All you need is a wormhole,
the Large Hadron Collider or a relatively big rocket that goes really, really
fast.
Hello. My name is Stephen
Hawking. Physicist, cosmologist and something of a dreamer. Although I cannot
move and I have to speak through a computer, in my mind I am free.
Free to explore the universe
and ask the big questions, such as: is time travel possible? Can we open a
portal to the past or find a shortcut to the future? Can we ultimately use the
laws of nature to become masters of time itself?
Time travel was once considered
scientific heresy. I used to avoid talking about it for fear of being labelled
a crank. But these days I’m not so cautious. In fact, I’m more like the people
who built Stonehenge. I’m obsessed by time. If I had a time machine I’d visit
Marilyn Monroe in her prime or drop in on Galileo as he turned his telescope to
the heavens. Perhaps I’d even travel to the end of the universe to find out how
our whole cosmic story ends.
To see how this might be
possible, we need to look at time as physicists do – at the fourth dimension.
It’s not as hard as it sounds. Every attentive schoolchild knows that all
physical objects, even me in my chair, exist in three dimensions. Everything
has a width and a height and a length.
But there is another kind
of length, a length in time. While a human may survive for 80 years, the stones
at Stonehenge, for instance, have stood around for thousands of years.
And the solar system will
last for billions of years. Everything has a length in time as well as space.
Travelling in time means travelling through this fourth dimension.
To see what that means,
let’s imagine we’re doing a bit of normal, everyday car travel. Drive in a
straight line and you’re travelling in one dimension. Turn right or left and
you add the second dimension. Drive up or down a twisty mountain road and that
adds height, so that’s travelling in all three dimensions. But how on Earth do
we travel in time? How do we find a path through the fourth dimension?
Let’s indulge in a little
science fiction for a moment. Time travel movies often feature a vast,
energy-hungry machine. The machine creates a path through the fourth dimension,
a tunnel through time. A time traveller, a brave, perhaps foolhardy individual,
prepared for who knows what, steps into the time tunnel and emerges who knows
when. The concept may be far-fetched, and the reality may be very different
from this, but the idea itself is not so crazy.
Physicists have been
thinking about tunnels in time too, but we come at it from a different angle.
We wonder if portals to the past or the future could ever be possible within
the laws of nature.
As it turns out, we think
they are. What’s more, we’ve even given them a name: wormholes. The truth is
that wormholes are all around us, only they’re too small to see. Wormholes are
very tiny. They occur in nooks and crannies in space and time. You might find
it a tough concept, but stay with me.
Nothing is flat or solid.
If you look closely enough at anything you’ll find holes and wrinkles in it.
It’s a basic physical principle, and it even applies to time. Even something as
smooth as a pool ball has tiny crevices, wrinkles and voids. Now it’s easy to
show that this is true in the first three dimensions. But trust me, it’s also
true of the fourth dimension. There are tiny crevices, wrinkles and voids in
time.
Down at the smallest of
scales, smaller even than molecules, smaller than atoms, we get to a place
called the quantum foam. This is where wormholes exist. Tiny tunnels or
shortcuts through space and time constantly form, disappear, and reform within
this quantum world. And they actually link two separate places and two
different times.
Unfortunately, these
real-life time tunnels are just a billion-trillion-trillionths of a centimetre
across. Way too small for a human to pass through – but here’s where the notion
of wormhole time machines is leading. Some scientists think it may be possible
to capture a wormhole and enlarge it many trillions of times to make it big
enough for a human or even a spaceship to enter.
Given enough power and
advanced technology, perhaps a giant wormhole could even be constructed in
space. I’m not saying it can be done, but if it could be, it would be a truly
remarkable device. One end could be here near Earth, and the other far, far
away, near some distant planet.
Theoretically, a time
tunnel or wormhole could do even more than take us to other planets. If both
ends were in the same place, and separated by time instead of distance, a ship
could fly in and come out still near Earth, but in the distant past.
Maybe dinosaurs would
witness the ship coming in for a landing. The fastest manned vehicle
in history was Apollo 10. It reached 25,000mph. But to travel in time we’ll
have to go more than 2,000 times faster.
Now, I realise that thinking
in four dimensions is not easy, and that wormholes are a tricky concept to wrap
your head around, but hang in there. I’ve thought up a simple experiment that
could reveal if human time travel through a wormhole is possible now, or even
in the future. I like simple experiments, and champagne.
So I’ve combined two of my
favourite things to see if time travel from the future to the past is possible.
Let’s imagine I’m throwing a
party, a welcome reception for future time travellers. But there’s a twist. I’m
not letting anyone know about it until after the party has happened. I’ve drawn
up an invitation giving the exact coordinates in time and space. I am hoping
copies of it, in one form or another, will be around for many thousands of
years. Maybe one day someone living in the future will find the information on
the invitation and use a wormhole time machine to come back to my party,
proving that time travel will, one day, be possible.
In the meantime, my time
traveller guests should be arriving any moment now. Five, four, three, two,
one. But as I say this, no one has arrived. What a shame. I was hoping at least
a future Miss Universe was going to step through the door. So why didn’t the
experiment work? One of the reasons might be because of a well-known problem
with time travel to the past, the problem of what we call paradoxes.
Paradoxes are fun to think
about. The most famous one is usually called the Grandfather paradox. I have a
new, simpler version I call the Mad Scientist paradox.
I don’t like the way
scientists in movies are often described as mad, but in this case, it’s true.
This chap is determined to create a paradox, even if it costs him his life.
Imagine, somehow, he’s built a wormhole, a time tunnel that stretches just one
minute into the past.
 |
| Hawking in a scene from Star Trek with dinner guests from the past, and future: (from left) Albert Einstein, Data and Isaac Newton |
Through the wormhole, the
scientist can see himself as he was one minute ago. But what if our scientist
uses the wormhole to shoot his earlier self? He’s now dead. So who fired the
shot? It’s a paradox. It just doesn’t make sense. It’s the sort of situation
that gives cosmologists nightmares.
This kind of time machine
would violate a fundamental rule that governs the entire universe – that causes
happen before effects, and never the other way around. I believe things can’t
make themselves impossible. If they could then there’d be nothing to stop the
whole universe from descending into chaos. So I think something will always
happen that prevents the paradox. Somehow there must be a reason why our
scientist will never find himself in a situation where he could shoot himself.
And in this case, I’m sorry to say, the wormhole itself is the problem.
In the end, I think a
wormhole like this one can’t exist. And the reason for that is feedback. If
you’ve ever been to a rock gig, you’ll probably recognise this screeching
noise. It’s feedback. What causes it is simple. Sound enters the microphone.
It’s transmitted along the wires, made louder by the amplifier, and comes out
at the speakers.
But if too much of the sound
from the speakers goes back into the mic it goes around and around in a loop
getting louder each time. If no one stops it, feedback can destroy the sound
system.
The same thing will happen
with a wormhole, only with radiation instead of sound. As soon as the wormhole
expands, natural radiation will enter it, and end up in a loop. The feedback
will become so strong it destroys the wormhole. So although tiny wormholes do
exist, and it may be possible to inflate one some day, it won’t last long
enough to be of use as a time machine. That’s the real reason no one could come
back in time to my party.
Any kind of time travel to
the past through wormholes or any other method is probably impossible,
otherwise paradoxes would occur. So sadly, it looks like time travel to the
past is never going to happen. A disappointment for dinosaur hunters and a
relief for historians.
But the story’s not over
yet. This doesn’t make all time travel impossible. I do believe in time travel.
Time travel to the future. Time flows like a river and it seems as if each of
us is carried relentlessly along by time’s current.
But time is like a river in
another way. It flows at different speeds in different places and that is the
key to travelling into the future. This idea was first proposed by Albert
Einstein over 100 years ago. He realized that there should be places where time
slows down, and others where time speeds up. He was absolutely right. And the
proof is right above our heads. Up in space.
This is the Global
Positioning System, or GPS. A network of satellites is in orbit around Earth.
The satellites make satellite navigation possible. But they also reveal that
time runs faster in space than it does down on Earth. Inside each spacecraft is
a very precise clock. But despite being so accurate, they all gain around a
third of a billionth of a second every day. The system has to correct for the
drift, otherwise that tiny difference would upset the whole system, causing
every GPS device on Earth to go out by about six miles a day. You can just
imagine the mayhem that that would cause.
The problem doesn’t lie
with the clocks. They run fast because time itself runs faster in space than it
does down below.
And the reason for this
extraordinary effect is the mass of the Earth. Einstein realised that matter
drags on time and slows it down like the slow part of a river. The heavier the
object, the more it drags on time. And this startling reality is what opens the
door to the possibility of time travel to the future.
Right in the centre of the
Milky Way, 26,000 light years from us, lies the heaviest object in the galaxy.
It is a supermassive black hole containing the mass of four million suns
crushed down into a single point by its own gravity. The closer you get to the black
hole, the stronger the gravity. Get really close and not even light can escape.
A black hole like this one has a dramatic effect on time, slowing it down far
more than anything else in the galaxy. That makes it a natural time machine.
I like to imagine how a
spaceship might be able to take advantage of this phenomenon, by orbiting it.
If a space agency were controlling the mission from Earth they’d observe that
each full orbit took 16 minutes. But for the brave people on board, close to
this massive object, time would be slowed down. And here the effect would be
far more extreme than the gravitational pull of Earth. The crew’s time would be
slowed down by half. For every 16-minute orbit, they’d only experience eight
minutes of time.
Around and around they’d go,
experiencing just half the time of everyone far away from the black hole. The
ship and its crew would be travelling through time. Imagine they circled the
black hole for five of their years. Ten years would pass elsewhere. When they
got home, everyone on Earth would have aged five years more than they had.
So a supermassive black hole
is a time machine. But of course, it’s not exactly practical. It has advantages
over wormholes in that it doesn’t provoke paradoxes. Plus it won’t destroy
itself in a flash of feedback. But it’s pretty dangerous. It’s a long way away
and it doesn’t even take us very far into the future. Fortunately there is
another way to travel in time. And this represents our last and best hope of
building a real time machine.
You just have to travel
very, very fast. Much faster even than the speed required to avoid being sucked
into a black hole. This is due to another strange fact about the universe.
There’s a cosmic speed limit, 186,000 miles per second, also known as the speed
of light. Nothing can exceed that speed. It’s one of the best established
principles in science. Believe it or not, travelling at near the speed of light
transports you to the future.
To explain why, let’s dream
up a science-fiction transportation system. Imagine a track that goes right
around Earth, a track for a superfast train. We’re going to use this imaginary
train to get as close as possible to the speed of light and see how it becomes
a time machine. On board 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 pretty fast. 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, just shy
of that ultimate speed.
Now 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.
This happens to protect the
speed limit, and it’s not hard to see why. Imagine a child running forwards 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. 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. And from that fact comes the possibility of travelling many
years into the future.
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.
Of course, building a train
that could reach such a speed is quite impossible. But we have built something
very like the train at the world’s largest particle accelerator at CERN in
Geneva, Switzerland.
Deep underground, in a
circular tunnel 16 miles long, is a stream of trillions of tiny particles. When
the power is turned on they accelerate from zero to 60,000mph 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 pi-mesons. Ordinarily, they disintegrate after
just 25 billionths of a second. But when they are accelerated to near-light speed
they last 30 times longer.
It really is that simple.
If we want to travel into the future, we just need to go fast. Really fast. And
I think the only way we’re ever likely to do that is by going into space. And I think the only way we’re ever likely to do that is
by going into space.
The fastest manned vehicle
in history was Apollo 10. It reached 25,000mph. But to travel in time we’ll
have to go more than 2,000 times faster. And to do that we’d need a much bigger
ship, a truly enormous machine. The ship would have to be big 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.
The initial acceleration
would be gentle because the ship would be so big and heavy. But gradually it
would pick up speed and soon would be covering massive distances. In one week
it would have reached the outer planets. After two years it would reach
half-light speed and be far outside our solar system. Two years later it would
be travelling at 90 per cent of the speed of light. Around 30 trillion miles
away from Earth, and four years after launch, the ship would begin to travel in
time. For every hour of time on the ship, two would pass on Earth. A similar
situation to the spaceship that orbited the massive black hole.
After another two years of
full thrust the ship would reach its top speed, 99 per cent of the speed of
light.
At this speed, a single day
on board is a whole year of Earth time. Our ship would be truly flying into the
future.
The slowing of time has
another benefit. It means we could, in theory, travel extraordinary distances
within one lifetime. A trip to the edge of the galaxy would take just 80 years.
But the real wonder of our journey is that it reveals just how strange the
universe is. It’s a universe where time runs at different rates in different
places. Where tiny wormholes exist all around us. And where, ultimately, we
might use our understanding of physics to become true voyagers through the
fourth dimension.