1 Space-time

For it to work, an object needs to follow a world line through space-time that returns it to the same coordinates that it was at before it set off.

2 Time travel loop

A closed timelike curve is a time travel loop. Einstein said this would create a paradox – such as going back and killing your father before your birth.

3 Black holes

A spinning black hole would create a powerful gravitational field that could warp the fabric of existence and loop space-time on itself.

4 Quantum mechanics

But quantum mechanics suggests small objects – perhaps matter or information – could enter a closed timelike curve without a paradox.