2. Wormholes
Easily one of the most well-known time travel theories, it was made even more popular with the recent release of the film Interstellar. Wormholes are essentially shortcuts in the universe, in both space and time. Unlike the Tipler cylinder which is infinite, a wormhole has a definite beginning and end. They are said to be portals to both the past and the future. However, they are tiny so that only small particles can pass through them.
