Tipler Cylinder

A Tipler Cylinder is a hypothetical cylinder of infinite length and rotating with a specific angular velocity, proposed by physicist Frank Tipler in 1974 as a solution to the equations of general relativity that permits closed timelike curves — paths through spacetime that return to their own past. Tipler showed that the gravitational field of a sufficiently massive, infinitely long rotating cylinder could warp spacetime in such a way that a particle moving around the cylinder could arrive at its own past, creating a scenario where time travel into the past is mathematically permitted by Einstein's equations.

The Tipler Cylinder is a thought experiment, not a realistic engineering proposal. An infinite cylinder is physically impossible to construct, and the rotation speeds required are extreme. The cylinder serves as a theoretical demonstration that general relativity does not forbid time travel at the level of the equations — it merely makes it extraordinarily difficult. This is one of the motivations for the Chronology Protection Conjecture, proposed by Stephen Hawking, which suggests that the laws of physics conspire to prevent macroscopic time travel even if it is mathematically possible.

The Tipler Cylinder is often dismissed as a mere curiosity because it requires an infinite object. But this dismissal misses the point. The cylinder is not a proposal for a time machine; it is a probe of the consistency of general relativity. If the equations permit time travel under any conditions, no matter how unrealistic, then the theory itself does not contain a prohibition against causality violation. The question is not whether we can build a Tipler Cylinder but whether causality is a built-in feature of spacetime or an emergent property that requires special conditions to protect it.