**Tara Magill**

U.S. astronomer Frank Tipler constructed a ‘simple’ method for time travel to the past: first, take a piece of material ten times the mass of the Sun, then squeeze it together and roll it into a long, thin, super-dense cylinder of infinite length. Then, spin the cylinder at a few billion revolutions per minute and see what happens. Easy, right?

His theory suggests that if a ship was to navigate in a perfect spiral around a Tipler cylinder, they could find themselves billions of years and several galaxies away from where they started. This ship would be on a closed, time-like curve. However, as you can imagine, producing an infinitely long cylinder that is as dense as a black hole has its problems. Namely, at our current level of technological development, it’s impossible.

If we consider this to be a viable theory, however, there are a few things to know before you take off for your journey to the past. Firstly, stay away from the ends of this theoretically infinite cylinder, as distortions here would have an extremely undesirable effect on whoever goes near them. Stick near the centre, and you should hopefully be able to survive the trip.

The way that the Tipler cylinders work is through creating a frame-dragging effect. This means that for objects near the rotating cylinder, their light cones become tilted and part of them turn backwards on the time axis of a space time diagram. This is what allows the spacecraft to travel backwards in time, as they move along this backward-facing light cone. I will be discussing more about light cones in a future entry.

Despite the seemingly obvious practicality issues, Stephen Hawking began attempting to create a more realistic Tipler Cylinder in 1992. However, he came to the conclusion that it *is* possible to build a Tipler cylinder (of finite length), however, it would have to be built in an area of space which has negative energy but no exotic matter. So, unfortunately, we are unable to travel back in time with this theory, as it’s not possible to create a Tipler Cylinder in the real world under these conditions.

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