Time travel hasn’t been cracked yet, but that hasn’t stopped physicists from imagining how it could happen. From twisting space-time with spinning cylinders to riding a wormhole through the universe, here are six of the most intriguing (and wildly speculative) ideas scientists have explored.

Wormholes

A shortcut through space-time, like a tunnel from one cosmic point to another. If one end experiences time differently (say, near a black hole), a wormhole might allow time travel. But you'd need exotic matter to keep it from collapsing.

Rotating Black Holes (Kerr Black Holes)

Unlike regular black holes, spinning ones could create loops in time, called closed timelike curves. The problem? Radiation and gravitational chaos make them more deadly than useful.

Cosmic Strings

These hypothetical, super-dense threads may be leftovers from the early universe. Two moving past each other at near-light speed could, in theory, bend time enough to form a loop. But no one’s ever found one.

The Warp Drive

Known as the Alcubierre Drive, this theoretical engine would bend space around a spacecraft. Travel faster than light? Maybe. Arrive before you left? Possibly. But it currently violates more laws of physics than it obeys.

The Tipler Cylinder

Imagine an infinitely long, spinning cylinder. According to math, you could orbit it just right and loop back in time. It’s elegant, but infinite objects are hard to come by.

Einstein’s Time Dilation

This one’s real. If you travel close to the speed of light, your time slows down compared to someone on Earth. You could return to find decades have passed—even if you’ve only aged a few years.