Time travel into the future is not only possible, it’s practically unstoppable. You might try traveling at near the speed of light or parking near a giant gravity well – but no matter what you try, your clock will keep ticking forward, no matter how slowly someone outside is watching it.
Time travel to the past is a whole different theoretical game. Science fiction writers and scientists have tried to think of ways in which this could be 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 in terms of mass and energy. One idea that stems from Einstein’s work is that “closed time-like curves” may be possible, where space-time is so distorted (intentionally or by nature) that an object or observer passing through it will be thrown back into your starting point.
A natural time-like curve (if it existed) would be a wormhole. These are structures allowed in the mathematics of general relativity, connecting two distinct regions of space and/or time. Physicists suggest that a black hole could be connected to a mirror white hole, with the wormhole forming a link. However, we have never observed a wormhole, and physicists have been tying themselves in knots trying to figure out whether they would be stable enough to pass through. After all, if you say “I found a time machine”, people will expect a bit more of a demonstration than you will be crushed to a fine pulp the second you step inside it.
Another idea for a time machine using closed time-like curves is a Tipler cylinder or Tipler time machine. This was first proposed in 1923, but gained popularity after a 1974 paper by physicist Frank Tipler.
The basic idea of the Tipler cylinder is to take a cylinder and spin it incredibly fast. That doesn’t sound too complicated considering the result would be a functional time machine – but before you go hunting for a used toilet roll, there are a few caveats.
The cylinder itself would have to be incredibly long and incredibly dense, probably requiring matter with at least ten times the mass of our Sun to make it work. Then you have to spin it to ridiculous speeds so that it spins several billion times per second. On another practical level, you would then need to be able to approach the tube – with its incredible gravitational pull – and cross it, hopefully not being blown away by the force of its own rotation.
But if you managed to overcome these problems, entering the cylinder and accelerating along the right path inside the cylinder-warped space-time should (according to some highly speculative mathematical calculations) allow you to exit thousands or billions of years away, and possibly a few galaxies from where it started.
“Your path, which normally moves you forward in time inexorably, changes because moving around the cylinder in the direction of rotation will move you backwards in time,” maths lecturer Steve Humble explains to The Conversation. “The machine causes the direction of time to shrink into the past, so the longer you follow the machine’s rotation, the further back in time you will go. To restore normal motion, simply move away from the cylinder, return to Earth, and you’ll be brought back to the present—albeit a present in the past.”
As fun as this sounds as a type II / type III civilization project, we’re not going to get our hopes up just yet. Like wormholes, it’s not clear that such a thing—with all its possible causality-breaking labels—could exist outside of interesting mathematical work. And if it could, it might require negative mass that we don’t know exists, or a cylinder that is infinitely long.
If we’ve mastered the making of this, we’re probably in an interesting enough time that we don’t want to go back in time anyway.
