A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 10 : Wormholes and Time Travel | Summary



It is in this chapter that Stephen Hawking tackles the exciting possibilities at the core of science fiction—wormholes that can tunnel a spaceship from one point at vast distance to another, and the possibilities of time travel. However, Hawking also points out that in order to make time travel a possibility, the curvature of the universe would have to be bent inward (concave, like the shape of a saddle) rather than convex (or bulging outward like a balloon). This means that time-travel engineers would have to find some way to warp local regions of space-time to allow it to happen.

Another possibility is wormholes, first theorized by Albert Einstein and Nathan Rosen in 1935. If one could travel in a wormhole, it might send the traveler not only to a distance place in the galaxy but also permit travel back into the past. A more advanced system of technology would be needed to keep a wormhole open long enough to permit passage because, as soon as an object enters one, there's a good chance that the entry of an object will have the effect of collapsing it.

Hawking continues with discussions of various paradoxes posed by the possibility of time travel, finding a possible resolution in the alternative history hypothesis. This hypothesis suggests there is not just one universe but actually multiple ones. Thus, supposing one could travel back in time, any changes to history would affect a separate universe from the one the traveler came from.

Hawking explores the possibilities for time travel, using Richard Feynman's sum over histories approach and quantum theory. He discusses how quantum mechanics allows, theoretically, for time travel of microscopic particles. But large-scale time travel (usable by humans) appears to be foreclosed for now by what Hawking calls the chronology protection conjecture.


As previously stated in A Brief History of Time, observations on the expansion of the universe indicate a model more like a balloon being inflated, on the surface of which objects have positions and movements comparable to dots. As the balloon inflates, the distances between those dots increase equally across its surface (Chapter 3).

A survey of all the creative science fiction solutions to the problems of traveling great distances of space in shortened time, and travel back and forth between the past and the future (without losing track of the present the traveler would leave from and return to) would fill a vast library. Amazon lists more than 1.2 million books available in this popular genre. Also, that doesn't include all the gaming versions, movies, and so on, making the intrigues of both time travel and the ability to visit distant stars and their planets perennial topics to pique our imaginations.

Despite the appeal of wormholes and time travel as we see them in science fiction, Hawking acknowledges that our best current models indicate these kinds of movement for humans are not possible. In any case, he jests that he's not "going to bet on it" because the other bettor may know the future.

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