Paradox: The Nine Greatest Enigmas in Physics by Al-Khalili Jim

Author:Al-Khalili, Jim [Al-Khalili, Jim]
Language: eng
Format: epub
Tags: Physics
ISBN: 9780307986801
Goodreads: 13642145
Publisher: Broadway
Published: 2012-10-23T00:00:00+00:00

6

THE PARADOX OF THE TWINS

By traveling very fast we can move into the future.

In this chapter we continue with the theme of paradoxes arising from the predictions of Einstein’s relativity theory. Here, however, we will be delving into the pleasingly mind-bending concepts associated with the nature of time itself and how it is affected by travel at close to the speed of light.

The story line of this paradox may sound like science fiction, but is in fact perfectly within the mainstream science taught to every physics student as an example of the implications of relativity, even if it is not technologically achievable just yet. It involves a spacecraft capable of reaching near light speed—which, while we have no means of developing such a craft at the moment, is nevertheless perfectly admissible in principle. And, because it doesn’t exceed the speed of light, there is no need to appeal to the rather more speculative ideas one finds in popular science fiction such as warp drives or short cuts through hyperspace.

Meet our heroes, the twins Alice and Bob, who design and build this spacecraft. While Bob remains behind on earth, Alice pilots the spacecraft, flying it from the Earth on a round-trip through the galaxy that takes her exactly one year to complete. On her return to Earth she is biologically one year older, she feels that one year has gone by, and all the clocks and timekeeping devices on board the spacecraft agree that a single year has elapsed since she left the Earth.

Bob, meanwhile, has been monitoring her journey throughout, and he witnesses one of the weird effects that manifest themselves as a result of her traveling at near light speed, and is predicted by Einstein’s theory of relativity: time on board the spacecraft appears to Bob to be running more slowly than it is on Earth. If he were to watch events inside the spacecraft through a camera he would see everything taking place in slow motion: clocks on board ticking more slowly, Alice moving and speaking more slowly, and so on. So a journey that seems to Alice on board the spacecraft to have taken just one year in fact lasts ten Earth years as far as Bob is concerned. Indeed, Alice arrives back on Earth to find that her twin has aged ten years, whereas she is biologically just one year older.

This in itself is not the source of the “paradox” of the chapter title. It may sound odd, but it is perfectly consistent with the predictions of Einstein’s theory. The times I have chosen are arbitrary and depend on just how fast the spacecraft has been traveling. Had Alice, for instance, nudged even closer to the speed of light, then quite straightforward mathematics for anyone with a pocket calculator would show that one year on the rocket could correspond to a million years on Earth or, equivalently, during a day’s travel by Alice’s reckoning thousands of years would go by on Earth. But let us stick

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