Colonising the future

British physicist Stephen Hawking invites us to imagine the possibility of time travel, writes Jonathan Leake

Humans may one day be able to use time travel to skip generations into the future, says Stephen Hawking. He has suggested humans could build spaceships capable of such high speeds that time itself would slow down for those on board. Such a spaceship could travel thousands of years into the future at close to the speed of light, reaching distant star systems within the lifetime of its crew.

In theory it could allow humans to "colonise the future" - perhaps even returning to repopulate Earth if a disaster caused extinction on this planet during the flight.

"Time travel was once considered scientific heresy and I used to avoid talking about it for fear of being labelled a crank. These days I'm not so cautious," Hawking said.

He makes his comments in Stephen Hawking's Universe, a documentary to be screened by the Discovery channel. A suggestion in another part of the series - that alien life is highly likely to exist but humans should try to avoid it - has already generated global interest.

Hawking's views on time travel could be just as provocative. He suggests humanity could build a giant "relativistic" spaceship, so called because it would exploit the science set out by Albert Einstein in his theories of relativity.

Einstein found that as objects accelerate through space, the rate at which time passes for them slows down. For objects such as cars and aircraft the effect is negligible, but Hawking's spaceship would exceed 98% of the speed of light, when such effects would be extremely powerful.

Hawking said such a ship could theoretically reach speeds of more than 650 million miles per hour (1046073600km/h), but would have to be built on a huge scale simply to carry all the fuel needed.

"It would take six years at full power just to reach these speeds. After the first two years it would reach half light speed and be far outside the solar system. After another two years it would be travelling at 90% of the speed of light," he said. "After another two years of full thrust, the ship would reach full speed, 98% of the speed of light, and each day on the ship would be a year on Earth. At such speeds a trip to the edge of the galaxy would take just 80 years for those on board."

Hawking dismisses the prospect of time travel into the past. Some scientists have suggested this could be done by exploiting wormholes, gateways linking different parts of the universe or which provide a short-cut backwards or forwards through time.

Theory suggests such wormholes do exist at the quantum scale, meaning they are far smaller even than atoms, so the challenge would be to enlarge them to a human scale.

Hawking dismisses this idea, pointing out that time travel into the past would create the "mad scientist paradox", where a researcher could travel back in time and shoot his past self, raising the question of who could have fired the shot.

"This kind of time machine would violate a fundamental rule that cause comes before effect," Hawking said. "I believe things cannot make themselves impossible. So it won't be possible to travel back to the past using wormholes or any other method."

Other physicists back Hawking's theories but also acknowledge the technical challenges. Among them is Brian Cox, professor of particle physics at Manchester University and presenter of the recent BBC television series Wonders of the Solar System.

"We can already see how time slows down for objects travelling at high speed by looking at what happens in particle accelerators," said Cox. "If we could build a spaceship that was fast enough, it could reach other stars in the lifetime of the crew - but maybe 2.5 million years would have passed by on Earth."