Physics

Stephen Hawking on Time Travel



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For decades, time travel has been one of the most common tropes of popular science fiction. Today, a growing number of physicists and cosmologists are beginning to suggest that it might eventually become a reality - although not necessarily the casual back-and-forth phenomenon we saw on television. One such physicist is Stephen Hawking, who believes our descendants will eventually be able to skip forward in time as many as a few million years at a time.

- Classic Time Travel: The Wormhole -

Physicists and science fiction writers agree on one thing: wormholes exist and they probably enable, at least in theory, transit between two points in time just as easily as transit between two points in space. The basic observation, enabled by quantum theory, is that if we look closely enough at anything in spacetime, its surface is broken and pockmarked, not smooth. We can produce this effect by looking at any seemingly smooth object under a powerful microscope. However, theoretical physics tells us that space and time are the same way. One of the consequences, at the level called quantum foam, is the wormhole: a tunnel between two different points.

However, Hawking thinks we will never actually be able to use these wormholes for time travel, or any other travel for that matter - which will come as a great disappointment to fans of series ranging from Star Trek to Stargate. The reason is that naturally occurring wormholes are far too tiny and short-lived. Even if we could somehow harness one and expand it so that it was large enough for a person to walk into, Hawking thinks the radiation feedback would collapse it instantly. So travel backward through time is theoretically possible at one level, but almost certainly impossible for human beings.

In the past, Hawking has also said that the fact that we are not being constantly visited by future time travellers is further evidence for his belief in this regard. This is a variant of the so-called Fermi Paradox: the observation that if the universe were filled with spacefaring alien civilizations, we ought to have seen some of them by now. However, like the paradox itself, it is a fairly weak point. There are any number of reasons why time travel is physically possible but we have not seen time travellers: they are sophisticated enough to hide their presence, they use it very rarely and under strict controls, they are aware of but uninterested in Earth, and so on. It is even possible, Hawking admits, that backwards time travel can occur but only in specially prepared, warped space, sort of a time travel equivalent of the preparation difficulties encountered by the theoretical Alcubierre warp drive.

- Relativity and Forward Time Travel -

In contrast, Hawking points out, forward time travel is actually comparatively easy. We already know precisely how to do it, and all physicists would agree on the basic precepts - although the technology is still lacking, of course.

Unlike back-and-forth time travel, which would have to rely on some sort of quantum trickery, forward time travel is made possible by relativity. We know that, as our velocity speeds up, our perception of the passage of time actually slows down. This effect has been verified by launching highly precise clocks at high speeds into orbit, and then comparing them with atomic clocks on Earth. In essence, as you approach the speed of light, time slows down. In theory, for an object travelling at the speed of light (which is believed to be impossible, anyways), time would stop.

Today, we can travel only at tiny fractions of the speed of light - and physics tells us that we will never be able to travel at the speed of light itself. However, it is conceivable that we could build a spacecraft capable of traveling at a very high percentage of the speed of light. For people (or instruments) onboard such a spacecraft, just a few seconds would pass while much longer periods of time occurred here on the surface of Earth. One could "return" to normal spacetime simply by slowing down or stopping the spacecraft at the desired point in time. How far into the future one could travel with this process would depend on how close to the speed of light one's spacecraft could go, and how long one waited before dropping back to "normal" speeds.

Of course, any such journey would have to be a one-way trip, because there would be no comparable way of going back in time. Still, Hawking thinks our descendants could use this forward time travel to "skip" forward in time, using the timeship as a sort of ark. They could leave an Earth where resources and life were being rapidly depleted, hop forward in time to see whether humanity had survived, and, if it had not, re-settle on the surface once the biosphere had recovered.

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