Page 78 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 10
travel. According to relativity, nothing can travel faster than light. If we therefore sent a spaceship to our nearest
neighboring star, Alpha Centauri, which is about four light-years away, it would take at least eight years before
we could expect the travelers to return and tell us what they had found. If the expedition were to the center of
our galaxy, it would be at least a hundred thousand years before it came back. The theory of relativity does
allow one consolation. This is the so-called twins paradox mentioned in Chapter 2.

Because there is no unique standard of time, but rather observers each have their own time as measured by
clocks that they carry with them, it is possible for the journey to seem to be much shorter for the space travelers
than for those who remain on earth. But there would not be much joy in returning from a space voyage a few
years older to find that everyone you had left behind was dead and gone thousands of years ago. So in order to
have any human interest in their stories, science fiction writers had to suppose that we would one day discover
how to travel faster than light. What most of thee authors don’t seem to have realized is that if you can travel
faster than light, the theory of relativity implies you can also travel back in the, as the following limerick says:
There was a young lady of Wight
Who traveled much faster than light.
She departed one day,
In a relative way,
And arrived on the previous night

The point is that the theory of relativity says hat there is no unique measure of time that all observers will agree
on Rather, each observer has his or her own measure of time. If it is possible for a rocket traveling below the
speed of light to get from event A (say, the final of the 100-meter race of the Olympic Games in 202) to event B
(say, the opening of the 100,004th meeting of the Congress of Alpha Centauri), then all observers will agree
that event A happened before event B according to their times. Suppose, however, that the spaceship would
have to travel faster than light to carry the news of the race to the Congress. Then observers moving at
different speeds can disagree about whether event A occurred before B or vice versa. According to the time of
an observer who is at rest with respect to the earth, it may be that the Congress opened after the race. Thus
this observer would think that a spaceship could get from A to B in time if only it could ignore the speed-of-light
speed limit. However, to an observer at Alpha Centauri moving away from the earth at nearly the speed of light,
it would appear that event B, the opening of the Congress, would occur before event A, the 100-meter race.
The theory of relativity says that the laws of physics appear the same to observers moving at different speeds.

This has been well tested by experiment and is likely to remain a feature even if we find a more advanced
theory to replace relativity Thus the moving observer would say that if faster-than-light travel is possible, it
should be possible to get from event B, the opening of the Congress, to event A, the 100-meter race. If one
went slightly faster, one could even get back before the race and place a bet on it in the sure knowledge that
one would win.

There is a problem with breaking the speed-of-light barrier. The theory of relativity says that the rocket power
needed to accelerate a spaceship gets greater and greater the nearer it gets to the speed of light. We have
experimental evidence for this, not with spaceships but with elementary particles in particle accelerators like
those at Fermilab or CERN (European Centre for Nuclear Research). We can accelerate particles to 99.99
percent of the speed of light, but however much power we feed in, we can’t get them beyond the speed-of-light
barrier. Similarly with spaceships: no matter how much rocket power they have, they can’t accelerate beyond
the speed of light.
That might seem to rule out both rapid space travel and travel back in time. However, there is a possible way
out. It might be that one could warp space-time so that there was a shortcut between A and B One way of doing
this would be to create a wormhole between A and B. As its name suggests, a wormhole is a thin tube of
space-time which can connect two nearly flat regions far apart.

There need be no relation between the distance through the wormhole and the separation of its ends in the
nearly Hat background. Thus one could imagine that one could create or find a wormhole that world lead from
the vicinity of the Solar System to Alpha Centauri. The distance through the wormhole might be only a few
million miles even though earth and Alpha Centauri are twenty million million miles apart in ordinary space. This
would allow news of the 100-meter race to reach the opening of the Congress. But then an observer moving
toward 6e earth should also be able to find another wormhole that would enable him to get from the opening of

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