direction of time to collapse into the past, so the longer you follow the machine’s rotation, the further back in time you go. To return movement to normal, simply move away from the war, return to Earth, and you will be returned to the present – albeit a presence in the past.”
While this sounds like a fun project for a Type II / III civilization, we still wouldn’t give up hope. As with wormholes, it’s not clear that such a thing—with all its possible tag-violating causalities—could exist outside of interesting mathematical work. And if it could, it’s possible that it would require negative mass that we don’t know exists, or a cylinder that’s infinitely long.
If we’ve made it, we’ll probably be in an interesting enough time that we won’t want to go back to the past anyway.