New Scientist, 10 December 2011
At 10pm on Saturday May 7 2005, some 400 people waited at the Massachusetts Institute of Technology for some very special guests to arrive. Time travellers from any and every point in the future had been invited to join the party in Cambridge. “The idea was a simple one that might at first seem trivial but was, in fact rather clever,” Brian Clegg explains in How to Build a Time Machine. “If time travel is possible, why not flag up a certain place and time in history and invite time travellers to attend?”
The organisers had tried to ensure that the relevant information seeped into the future, and hoped a combination of the internet, print media and TV coverage would do the job.
How could any curious, party-loving time traveller resist?
The no-show raises a simple question about the possibility
of travelling back in time: We may not yet have the technology to
move freely through the ages, but if time machines are going to be built at some
point in the future, why hasn't anyone come back to visit us?
According to the theory of special relativity, as one moves faster and faster
and approaches the speed of light, time slows down. At the speed of light, time
stands still. If one could go faster than light, then in principle it is
possible to travel back in time. So, Clegg asks, “Where are the time
travellers?”
Accelerating
beyond light speed to go back to the future requires an infinite of energy, so
is practically ruled out (though a huge question mark hangs over faster-than-light neutrinos).
However, general relativity does permit the construction of a time machine if space-time is twisted to create a loop, allowing a traveler heading into the future to circle back to an event in his or her own past. This
is possible in curved space-time because it’s like a rollercoaster with a loop-the-loop: the cars always go forward but the track circles back to a previous point.
If a time machine is constructed in the year 2100, for example, it means the loop in space-time starts then: the time machine can be used to go back to 2100 but not to a time before. This feature of time machines has been suggested by physicists J. Richard Gott and Kip Thorne - the former using cosmic strings and the latter, wormholes. Time travel is possible machine when strings cross or wormhole mouths are moved.
If a time machine is constructed in the year 2100, for example, it means the loop in space-time starts then: the time machine can be used to go back to 2100 but not to a time before. This feature of time machines has been suggested by physicists J. Richard Gott and Kip Thorne - the former using cosmic strings and the latter, wormholes. Time travel is possible machine when strings cross or wormhole mouths are moved.
Despite its impracticality, Clegg believes
it’s never too soon to consider the potential social and ethical impact of a
functioning time machine. He devotes a chapter to the classic “grandfather
paradox” - travelling back in time to kill your grandfather before he
ever meets your grandmother, rubbing yourself out of existence. It was to
handle such conundrums that Stephen Hawking suggested the chronology protection conjecture - the laws of physics conspire to prevent time travel to
the past on a macroscopic scale.
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| H G Wells |
