The keys to time travel are black holes,
objects so dense that not even light can escape their gravitational grip.
"A black hole ... has a dramatic effect
on time, slowing it down far more than anything else in the galaxy. That makes
it a natural time machine," Hawking writes.
Here's how it might work:
Imagine a spaceship orbiting the
super-massive black hole at the center of the Milky Way galaxy, 26,000 light
years away. From Earth, it would look like the ship is making one orbit every
16 minutes, Hawking writes.
"But for the brave people on board,
close to this massive object, time would be slowed down," Hawking writes.
"For every 16-minute orbit, they'd only experience eight minutes of
time."
If they circled for five years, local time,
10 years would have passed back on Earth.
This scenario doesn't produce the paradoxes
inherent in wormhole travel, but it's still pretty impractical, Hawking
acknowledges.
But there's one more possibility: traveling
super fast.
"This is due to another strange fact
about the universe," writes Hawking -- the cosmic speed limit: 186,000
miles per second, or the speed of light.
"Nothing can exceed that speed. It's
one of the best established principles in science," writes Hawking, but
"believe it or not, traveling at near the speed of light transports you to
the future."
"Imagine a track that goes right around
Earth, a track for a super-fast train. Onboard are passengers with a one-way
ticket to the future. The train begins to accelerate, faster and faster. Soon
it's circling the Earth over and over again.
"To approach the speed of light means
circling the Earth seven times a second. But no matter how much power the train
has, it can never quite reach the speed of light, since the laws of physics
forbid it.
"Instead, let's say it gets
close," writes Hawking. "Something extraordinary happens: Time starts
flowing slowly on board relative to the rest of the world, just like near the
black hole, only more so. Everything on the train is in slow motion."
Speed of Light Protection
This happens to protect the cosmic speed
limit, Hawking said. Here's why: Say there's a child running forward up the
train. "Her forward speed is added to the speed of the train, so couldn't
she break the speed limit simply by accident? The answer is no," writes Hawking.
"The laws of nature prevent the possibility by slowing down time onboard.
Now she can't run fast enough to break the limit. Time will always slow down
just enough to protect the speed limit." This is the essence of why time
travel into the future is possible. "Imagine that the train left the
station on January 1, 2050. It circles Earth over and over again for 100 years
before finally coming to a halt on New Year's Day, 2150. The passengers will
have only lived one week because time is slowed down that much inside the
train. When they got out they'd find a very different world from the one they'd
left. In one week they'd have travelled 100 years into the future,"
Hawking writes.
Right now, the fastest motion on Earth is
taking place in the circular tunnels of the world's largest particle
accelerator at CERN, in Geneva.
"When the power is turned on
(particles) accelerate from zero to 60,000 mph in a fraction of a second.
Increase the power and the particles go faster and faster, until they're
whizzing around the tunnel 11,000 times a second, which is almost the speed of
light. But just like the train, they never quite reach that ultimate speed.
They can only get to 99.99 per cent of the limit. When that happens, they too
start to travel in time. We know this because of some extremely short-lived
particles, called pimesons. Ordinarily, they disintegrate after just 25
billionths of a second. But when they are accelerated to near-light speed they
last 30 times longer." To accelerate humans to that speed, we'll need to
be in space, concludes Hawking, noting that so far, the fastest that people
have traveled is 25,000 mph aboard Apollo 10. "To travel in time we'll
have to go more than 2,000 times faster (than Apollo 10). And to do that we'd
need a much bigger ship, a truly enormous machinebig enough to carry a huge
amount of fuel, enough to accelerate it to nearly the speed of light. Getting
to just beneath the cosmic speed limit would require six whole years at full
power.
