Stephen Hawking: We all have questions...
Big questions.
Norman: Can I journey into the past?
Hawking: It's part of what it means to be human.
Paul: Is time travel just science fiction?
Hawking: My name is Stephen Hawking.
And I believe that anyone
can answer big questions for themselves.
So with the help of some ordinary people...
Paul: Wow!
Marisol: This is awesome!
Hawking: And a team of experts...
Alex filippenko: You can't get a place in the universe
where gravity is stronger.
Hawking: We are going on the ultimate voyage...
Norman: Pulling a lot of gs!
Hawking: A quest to answer
the greatest mysteries of the universe...
Using the power of the human mind.
[Participants cheering]
Hawking: Because anyone
can think like a genius.
Participants: Can we time travel?
Hawking: Time travel is one of my favorite ideas.
Most of us have dreamt of visiting a different age...
Perhaps a peek into our own future...
[Cheering]
Or to see a monumental event in history.
For decades, time travel has proved irresistible
to science fiction writers
and Hollywood directors.
But can we work out if it is really possible
to travel through time?
I have asked 3 ordinary people to come on a journey
of discovery...
A series of puzzles and challenges await them.
They'll be given equipment and instructions to help.
Paul: Wow!
Marisol: Ha ha! Nice.
Hawking: Can they solve the mysteries
and find the truth about time travel?
Can I travel in time?
Marisol: I don't know. I don't-- i don't know where to start.
I feel excited to contemplate the issue of time travel.
I'm going to say no.
But--technology now-- possibly in the future.
I don't know how or what the actual mechanics of it are,
but i--i think it's possible.
Yes, I believe it is possible.
Hawking: I have brought my volunteers to New York City.
What better place to begin their quest than Times Square?
Hawking: I want to see if my team can grasp how science
began to explore this elusive thing we call time.
I have lined up a surprise.
Norman: Oh! Look, look, look.
[Paul laughs]
Hawking: Hello, my budding geniuses.
Paul: Wow!
This is awesome!
Hawking: Welcome to New York.
I hope you are ready for some fun.
H--hey! Ha ha!
I did not expect Stephen Hawking. It was awesome.
It was a complete surprise and complete shock.
Paul: Oh, I was flabbergasted to see his face there.
Hawking: I'd like you to come to a party.
Using these 3 numbers, can you
work out how to get there?
Good luck.
Norman: 48, 16...
All: 11.
Paul: Can we make it to a party
using those 3 digits?
11? 48...
48, 16... 48, 16...
11. 11.
Hawking: Time to get thinking.
Marisol: What do those numbers have...
Paul: In common? Yeah.
What comes in 3 numbers?
A triangle.
Paul: 48, 16, 11.
Distance and width.
I can't think of a mathematical relation.
Norman: I can't picture it, no.
Could be, um...
Like longitude and latitude.
Marisol: Or is it streets?
In time-- good thinking.
Yeah.
Is there a 48th street?
There is a 48th street.
16th Avenue would put you into the Hudson.
Well! They stop there, do they?
The numbers stop there.
Norman: Yeah. 'Cause there's a river.
Also, 48th street, 11th Avenue.
And maybe once we get
to 48th and 11th,
the 16 will present itself.
Paul: Yes.
We can ponder it on the way.
Marisol: Let's go.
Hawking: I have asked the volunteers to make
a video diary of their journey.
They will need this later.
Paul: Ok, Marisol,
what are we up to?
Ha ha! We're gonna go to a party. Ha ha!
Norman. A party, bro.
We're on a mission.
We might be wrong, but we took that address
to be the one thing we could take action on.
Marisol: All right. What street are we on?
Norman: This is, uh-- what is that?
Paul: 46th.
Marisol: And this is 7th.
7th.
Marisol, voice-over: It made sense to follow the grid.
Like, to follow the streets.
Marisol: We are, 48th-- the first number.
The first digit. Excellent!
Show what we got here?
Marisol: Selfie. Selfie. Selfie.
Paul: Where are we? 48th.
48th.
It's us. And there's a sign.
Now off to 11th.
Norman: So, yeah. 11th is that way.
Norman: Hopefully, 48th and 11th was gonna pan out.
Or were we gonna have to start all over?
Norman: Here we go.
48th street and 10th Avenue.
Paul: One more Avenue to go.
Paul, voice-over: We were really hoping that
we would be--we would come across some kind of
[chuckling] Divine inspiration.
Norman: Here we go. 11th and 48th.
48th and 11th.
Paul, in high-pitched voice: ♪ ah!
Hawking: They're right about
the intersection of 48th and 11th.
But can they use the third number to get them to the party?
I don't see anything in particular.
We got the 48. We got the 11.
That just leaves 16.
Marisol: First just saw the building.
Nothing was screaming "16."
But it was just, like, a really tall building.
1, 2, 3, 4, 5, 6, 7...
16th floor?
Marisol, voice-over: 16 floors. It had to be a level.
So let's check it out, yeah?
Yeah. Let's do it.
All right. Let's do it.
Hawking: Traveling through anything,
be it space or New York City,
means going from one place to another.
Thanks to a genius who loved to lie in bed,
science has a way to describe what this means.
Greg radick: In Europe
in the first half of the 17th century,
what became the scientific revolution began to gather pace
as a new generation came along.
Among their number was a frenchman, Rene descartes.
He was esteemed, in particular, for his work
in the study of nature,
above all the mathematical study of nature.
The story goes like this--
that descartes was a sickly child,
and so he was allowed by an enlightened teacher
to spend his mornings in bed.
And, indeed, descartes found this so useful, uh,
that he kept it up for the rest of his life.
He found often his best thoughts came while he was in bed.
Hawking: Descartes found inspiration in ordinary things.
On one occasion, it was thought to be a common housefly.
Radick: One day, descartes was in bed,
and he was idly watching a fly up on the ceiling.
And as the fly wanders around, it occurs to descartes
that with just 2 numbers...
One to describe its position along a horizontal axis,
another to describe its position
along a vertical axis.
He can capture mathematically the position of the fly
at any given moment.
So there's descartes watching the fly up on the ceiling.
And at a certain moment, it drops down.
Now, on the ceiling, the fly needed just 2 numbers
for its position to be uniquely described.
When it drops down, it leaves 2 dimensions
and enters into 3 dimensions.
Now we need 3 numbers.
[Fly buzzing]
And it turns out that anything can be positioned
using just 3 numbers.
[Fly buzzing]
Hawking: In this simple
stroke of genius, descartes laid
the foundation for
modern mathematical science.
Radick: And this insight
becomes the germ of
what we now call the cartesian coordinate system.
Hawking: The cartesian coordinate system is
still in use today, especially in New York City.
Marisol: All right. 16th floor?
Yup. Go for it.
Norman: That's the last of our coordinates.
Hawking: It allows us to precisely pinpoint a location,
which is what you need for a party.
Paul: Stephen Hawking--he's up there waiting for us.
Probably. That would be a surprise.
That would be good. That would be good.
Yeah! That would be a party.
It would be a party!
[Laughter]
Paul: Ok, guys, we're here.
All right. What?
Norman, voice-over: We walk out onto the 16th floor,
and there's a party...
Paul: Oh, ho! Wow!
Norman: Hey! It looks like we got the right spot.
Norman, voice-over: That had finished up some time ago.
There was a party.
There was! It's over.
There was a party, yeah.
We missed it.
Norman: It looks like they had a good time.
Paul, voice-over: My initial feeling was
one of massive disappointment to have missed what looked like
it had been a great tear-up.
Paul: Sigh.
Marisol: Shucks.
Well, at least we're on the right path.
Hawking: My invitation wasn't complete.
So my volunteers had missed the party.
Can they figure out why?
"I'm sorry I had to go.
"How many pieces of information do you need
to make it to a party?"
Paul: "How many pieces of information do you need
to make it to a party?"
48, 16, and 11.
So a point.
So, like, an intersection.
We got 48...
So 2 numbers for the point.
That's it. Two dimensions.
Then your third dimension is in height.
So we went--went that way, that way, that way.
So that's 3 points of information.
And the fourth piece of information would be time.
So you'd need 4 pieces of information.
And we only had 3.
The additional one being
the fourth dimension--time.
Paul: Yes.
So time is a dimension
that we're traveling through,
but we don't really realize it because it's always present.
Paul: So that's why we missed it.
That's why we missed it. We missed--
we didn't have all the information we needed.
Hawking: This was a mean trick to play
but they're right.
You need 3 numbers to describe a place,
but you need a fourth to describe an event,
and that fourth is time.
[Cheering]
So we made it. Ha ha!
Norman: It was more of a revelation
than a disappointment.
Hey!
Awesome!
Norman, voice-over: We're used to what we can see.
And you give me a height, distance, place,
I can work with those because I can see it.
Time is there working, but you're not aware of it.
Things are always happening
in that fourth dimension, as well.
I guess this goes beyond being fashionably late.
[Chuckles]
Cheers, guys. Cheers.
Cheers. Congratulations.
Hawking: To travel in time,
we need to find a means of traveling
through the fourth dimension.
But what does this really mean?
To help understand,
I am giving my volunteers a tough new challenge.
In the next phase of their quest,
to discover if time travel is possible,
I hope the volunteers will grasp a concept
that revolutionized our understanding of the universe.
Marisol: Hmm?
Paul: Serious?
Hawking: If they can understand it,
then they really will be thinking like a genius.
Marisol: Thank you. Paul: Thank you.
Norman: My pleasure.
Norman, voice-over: We're walking in.
And at first, really,
I wasn't sure what I'm looking at.
Marisol: It's where we were.
Paul: Times Square.
Norman, voice-over: And then we all see
the maps of New York.
Paul: They're all the same map, aren't they?
Look, Broadway, Broadway, Broadway...
Marisol: At different times.
At different times.
Paul, voice-over: The maps were identical,
save for the time.
Marisol: This one says 50 minutes, 10 minutes,
+20,
+30, +40 minutes.
Hawking: Each map covers the area of New York
they traveled through to get to the party.
And each represents a moment in time.
The map on the light box has their start time
in Times Square.
Norman, look, it's you with hair.
Marisol: Ha ha! Look! Ha ha!
Paul: And mine's far too pretty.
[Marisol laughing]
Hawking: I have asked them to visualize their journey
from Times Square to the party in both space and time.
Let's see if they can get the purpose of this.
Paul: Oh! I've got it.
Each of these maps is a point in time, isn't it?
Mm-hmm. Right.
And we can put our positions on the maps
where we were at that point in time.
Norman, voice-over: Seven maps,
7 figures for each of us.
So it was pretty clear that each of us will represent
a certain spot during the journey
at a certain time.
Wh--where do we start, guys?
Right here. Times Square, 43rd.
We're going this way.
Here on the...
Paul: Should we see where we were
10 minutes later?
Hawking: The camera was not just for selfies.
Marisol on video: We're gonna go to a party!
[Laughter]
Hawking: Hopefully, it will come in handy.
Norman, voice-over: We were able to plot where we were
because we had the time signature
on the video.
Do you see the street sign?
Paul on video: 46th.
Marisol: So at 10 minutes,
we got to 46th street.
Let's do it.
You--you take your time, norm.
Ha ha!
Marisol, voice-over: It just kind of seemed natural
to stack the glass grids of Manhattan into layers
and to place us in each time label
that was on each table.
All right.
Then the next one, I think it's +20.
So if you guys want to grab it.
Yup, sure.
To me to you, to me to you.
Paul on video: Where are we?
48th. 48th.
48th. It's us...
This is the end of our first block.
Norman: 48th street.
Marisol: That's when we turned left.
Norman: And then we go to 30, +30.
Marisol: 30. Good job, guys.
Precision engineering, Norman.
Norman: We'll have to update our resumes.
Marisol: So 40 minutes. There we go.
Norman: 45.
All right. So we are on...
Paul: Destination!
Marisol: 11th and 48th...
Norman: And here's 48th. Yup.
At the party. There we go.
That's us!
Marisol: We did it, guys.
Awesome.
Paul: We've built a path of where we started
and where we finished
in space and time.
So you can think of each layer
as the spatial dimension, couldn't you,
and each level, if you like, as the time dimension.
Which is the fourth dimension.
Paul and Marisol: The fourth dimension.
Yeah. So we've kind of plotted
the fourth dimension.
So where does that get us?
[Chuckling]
Hawking: The conclusion I hope they will reach is one
that was dreamt up by a clerk in Switzerland.
He would become the most influential physicist
in the 20th century.
Radick: If you were placing bets
on who was about to revolutionize physics,
you wouldn't have bet on Albert Einstein.
26 years old, a violin playing bohemian
working not in one of the great labs,
but in the Swiss patent office in bern,
where he was a lowly patent examiner.
But Einstein toiled away
on some of the most fundamental problems of the day...
Because in Europe at that time,
one of the most pressing practical problems was
about how to coordinate the clocks
so that the trains ran on time.
[Clock ticking]
And Einstein was examining systems
which purported to do that.
[Whistle blows]
Hawking: Perhaps this work inspired him
to start thinking about time.
In 1905, Einstein published a paper
which changed the way we see space and time.
[Clock ticking]
Radick: Before Einstein, the conventional view was that
space and time were two completely different things.
Space was a kind of grand container
holding objects and events...
Whereas time was a universal flow carrying everything
from the present into the future.
Einstein came to a completely different view
on space and time.
They were not independent of one another.
They were aspects of a single thing--
a single fabric of space-time...
[Clock ticking]
Everything unfolding within those 4 dimensions always.
Hawking: Before the volunteers can really understand
how we might travel through time,
they'll need to make the same intellectual leap
that Einstein made.
Paul: We realized it was a model of our journey.
I think the realization of what it meant
came quite a bit later on, though. Ha ha!
Marisol: Traveling through time with this visual,
it seems like moving up, moving forward.
I mean, ultimately, i think we want to--
to be able to manipulate where we are
and when we are
and that the two are inter--intertwined.
Now with this you can see
you can't separate space from time,
even though we
don't typically look at it that way.
Are you saying it's not space and time? It's...
Norman and Marisol: Space-time.
Paul: Wow! Space-time.
It's no longer a separation,
for now we have a space-time map.
So both are all in the same...
Norman, we've brought together the 4 dimensions!
[Laughter]
All right!
What a stunning achievement!
Hawking: They've got it.
They have seen that their journey through space...
And their passage through time...
Are inextricably linked...
Time and space do not exist separately.
They are unified.
Hawking: Space-time fills every corner of the universe
and always has.
Every event in our lives has a unique reference point
within it,
both a physical location and a specific moment in time.
The same is true for every particle in the cosmos
and every moment in history.
I won't look at it the same way anymore
because I am where I am
and I'm also when I am.
Marisol: Space-time, it's the same thing.
And you can't separate them.
So we'd have to manipulate both to travel through time.
Hawking: Most people think of time travel
as a sudden disappearance from one moment in time...
And an instantaneous arrival in another...
Usually involving
a cleverly engineered time machine.
I hate to be a killjoy, but i doubt it will ever be possible
to jump through space-time in this way.
To understand time travel,
we need to think of it as a journey--
an expedition either forwards or backwards
along the fourth dimension.
Having grasped the concept of space-time, the volunteers are
ready to come to grips with time travel.
For their next time travel challenge,
my team will need plenty of room,
so we've left New York and come
to the beautiful desert of Arizona.
I want them to imagine traveling backwards in time.
Paul: Yeah. Look. Lines.
Hawking: And there's another surprise in store.
Marisol: So I'm walking in.
And there's a dirt area.
And there's 3 tracks.
Norman: Look at this.
Arrows.
Norman: We got 2 "forwards."
What's that one say down there?
Marisol: Yeah. Let's check it out.
Norman, voice-over: There's a Lane
with an arrow pointing away from us that says "forward."
Paul: The second Lane says "back"
and was labeled back.
Marisol: And then the next one
is a third track with "forward" again.
Paul: Two "forward," one "back."
Hawking: Imagine this is a magical parking lot
that will allow them to travel through the dimension of time.
But to do that, they'll need a time machine.
[Paul laughing]
Norman: Oh, look at that!
Hey! Ha ha! Well...
All right!
Marisol: A delorean.
Paul, voice-over: To our absolute amazement,
a delorean came spinning round the corner.
[Excited chatter, laughter]
Paul, voice-over: And then the driver jumps out,
wangled keys in front of us.
And no right-minded person confronted with delorean keys
would not grab them.
Hey!
Ha ha! Marisol: Awesome.
Instructions.
"Imagine this is a magical parking lot."
Hawking: Imagine each Lane represents
the same passage through space-time.
In the first Lane, they will be traveling forwards with time
in the usual way just like the rest of us.
In the second, they become time travelers
as they retrace their route backwards
through space-time to the moment they began.
[Clock ticking]
In the third Lane, they will be traveling forwards with time
in the usual way once again.
[Clock ticking]
Marisol: "You must complete all 3 lanes of the course.
"But before you set off down each Lane,
think about what you will see."
Hawking: Their challenge is to predict...
All 3 lanes.
What it would look like to travel backwards in time.
The first time we go down,
we'll see nothing out of the ordinary,
except us in the delorean.
We get to the end.
We come back in time.
And at some stage,
we will pass ourselves going forward in time, won't we?
Yeah.
Norman: Are you sure?
Hawking: Norman is right to question this plan.
It has a flaw.
Paul: Here's a radio!
We'll call you from the future.
All right.
Hawking: But perhaps once they're underway,
they'll spot their mistake.
Paul: Time to fire up the delorean.
[Engine revs]
Marisol: Fire it up! Ha ha!
Bye! Ha ha!
We're in a delorean. Into the future.
Awesome.
Paul, voice-over: Our initial thought was,
when we came back, we thought that we would pass ourselves
or--or we'd pass ourselves at about the halfway point.
We hadn't considered at that point what we had missed.
Paul: Let's slide out,
ready to go back to the past.
All right. Ok.
Paul: All right. Let's give Norman a call.
Marisol: Hey, norm,
what we should expect to see when we go backwards
through the space-time is ourselves.
There's going to be another delorean
just passing by.
What do you think?
Norman: I--i don't know.
Hawking: They're right about one thing.
If they journey back through space-time,
they would see themselves.
So for that, they're going to need another car.
[Paul laughing]
Paul, voice-over: All of a sudden,
another delorean appears where we were.
And to our utter amazement, now, there's not only--
now there's not one delorean.
Now there's two deloreans!
Norm, who is that? Is that us?
Norman: Yeah. Yup, that is you.
[Both laughing]
Let's go back in time. Let's go back in time.
Hawking: Let's see if coming face-to-face with themselves
helps them realize their mistake.
[Marisol and Paul laughing]
It's us! It's us!
[Laughter]
Marisol, voice-over: It was actually us.
Like, our faces were there--ha ha!--
just staring at us.
Paul, voice-over: We started to see flaws in our logic
at that point.
Hawking: I think this crossing of paths is
what most people would predict.
But I don't see backward time travel this way.
They must think again.
Paul: Now, what would you expect to see
if you retraced your steps
back through space-time? Well, think about it.
You would see--no. You would--
both: See yourself...
Go. Going what?
You'd see yourself going backwards, wouldn't you?
Imagine--at the top of that space-time model and coming down it.
You'd see yourself going backwards, wouldn't you?
Marisol: Yeah.
Hawking: Paul has spotted their error.
If you imagine going forwards
through the time dimension one way,
then returning back the other,
you would not cross paths with yourself.
Instead, you would see yourself going backwards.
Once you returned,
you would continue to travel forwards with time once again...
Only now there would be three of you...
Which calls for another car.
Marisol: Awesome!
[Marisol and Paul laughing]
Marisol: Look at us! Oh, my god!
[Laughter]
Norman, voice-over: When we left,
we created a new future.
And that's when the third car pops up.
[Laughter]
Hawking: To make this clear,
let's try it once more from the beginning.
All right.
We're gonna drive into the future.
From forwards to back to forwards.
Marisol: And here we go again.
Oh, this is trippy.
[Laughter]
Hawking: In a linear journey through space-time,
Paul and Marisol would see their future selves
traveling back in time and moving forwards with time
after their backward journey.
Paul: Three deloreans! 1, 2, 3.
[Laughter]
Paul, voice-over: When you move forward in time,
lanes one and 3 are going in the same direction,
relative to Lane 2.
It's going backwards.
Hawking: As they journey back through time,
they would be flanked by
their forward-traveling past selves
and their forward-traveling future selves.
Norman: Everything is happening in sync.
It all has to happen together. It all...
You're moving forward and backwards together.
Hawking: Once they'd stop time traveling
and time passed normally for them once more,
they'd see both sets of their past selves
still traveling through time.
Marisol: It's so weird looking at--at myself.
You look so serious.
Paul: And to them, it's gonna look like
we've gone backwards in time.
[Laughing]
Hawking: There's a fundamental issue with backward time travel.
Let's see if my volunteers have spotted it.
Paul: Coming back starts to get
a bit problematic
because we witnessed ourselves
going forward in time, didn't we?
So at that point, there's now two of us.
Something has to pop into existence
and pop out and...
At the moment, well, if you just think, it's not possible,
because I don't see how we can have two of us
in existence at the same time.
Marisol: That-- that's what I'm saying. It always seems like...
Norman, voice-over: You can't be making things
out of thin air,
that there's living, breathing creatures
and mass and energy showing up essentially out of nothing.
That's not gonna happen.
Norman: If you go back,
there's--you've created something that was
never there in the first place.
You can't go back.
Marisol, voice-over: We came to the conclusion
that most likely
traveling back in time was impossible.
So it was nice to actually have it checked off the list.
Hawking: They're right.
The laws of nature do not support
the sudden appearance
of such large quantities of new matter.
In other words, duplicates of time machines and people
could not flash into existence.
So backward time travel is unlikely to work.
It's disappointing, but I prefer to look on the bright side.
At least history is safe
from meddling time-traveling tourists.
[Crowd cheering]
Now that they've ruled out traveling backwards in time,
my volunteers need to think about traveling to the future.
We've come to the university of Arizona.
The volunteers will be surrounded
by inquisitive minds,
as they endeavor to find out if it's possible to travel
forwards through time.
Marisol: I see a line of clocks
with no numbers and a chest.
It almost looks like a treasure chest, a box.
Paul: Dare we? Marisol: Do we?
1...
2...
What the heck?
Norman: Timers. Ha ha!
Paul, voice-over: There were several counters.
They were counting devices.
It's a parachute.
Let's get it out.
Norman, voice-over: In the beginning, we just figured
it was packing to hold the counters,
and then some inspiration came to us.
Marisol: There's a plug over here.
Do you want to give it a whirl, then?
Definitely! Gosh.
There we go. Ready?
Yup. Coming on.
All right.
It inflates.
Paul: Shall I get underneath it?
Norman: Yes!
[Marisol laughing]
Marisol: Have you got it?
Yeah, b--because it's gonna come up.
Hawking: Once inflated, they will have
everything they need to work out
how to travel to the future.
Hawking: Switching the clocks on may help.
Paul: Ah!
Norman: They're moving at different speeds.
Marisol: They're all going different speeds.
Norman: Yeah, they are.
Paul, voice-over: At first it looked like
they were running in some kind of random order.
But it became apparent when you stood back
they were actually running slower and slower and slower
the further away you got from the first one.
Marisol: What do you guys think?
Paul: There seems to be a pattern,
doesn't there, in the...
This one's whizzing round... Yeah.
This one's going a little bit slower
than that one. Right.
Hawking: Time is behaving strangely
near their inflatable.
But will this help them work out what it represents?
Paul, voice-over: We wasn't too sure
what the significance of it was.
It looked like a planet at first,
and then realized it was--it was a black hole.
Black hole?
Black hole.
Hawking: Black holes are stranger
than anything dreamt up by science fiction writers,
but they are firmly science fact.
A black hole is a star with an extreme amount of mass
compacted into a relatively small amount of space.
This mass creates the strongest gravitational field
in the universe...
A field so strong that the star's own light
cannot escape,
which is why they are called black holes.
Albert Einstein discovered how they function
when he turned his attention
to something that had been puzzling him--
gravity.
[Motor running]
Filippenko: So Einstein said, you know,
"none of you guys...
"None of you physicists really has
"an adequate theory of gravity. And I don't either,
but I'm gonna come up with one."
What Einstein came up with is that the presence of mass
actually bends or warps the shape
of both space and time in its surroundings.
And other objects then follow
their natural paths through this intrinsically curved space.
That's what the sun does, so earth orbits it.
That's what earth does,
so the moon orbits it.
That's what the moon does,
so rockets that we send up there orbit it.
Hawking: Einstein said this warping of space-time is
what creates gravity
and that it also alters time itself.
[Clock ticking]
And that time passes at different speeds
depending on how strong the gravity is.
The stronger the gravity, the more pronounced the effect.
[Clock ticking speed slows down]
He called his theory general relativity.
Well, most people reacted
in a very puzzled and incredulous way
when Einstein first said that this is
what gravity is, you know.
"What the heck?" You know, "what are you talking about?"
Hawking: But when we apply general relativity
to black holes, things get truly strange.
Filippenko: A black hole's a place where gravity is
at its strongest.
You can't get a place in the universe where gravity
is stronger than what's in a black hole.
In Einstein's general relativity,
the passage of time slows down to a halt
at the boundary of a black hole.
Regions near the mass are bent or curved more.
And in a similar way, time is slowed down more.
Hawking: This is a hard concept to grasp.
Let's try an experiment to make it clear.
Can the volunteers work out how warped time near a black hole
can be used to travel to the future?
Can you use a black hole to travel to the future?
Norman: Travel to the future? Yeah.
Paul: So if you were standing at the first clock,
time would appear to be going very slowly to you,
wouldn't it, compared to somebody here.
Mm-hmm. Yes.
If you came back from that point to here...
Time would already have been going ahead,
so you're in the future.
Paul: Exactly. Norman: Right.
Marisol: Ok.
Norman: We staged ourselves
one at the nearest clock and one at the furthest
and one in the middle.
And we did counts of how many times we went past m--midnight.
Norman: We'll synch on 12.
And go.
1...
2...
3...
[Laughing] 4...
5...
6...
Hawking: They are getting the idea.
The speed at which time is passing
is relative to their proximity to the black hole.
3.
Marisol, voice-over: I was counting on the fastest clock.
And when I got to 20, we all stopped.
19, 20.
Stop! Paul: Stop!
Paul: We're coming back to the present. Ha ha!
All right, guys. Ha ha!
All right, guys. What did you get?
Paul: This is how many
units of time have passed.
Let's call it days. Days.
Ok, after 3. Norman: All right.
1, 2, 3. ...2, 3.
How many have you got?
1. 20.
Paul: Well I'm on 5.
So let me get in the middle. All right.
So I saw one day; You saw 5; And you saw 20.
I saw 20.
So we can use a black hole to travel to the future.
Awesome.
Hawking: It seems counterintuitive,
but hanging out in a place where time passes slowly
allows us to travel to the future.
When we return to where our journey began,
we will have experienced less time
relative to those we left behind.
But to do this in reality, we would require a spacecraft.
[Participants laughing]
Paul: I'll give you a countdown.
Norman: Countdown!
5, 4, 3, 2, 1.
Engines go! [Norman yells]
Take off. [Marisol laughing]
Norman, voice-over: We were outfitted
with the latest and greatest equipment
for making the trip to the black hole.
Marisol: Good timing, captain.
[Both mock yelling]
Norman: Pulling a lot of gs, pulling a lot of gs!
Norman, voice-over: You could look at it
as just driving around in circles on a golf cart,
or, you know, you're really focused.
You're in a spaceship going around a black hole.
Marisol: We're at 2.
Hawking: Imagine Norman and Marisol
have traveled 26,000 light years
to the super massive black hole at the center of our galaxy.
If they managed to go into orbit without getting sucked in,
time would pass normally for them.
But they might come back millions,
even billions of years later.
[Imitates gravelly voice] Oh! Welcome back
to earth, guys.
You've been gone 70 years!
Norman: Wow.
Marisol: Really. Yeah! We got 3.
Paul: Oh, so, you guys have come into the future.
Welcome to the future.
Thank you. Thank you.
Only 3 years have gone by.
Wow.
Norman: Ok. Well, there you go.
We can use a black hole
to travel forward in time.
Hawking: Though it is not the jump through time
that popular culture prefers,
this is a method of time travel
that is theoretically achievable.
But can my volunteers work out
why time behaves this way around a black hole?
Paul, voice-over: As I think about it,
as the gravitational strength gets stronger and stronger,
I would expect time to actually speed up,
not slow down.
T--oh! Of course, of course.
You've got time, and it's being stretched.
So, of course, if I take one unit and stretch it,
it's worth more of the other units
the further you go.
And, of course, the closer and closer you get
to the black hole, the more and more time gets stretched.
That's the explanation, isn't it?
Hawking: Paul's got it.
Norman: It's amazing to see it
play out like this...
Marisol: Yeah. To actually be in it physically.
Norman: Yeah.
Awesome. Good job, guys.
Thank you. Whoo!
Marisol, voice-over: So it's not so much
as learning how to manipulate space-time.
It is just finding the location
where space-time is already distorted and altered
and then using it to our advantage
in order to go into the future.
Paul: If you can imagine a theoretical space journey
for a length of time,
come back younger than your children--
so, yeah, it's a--it is a crazy idea
to think that, you know,
you will literally be traveling in time.
[Clock winding down]
Hawking: Now they've worked out that we can use
the warping of space-time to travel into the future,
they are ready to attempt this for real...
Here on earth.
As my volunteers try to discover
whether we can travel through time,
I've arranged for them to attempt
a highly unusual experiment.
Over the next 24 hours,
their challenge is to send one of them into the future
right here in Arizona.
And yesterday's challenge was using a black hole
to travel to the future.
The use of the black hole for traveling through time
was because of the intense gravity there.
Paul: Sure.
So we do have gravity here.
Hawking: This is the perfect place to try time travel.
Let's see if they can work out why.
Paul: I would guess we can use
our physical locations to give us 2 reference points.
We just need to go higher.
And I can see something right in front of us
that will let us do that.
Like what, Paul?
Paul: Like that mountain over there, Norman!
[Laughter]
Paul: You could have one at the top of the mountain,
one at the bottom of the mountain.
That gives us the same thing we had yesterday.
We need clickers basically. Yeah, we need some...
We need clickers at each location.
Really good clickers. Yeah. That's what we need.
Hawking: They are on the right track.
Just like with a black hole,
gravity is stronger nearer earth's mass,
making time pass more slowly.
High on a mountain, further away from earth's mass,
gravity is weaker, and time there passes faster.
To see the time difference,
I've arranged for a world expert on clocks
to loan them some impressive equipment.
So tom opens the back of his trunk.
Norman: H--hey! All right. Wow!
Marisol, voice-over: And it looks like old--
like 1980s computer servers, all with a clock,
six of them going at the same time.
Like, exactly at the same time.
These are cesium atomic clocks.
Atomic clocks! They are.
They are so accurate
that it takes millions of years for there to be
one second of error.
Perfect.
So we've got a challenge
that's going to require us to have a clock
with a higher elevation.
So if you could lend us a couple of clocks...
Sure! You need at least two clocks. No problem.
At least two. Yeah.
Hawking: The clocks have been set
to coordinated universal time.
Right now, they are all perfectly synchronized
to each other.
Marisol: Perfect fit. Oh, yeah. Good to go.
Hawking: Though they are stunningly precise,
cesium clocks are fragile.
And it's going to be cold where this one's going.
[Norman laughs]
Norman: Ok, watch out, Marisol.
[Indistinct chatter]
Ok. All right. Well,
we're gonna go up. You stay down.
And, uh, we'll see you at the end.
Hawking: By climbing to a high elevation,
Norman and Marisol should experience
a different amount of time to Paul,
who will remain closer to the mass of the earth.
It's gonna be fun.
Marisol: Oh, my god. Ha ha!
Hawking: Tomorrow Paul can join them on the mountain
to compare their clocks
and see how gravity has warped space-time.
Marisol, voice-over: The landscape is beautiful
and keeps on changing.
We get higher and higher.
And then suddenly the cactus around us
disappeared, and then turn into, like, alpines and snow.
Norman: It's a, uh, world of difference,
being up and out of the desert.
Marisol: A completely different world.
Hawking: The higher they climb,
the weaker the earth's gravitational pull becomes.
It's not by much, but time is passing faster as they go.
I don't know how much higher we can get.
But I think we are doing a good job about
reaching the highest elevation we can.
Yeah, looks like we're almost at the top.
Norman: Oh! Look at the dome!
Marisol: Nice. Ha ha!
Norman, voice-over: We saw the tower was the--
the highest point.
Marisol: All right. Let's get this show on the road.
Norman, voice-over: And it's like we've already come up,
you know, 6,000 feet or so.
Do we really need to bring it up that last little bit
to get it up the tower?
But, you know, if you're going to do it, then do it.
All right. God.
Marisol: Oh, my gosh. That is heavy.
This better work, 'cause I'm not doing that again.
All right. Let's see.
So we got it up here.
And we are at...
What have we got? 9,091 feet.
Nice!
Hawking: At the peak of mount lemmon,
they've reached 9,091 feet.
Below, Paul is at 2,357.
The differing strengths of gravity over this range
should be enough to allow them to time travel on earth.
Hawking: Their task for the next 24 hours is simple--
sit back and wait,
perhaps with some light reading,
while the warping of space-time takes effect.
[Ring]
Norman: Paul!
Norman, Norman, how are you?
I'm fine, Paul. I could not be better.
Ha ha!
Paul: I'm glad to hear
you've not been eaten by a mountain lion.
That's right. So far. It's still early.
Ha ha!
Marisol: Do you have an idea of what possibly
is gonna happen tomorrow?
Paul: Uh, well, I'd expect my clock
to run a little slower than yours. You know,
I'm closer to the center of the earth than you are.
I don't really know what the difference is gonna be.
Um, so, yeah, I'm curious to see--
I'm interested to see what the difference is...
Hawking: In their attempt to travel
into the future here on earth,
my volunteers' 24-hour wait is over.
By comparing their clocks,
we can see if they've accomplished their goal.
Paul: Good morning! Guys, how are you?
Good. How was it?
Norman: Good.
Paul: This is going to be very tricky.
Marisol: We can do it!
So 24 hours later, we transfer
Paul's atomic clock up to the tower.
Norman: Then it was just kind of waiting
to open our Christmas present to see what was in there.
Are you ready for your results?
Marisol: Yes!
Norman: Am I ever!
Here's the clock that we brought up yesterday.
Yup. Yup.
It's been up here
probably running faster, gaining time.
Here's the clock we left down at the hotel
for the whole day.
We think this clock should be ahead of this clock.
And the answer is...
20.485 nanoseconds.
Marisol and Norman: Nanoseconds!
Tom: 20 billionths of a second.
Paul: Wow. Norman: All right.
Just seeing the number was amazing
because it actually did happen.
Marisol: Oh, my gosh!
That's the difference between these two clocks.
And that was time travel.
That was time travel.
Marisol, voice-over: Um, it's kind of unbelievable
and, um, amazing that it can actually be calculated
and it can actually be done within 24 hours.
Hawking: Having joined them at the mountaintop,
Paul has traveled into their future
by 20 nanoseconds.
Paul, voice-over: It's incredible.
I was 20 billionths of a second behind the other clock.
So really I am 20 billionths of a second younger
than the other two guys.
Hawking: Just like with a black hole,
by being nearer the mass of the earth,
Paul has experienced a slower passing of time
than Marisol and Norman.
Wow! Look at the view!
Marisol: Oh! Ah, ooh. Thank you...
Hawking: They are truly time travelers.
Norman: Wow. Look at that. Thank you.
Marisol: It's pretty unbelievable
that you were just down there,
and we just took a quick trip up the mountain.
And time sped up, our clock was faster up here.
So it's just cool that we can have, like,
binoculars and...Just look at people down there.
And technically they're in the past.
Norman: The idea of time travel where it's always seemed
as this far-fetched thing,
it's happening all the time, albeit
in little, tiny increments.
The world is working in a-- in a way
that you hadn't really paid attention to.
Marisol: Lower.
Norman: You, sir, have traveled into the future.
So you've allowed me to join you.
I've time traveled to join you in your present.
[Laughter]
Time traveler.
Paul: Time traveler.
Uh, is that worth a high five?
Marisol: Yeah! Ha ha!
Whoo!
Paul, voice-over: It's an incredible demonstration
of how gravity affects time,
how you can by definition move forwards through time.
Stephen: Hello, my budding geniuses.
Marisol, voice-over: In the beginning I was thinking,
time travel
is just like a ridiculous idea,
so it was pretty amazing to actually get an answer.
Tom: 20 billionths of a second.
Paul: Wow.
Yes, we can travel through time.
We did it. We actually did it.
Hawking: The volunteers have come
a long way on their journey.
They've understood that the fourth dimension of time
is unified with space
into the single entity of space-time.
Traveling to the past has been ruled out.
But they've learned how to use gravity's warping of space-time
to travel to the future.
Norman: Each challenge has helped build
a better understanding of how time works,
not just what it takes for time travel,
but how it actually works.
And it's good to have.
You've been gone 70 years!
Norman: Wow.
So we've come to the conclusion at the end of the week
that it is possible to travel forward through time.
Wow!
Paul, voice-over: And, uh, yeah.
It's a lovely feeling to draw that conclusion--
to have those Eureka moments.
It's got to be worth one, then! Come on!
[Laughter]
Hawking: We all travel in time all the time.
When you fly in an airplane, climb a mountain,
or even use an elevator,
you are altering the speed at which you get to the future.
As for me, it's time to go.
Hawking: Are we alone
in the universe?
♪