Cosmos (1980): Season 1, Episode 8 - Journeys in Space and Time - full transcript
Carl Sagan explains scientific concepts like relativity and examines the possibility of time travel.
SAGAN: We are drifting in
a great ocean of space and time.
In that ocean, the events
that shape the future...
...are working themselves out.
Each creature and every world,
to the remotest star...
...owe their existence to...
...the great, coursing,
implacable forces of nature...
...but also, to minor happenstance.
We are carried with our planet
around the sun.
The Earth has made more than
4 billion circuits of our star...
...since its origin.
The sun itself travels about
the core of the Milky Way galaxy.
Our galaxy is moving
among the other galaxies.
We have always been space travelers.
These fine sand grains are all,
more or less, uniform in size.
They're produced from bigger rocks
through ages of...
...jostling and rubbing,
abrasion and erosion.
Driven in part by
the distant moon and sun.
So the roots of the present
lie buried in the past.
We are also travelers in time.
But trapped on Earth...
...we've had little to say about
where we go in time and space...
...or how fast.
But now we're thinking
about true journeys in time...
...and real voyages
to the distant stars.
A handful of sand contains
about 10,000 grains...
...more than all the stars
we can see...
...with the naked eye
on a clear night.
But the number of stars we can see...
...is only the tiniest fraction
of the number of stars that are.
What we see at night
is the merest smattering...
...of the nearest stars...
...with a few more distant bright
stars thrown in for good measure.
Meanwhile, the cosmos
is rich beyond measure.
The number of stars
in the universe...
...is larger than all the grains
of sand on all the beaches...
...of the planet Earth.
Long ago, before we had figured out
that the stars are distant suns...
...they seemed to us
to make pictures in the sky.
Just follow the dots.
The Big Dipper constellation
today in North America...
...has had many other incarnations.
Every culture, ancient and modern...
...has placed its totems
and concerns among the stars.
From a Chinese bureaucrat
to a German wagon.
But very ancient cultures would have
seen different constellations...
...because the stars move
with respect to one another.
We can give a computer the present
positions and motions of stars...
...and then run the patterns
back into time.
Every constellation is a single frame
in a cosmic movie...
...but because our lives
are so short...
...because star patterns
change slowly...
...we tend not to notice
it's a movie.
A million years ago,
there was no Big Dipper.
Our ancestors, looking up
and wondering about the stars...
...saw some other pattern
in the northern skies.
We can also run a constellation,
Leo the Lion, say, forward in time...
...and see what the patterns
in the stars will be in the future.
A million years from now,
Leo might be renamed...
...the constellation
of the Radio Telescope.
Although I suspect radio telescopes
then will be as obsolete...
...as stone spears are now.
Or, here's the constellation
of Cetus the Whale.
A million years ago, it may have
been called something else.
Perhaps the Spear.
Now, let's run fast-forward
through a billion nights.
Millions of years from now...
...some other very different image
will be featured in this cosmic movie.
In Orion the Hunter,
things are changing...
...not only because
the stars are moving...
...but also because
the stars are evolving.
Many of Orion's stars are
hot, young and short-lived.
They're born, live and die within
a span of only a few million years.
If we run Orion forward in time...
...we see the births
and explosive deaths...
...of dozens of stars...
...flashing on and winking off
like fireflies in the night.
If we wait long enough,
we see the constellations change.
But if we go far enough,
we also see the star patterns alter.
Two-dimensional constellations...
...are only the appearance of stars
strewn through three dimensions.
Some are dim and near,
others are bright but farther away.
Could a space traveler
actually see...
...the patterns of
the constellations change?
For that, you must travel roughly as
far as the constellation is from us.
Here, we're traveling
hundreds of light-years...
...circling all the way around
the stars of the Big Dipper.
Inhabitants of planets
around other stars...
...will see different constellations
than us...
...because their vantage points
are different.
Here we are
in the constellation Andromeda...
...or at least a model of it
next to the constellation Perseus.
Andromeda, in the Greek myth...
...was the maiden
who was saved by Perseus...
...from a sea monster.
This star just above me
is Beta Andromedae...
...the second brightest star
in the constellation...
...75 light-years from the Earth.
The light by which we see this star...
...has spent 75 years
traversing interstellar space...
...on its journey to the Earth.
In the unlikely event
that Beta Andromedae...
...blew itself up
a week ago Tuesday...
...we will not know of it
for another 75 years...
...as this interesting information,
traveling at the speed of light...
...crosses the enormous
interstellar distances.
When the light we see
from this star set out...
...on its long
interstellar voyage...
...the young Albert Einstein...
...working as a Swiss patent clerk...
...had just published his epochal
special theory of relativity...
...here on Earth.
We see...
...that space and time
are intertwined.
We cannot look out into space...
...without looking back into time.
The speed of light is very fast...
...but space is very empty...
...and the stars are very far apart.
The distances that we've been
talking about up to now...
...are very small
by the usual astronomical standards.
In fact, the distance
from the Earth...
...to the center
of the Milky Way galaxy...
...is 30,000 light-years.
From our galaxy to the nearest
spiral galaxy like our own...
...called M31 ...
...and which is also within,
that means behind...
...the constellation Andromeda...
...is 2 million light-years.
When the light we see today
from M31 ...
...left on its journey for Earth...
...there were no human beings...
...although our ancestors
were nicely evolving...
...and very rapidly,
to our present form.
There are much greater distances
in astronomy.
The distance from the Earth
to the most distant quasars...
...is 8 or 10 billion light-years.
We see them as they were before
the Earth itself accumulated...
...before the Milky Way galaxy
was formed.
The fastest space vehicles ever
launched by the human species...
...are the Voyager spacecraft.
They are traveling so fast...
...that it's only
10,000 times slower...
...than the speed of light.
The Voyager spacecraft
will take 40,000 years...
...to go the distance
to the nearest stars...
...and they're not even headed
towards the nearest stars.
But is there a method
by which we could travel...
...in a conveniently short time
to the stars?
Can we travel close
to the speed of light?
And what's magic
about the speed of light?
Can't we travel faster than that?
It turns out that
there is something very strange...
...about the speed of light.
Something that provides the key...
...to our understanding
of time and space.
The story of its discovery...
...takes us to Tuscany
in northern Italy.
There's something timeless
about this place.
A century ago, it probably
looked very much the same.
If you had traveled these roads
in the summer of 1895...
...you might have come upon a
16-year-old German high-school dropout.
His teacher told him that
he'd never amount to anything...
...that his attitude destroyed
classroom discipline...
...that he should drop out.
So he left and came here...
...where he enjoyed
wandering these roads...
...and giving his mind
free rein to explore.
One day, he began
to think about light...
...about how fast it travels.
We always measure
the speed of a moving object...
...relative to something else.
I'm moving at about 10 kilometers
an hour relative to the ground.
But the ground isn't at rest.
The Earth is turning at more
than 1600 kilometers an hour.
The Earth itself is
in orbit around the sun.
The sun is moving among
the drifting stars, and so on.
It was hard for the young man
to imagine some absolute standard...
...to measure all these
relative motions against.
He knew that sound waves are
a vibration of the air...
...and their speed is measured
relative to the air itself.
But sunlight travels across
the vacuum of empty space.
"Do light waves move
relative to something else?
And if so," he wondered,
"relative to what?"
That teenage dropout's name...
...was Albert Einstein.
And his ruminations changed the world.
He had been fascinated
by Bernstein's 1869...
...People's Book of Natural Science.
Here, on its very first page...
...it describes the astonishing speed
of electricity through wires...
...and light through space.
Einstein wondered, perhaps for
the first time, in northern Italy...
...what the world would look like if
you could travel on a wave of light.
To travel at the speed of light.
What an engaging and magical thought
for a teenage boy on the road...
...where the countryside is dappled
and rippling in sunlight.
You couldn't tell you were on a light
wave if you were traveling with it.
If you started on a wave crest...
...you would stay on the crest and
lose all notion of it being a wave.
Something funny happens
at the speed of light.
The more Einstein thought about it,
the more troubling it became.
Paradoxes seemed to pop up all over...
...if you could travel
at the speed of light.
Certain ideas had been
accepted as true...
...without sufficiently
careful thought.
One of those ideas had to do
with the light from a moving object.
The images by which we see the world
are made of light...
...and are carried
at the speed of light...
...300,000 kilometers a second.
You might think that the image of me
should be moving out ahead of me...
...at the speed of light
plus the speed of the bicycle.
If I'm moving towards you
faster than a horse-and-cart...
...then my image should be
approaching you that much faster.
My image ought to arrive earlier.
But in reality
you don't see any time delay.
In a near collision, for example,
you see everything happen at once.
Horse, cart, swerve, bicycle.
All simultaneous.
But how would it look if
it were proper to add the velocities?
Since I'm heading toward you, you'd
add my speed to the speed of light.
So my image ought to arrive before
the image of the horse-and-cart.
I'd be cycling towards you
quite normally.
To me, a collision
would seem imminent.
But you'd see me swerve
for no apparent reason...
...and have a collision with nothing.
Now, the horse-and-cart
aren't headed towards you.
Their image would arrive only
at the speed of light.
Could it seem to me that
I just missed colliding...
...while to you it wasn't even close?
In precise laboratory experiments...
...scientists have never observed
any such thing.
If the world is to be understood...
...if we are to avoid logical paradoxes
when traveling at high speeds...
...then there are rules
which must be obeyed.
Einstein called these rules
the special theory of relativity.
Light from a moving object
travels at the same speed...
...no matter whether the object
is at rest or in motion.
"Thou shalt not add my speed
to the speed of light."
Also, no material object can travel
at or beyond the speed of light.
Nothing in physics prevents you from
traveling close to the speed of light.
99.9 percent the speed of light
is just fine.
But no matter how hard you try...
...you can never gain
that last decimal point.
For the world
to be logically consistent...
...there must be a cosmic speed limit.
The crack of a whip is,
due to its tip...
...moving faster
than the speed of sound.
It makes a shock wave...
...a small sonic boom
in the Italian countryside.
A thunderclap has a similar origin.
So does the sound of
a supersonic airplane.
So why is the speed of light a barrier
any more than the speed of sound?
The answer is not just that...
...light travels a million times
faster than sound.
It's not merely an engineering problem
like the supersonic airplane.
Instead, the light barrier is
a fundamental law of nature...
...as basic as gravity.
Einstein found his absolute framework
for the world:
This sturdy pillar among all
the relative motions of the cosmos.
Light travels just as fast,
no matter how its source is moving.
The speed of light is constant,
relative to everything else.
Nothing can ever catch up with light.
Einstein's prohibition against
traveling faster than light...
...seems to clash with
our common sense notions.
But why should we expect
our common sense notions...
...to have any reliability
in a matter of this sort?
Why should our experience
at 10 kilometers an hour...
...constrain the laws of nature...
...at 300,000 kilometers a second?
Relativity sets limits...
...on what humans ultimately can do.
The universe is not required...
...to be in perfect harmony
with human ambition.
Imagine a place
where the speed of light...
...isn't its true value
of 300,000 kilometers a second...
...but something a lot less.
Let's say, 40 kilometers an hour...
...and strictly enforced.
Just as in the real world we can
never reach the speed of light...
...the commandment here is still...
..."Thou shalt not travel
faster than light."
We can do thought experiments on
what happens near the speed of light...
...here 40 kilometers per hour,
the speed of a motor scooter.
You can't break the laws of nature.
There are no penalties for doing so.
The real world and this one...
...are merely so arranged
that transgressions can't happen.
The job of physics is to find out
what those laws are.
Before Einstein,
physicists thought that...
...there were privileged frames
of reference...
...some special places and times...
...against which everything else
had to be measured.
Einstein encountered
a similar notion in human affairs.
The idea that the customs
of a particular nation...
...his native Germany
or Italy or anywhere...
...are the standard which all
other societies must be measured.
But Einstein rejected the strident
nationalism of his time.
He believed every culture
had its own validity.
Also in physics,
he understood that...
...there are no privileged
frames of reference.
Every observer,
in any place, time or motion...
...must deduce
the same laws of nature.
(SPEAKING IN ITALIAN)
A speed is simply how much space
you cover in a given time...
...as any kid on
a motor scooter knows.
Since near the velocity of light...
...we cannot simply add speeds...
...the familiar notions of
absolute space and absolute time...
...independent of your
relative motion, must give way.
That's why, as Einstein showed...
...funny things have to happen
close to the speed of light.
There, our conventional perspectives
of space and time...
...strangely change.
Your nose is just a little closer
to me than your ears.
Light reflected off your nose
reaches me...
...an instant in time
before your ears.
But suppose I had a magic camera...
...so that I could see
your nose and your ears...
...at precisely the same instant?
(SCOOTER STARTS UP)
(SCOOTER HONKS)
With such a camera you could take
some pretty interesting pictures.
Paolo says goodbye to
his little brother, Vincenzo...
-Ciao, Vincenzo.
-Ciao, Paolo.
...and rides off.
He's now going more than
half the speed of light.
He is almost catching up
with his own light waves.
This compresses the light waves
in front of him...
...and his image becomes blue.
The shorter wavelength is
what makes blue light waves blue.
Also Paolo becomes skinny
in the direction of motion.
This isn't just some optical illusion.
It really happens when you travel
near the speed of light.
As he roars away, he leaves his own
light waves stretched out behind him.
Long light waves are red.
We say that his receding image
is red-shifted.
Now Paolo leaves for
a short tour of the countryside.
He experiences something
even stranger.
Everything he can see is squeezed...
...into a moving window
just ahead of him...
...blue-shifted at the center,
red-shifted at the edges.
To a passerby, Paolo appears
blue-shifted when approaching...
...red-shifted when receding.
But to him, the entire world
is both coming and going...
...at nearly the speed of light.
Roadside houses and trees
that has already gone past...
...still appear to him at the edge
of his forward field of view...
...but distorted and red-shifted.
When he slows down,
everything again looks normal.
Only very close
to the speed of light...
...does the visible world
get squeezed into a kind of tunnel.
You'd see these distortions if you
traveled near the speed of light.
Someday, perhaps,
interstellar navigators...
...will take their bearings
on stars behind them...
...whose images have all crowded
together on the forward view screen.
The most bizarre aspect of traveling
near the speed of light...
...is that time slows down.
All clocks,
mechanical and biological...
...tick more slowly
near the speed of light.
But stationary clocks tick
at their usual rate.
If we travel close to light speed...
...we age more slowly
than those we left behind.
Paolo's watch and his internal
sense of time show...
...that he has been gone from
his friends for only a few minutes.
But from their point of view,
he has been away for many decades.
His friends have grown up,
moved on and died.
And his younger brother has been...
...patiently waiting
for him all this time.
The two brothers experience
the paradox of time dilation.
They've encountered
Einstein's special relativity.
Vincenzo.
This was just a thought experiment.
But atomic particles traveling
near the speed of light...
...do decay more slowly
than stationary particles.
As strange and counterintuitive
as it seems...
...time dilation is a law of nature.
Traveling close
to the speed of light...
...is a kind of elixir of life.
Because time slows down
close to the speed of light...
...special relativity provides us...
...with a means of going to the stars.
This region of northern Italy
is not only the caldron...
...of some of the thinking
of the young Albert Einstein...
...it is also the home
of another great genius...
...who lived 400 years earlier.
Leonardo da Vinci.
Leonardo delighted
in climbing these hills...
...and viewing the ground
from a great height...
...as if he were soaring like a bird.
He drew the first aerial views...
...of landscapes, villages,
fortifications.
I've been talking about Einstein
in and around this town of Vinci...
...in which Leonardo grew up.
Einstein greatly respected Leonardo...
...and their spirits, in some sense...
...inhabit this countryside still.
Among Leonardo's
many accomplishments...
...in painting, sculpture,
architecture, natural history...
...anatomy, geology,
civil and military engineering...
...he had a great passion.
He wished to construct a machine...
...which would fly.
He made sketches of such machines,
built miniature models...
...constructed great,
full-scale prototypes.
And not a one of them ever worked.
There were no machines of adequate
capacity available in his time.
The technology was just not ready.
The designs, however, were brilliant.
For example, this bird-like machine...
...here in the Leonardo Museum
in the town of Vinci.
Leonardo's great designs encouraged
engineers in later epochs...
...although Leonardo himself
was very depressed at these failures.
But it's not his fault...
...he was trapped in the 15th century.
A somewhat similar case
occurred in 1939...
...when a group of engineers called
the British Interplanetary Society...
...decided to design a ship...
...which would carry people
to the moon.
Now, it was by no means
the same design...
...as the Apollo ship which actually
took people to the moon years later.
But that design suggested that...
...a mission to the moon
might one day...
...be a practical
engineering possibility.
Today...
...we have preliminary
designs of ships...
...which will take people
to the stars.
They are constructed in Earth orbit
and from there...
...they venture on their great
interstellar journeys.
One of them...
...is called Project Orion.
It utilizes nuclear weapons...
...hydrogen bombs
against an inertial plate.
Each explosion providing
a kind of "putt-putt"...
...a vast nuclear motorboat in space.
Orion seems entirely practical...
...and was under development
in the U.S...
...until the signing
of the international treaty...
...forbidding nuclear weapons
explosions in space.
I think, the Orion starship
is the best use of nuclear weapons...
...provided the ships don't depart
from very near the Earth.
Project Daedalus is
a recent design...
...of the British
Interplanetary Society.
It assumes the existence
of a nuclear fusion reactor...
...something much safer
and more efficient...
...than the existing nuclear
fission power plants.
We do not yet have fusion reactors.
One day, quite soon, we may.
Orion and Daedalus might go...
...10 percent the speed of light.
So a trip to Alpha Centauri,
4 1/2 light-years away...
...would take 45 years,
less than a human lifetime.
Such ships could not travel
close enough to the speed of light...
...for the time-slowing effects
of special relativity...
...to become important.
It does not seem likely
that such ships...
...would be built before
the middle of the 21 st century...
...although we could build
an Orion starship now.
For voyages beyond the nearest stars,
something must be added.
Perhaps they could be used
as multigeneration ships...
...so those arriving would be
the remote descendants...
...of those who had originally
set out centuries before.
Or perhaps some safe means
of human hibernation might be found...
...so that space travelers might be
frozen and then thawed out...
...when they arrive at
the destination centuries later.
But fast interstellar space flight
approaching the speed of light...
...is much more difficult.
That's an objective
not for a hundred years...
...but for a thousand
or for 10 thousand...
...but it also is possible.
A kind of interstellar ramjet
has been proposed...
...which scoops up
the hydrogen atoms...
...which float between the stars...
...accelerates them into an engine
and spits them out the back.
But in deep space,
there is one atom...
...for every 10 cubic centimeters
of space.
For the ramjet to work...
...it has to have a frontal scoop...
...hundreds of kilometers across.
Reaching relativistic velocities,
the hydrogen atoms will be moving...
...with respect
to the interstellar spaceship...
...at close to the speed of light.
If precautions aren't taken...
...the passengers will be fried
by these induced cosmic rays.
There's a proposed solution:
A laser is used to strip
electrons off the atoms...
...and electrically charge them
while they're some distance away.
And an extremely strong
magnetic field...
...is used to deflect
the charged atoms into the scoop...
...and away from the spacecraft.
This is engineering...
...on a scale so far
unprecedented on the Earth.
We are talking of engines
the size of small worlds.
Suppose that the spacecraft is
designed to accelerate at 1 g...
...so we'd be comfortable aboard it.
We'd go closer and closer
to the speed of light...
...until the midpoint of the journey.
Then the spacecraft is
turned around...
...and we decelerate at 1 g
to the destination.
For most of the trip, the velocity
would be close to the speed of light...
...and time would
slow down enormously.
By how much?
Barnard's Star could be reached
by such a ship...
...in eight years, ship time.
The center of the Milky Way galaxy
in 21 years.
The Andromeda galaxy in 28 years.
Of course, the people
left behind on the Earth...
...would see things
somewhat differently.
Instead of 21 years to the galaxy...
...they would measure it
as 30,000 years.
When we got back...
...very few of our friends
would be around to greet us.
In principle, such a journey...
...mounting the decimal points closer
and closer to the speed of light...
...would even permit us to
circumnavigate the known universe...
...in 56 years, ship time.
We would return tens
of billions of years...
...in the far future...
...with the Earth a charred cinder...
...and the sun dead.
Relativistic space flight makes
the universe accessible...
...to advanced civilizations...
...but only to those
who go on the journey...
...not to those who stay home.
These designs are probably further...
...from the actual interstellar
spacecraft of the future...
...than Leonardo's models are...
...from the supersonic transports
of the present.
But if we do not destroy ourselves...
...I believe that we will,
one day, venture to the stars.
When our solar system
is all explored...
...the planets of other stars
will beckon.
Space travel and time travel
are connected.
To travel fast into space...
...is to travel fast into the future.
We travel into the future,
although slowly, all the time.
But what about the past?
Could we journey into yesterday?
Many physicists think this is
fundamentally impossible...
...that we could
not build a device...
...which would carry us
backwards into time.
Some say that even if we were
to build such a device...
...it wouldn't do much good.
We couldn't significantly
affect the past.
For example, suppose you
traveled into the past...
...and somehow or other prevented...
...your own parents from meeting.
Why, then you would probably
never have been born...
...which is something
of a contradiction, isn't it...
...since you are clearly there.
Other people think that...
...the two alternative histories
have equal validity...
...that they're parallel threads,
skeins of time...
...that they could exist side by side.
The history in which
you were never born...
...and the history that
you know all about.
Perhaps time itself has
many potential dimensions...
...despite the fact that
we are condemned to experience...
...only one of those dimensions.
Now, suppose you could go back
into the past...
...and really change it by,
let's say something like...
...persuading Queen Isabella not
to bankroll Christopher Columbus.
Then you would have set into motion...
...a different sequence
of historical events...
...which those people
you left behind you in our time...
...would never get to know about.
If that kind of time travel
were possible...
...then every imaginable sequence...
...of alternative history...
...might in some sense really exist.
Would it be possible
for a time traveler...
...to change the course of history
in a major way?
Well, let's think about that.
History consists for the most part...
...of a complex multitude
of deeply interwoven threads...
...biological, economic
and social forces...
...that are not so easily unraveled.
The ancient Greeks imagined the course
of human events to be a tapestry...
...created by three goddesses:
the Fates.
Random minor events generally
have no long-range consequences.
But some which occur
at critical junctures...
...may alter the weave of history.
There may be cases where
profound changes can be made...
...by relatively trivial adjustments.
The further in the past such an event
is, the more powerful its influence.
What if our time traveler had
persuaded Queen Isabella that...
...Columbus' geography was wrong?
Almost certainly, some other European
would have sailed to the New World.
There were many inducements:
The lure of the spice trade,
improvements in navigation...
...competition among
rival European powers.
The discovery of America
around 1500 was inevitable.
Of course, there wouldn't be any
postage stamps showing Columbus...
...and the Republic of Colombia
would have another name.
But the big picture would have
turned out more or less the same.
In order to affect
the future profoundly...
...a time traveler
has to pick and choose.
He'd probably have to intervene
in a number of events...
...which are
very carefully selected...
...so he could change
the weave of history.
It's a lovely fantasy...
...to explore those other worlds
that never were.
If you had H.G. Wells' time machine...
...maybe you could understand
how history really works.
If an apparently pivotal person
had never lived...
...Paul the Apostle or Peter the Great
or Pythagoras...
...how different would
the world really be?
What if the scientific tradition...
...of the ancient Ionian Greeks...
...had prospered and flourished?
It would have required
many social factors at the time...
...to have been different...
...including the common feeling...
...that slavery was right and natural.
But what if that light
that had dawned...
...on the eastern Mediterranean
some 2500 years ago...
...had not flickered out?
What if scientific method
and experiment...
...had been vigorously pursued...
...2000 years before
the industrial revolution...
...our industrial revolution?
What if the power of this new mode
of thought, the scientific method...
...had been generally appreciated?
I think we might have saved
10 or 20 centuries.
Perhaps the contributions
that Leonardo made...
...would have been made
1000 years earlier...
...and the contributions
of Einstein 500 years ago.
Not that it would have
been those people...
...who would've made
those contributions...
...because they lived only
in our timeline.
If the Ionians had won...
...we might by now, I think,
be going to the stars.
We might at this moment have
the first survey ships...
...returning with astonishing results
from Alpha Centauri...
...and Barnard's Star,
Sirius and Tau Ceti.
There would now be great fleets...
...of interstellar transports...
...being constructed in Earth orbit...
...small, unmanned survey ships...
...liners for immigrants, perhaps...
...great trading ships...
...to ply the spaces
between the stars.
On all these ships
there would be symbols...
...and inscriptions on the sides.
The inscriptions,
if we looked closely...
...would be written in Greek.
The symbol...
...perhaps, would be the dodecahedron.
And the inscription on the sides
of the ships to the stars...
...something like:
"Starship Theodorus
of the Planet Earth."
If you were a really
ambitious time traveler...
...you might not dally
with human history...
...or even pause to examine
the evolution on Earth.
Instead, you would journey back...
...to witness the origin
of our solar system...
...from the gas and dust
between the stars.
Five billion years ago...
...an interstellar cloud was
collapsing to form our solar system.
Most clumps of matter
gravitated towards the center...
...and were destined
to form the sun.
Smaller peripheral clumps
would become the planets.
Long ago, there was a kind of
natural selection among the worlds.
Those on highly elliptical orbits
tended to collide and be destroyed...
...but planets in circular orbits
tended to survive.
But if events had been
a little different...
...the Earth would never have formed...
...and another planet at another
distance from the sun would be around.
We owe the existence of our world...
...to random collisions
in a long-vanished cloud.
Soon, the central mass
became very hot.
Thermonuclear reactions were initiated
and the sun turned on...
...flooding the solar system
with light.
But the growing smaller lumps...
...would never achieve
such high temperatures...
...and would never generate
thermonuclear reactions.
They would become
the Earth and the other planets...
...heated not from within,
but mainly by the distant sun.
The accretion continued until...
...almost all the gas and dust
and small worldlets...
...were swept up
by the surviving planets.
Our time traveler would witness...
...the collisions
that made the worlds.
Except for the comets and asteroids...
...the chaos of the early
solar system was reduced...
...to a remarkable simplicity:
Nine or so principal planets
in almost circular orbits...
...and a few dozen moons.
Now, let's take a different look.
If we view the solar system edge on...
...and move the sun
off-screen to the left...
...we see that
the small terrestrial planets...
...the ones about as massive as Earth,
tend to be close to the sun.
The big Jupiter-like planets tend
to be much further from the sun.
But is that the way it has to be?
Computer studies suggest...
...that there may be many
similar systems about stars...
...with the terrestrials in close
and the Jovian planets further away.
But some systems might have Jovians
and terrestrials mixed together.
There may be great worlds
like Jupiter looming in other skies.
Rarely, the Jovian planets
may form close to the star...
...the terrestrials trailing away
towards interstellar space.
Our familiar arrangement of planets...
...is only one,
perhaps typical, case...
...in the vast expanse of systems.
Often, one fledgling planet
accumulates so much gas and dust...
...that thermonuclear reactions
do occur.
It becomes a second sun.
A binary star system has formed.
From most of these worlds,
the vistas will be dazzling.
Not one of them will be
identical to the Earth.
A few will be hospitable.
Many will appear hostile.
Where there are two suns in the sky...
...every object will cast two shadows.
What wonders are waiting for us...
...on the planets of the nearby stars?
Are there radically
different kinds of worlds...
...unimaginably exotic forms of life?
Perhaps in another century or two...
...when our solar system
is all explored...
...we will also have put
our own planet in order.
Then we will set sail for the stars...
...and the beckoning worlds
around them.
In that day, our machines
and our descendants...
...approaching the speed of light,
will skim the light-years...
...leaping ahead through time,
seeking new worlds.
Einstein has shown us
that it's possible.
We will journey simultaneously...
...to distant planets
and to the far future.
Some worlds, like this one...
...will look out onto
a vast gaseous nebula...
...the remains of a star...
...that once was and is no longer.
In all those skies,
rich and distant...
...and exotic constellations...
...there may be a faint yellow star...
...perhaps barely visible
to the naked eye...
...perhaps seen only
through the telescope.
The home star of a fleet
of interstellar transports...
...exploring this tiny region...
...of the great Milky Way galaxy.
The themes of space and time
are intertwined.
Worlds and stars, like people...
...are born, live and die.
The lifetime of a human being
is measured in decades.
But the lifetime of the sun...
...is a hundred million times longer.
Matter is much older than life.
Billions of years before
the sun and Earth even formed...
...atoms were being synthesized
in the insides of hot stars...
...and then returned to space
when the stars blew themselves up.
Newly formed planets were
made of this stellar debris.
The Earth and every living thing
are made of star stuff.
But how slowly, in our human
perspective, life evolved...
...from the molecules of the early
oceans to the first bacteria.
Evolution is not immediately
obvious to everybody...
...because it moves
so slowly and takes so long.
How can creatures who
live for only 70 years...
...detect events that
take 70 million years to unfold?
Or 4 billion?
By the time
one-celled animals had evolved...
...the history of life
on Earth was half over.
Not very far along to us,
you might think...
...but by now almost all
the basic chemistry of life...
...had been established.
Forget our human time perspective.
From the point of view of a star...
...evolution was weaving
intricate new patterns...
...from the star stuff on
the planet Earth, and very rapidly.
Most evolutionary lines
became extinct.
Many lines became stagnant.
If things had gone
a bit differently...
...a small change of climate,
say, or...
...a new mutation...
...or the accidental death
of a different humble organism...
...the entire future history of life
might have been very different.
Maybe the line to an intelligent
technological species...
...would have passed through worms.
Maybe the present masters
of the planet...
...would have had ancestors
who were tunicates.
We might not have evolved.
Someone else,
someone very different...
...would be here now in our stead,
maybe pondering their origins.
But that's not what happened.
There's a particular sequence
of environmental accidents...
...and random mutations
in the hereditary material.
One particular timeline
for life on Earth...
...in this universe.
As a result, the dominant organisms
on the planet today...
...come from fish.
Along the way, many more species
became extinct than now exist.
If history had
a slightly different weave...
...some of those extinct organisms
might have survived and prospered.
But occasionally, a creature
thought to have become extinct...
...hundreds of millions
of years ago...
...turns out to be alive and well.
The coelacanth, for example.
For 3 1/2 billion years, life had
lived exclusively in the water.
But now, in a great
breathtaking adventure...
...it took to the land.
But if things had gone
a little differently...
...the dominant species might
still be in the ocean...
...or developed spaceships to
carry them off the planet altogether.
From our ancestors, the reptiles...
...there developed
many successful lines...
...including the dinosaurs.
Some were fast, dexterous
and intelligent.
A visitor from
another world or time...
...might have thought them
the wave of the future.
But after nearly 200 million years,
they were suddenly all wiped out.
Perhaps it was a great meteorite
colliding with the Earth...
...spewing debris into the air,
blotting out the sun...
...and killing the plants
that the dinosaurs ate.
I wonder when they first sensed
that something was wrong.
The successors of the dinosaurs
came from the same reptilian stock...
...but they survived the catastrophe
that destroyed their cousins.
Again, there were many branches
which became extinct.
And had events been
a little different...
...those branches might have led
to the dominant form today.
For 40 million years, a visitor
would not have been impressed...
...by these timid little creatures...
...but they led to all
the familiar mammals of today.
And that includes the primates.
About 20 million years ago,
a space time traveler...
...might have recognized
these guys as promising...
...bright, quick, agile,
sociable, curious.
Their ancestors were once
atoms made in stars...
...then simple molecules,
single cells...
...polyps stuck to the ocean floor...
...fish, amphibians, reptiles, shrews.
But then they came down
from the trees and stood upright.
They grew an enormous brain...
...they developed culture,
invented tools...
...domesticated fire.
They discovered language and writing.
They developed agriculture.
They built cities and forged metal.
And ultimately,
they set out for the stars...
...from which they had come
5 billion years earlier.
We are star stuff...
...which has taken its destiny
into its own hands.
The loom of time and space...
...works the most astonishing
transformations of matter.
Our own planet is only a tiny part...
...of the vast cosmic tapestry...
...a starry fabric
of worlds yet untold.
Those worlds in space
are as countless...
...as all the grains of sand
on all the beaches of the Earth.
Each of those worlds
is as real as ours.
In every one of them,
there's a succession of...
...incidents, events, occurrences
which influence its future.
Countless worlds,
numberless moments...
...an immensity of space and time.
And our small planet,
at this moment...
...here, we face
a critical branchpoint in history.
What we do with our world right now...
...will propagate down
through the centuries...
...and powerfully affect
the destiny of our descendants.
It is well within our power
to destroy our civilization...
...and perhaps our species as well.
If we capitulate to superstition...
...or greed or stupidity...
...we can plunge our world into
a darkness deeper than the time...
...between the collapse of classical
civilization and Italian Renaissance.
But we are also capable...
...of using our compassion
and our intelligence...
...our technology and our wealth...
...to make an abundant
and meaningful life...
...for every inhabitant
of this planet...
...to enhance enormously
our understanding of the universe...
...and to carry us to the stars.
In our motorbike sequence...
...we showed how
the landscape might look...
...if we barreled through it
at close to light speed.
Since then,
inspired by this sequence...
...Ping-Kang Hsiung
at Carnegie Mellon University...
...produced an exact
computer animation.
This is what you'd see if you
traveled at ordinary speeds...
...through this red and white lattice.
But this is how it would appear...
...if you were traveling
at close to the speed of light.
We're probably many centuries away
from traveling close to light speed...
...and experiencing time dilation.
But even then,
it might not be fast enough...
...if we wanted to travel
to some distant place in the galaxy...
...and then come back to Earth
in our own epoch.
Some years after completing Cosmos...
...I took time out from
my scientific work to write a novel.
A novel about travel...
...to the center
of the Milky Way galaxy.
I was willing to imagine
beings and civilizations...
...far more advanced than we...
...but I wasn't willing
to ignore the laws of physics.
Was there, even in principle,
a way to get very quickly...
...to 30,000 light-years from Earth?
So I asked my friend...
...Kip Thorne of the California
Institute of Technology.
He's a leading expert
on the nature of space and time.
Kip thought about it for a while...
...and then answered with
about 50 lines of equations...
...which showed that
a really advanced civilization...
...might establish
and hold open wormholes...
...which we might think of as tubes
through the fourth dimension...
...which connect the Earth
with another place...
...without having to traverse
the intervening distance.
Something like crawling
through a wormhole in an apple.
I was happy with this result...
...and used it as
a key plot device in Contact.
But such wormholes through space...
...would also be time machines,
it seemed to me.
And I used that notion
in my novel Contact as well.
Kip Thorne and his colleagues
later proved, or so it seemed...
...that time travel
of this sort was possible.
Here, look at this.
The key question being explored now...
...is whether such time travel
can be done consistently...
...with causes preceding effects, say,
rather than following them.
Does nature contrive it...
...so that even with a time machine,
you can't intervene...
...to prevent your own conception,
for example?
Even if time travel of this sort
is really possible...
...it's far in
our technological future.
But maybe other beings
much more advanced than we...
...are voyaging to the far future
and the remote past...
...not a measly 40 years ago
on Earth...
...but to witness
the death of the sun, say...
...or the origin of the cosmos.
a great ocean of space and time.
In that ocean, the events
that shape the future...
...are working themselves out.
Each creature and every world,
to the remotest star...
...owe their existence to...
...the great, coursing,
implacable forces of nature...
...but also, to minor happenstance.
We are carried with our planet
around the sun.
The Earth has made more than
4 billion circuits of our star...
...since its origin.
The sun itself travels about
the core of the Milky Way galaxy.
Our galaxy is moving
among the other galaxies.
We have always been space travelers.
These fine sand grains are all,
more or less, uniform in size.
They're produced from bigger rocks
through ages of...
...jostling and rubbing,
abrasion and erosion.
Driven in part by
the distant moon and sun.
So the roots of the present
lie buried in the past.
We are also travelers in time.
But trapped on Earth...
...we've had little to say about
where we go in time and space...
...or how fast.
But now we're thinking
about true journeys in time...
...and real voyages
to the distant stars.
A handful of sand contains
about 10,000 grains...
...more than all the stars
we can see...
...with the naked eye
on a clear night.
But the number of stars we can see...
...is only the tiniest fraction
of the number of stars that are.
What we see at night
is the merest smattering...
...of the nearest stars...
...with a few more distant bright
stars thrown in for good measure.
Meanwhile, the cosmos
is rich beyond measure.
The number of stars
in the universe...
...is larger than all the grains
of sand on all the beaches...
...of the planet Earth.
Long ago, before we had figured out
that the stars are distant suns...
...they seemed to us
to make pictures in the sky.
Just follow the dots.
The Big Dipper constellation
today in North America...
...has had many other incarnations.
Every culture, ancient and modern...
...has placed its totems
and concerns among the stars.
From a Chinese bureaucrat
to a German wagon.
But very ancient cultures would have
seen different constellations...
...because the stars move
with respect to one another.
We can give a computer the present
positions and motions of stars...
...and then run the patterns
back into time.
Every constellation is a single frame
in a cosmic movie...
...but because our lives
are so short...
...because star patterns
change slowly...
...we tend not to notice
it's a movie.
A million years ago,
there was no Big Dipper.
Our ancestors, looking up
and wondering about the stars...
...saw some other pattern
in the northern skies.
We can also run a constellation,
Leo the Lion, say, forward in time...
...and see what the patterns
in the stars will be in the future.
A million years from now,
Leo might be renamed...
...the constellation
of the Radio Telescope.
Although I suspect radio telescopes
then will be as obsolete...
...as stone spears are now.
Or, here's the constellation
of Cetus the Whale.
A million years ago, it may have
been called something else.
Perhaps the Spear.
Now, let's run fast-forward
through a billion nights.
Millions of years from now...
...some other very different image
will be featured in this cosmic movie.
In Orion the Hunter,
things are changing...
...not only because
the stars are moving...
...but also because
the stars are evolving.
Many of Orion's stars are
hot, young and short-lived.
They're born, live and die within
a span of only a few million years.
If we run Orion forward in time...
...we see the births
and explosive deaths...
...of dozens of stars...
...flashing on and winking off
like fireflies in the night.
If we wait long enough,
we see the constellations change.
But if we go far enough,
we also see the star patterns alter.
Two-dimensional constellations...
...are only the appearance of stars
strewn through three dimensions.
Some are dim and near,
others are bright but farther away.
Could a space traveler
actually see...
...the patterns of
the constellations change?
For that, you must travel roughly as
far as the constellation is from us.
Here, we're traveling
hundreds of light-years...
...circling all the way around
the stars of the Big Dipper.
Inhabitants of planets
around other stars...
...will see different constellations
than us...
...because their vantage points
are different.
Here we are
in the constellation Andromeda...
...or at least a model of it
next to the constellation Perseus.
Andromeda, in the Greek myth...
...was the maiden
who was saved by Perseus...
...from a sea monster.
This star just above me
is Beta Andromedae...
...the second brightest star
in the constellation...
...75 light-years from the Earth.
The light by which we see this star...
...has spent 75 years
traversing interstellar space...
...on its journey to the Earth.
In the unlikely event
that Beta Andromedae...
...blew itself up
a week ago Tuesday...
...we will not know of it
for another 75 years...
...as this interesting information,
traveling at the speed of light...
...crosses the enormous
interstellar distances.
When the light we see
from this star set out...
...on its long
interstellar voyage...
...the young Albert Einstein...
...working as a Swiss patent clerk...
...had just published his epochal
special theory of relativity...
...here on Earth.
We see...
...that space and time
are intertwined.
We cannot look out into space...
...without looking back into time.
The speed of light is very fast...
...but space is very empty...
...and the stars are very far apart.
The distances that we've been
talking about up to now...
...are very small
by the usual astronomical standards.
In fact, the distance
from the Earth...
...to the center
of the Milky Way galaxy...
...is 30,000 light-years.
From our galaxy to the nearest
spiral galaxy like our own...
...called M31 ...
...and which is also within,
that means behind...
...the constellation Andromeda...
...is 2 million light-years.
When the light we see today
from M31 ...
...left on its journey for Earth...
...there were no human beings...
...although our ancestors
were nicely evolving...
...and very rapidly,
to our present form.
There are much greater distances
in astronomy.
The distance from the Earth
to the most distant quasars...
...is 8 or 10 billion light-years.
We see them as they were before
the Earth itself accumulated...
...before the Milky Way galaxy
was formed.
The fastest space vehicles ever
launched by the human species...
...are the Voyager spacecraft.
They are traveling so fast...
...that it's only
10,000 times slower...
...than the speed of light.
The Voyager spacecraft
will take 40,000 years...
...to go the distance
to the nearest stars...
...and they're not even headed
towards the nearest stars.
But is there a method
by which we could travel...
...in a conveniently short time
to the stars?
Can we travel close
to the speed of light?
And what's magic
about the speed of light?
Can't we travel faster than that?
It turns out that
there is something very strange...
...about the speed of light.
Something that provides the key...
...to our understanding
of time and space.
The story of its discovery...
...takes us to Tuscany
in northern Italy.
There's something timeless
about this place.
A century ago, it probably
looked very much the same.
If you had traveled these roads
in the summer of 1895...
...you might have come upon a
16-year-old German high-school dropout.
His teacher told him that
he'd never amount to anything...
...that his attitude destroyed
classroom discipline...
...that he should drop out.
So he left and came here...
...where he enjoyed
wandering these roads...
...and giving his mind
free rein to explore.
One day, he began
to think about light...
...about how fast it travels.
We always measure
the speed of a moving object...
...relative to something else.
I'm moving at about 10 kilometers
an hour relative to the ground.
But the ground isn't at rest.
The Earth is turning at more
than 1600 kilometers an hour.
The Earth itself is
in orbit around the sun.
The sun is moving among
the drifting stars, and so on.
It was hard for the young man
to imagine some absolute standard...
...to measure all these
relative motions against.
He knew that sound waves are
a vibration of the air...
...and their speed is measured
relative to the air itself.
But sunlight travels across
the vacuum of empty space.
"Do light waves move
relative to something else?
And if so," he wondered,
"relative to what?"
That teenage dropout's name...
...was Albert Einstein.
And his ruminations changed the world.
He had been fascinated
by Bernstein's 1869...
...People's Book of Natural Science.
Here, on its very first page...
...it describes the astonishing speed
of electricity through wires...
...and light through space.
Einstein wondered, perhaps for
the first time, in northern Italy...
...what the world would look like if
you could travel on a wave of light.
To travel at the speed of light.
What an engaging and magical thought
for a teenage boy on the road...
...where the countryside is dappled
and rippling in sunlight.
You couldn't tell you were on a light
wave if you were traveling with it.
If you started on a wave crest...
...you would stay on the crest and
lose all notion of it being a wave.
Something funny happens
at the speed of light.
The more Einstein thought about it,
the more troubling it became.
Paradoxes seemed to pop up all over...
...if you could travel
at the speed of light.
Certain ideas had been
accepted as true...
...without sufficiently
careful thought.
One of those ideas had to do
with the light from a moving object.
The images by which we see the world
are made of light...
...and are carried
at the speed of light...
...300,000 kilometers a second.
You might think that the image of me
should be moving out ahead of me...
...at the speed of light
plus the speed of the bicycle.
If I'm moving towards you
faster than a horse-and-cart...
...then my image should be
approaching you that much faster.
My image ought to arrive earlier.
But in reality
you don't see any time delay.
In a near collision, for example,
you see everything happen at once.
Horse, cart, swerve, bicycle.
All simultaneous.
But how would it look if
it were proper to add the velocities?
Since I'm heading toward you, you'd
add my speed to the speed of light.
So my image ought to arrive before
the image of the horse-and-cart.
I'd be cycling towards you
quite normally.
To me, a collision
would seem imminent.
But you'd see me swerve
for no apparent reason...
...and have a collision with nothing.
Now, the horse-and-cart
aren't headed towards you.
Their image would arrive only
at the speed of light.
Could it seem to me that
I just missed colliding...
...while to you it wasn't even close?
In precise laboratory experiments...
...scientists have never observed
any such thing.
If the world is to be understood...
...if we are to avoid logical paradoxes
when traveling at high speeds...
...then there are rules
which must be obeyed.
Einstein called these rules
the special theory of relativity.
Light from a moving object
travels at the same speed...
...no matter whether the object
is at rest or in motion.
"Thou shalt not add my speed
to the speed of light."
Also, no material object can travel
at or beyond the speed of light.
Nothing in physics prevents you from
traveling close to the speed of light.
99.9 percent the speed of light
is just fine.
But no matter how hard you try...
...you can never gain
that last decimal point.
For the world
to be logically consistent...
...there must be a cosmic speed limit.
The crack of a whip is,
due to its tip...
...moving faster
than the speed of sound.
It makes a shock wave...
...a small sonic boom
in the Italian countryside.
A thunderclap has a similar origin.
So does the sound of
a supersonic airplane.
So why is the speed of light a barrier
any more than the speed of sound?
The answer is not just that...
...light travels a million times
faster than sound.
It's not merely an engineering problem
like the supersonic airplane.
Instead, the light barrier is
a fundamental law of nature...
...as basic as gravity.
Einstein found his absolute framework
for the world:
This sturdy pillar among all
the relative motions of the cosmos.
Light travels just as fast,
no matter how its source is moving.
The speed of light is constant,
relative to everything else.
Nothing can ever catch up with light.
Einstein's prohibition against
traveling faster than light...
...seems to clash with
our common sense notions.
But why should we expect
our common sense notions...
...to have any reliability
in a matter of this sort?
Why should our experience
at 10 kilometers an hour...
...constrain the laws of nature...
...at 300,000 kilometers a second?
Relativity sets limits...
...on what humans ultimately can do.
The universe is not required...
...to be in perfect harmony
with human ambition.
Imagine a place
where the speed of light...
...isn't its true value
of 300,000 kilometers a second...
...but something a lot less.
Let's say, 40 kilometers an hour...
...and strictly enforced.
Just as in the real world we can
never reach the speed of light...
...the commandment here is still...
..."Thou shalt not travel
faster than light."
We can do thought experiments on
what happens near the speed of light...
...here 40 kilometers per hour,
the speed of a motor scooter.
You can't break the laws of nature.
There are no penalties for doing so.
The real world and this one...
...are merely so arranged
that transgressions can't happen.
The job of physics is to find out
what those laws are.
Before Einstein,
physicists thought that...
...there were privileged frames
of reference...
...some special places and times...
...against which everything else
had to be measured.
Einstein encountered
a similar notion in human affairs.
The idea that the customs
of a particular nation...
...his native Germany
or Italy or anywhere...
...are the standard which all
other societies must be measured.
But Einstein rejected the strident
nationalism of his time.
He believed every culture
had its own validity.
Also in physics,
he understood that...
...there are no privileged
frames of reference.
Every observer,
in any place, time or motion...
...must deduce
the same laws of nature.
(SPEAKING IN ITALIAN)
A speed is simply how much space
you cover in a given time...
...as any kid on
a motor scooter knows.
Since near the velocity of light...
...we cannot simply add speeds...
...the familiar notions of
absolute space and absolute time...
...independent of your
relative motion, must give way.
That's why, as Einstein showed...
...funny things have to happen
close to the speed of light.
There, our conventional perspectives
of space and time...
...strangely change.
Your nose is just a little closer
to me than your ears.
Light reflected off your nose
reaches me...
...an instant in time
before your ears.
But suppose I had a magic camera...
...so that I could see
your nose and your ears...
...at precisely the same instant?
(SCOOTER STARTS UP)
(SCOOTER HONKS)
With such a camera you could take
some pretty interesting pictures.
Paolo says goodbye to
his little brother, Vincenzo...
-Ciao, Vincenzo.
-Ciao, Paolo.
...and rides off.
He's now going more than
half the speed of light.
He is almost catching up
with his own light waves.
This compresses the light waves
in front of him...
...and his image becomes blue.
The shorter wavelength is
what makes blue light waves blue.
Also Paolo becomes skinny
in the direction of motion.
This isn't just some optical illusion.
It really happens when you travel
near the speed of light.
As he roars away, he leaves his own
light waves stretched out behind him.
Long light waves are red.
We say that his receding image
is red-shifted.
Now Paolo leaves for
a short tour of the countryside.
He experiences something
even stranger.
Everything he can see is squeezed...
...into a moving window
just ahead of him...
...blue-shifted at the center,
red-shifted at the edges.
To a passerby, Paolo appears
blue-shifted when approaching...
...red-shifted when receding.
But to him, the entire world
is both coming and going...
...at nearly the speed of light.
Roadside houses and trees
that has already gone past...
...still appear to him at the edge
of his forward field of view...
...but distorted and red-shifted.
When he slows down,
everything again looks normal.
Only very close
to the speed of light...
...does the visible world
get squeezed into a kind of tunnel.
You'd see these distortions if you
traveled near the speed of light.
Someday, perhaps,
interstellar navigators...
...will take their bearings
on stars behind them...
...whose images have all crowded
together on the forward view screen.
The most bizarre aspect of traveling
near the speed of light...
...is that time slows down.
All clocks,
mechanical and biological...
...tick more slowly
near the speed of light.
But stationary clocks tick
at their usual rate.
If we travel close to light speed...
...we age more slowly
than those we left behind.
Paolo's watch and his internal
sense of time show...
...that he has been gone from
his friends for only a few minutes.
But from their point of view,
he has been away for many decades.
His friends have grown up,
moved on and died.
And his younger brother has been...
...patiently waiting
for him all this time.
The two brothers experience
the paradox of time dilation.
They've encountered
Einstein's special relativity.
Vincenzo.
This was just a thought experiment.
But atomic particles traveling
near the speed of light...
...do decay more slowly
than stationary particles.
As strange and counterintuitive
as it seems...
...time dilation is a law of nature.
Traveling close
to the speed of light...
...is a kind of elixir of life.
Because time slows down
close to the speed of light...
...special relativity provides us...
...with a means of going to the stars.
This region of northern Italy
is not only the caldron...
...of some of the thinking
of the young Albert Einstein...
...it is also the home
of another great genius...
...who lived 400 years earlier.
Leonardo da Vinci.
Leonardo delighted
in climbing these hills...
...and viewing the ground
from a great height...
...as if he were soaring like a bird.
He drew the first aerial views...
...of landscapes, villages,
fortifications.
I've been talking about Einstein
in and around this town of Vinci...
...in which Leonardo grew up.
Einstein greatly respected Leonardo...
...and their spirits, in some sense...
...inhabit this countryside still.
Among Leonardo's
many accomplishments...
...in painting, sculpture,
architecture, natural history...
...anatomy, geology,
civil and military engineering...
...he had a great passion.
He wished to construct a machine...
...which would fly.
He made sketches of such machines,
built miniature models...
...constructed great,
full-scale prototypes.
And not a one of them ever worked.
There were no machines of adequate
capacity available in his time.
The technology was just not ready.
The designs, however, were brilliant.
For example, this bird-like machine...
...here in the Leonardo Museum
in the town of Vinci.
Leonardo's great designs encouraged
engineers in later epochs...
...although Leonardo himself
was very depressed at these failures.
But it's not his fault...
...he was trapped in the 15th century.
A somewhat similar case
occurred in 1939...
...when a group of engineers called
the British Interplanetary Society...
...decided to design a ship...
...which would carry people
to the moon.
Now, it was by no means
the same design...
...as the Apollo ship which actually
took people to the moon years later.
But that design suggested that...
...a mission to the moon
might one day...
...be a practical
engineering possibility.
Today...
...we have preliminary
designs of ships...
...which will take people
to the stars.
They are constructed in Earth orbit
and from there...
...they venture on their great
interstellar journeys.
One of them...
...is called Project Orion.
It utilizes nuclear weapons...
...hydrogen bombs
against an inertial plate.
Each explosion providing
a kind of "putt-putt"...
...a vast nuclear motorboat in space.
Orion seems entirely practical...
...and was under development
in the U.S...
...until the signing
of the international treaty...
...forbidding nuclear weapons
explosions in space.
I think, the Orion starship
is the best use of nuclear weapons...
...provided the ships don't depart
from very near the Earth.
Project Daedalus is
a recent design...
...of the British
Interplanetary Society.
It assumes the existence
of a nuclear fusion reactor...
...something much safer
and more efficient...
...than the existing nuclear
fission power plants.
We do not yet have fusion reactors.
One day, quite soon, we may.
Orion and Daedalus might go...
...10 percent the speed of light.
So a trip to Alpha Centauri,
4 1/2 light-years away...
...would take 45 years,
less than a human lifetime.
Such ships could not travel
close enough to the speed of light...
...for the time-slowing effects
of special relativity...
...to become important.
It does not seem likely
that such ships...
...would be built before
the middle of the 21 st century...
...although we could build
an Orion starship now.
For voyages beyond the nearest stars,
something must be added.
Perhaps they could be used
as multigeneration ships...
...so those arriving would be
the remote descendants...
...of those who had originally
set out centuries before.
Or perhaps some safe means
of human hibernation might be found...
...so that space travelers might be
frozen and then thawed out...
...when they arrive at
the destination centuries later.
But fast interstellar space flight
approaching the speed of light...
...is much more difficult.
That's an objective
not for a hundred years...
...but for a thousand
or for 10 thousand...
...but it also is possible.
A kind of interstellar ramjet
has been proposed...
...which scoops up
the hydrogen atoms...
...which float between the stars...
...accelerates them into an engine
and spits them out the back.
But in deep space,
there is one atom...
...for every 10 cubic centimeters
of space.
For the ramjet to work...
...it has to have a frontal scoop...
...hundreds of kilometers across.
Reaching relativistic velocities,
the hydrogen atoms will be moving...
...with respect
to the interstellar spaceship...
...at close to the speed of light.
If precautions aren't taken...
...the passengers will be fried
by these induced cosmic rays.
There's a proposed solution:
A laser is used to strip
electrons off the atoms...
...and electrically charge them
while they're some distance away.
And an extremely strong
magnetic field...
...is used to deflect
the charged atoms into the scoop...
...and away from the spacecraft.
This is engineering...
...on a scale so far
unprecedented on the Earth.
We are talking of engines
the size of small worlds.
Suppose that the spacecraft is
designed to accelerate at 1 g...
...so we'd be comfortable aboard it.
We'd go closer and closer
to the speed of light...
...until the midpoint of the journey.
Then the spacecraft is
turned around...
...and we decelerate at 1 g
to the destination.
For most of the trip, the velocity
would be close to the speed of light...
...and time would
slow down enormously.
By how much?
Barnard's Star could be reached
by such a ship...
...in eight years, ship time.
The center of the Milky Way galaxy
in 21 years.
The Andromeda galaxy in 28 years.
Of course, the people
left behind on the Earth...
...would see things
somewhat differently.
Instead of 21 years to the galaxy...
...they would measure it
as 30,000 years.
When we got back...
...very few of our friends
would be around to greet us.
In principle, such a journey...
...mounting the decimal points closer
and closer to the speed of light...
...would even permit us to
circumnavigate the known universe...
...in 56 years, ship time.
We would return tens
of billions of years...
...in the far future...
...with the Earth a charred cinder...
...and the sun dead.
Relativistic space flight makes
the universe accessible...
...to advanced civilizations...
...but only to those
who go on the journey...
...not to those who stay home.
These designs are probably further...
...from the actual interstellar
spacecraft of the future...
...than Leonardo's models are...
...from the supersonic transports
of the present.
But if we do not destroy ourselves...
...I believe that we will,
one day, venture to the stars.
When our solar system
is all explored...
...the planets of other stars
will beckon.
Space travel and time travel
are connected.
To travel fast into space...
...is to travel fast into the future.
We travel into the future,
although slowly, all the time.
But what about the past?
Could we journey into yesterday?
Many physicists think this is
fundamentally impossible...
...that we could
not build a device...
...which would carry us
backwards into time.
Some say that even if we were
to build such a device...
...it wouldn't do much good.
We couldn't significantly
affect the past.
For example, suppose you
traveled into the past...
...and somehow or other prevented...
...your own parents from meeting.
Why, then you would probably
never have been born...
...which is something
of a contradiction, isn't it...
...since you are clearly there.
Other people think that...
...the two alternative histories
have equal validity...
...that they're parallel threads,
skeins of time...
...that they could exist side by side.
The history in which
you were never born...
...and the history that
you know all about.
Perhaps time itself has
many potential dimensions...
...despite the fact that
we are condemned to experience...
...only one of those dimensions.
Now, suppose you could go back
into the past...
...and really change it by,
let's say something like...
...persuading Queen Isabella not
to bankroll Christopher Columbus.
Then you would have set into motion...
...a different sequence
of historical events...
...which those people
you left behind you in our time...
...would never get to know about.
If that kind of time travel
were possible...
...then every imaginable sequence...
...of alternative history...
...might in some sense really exist.
Would it be possible
for a time traveler...
...to change the course of history
in a major way?
Well, let's think about that.
History consists for the most part...
...of a complex multitude
of deeply interwoven threads...
...biological, economic
and social forces...
...that are not so easily unraveled.
The ancient Greeks imagined the course
of human events to be a tapestry...
...created by three goddesses:
the Fates.
Random minor events generally
have no long-range consequences.
But some which occur
at critical junctures...
...may alter the weave of history.
There may be cases where
profound changes can be made...
...by relatively trivial adjustments.
The further in the past such an event
is, the more powerful its influence.
What if our time traveler had
persuaded Queen Isabella that...
...Columbus' geography was wrong?
Almost certainly, some other European
would have sailed to the New World.
There were many inducements:
The lure of the spice trade,
improvements in navigation...
...competition among
rival European powers.
The discovery of America
around 1500 was inevitable.
Of course, there wouldn't be any
postage stamps showing Columbus...
...and the Republic of Colombia
would have another name.
But the big picture would have
turned out more or less the same.
In order to affect
the future profoundly...
...a time traveler
has to pick and choose.
He'd probably have to intervene
in a number of events...
...which are
very carefully selected...
...so he could change
the weave of history.
It's a lovely fantasy...
...to explore those other worlds
that never were.
If you had H.G. Wells' time machine...
...maybe you could understand
how history really works.
If an apparently pivotal person
had never lived...
...Paul the Apostle or Peter the Great
or Pythagoras...
...how different would
the world really be?
What if the scientific tradition...
...of the ancient Ionian Greeks...
...had prospered and flourished?
It would have required
many social factors at the time...
...to have been different...
...including the common feeling...
...that slavery was right and natural.
But what if that light
that had dawned...
...on the eastern Mediterranean
some 2500 years ago...
...had not flickered out?
What if scientific method
and experiment...
...had been vigorously pursued...
...2000 years before
the industrial revolution...
...our industrial revolution?
What if the power of this new mode
of thought, the scientific method...
...had been generally appreciated?
I think we might have saved
10 or 20 centuries.
Perhaps the contributions
that Leonardo made...
...would have been made
1000 years earlier...
...and the contributions
of Einstein 500 years ago.
Not that it would have
been those people...
...who would've made
those contributions...
...because they lived only
in our timeline.
If the Ionians had won...
...we might by now, I think,
be going to the stars.
We might at this moment have
the first survey ships...
...returning with astonishing results
from Alpha Centauri...
...and Barnard's Star,
Sirius and Tau Ceti.
There would now be great fleets...
...of interstellar transports...
...being constructed in Earth orbit...
...small, unmanned survey ships...
...liners for immigrants, perhaps...
...great trading ships...
...to ply the spaces
between the stars.
On all these ships
there would be symbols...
...and inscriptions on the sides.
The inscriptions,
if we looked closely...
...would be written in Greek.
The symbol...
...perhaps, would be the dodecahedron.
And the inscription on the sides
of the ships to the stars...
...something like:
"Starship Theodorus
of the Planet Earth."
If you were a really
ambitious time traveler...
...you might not dally
with human history...
...or even pause to examine
the evolution on Earth.
Instead, you would journey back...
...to witness the origin
of our solar system...
...from the gas and dust
between the stars.
Five billion years ago...
...an interstellar cloud was
collapsing to form our solar system.
Most clumps of matter
gravitated towards the center...
...and were destined
to form the sun.
Smaller peripheral clumps
would become the planets.
Long ago, there was a kind of
natural selection among the worlds.
Those on highly elliptical orbits
tended to collide and be destroyed...
...but planets in circular orbits
tended to survive.
But if events had been
a little different...
...the Earth would never have formed...
...and another planet at another
distance from the sun would be around.
We owe the existence of our world...
...to random collisions
in a long-vanished cloud.
Soon, the central mass
became very hot.
Thermonuclear reactions were initiated
and the sun turned on...
...flooding the solar system
with light.
But the growing smaller lumps...
...would never achieve
such high temperatures...
...and would never generate
thermonuclear reactions.
They would become
the Earth and the other planets...
...heated not from within,
but mainly by the distant sun.
The accretion continued until...
...almost all the gas and dust
and small worldlets...
...were swept up
by the surviving planets.
Our time traveler would witness...
...the collisions
that made the worlds.
Except for the comets and asteroids...
...the chaos of the early
solar system was reduced...
...to a remarkable simplicity:
Nine or so principal planets
in almost circular orbits...
...and a few dozen moons.
Now, let's take a different look.
If we view the solar system edge on...
...and move the sun
off-screen to the left...
...we see that
the small terrestrial planets...
...the ones about as massive as Earth,
tend to be close to the sun.
The big Jupiter-like planets tend
to be much further from the sun.
But is that the way it has to be?
Computer studies suggest...
...that there may be many
similar systems about stars...
...with the terrestrials in close
and the Jovian planets further away.
But some systems might have Jovians
and terrestrials mixed together.
There may be great worlds
like Jupiter looming in other skies.
Rarely, the Jovian planets
may form close to the star...
...the terrestrials trailing away
towards interstellar space.
Our familiar arrangement of planets...
...is only one,
perhaps typical, case...
...in the vast expanse of systems.
Often, one fledgling planet
accumulates so much gas and dust...
...that thermonuclear reactions
do occur.
It becomes a second sun.
A binary star system has formed.
From most of these worlds,
the vistas will be dazzling.
Not one of them will be
identical to the Earth.
A few will be hospitable.
Many will appear hostile.
Where there are two suns in the sky...
...every object will cast two shadows.
What wonders are waiting for us...
...on the planets of the nearby stars?
Are there radically
different kinds of worlds...
...unimaginably exotic forms of life?
Perhaps in another century or two...
...when our solar system
is all explored...
...we will also have put
our own planet in order.
Then we will set sail for the stars...
...and the beckoning worlds
around them.
In that day, our machines
and our descendants...
...approaching the speed of light,
will skim the light-years...
...leaping ahead through time,
seeking new worlds.
Einstein has shown us
that it's possible.
We will journey simultaneously...
...to distant planets
and to the far future.
Some worlds, like this one...
...will look out onto
a vast gaseous nebula...
...the remains of a star...
...that once was and is no longer.
In all those skies,
rich and distant...
...and exotic constellations...
...there may be a faint yellow star...
...perhaps barely visible
to the naked eye...
...perhaps seen only
through the telescope.
The home star of a fleet
of interstellar transports...
...exploring this tiny region...
...of the great Milky Way galaxy.
The themes of space and time
are intertwined.
Worlds and stars, like people...
...are born, live and die.
The lifetime of a human being
is measured in decades.
But the lifetime of the sun...
...is a hundred million times longer.
Matter is much older than life.
Billions of years before
the sun and Earth even formed...
...atoms were being synthesized
in the insides of hot stars...
...and then returned to space
when the stars blew themselves up.
Newly formed planets were
made of this stellar debris.
The Earth and every living thing
are made of star stuff.
But how slowly, in our human
perspective, life evolved...
...from the molecules of the early
oceans to the first bacteria.
Evolution is not immediately
obvious to everybody...
...because it moves
so slowly and takes so long.
How can creatures who
live for only 70 years...
...detect events that
take 70 million years to unfold?
Or 4 billion?
By the time
one-celled animals had evolved...
...the history of life
on Earth was half over.
Not very far along to us,
you might think...
...but by now almost all
the basic chemistry of life...
...had been established.
Forget our human time perspective.
From the point of view of a star...
...evolution was weaving
intricate new patterns...
...from the star stuff on
the planet Earth, and very rapidly.
Most evolutionary lines
became extinct.
Many lines became stagnant.
If things had gone
a bit differently...
...a small change of climate,
say, or...
...a new mutation...
...or the accidental death
of a different humble organism...
...the entire future history of life
might have been very different.
Maybe the line to an intelligent
technological species...
...would have passed through worms.
Maybe the present masters
of the planet...
...would have had ancestors
who were tunicates.
We might not have evolved.
Someone else,
someone very different...
...would be here now in our stead,
maybe pondering their origins.
But that's not what happened.
There's a particular sequence
of environmental accidents...
...and random mutations
in the hereditary material.
One particular timeline
for life on Earth...
...in this universe.
As a result, the dominant organisms
on the planet today...
...come from fish.
Along the way, many more species
became extinct than now exist.
If history had
a slightly different weave...
...some of those extinct organisms
might have survived and prospered.
But occasionally, a creature
thought to have become extinct...
...hundreds of millions
of years ago...
...turns out to be alive and well.
The coelacanth, for example.
For 3 1/2 billion years, life had
lived exclusively in the water.
But now, in a great
breathtaking adventure...
...it took to the land.
But if things had gone
a little differently...
...the dominant species might
still be in the ocean...
...or developed spaceships to
carry them off the planet altogether.
From our ancestors, the reptiles...
...there developed
many successful lines...
...including the dinosaurs.
Some were fast, dexterous
and intelligent.
A visitor from
another world or time...
...might have thought them
the wave of the future.
But after nearly 200 million years,
they were suddenly all wiped out.
Perhaps it was a great meteorite
colliding with the Earth...
...spewing debris into the air,
blotting out the sun...
...and killing the plants
that the dinosaurs ate.
I wonder when they first sensed
that something was wrong.
The successors of the dinosaurs
came from the same reptilian stock...
...but they survived the catastrophe
that destroyed their cousins.
Again, there were many branches
which became extinct.
And had events been
a little different...
...those branches might have led
to the dominant form today.
For 40 million years, a visitor
would not have been impressed...
...by these timid little creatures...
...but they led to all
the familiar mammals of today.
And that includes the primates.
About 20 million years ago,
a space time traveler...
...might have recognized
these guys as promising...
...bright, quick, agile,
sociable, curious.
Their ancestors were once
atoms made in stars...
...then simple molecules,
single cells...
...polyps stuck to the ocean floor...
...fish, amphibians, reptiles, shrews.
But then they came down
from the trees and stood upright.
They grew an enormous brain...
...they developed culture,
invented tools...
...domesticated fire.
They discovered language and writing.
They developed agriculture.
They built cities and forged metal.
And ultimately,
they set out for the stars...
...from which they had come
5 billion years earlier.
We are star stuff...
...which has taken its destiny
into its own hands.
The loom of time and space...
...works the most astonishing
transformations of matter.
Our own planet is only a tiny part...
...of the vast cosmic tapestry...
...a starry fabric
of worlds yet untold.
Those worlds in space
are as countless...
...as all the grains of sand
on all the beaches of the Earth.
Each of those worlds
is as real as ours.
In every one of them,
there's a succession of...
...incidents, events, occurrences
which influence its future.
Countless worlds,
numberless moments...
...an immensity of space and time.
And our small planet,
at this moment...
...here, we face
a critical branchpoint in history.
What we do with our world right now...
...will propagate down
through the centuries...
...and powerfully affect
the destiny of our descendants.
It is well within our power
to destroy our civilization...
...and perhaps our species as well.
If we capitulate to superstition...
...or greed or stupidity...
...we can plunge our world into
a darkness deeper than the time...
...between the collapse of classical
civilization and Italian Renaissance.
But we are also capable...
...of using our compassion
and our intelligence...
...our technology and our wealth...
...to make an abundant
and meaningful life...
...for every inhabitant
of this planet...
...to enhance enormously
our understanding of the universe...
...and to carry us to the stars.
In our motorbike sequence...
...we showed how
the landscape might look...
...if we barreled through it
at close to light speed.
Since then,
inspired by this sequence...
...Ping-Kang Hsiung
at Carnegie Mellon University...
...produced an exact
computer animation.
This is what you'd see if you
traveled at ordinary speeds...
...through this red and white lattice.
But this is how it would appear...
...if you were traveling
at close to the speed of light.
We're probably many centuries away
from traveling close to light speed...
...and experiencing time dilation.
But even then,
it might not be fast enough...
...if we wanted to travel
to some distant place in the galaxy...
...and then come back to Earth
in our own epoch.
Some years after completing Cosmos...
...I took time out from
my scientific work to write a novel.
A novel about travel...
...to the center
of the Milky Way galaxy.
I was willing to imagine
beings and civilizations...
...far more advanced than we...
...but I wasn't willing
to ignore the laws of physics.
Was there, even in principle,
a way to get very quickly...
...to 30,000 light-years from Earth?
So I asked my friend...
...Kip Thorne of the California
Institute of Technology.
He's a leading expert
on the nature of space and time.
Kip thought about it for a while...
...and then answered with
about 50 lines of equations...
...which showed that
a really advanced civilization...
...might establish
and hold open wormholes...
...which we might think of as tubes
through the fourth dimension...
...which connect the Earth
with another place...
...without having to traverse
the intervening distance.
Something like crawling
through a wormhole in an apple.
I was happy with this result...
...and used it as
a key plot device in Contact.
But such wormholes through space...
...would also be time machines,
it seemed to me.
And I used that notion
in my novel Contact as well.
Kip Thorne and his colleagues
later proved, or so it seemed...
...that time travel
of this sort was possible.
Here, look at this.
The key question being explored now...
...is whether such time travel
can be done consistently...
...with causes preceding effects, say,
rather than following them.
Does nature contrive it...
...so that even with a time machine,
you can't intervene...
...to prevent your own conception,
for example?
Even if time travel of this sort
is really possible...
...it's far in
our technological future.
But maybe other beings
much more advanced than we...
...are voyaging to the far future
and the remote past...
...not a measly 40 years ago
on Earth...
...but to witness
the death of the sun, say...
...or the origin of the cosmos.