Time travel into the future is not only possible, it’s virtually unstoppable. You can try traveling at near-light speed, or parking yourself in a giant gravity well—but no matter what you try, your clock will always keep ticking, no matter how slowly someone on the outside looks at it.

Time travel into the past is a whole other theoretical game. Science fiction writers and scientists have tried to imagine ways in which it could be possible within the laws of physics as we know them.

A few possibilities have been proposed based on Einstein’s field equations that describe the curvature of spacetime by mass and energy. One idea that comes from Einstein’s work is that “closed timelike curvatures” might be possible, where spacetime is so warped (either intentionally or naturally) that an object or observer traveling through it would return to their starting point.

A natural timelike curve (if one existed) would be a wormhole. These are structures allowed in the mathematics of general relativity that connect 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 gone to great lengths to figure out whether they would be stable enough to be traversed. After all, when you say “I found a time machine,” people expect more of a demonstration than to be ground to a fine pulp the moment you step inside.

Another idea for a time machine using closed timelike curves is a Tipler cylinder, or Tipler time machine. This was first proposed in 1923, but became popular after a 1974 paper by physicist Frank Tipler.

The basic idea of the Tipler Cylinder is to take a cylinder and make it rotate incredibly fast. 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 would have to be incredibly long and incredibly dense, and probably require matter at least ten times the mass of our sun to work. Then you would have to rotate it to absurd speeds, so that it would spin a few billion times a second. On another practical level, you would then be able to approach the tube – with its incredible gravity – and travel through it, while hopefully not having it blown apart by the force of its own rotation.

But if you could overcome those problems, entering the cylinder and accelerating on the right path in cylindrical spacetime should (according to some highly speculative mathematics) take you thousands or billions of years further away, and possibly even several galaxies away from where you started.

“Your path, which normally takes you inextricably forward in time, changes because moving around the cylinder in the direction of its rotation takes you backward in time,” maths teacher Steve Humble explains in The Conversation. “The machine causes the direction of time to disappear into the past, so the longer you follow the rotation of the machine, the further back in time you go. To return the motion to normal, all you have to do is move away from the cylinder, come back to Earth, and you’re back in 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 wouldn’t get our hopes up. As with wormholes, it’s unclear whether something like this – with all its potential causality-breaking add-ons – could exist outside of an interesting mathematical paper. And if it did, it’s possible that it would require negative mass, which we don’t know exists, or a cylinder that’s infinitely long.

Once we master that, we will likely find ourselves in a time so interesting that we may not want to go back to the past after all.