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Dr Who Meets Professor Heisenberg

Example of a CTC.

A space-time structure exhibiting closed time-like curves. Here a wormhole connects two points at the same location in space (horizontal) but at different times (vertical). A quantum particle travelling on such a path might interact with its older self.

By Martin Ringbauer

Researchers have simulated in the laboratory how quantum particles could overcome the “grandfather paradox” of time travel.

The full text of this article can be purchased from Informit.

From HG Wells through to Dr Who, the possibility of time travel is ubiquitous in science fiction. Yet it poses puzzling questions for physicists and philosophers alike.

According to Einstein’s theory of general relativity, space and time are not two separate concepts but one and the same thing: coordinates in four-dimensional space-time. Gravity is a consequence of the curvature of this space-time. A very heavy body, such as a star or a black hole, can bend space-time around it, causing other nearby objects to fall towards it.

General relativity also tells us that nothing can move faster than the speed of light. As a consequence, all the paths that a photon can take from a given point lie on the surface of a four-dimensional “light cone”. Since nothing can move faster than light, any massive object must follow a path within this light cone.

In normal space-time this means that we can always only go forward in time following a time-like path. But if we are close to a heavy object, space-time is bent a little and our light cone tilts towards this object. If at the same time this object spins very fast then it can drag space-time, including our light cone, with it – just like a wisp of smoke caught in a current around a fan. These effects can be so extreme that they cause time-like paths to wrap back onto themselves, creating a closed time-like curve (...

The full text of this article can be purchased from Informit.