Time travel into the future is not only possible, it is practically unstoppable. You may try traveling at close to the speed of light, or parking yourself near a giant gravitational well – but no matter what you try, your clock will keep ticking ever onward, no matter how slowly someone from the outside observes it.

Time travel to the past is a whole different theoretical ballgame. Science fiction writers and scientists alike have attempted to come up with ways it could be possible within the laws of physics as we currently understand them. 

A few possibilities have been suggested based on Einstein’s field equations describing the curvature of spacetime by mass and energy. One idea that comes out of Einstein’s work is that “closed timelike curves” could be possible, where spacetime is so warped (deliberately or by nature) that an object or observer traversing it would be returned to their starting point.

A natural timelike curve (if it existed) would be a wormhole. These are structures allowed for in the math 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 the wormhole forming a connection. However we have never observed a wormhole, and physicists have tied themselves in knots trying to figure out if they would be stable enough to be traversable. After all, if you say “I have found a time machine”, people will expect a little more from a demonstration than you getting crushed into a fine pulp the second you step foot in it.

Another idea for a time machine using closed timelike curves is a Tipler cylinder, or Tipler time machine. This was first suggested in 1923, but gained popularity following a 1974 paper by physicist Frank Tipler.

The basic idea of the Tipler cylinder is to take a cylinder and rotate it incredibly quickly. This 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 must be incredibly long and incredibly dense, likely requiring the matter of at least ten times our Sun’s mass to work. Then you have to rotate it to absurd speeds, so that it is spinning a few billion times a second. On another practical level, you would then have to be able to approach the tube – with its incredible gravitational pull – and traverse it, while it hopefully is not blown apart by the force of its own rotation. 

But if you were able to overcome those problems, entering the cylinder and accelerating on the right path inside the cylinder-warped spacetime should (according to some highly speculative math) let you emerge thousands or billions of years away, and possibly several galaxies away from where you started.

“Your path, which normally inextricably moves you forward in time, changes, since moving around the cylinder in the direction of rotation will shift you back in time,” math educator Steve Humble explains in The Conversation. “The machine makes the direction of time collapse into the past, so the longer you follow the machine’s spin, the further back in time you will go. To reset the movement to normal, simply move away from the cylinder, go back to Earth and you will be returned to the present – albeit a present in the past.”

As fun as that sounds as a project for a Type II / Type III civilization, we still would not get our hopes up. Like with wormholes, it is unclear if such a thing – with all its possible causality-breaking tag-ons – could exist outside of an interesting math paper. And if it could, it’s possible that it would require negative mass, which we do not know exists, or a cylinder that is infinitely long. 

If we have mastered creating that, we will probably be in an interesting enough time that we might not want to go back to the past anyway.