Is Time Travel Possible in String Theory?
The concept of time travel is often closely tied to infinities in the curvature of space-time, such as that within black holes. In fact, the discoveries of mathematically possible time travel were found in the general relativity equations containing extreme space-time curvature.
Stephen Hawking, one of the most renowned experts in looking at space-time curvature, believes that time travel is impossible and has proposed a chronology protection conjecture that some mechanism must exist to prevent time travel.
When black holes were first proposed as solutions to Einstein’s field equations, neither Einstein nor Eddington believed they were real. In a speech to a Royal Astronomical Society, Eddington said of black hole formation, I think there should be a law of nature to prevent a star from behaving in this absurd way!
Although Hawking is certainly comfortable with the idea of black holes, he objects to the idea of time travel. He proposed his chronology protection conjecture, which states that there must be something in the universe that prevents time travel.
Hawking’s sometimes collaborator, Oxford physicist Roger Penrose, made the much more guarded claim that all singularities would be protected by an event horizon, which would shield them from direct interaction with our normal space-time, known as the cosmic censorship conjecture. This would also potentially prevent many forms of time travel from being accessible to the universe at large.
One major reason time travel causes so much trouble for physics (and must therefore be prohibited, according to Einstein and Hawking) is that you could create a way of generating an infinite amount of energy.
Say you had a portal into the past and shone a laser into it. You set up mirrors so the light coming out of the portal is deflected back around to go into the portal again, in tandem with the original beam you have set up.
Now the total intensity of light coming out of the portal (in the past) would be (or have been) twice the original laser light going in. This laser light is sent back through the portal, yielding an output of four times as much light as originally transmitted. This process could be continued, resulting in literally an infinite amount of energy created instantaneously.
Obviously, such a situation is just one of many examples why physicists tend to doubt the possibility of time travel (with a few notable exceptions, which I cover throughout this chapter). If time travel were possible, then the predictive power of physics is lost, because the initial conditions are no longer trustworthy! The predictions based on those conditions would, therefore, be completely meaningless.