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There's an aspect of our lives
that we all take for granted.

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It's something so familiar,
you usually don't
give it a second thought.

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One of the key elements
of being a human being is that
we can tell the time.

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It allows us to make sense
of the world -

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past, present and the future.

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But it's nowhere near as
straightforward as you might think.

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I'm Professor Brian Cox and I want
to find out what makes time tick.

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We have ways to measure the passing
of time but we don't know what
happens when time passes.

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I can't even tell you at the
moment what at the moment means.

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Even momentarily.

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Knowing the time is not easy.

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I' damned sure that flow of time is
ultimately an illusion.

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There's no big clock in the sky
which ticks away at the same rate

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for everybody, no matter where
they are or what they're doing.

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Did time have a beginning?

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Why does time tick?

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And does our future already exist?

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I'm going to try and
answer one of the simplest
questions you could ask -

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"What time is it?"

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So, what time is it now? It's 5:58.
What time have we got to be there?

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The sun rises at 6:11.

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So we've got about 13 minutes.

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"What time is it?"
is a question that humanity
has asked for millennia.

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Many of our ancient civilisations
went to great lengths

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to try and answer
this simple question.

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None more so than the Maya.

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I've come to the Yucatan Peninsula
of Mexico to see the ancient
Mayan temples at Chichen Itza.

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You're not usually allowed
to do this. But because it's
6am and there's nobody here,

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we've got permission
to walk up the pyramid.

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It's just absolutely incredible.

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The Maya had an obsession
with time, working out intricate
ways to track its passing.

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The Maya didn't have clocks, they
didn't know how to build timepieces.

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But they used the world around them
to keep time extremely precisely.

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And they had interlocking cycles
of time, incredibly complicated,
that were based on the movement of

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all the bright celestial
objects you can see in the sky.

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If you think our lives are ruled by
time, the Maya took time worship
to a whole new level.

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They didn't just track its passing.

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They believed they had to help
time pass.

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They thought that
time actually created space

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and they thought that they had to
feed the sun, to help it go through
the sky, in order to make the space,

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you know, make their world.

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And they fed the sun with
blood, with human blood.

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So they made blood sacrifices
to the sun to kind of sustain it
on its journey across the sky.

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Such was their fixation with
knowing the time,

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that time itself is manifest
in Mayan architecture.

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Everything about this building
is about time.

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There are 91 steps
on the way up and there are
four sides to the building.

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That's four 91s, 364, and then
one step at the top to make 365.

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So the building itself is

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a visual representation
of the passing of time.

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This is the first stop
on my journey

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to try and answer what appears at
first to be a very simple question.

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Telling the time was something
the Maya thought they'd mastered.

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But it's not as easy
as you might think.

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I'd like to understand what time is.

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And it's a very tall order.

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Some of the best physicists in the
world are working on that really
simple question - what is time?

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And the Maya thought they knew,

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2,000 years ago.

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Newton thought he knew
400 years ago.

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Einstein kind of thought he knew
for about six months in 1915 and
then changed his mind in 1916.

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And it's a problem that's
confounded us to this day.

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This is truly the unknown.

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What time is it?

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I don't know.

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Do you know?

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Since the dawn of civilisation,
every culture has tracked
the passing of time by looking up.

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As the earth rotates, the sun,
moon and stars move across the sky,

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giving us a daily rhythm -
an instinctive way to tell the time.

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So if you want to know what time
it is, you don't need a watch,
you don't need a clock.

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You just need to look up at the sun,
that regular rising and setting, the
clockwork of the passing of the day.

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But actually, it turns out that that
regularity, the length of the day,

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isn't quite as precise
as you might think.

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For the last four billion years
or so, the speed our planet spins
has been gradually slowing down.

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The length of a day has been
steadily getting longer.

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It's all down to the moon.

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You see, the moon holds the water
on the surface of the earth towards
it so you get a tidal bulge.

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And the earth spins underneath
that bulge and that's what you see
as the tide's coming in and out.

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And there's friction between the
surface of the earth and the water

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which means that the spin,
or the rate of spin of
the earth, slows down.

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And that causes the day to lengthen.

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600 million years ago, the day was
just under 22 hours long.

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But the earth isn't just slowing
down over millions of years.

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The very thing that we relied
on for telling the time,

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the fact that the earth takes
24 hours to spin once on its axis,

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is something we now realise we
can't rely on even from day to day.

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For the last 30 years, measuring
the exact length of each day
has been the job of scientists

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at the Westford Observatory,
just north of Boston, Massachusetts.

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There are lots of different ways
of measuring the spin of the earth

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but strangely, one of
the best is to look for light

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that began its journey
a long time ago,

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from a galaxy far, far away.

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Unlike the Maya, the scientists
here don't rely on the sun
to track the spin of the earth.

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To make these measurements, you
need something that's as steady as
possible, as far away as possible.

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So, as bright as possible.
And the brightest things

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we know of in the universe
are these things called quasars.

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They're, we think,
the nuclei of galaxies.

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So, a big, huge black hole, sucking
loads of dust and stars into them,
throwing out radio waves.

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And it's those radio waves
that we actually use, remarkably,
to tell the time on the earth.

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Thanks.

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Arthur Niell is the man
who keeps track of time.

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Oh yeah! Mind your ears,
probably pops just now.

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This is like an airlock.
It is an airlock.

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How do you make this measurement?

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What we actually use are two
radio telescopes,

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actually two or more, but with two
radio telescopes, we can do this.

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And what we measure is this
radio wave coming in from space.

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As it comes in, we look to see the
instant of time when this radio wave

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arrives at the same time
at two different telescopes.

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When a quasar is directly overhead,
the radio waves arrive at the two

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separate telescopes in synch
and the stopwatch is set running.

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As the earth spins, the radio waves
drift out of synch,

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taking different times
to reach each telescope.

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But after one full rotation,

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the two telescopes are back in
synch and the clock is stopped.

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This gives you the exact time for
one rotation of our planet -

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one day.

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But it's never quite the same.

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Sometimes, you get there a little
early and sometimes you get there

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a little bit late relative to
the clock we have on earth that's
beating out very regular seconds.

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So, 24 hours...

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You come round 24 hours
and you think,

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"Uh-oh, we're not quite there yet. "

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And so, the earth rotates
a little bit more.

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Sometimes, it speeds up a little
and gets there a earlier

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compared to the time
that you expect it to.

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So the day is, in a sense,
the length of day is wobbling?

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Yes, the length of the day is not
24 hours exactly.

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Surprisingly,
earth's daily erratic speed changes

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are mostly caused by wind.

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As the winds speed up or slow down,
by the way they push on the mountains

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or the friction on the surface of
the earth, they then slow down or
speed up the solid part of the earth.

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Down in the control room, Arthur has
the most recent timing for a day.

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The latest measurement that came
back was from just two days ago,

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and the length of day was about
1.9 milliseconds more than 24 hours.

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So, 24 hours, 1.9 milliseconds?

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Right.
Makes a difference, it all adds up.

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Because the earth doesn't spin at
a steady rate, using the movement

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of the sun or other nearby celestial
bodies to tell you what time it is,

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is not going to be very reliable.

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The earth's spin's erratic.

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It changes from day to day,

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so you can't use that
age-old way of telling the time.

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To know what time it is, you have
to stop looking into the sky

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and looking at the stars and
the planets, and you have to look
down into the world of the small.

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You have to look into the world
of the atom to tell the time.

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The tree's in the way!

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If there's one place on earth where
you can come to find the time,

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it's here in Washington DC.

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This is the home of
the US Naval Observatory,

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one of a select bunch of time lords

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dotted across the planet who are
the keepers of time on earth...

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Universal Time,
19 hours, 54 minutes exactly.

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.. a time now derived
from atomic clocks.

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Dennis McCarthy, director of time.

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We defined a time in the 1950s
based on the position of the moon
with respect to the stars.

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We needed a more
accurate kind of time.

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Not only does it have to
be more accurate

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but it has to be more accessible.

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That accessible kind of time
is provided by atomic clocks.

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What we need to tell time is
something which repeats with great
regularity, that you can count on.

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How well we can tell time
depends on which atom you're using.

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So we choose certain ones
to use for keeping atomic time.

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So, how can atoms give us the time?

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What you can do...

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An atom is

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very crudely...

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The atomic nucleus sits
there and electrons sit
in orbit around the nucleus.

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But they only sit
in specific places,
so you can't have them anywhere.

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You can have them here
and here and here.

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And when they jump up to there,

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and then back down to there again,
they emit light.

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And that light has
a particular frequency.

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And it's this light that allows
us to tell the time so precisely.

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Inside the clocks are the atoms
of a rare metal called caesium.

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The electrons in the caesium atoms
are made to jump up.

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Then, as they fall back down,
they give out light.

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These light waves peak over
nine billion times every second.

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And it's this light
that drives the clock,

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effectively producing nine billion
ticks for each atomic second.

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This number never
changes, never alters,
and that's why it's so accurate.

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So, the atomic clock is actually
putting out an electronic signal

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which is essentially analogous to
the ticking of a pendulum clock,

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you know, a pendulum clock which
might tick once every second or
once every couple of seconds.

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This thing is providing us
something which is going
nine billion times per second.

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So it provides us with a very
fine definition of time.

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We actually have a number of
clocks at the Naval Observatory
located all over the grounds.

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Here's one of them.

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This is the Master Clock System One.

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This is the Master Clock System Two.

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So if I want an answer to
the question, "What time is it?",

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there it is? That's it.

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'Universal time,
20 hours, 0 minutes exactly. '

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Atomic time is the heartbeat
driving modern life.

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It acts as a single global
time frame, a way to synchronise

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the world with a time stamp
accurate to a billionth of a second.

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It plays a key role in everything
from navigation and scheduling
to global communication.

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It is THE definition of
time on earth.

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Problem solved!

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Well, not quite.

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Knowing atomic time is actually
somewhat irrelevant to answering
the question, "What time is it?"

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The labels you give to time
2008, May the 21st - it's...

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00:16:25,035 --> 00:16:26,617
What day is it today?

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12th. 12th, right.

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To us, this is the year 2008
because a Pope defined it
to be so about 500 years ago.

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In the Islamic calendar, it's 1429.

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00:16:39,532 --> 00:16:42,645
In the Jewish calendar, it's 5768.

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00:16:42,680 --> 00:16:49,000
And in 2012, in the Mayan
calendar, it'll be the end
of the long count - 130000.

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All different, all arbitrary.

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So, if you really want to know
what time it is, then we're going
to have to go a little deeper.

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The time that ticks on
your watch is the time now,
the time of the present.

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00:17:08,480 --> 00:17:16,400
But the feeling we experience as
the present time is something we
shouldn't take for granted.

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00:17:16,435 --> 00:17:21,600
Much of what we believe is in
the present is drawn from the past.

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What we feel is happening now
happened a little while ago.

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We feel that we experience a now
around us -

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everything we see happen now.

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00:17:33,280 --> 00:17:39,560
But actually, the light coming
from distant things into your
eye takes time to get there.

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So, you look at the sun.

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The sun is 93 million miles away.

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00:17:47,800 --> 00:17:53,760
That means light takes over eight
minutes to get from it into my eyes.

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00:17:53,795 --> 00:17:57,517
So I'm seeing the sun as it was
eight minutes in the past.

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It could explode and I
wouldn't notice for eight minutes.

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I'd just see that beautiful
image of the setting sun.

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00:18:06,400 --> 00:18:12,445
To answer the question what time is
it, we need to know when time began.

225
00:18:12,480 --> 00:18:18,845
The fact that light
travels at a finite speed
offers us a unique opportunity.

226
00:18:18,880 --> 00:18:25,920
It allows us to look back,
not just eight minutes, but
millions, even billions of years.

227
00:18:31,200 --> 00:18:36,365
I've come to Baltimore
to look back in time.

228
00:18:36,400 --> 00:18:41,280
Former director of the Hubble
space telescope Steve Beckwith

229
00:18:41,315 --> 00:18:45,720
was responsible for taking
an extraordinary photograph.

230
00:18:45,755 --> 00:18:47,885
As a director,
I had at my discretion

231
00:18:47,920 --> 00:18:51,480
10% of the telescope time per year
that I could use for anything.

232
00:18:51,515 --> 00:18:55,005
One year, I took all of my time,
in fact I took a little bit more

233
00:18:55,040 --> 00:19:00,240
than all of my time and decided that
we would devote it to the deepest
picture ever taken of the universe.

234
00:19:02,080 --> 00:19:07,360
In 2004, Steve pointed
the Hubble telescope at a tiny
piece of the night sky

235
00:19:07,395 --> 00:19:12,160
and took a picture
called the Ultra-Deep Field.

236
00:19:14,120 --> 00:19:17,760
It took a million
seconds of exposure on
the Hubble space telescope,

237
00:19:17,795 --> 00:19:20,605
the world's most powerful
telescope.

238
00:19:20,640 --> 00:19:25,160
And in this image, we can look
back in time 13 billion years.

239
00:19:25,195 --> 00:19:28,360
It's a difficult picture
almost to comprehend, isn't it?

240
00:19:28,395 --> 00:19:30,285
Because in some sense, it's...

241
00:19:30,320 --> 00:19:33,565
3D is the wrong word,
but it's some sense...

242
00:19:33,600 --> 00:19:37,200
Oh, no, it IS the right word. It
IS 3D. We are looking back in time.

243
00:19:39,440 --> 00:19:44,085
Every single galaxy in this image
can be dated.

244
00:19:44,120 --> 00:19:48,960
This galaxy emitted its
light when the universe
was 8.8 billion years old.

245
00:19:48,995 --> 00:19:52,400
Then, as you go back in time, this
is a galaxy that emitted its light

246
00:19:52,435 --> 00:19:54,685
when the universe was
3.3 billion years old.

247
00:19:54,720 --> 00:19:58,365
You can see it looks completely
different. It's really very chaotic.

248
00:19:58,400 --> 00:20:04,480
And this is one billion
years after the Big Bang,
very red, a little tail, very small.

249
00:20:06,160 --> 00:20:12,200
The most distant galaxy in the
Ultra-Deep Field is a red one that's
right over here. This one here?

250
00:20:12,235 --> 00:20:17,960
The light from that was emitted
when the universe was 700
to 800 million years old.

251
00:20:17,995 --> 00:20:19,765
So, really,

252
00:20:19,800 --> 00:20:24,360
this is one of the first structures
that formed in the universe?

253
00:20:24,395 --> 00:20:28,920
One of the first formed and one of
the first we've been able to see.

254
00:20:28,955 --> 00:20:32,717
In a sense, you see this almost...

255
00:20:32,752 --> 00:20:36,445
I was going to say paradoxical,

256
00:20:36,480 --> 00:20:41,240
but strange behaviour of the
universe as revealed by astronomy.

257
00:20:41,275 --> 00:20:46,000
Because I'm trying to say, "Well,
what does that look like now?"

258
00:20:46,035 --> 00:20:48,245
"What would that look like now?"

259
00:20:48,280 --> 00:20:54,400
In a sense, it's the wrong
language to use, isn't it?
That's what it looks like now.

260
00:20:54,435 --> 00:20:55,920
That's right.

261
00:20:57,200 --> 00:21:03,920
Steve has turned his photo into
a movie to journey back in time.

262
00:21:03,955 --> 00:21:06,605
You see these little
pieces coming at us?

263
00:21:06,640 --> 00:21:10,200
We're going back in time. You can
see the three-dimensional effect.

264
00:21:10,235 --> 00:21:12,880
Some of these others are
a little farther back,

265
00:21:12,915 --> 00:21:14,325
but here we're going back,

266
00:21:14,360 --> 00:21:18,600
we're probably back now about three
billion years from the present.

267
00:21:18,635 --> 00:21:22,077
As we keep going and
you get to the smaller ones,

268
00:21:22,112 --> 00:21:25,485
you get back to about
eight or nine billion years.

269
00:21:25,520 --> 00:21:29,560
And then, when we get to
the very tiny, most distant ones,

270
00:21:29,595 --> 00:21:33,565
we'll be back probably
10 or 11 billion years in time.

271
00:21:33,600 --> 00:21:37,000
We're deep into the universe,
looking at the smallest structures,

272
00:21:37,035 --> 00:21:39,560
back in time to about
13 billion years.

273
00:21:39,595 --> 00:21:42,240
And, suddenly, we run out.

274
00:21:43,600 --> 00:21:46,925
Steve's amazing photo allows us

275
00:21:46,960 --> 00:21:51,800
to travel back through almost
the entire history of the universe.

276
00:21:56,800 --> 00:21:59,605
But if we want to know
what time it is right now,

277
00:21:59,640 --> 00:22:04,740
we have to go back a little further
than Steve's picture allows.

278
00:22:04,775 --> 00:22:09,840
We need to get back to the point
when time itself started ticking,

279
00:22:09,875 --> 00:22:17,360
back to the moment
the universe began.

280
00:22:22,120 --> 00:22:29,800
The Big Bang, so the theory goes,
was the beginning of everything -
including time.

281
00:22:29,835 --> 00:22:33,045
If you think about it, that's
a remarkable thing to say.

282
00:22:33,080 --> 00:22:40,160
It means that the first day of the
universe didn't have a yesterday.

283
00:22:40,195 --> 00:22:44,480
If time began at the Big Bang,
then there was no yesterday.

284
00:22:44,515 --> 00:22:47,897
There was nothing, no time at all.

285
00:22:47,932 --> 00:22:51,280
And then the Big Bang happened
and...

286
00:22:51,315 --> 00:22:53,560
all this appeared.

287
00:22:54,880 --> 00:23:00,840
To know the time, we need to know
precisely, how old is the universe?

288
00:23:04,400 --> 00:23:09,480
Since the moment of the Big Bang,
the universe has been expanding.

289
00:23:09,515 --> 00:23:14,560
If we could work out exactly how
fast the universe is stretching,

290
00:23:14,595 --> 00:23:18,680
then we could imagine
rewinding this expansion

291
00:23:18,715 --> 00:23:21,560
all the way back to the beginning.

292
00:23:27,600 --> 00:23:32,920
I've come to Berkeley,
just east of San Francisco,

293
00:23:32,955 --> 00:23:35,645
to meet Saul Perlmutter.

294
00:23:35,680 --> 00:23:41,320
A few years ago, he worked out
a clever way to calculate
when it all began.

295
00:23:43,040 --> 00:23:47,280
So, could you describe to the
nuts and bolts of the measurement?

296
00:23:47,315 --> 00:23:51,360
The tool that we started
to work with is a...

297
00:23:51,395 --> 00:23:53,885
kind of exploding star,
a supernova.

298
00:23:53,920 --> 00:23:57,725
And there's one kind of supernova,
we call it the type 1A,

299
00:23:57,760 --> 00:24:03,120
which, when they explode,
they always seem to reach the exact
same brightness and then fade away.

300
00:24:05,520 --> 00:24:08,565
Saul uses these special supernovae

301
00:24:08,600 --> 00:24:12,725
to measure how fast
the universe is expanding.

302
00:24:12,760 --> 00:24:19,280
These particular stellar phenomena
always explode the same way,

303
00:24:19,315 --> 00:24:22,525
giving out light with
a particular blue colour.

304
00:24:22,560 --> 00:24:27,280
But it takes time for this light
to reach us and, in that time,

305
00:24:27,315 --> 00:24:30,125
the universe continues to expand.

306
00:24:30,160 --> 00:24:34,600
As the universe expands, the wave
length of the light gets stretched.

307
00:24:34,635 --> 00:24:40,885
This turns the original blue colour
into red.

308
00:24:40,920 --> 00:24:45,720
In an expanding universe, the
amount that the universe stretches
is exactly the same amount

309
00:24:45,755 --> 00:24:51,077
that the light from the supernova
has its wavelength stretched
while it's travelling to us.

310
00:24:51,112 --> 00:24:56,400
Ah, so we look back and we see that
the nearby ones are pretty much
the colour that they were.

311
00:24:56,435 --> 00:25:00,960
That's right, they look blue, right?
And then further away...
Redder and redder.

312
00:25:00,995 --> 00:25:04,297
They go redder and redder as
the light is stretched. Exactly.

313
00:25:04,332 --> 00:25:07,600
And now you can start asking,
"Let me now map out the history

314
00:25:07,635 --> 00:25:10,677
"of all those different stretches,
all those different

315
00:25:10,712 --> 00:25:13,685
"times in history, how much
the universe has expanded. "

316
00:25:13,720 --> 00:25:17,160
And back calculate
when was everything right
on top of each other?

317
00:25:17,195 --> 00:25:19,245
When were all the distances at zero?

318
00:25:19,280 --> 00:25:22,320
And that's what we call
the beginning of the universe.

319
00:25:24,920 --> 00:25:29,260
Using Saul's observations,
we can reverse the expansion

320
00:25:29,295 --> 00:25:33,565
of the universe
to arrive at the point it all began.

321
00:25:33,600 --> 00:25:38,520
We now know this time was roughly
13.7 billion years ago,

322
00:25:38,555 --> 00:25:42,600
the start of space
and the start of time.

323
00:25:46,280 --> 00:25:49,205
So the answer to the question,
"What time is it?"

324
00:25:49,240 --> 00:25:54,240
is not 2008 or any other
arbitrary number on a calendar.

325
00:25:54,275 --> 00:25:58,400
It's 13.7 billion years.

326
00:25:58,435 --> 00:26:00,485
Or is it?

327
00:26:00,520 --> 00:26:02,925
You get these very
precise sounding numbers

328
00:26:02,960 --> 00:26:07,240
like 13.7 plus or minus a percent.
And that sounds very impressive.

329
00:26:07,275 --> 00:26:10,125
But that doesn't have to be
the beginning of everything.

330
00:26:10,160 --> 00:26:13,520
That's just the beginning of
this period that we can learn about

331
00:26:13,555 --> 00:26:16,200
by watching the expansion
of the universe.

332
00:26:19,680 --> 00:26:23,245
The idea that everything began
at the Big Bang

333
00:26:23,280 --> 00:26:27,920
leads to one of the most
profound questions in science -

334
00:26:27,955 --> 00:26:32,560
what exactly triggered
the beginning of the universe?

335
00:26:32,595 --> 00:26:36,440
If there was something
before the Big Bang,

336
00:26:36,475 --> 00:26:39,685
might that something include time?

337
00:26:39,720 --> 00:26:43,160
The orthodox view today
is that there was no time.

338
00:26:43,195 --> 00:26:46,800
Time began at the Big Bang,
time zero.

339
00:26:48,520 --> 00:26:55,760
But there are theories that
suggest that maybe the universe
existed before then in some sense.

340
00:26:55,795 --> 00:27:00,080
Maybe time has existed for ever,
and what we see as the Big Bang

341
00:27:00,115 --> 00:27:04,960
is just the creation of our
little bits of space and time.

342
00:27:08,400 --> 00:27:13,460
I've come to Cambridge to find out
what happened before the Big Bang.

343
00:27:13,495 --> 00:27:19,407
Neil Turok is one of the world's
leading cosmologists and he believes

344
00:27:19,442 --> 00:27:25,320
the accepted view that time began
at the Big Bang is completely wrong.

345
00:27:25,355 --> 00:27:30,445
I would say the standard hypothesis,

346
00:27:30,480 --> 00:27:33,800
that the universe sprang into
existence 13.7 billion years ago,

347
00:27:33,835 --> 00:27:35,125
doesn't make any sense.

348
00:27:35,160 --> 00:27:39,880
Something mysterious happened
13.7 billion years ago

349
00:27:39,915 --> 00:27:42,840
and we do not yet know what that was.

350
00:27:44,440 --> 00:27:48,285
'Neil has a theory for
what caused the Big Bang.

351
00:27:48,320 --> 00:27:55,760
'If he's right, it would mean that
time has been around a lot longer
than originally thought. '

352
00:27:55,795 --> 00:28:00,240
If you want to explain the Big Bang,
the simplest option is that
something caused it.

353
00:28:00,275 --> 00:28:03,880
And, if something caused it, there
was a time before the Big Bang.

354
00:28:03,915 --> 00:28:08,525
So, if you look at the...
the universe,

355
00:28:08,560 --> 00:28:13,205
and if we try to draw
a graph of time and space,

356
00:28:13,240 --> 00:28:19,680
and we follow the particles
emerging from the Big Bang,

357
00:28:19,715 --> 00:28:26,120
then they come out of this event
13.7 billion years ago.

358
00:28:26,155 --> 00:28:29,605
And, at that event,
all the particles were

359
00:28:29,640 --> 00:28:34,240
on top of each other and space
had shrunk to no size at all.

360
00:28:34,275 --> 00:28:38,840
So the density was infinity,
there is no space to talk about.

361
00:28:38,875 --> 00:28:43,040
So, that's the point
at which time began?

362
00:28:43,075 --> 00:28:45,960
No! Not at all.

363
00:28:48,400 --> 00:28:52,965
'Neil has come up with a clever,
if mind-boggling, solution.

364
00:28:53,000 --> 00:28:58,600
'With maths drawn from
the almost unintelligible
realm of string theory,

365
00:28:58,635 --> 00:29:03,957
'he believes the solution lies
in additional dimensions of space

366
00:29:03,992 --> 00:29:09,280
'and the existence of parallel
worlds that he calls membranes,

367
00:29:09,315 --> 00:29:11,245
'or branes for short. '

368
00:29:11,280 --> 00:29:17,400
How, then, can we picture this,
the beginning of our bit of space

369
00:29:17,435 --> 00:29:20,045
if time has gone on for ever?

370
00:29:20,080 --> 00:29:24,820
The first concrete model we
could come up with, coming out of

371
00:29:24,855 --> 00:29:29,525
string theory, was a particular
set-up called brane worlds.

372
00:29:29,560 --> 00:29:36,260
And what happens in a brane world
is that the three dimensions
of space we live in,

373
00:29:36,295 --> 00:29:42,960
which I'll draw as a two-dimensional
sheet, just so that I can
draw a picture of it.

374
00:29:42,995 --> 00:29:46,685
You're to imagine that this
is our three-dimensional world

375
00:29:46,720 --> 00:29:51,120
and we're made out of particles
which can travel within that world.

376
00:29:51,155 --> 00:29:56,440
Literally, well, you and me
and everything? Our universe.

377
00:29:56,475 --> 00:29:58,405
Yes, is within this world.

378
00:29:58,440 --> 00:30:02,200
So, what string theory says
is that there's not just this

379
00:30:02,235 --> 00:30:05,925
three-dimensional world or
sheet in my picture of it.

380
00:30:05,960 --> 00:30:11,440
There's another one, separated
from ours by a very tiny gap

381
00:30:11,475 --> 00:30:15,085
and that tiny gap
is a fourth dimension of space.

382
00:30:15,120 --> 00:30:21,840
So what can happen
is that these two brane worlds can
move towards each other and hit.

383
00:30:21,875 --> 00:30:26,160
So as they move towards each other,
they become one.

384
00:30:29,600 --> 00:30:36,440
In Neil's model,
everything we see around us exists
entirely on one of his branes.

385
00:30:38,040 --> 00:30:40,605
But there are other
branes in the universe,

386
00:30:40,640 --> 00:30:47,360
separated from us by an additional
unseen dimension of space.

387
00:30:47,395 --> 00:30:50,605
If we go back in time
13.7 billion years,

388
00:30:50,640 --> 00:30:55,320
it was the collision of
two branes that created the event

389
00:30:55,355 --> 00:30:57,405
we know as the Big Bang,

390
00:30:57,440 --> 00:31:01,880
and it was this that brought
the universe we see today

391
00:31:01,915 --> 00:31:04,245
into existence.

392
00:31:04,280 --> 00:31:09,920
And so the radiation and
matter of the Big Bang was
the energy of the collision.

393
00:31:09,955 --> 00:31:11,445
Um...

394
00:31:11,480 --> 00:31:16,000
And as they separated again,
the two branes were now filled...

395
00:31:16,035 --> 00:31:18,645
or replenished...
with matter and radiation.

396
00:31:18,680 --> 00:31:26,360
So literally in this picture,
the universe, which is all these
branes, exists for ever? Yes.

397
00:31:26,395 --> 00:31:29,245
There's no beginning of time?
Yes, that's right.

398
00:31:29,280 --> 00:31:35,165
I think time probably has always
been there and always will be there,

399
00:31:35,200 --> 00:31:40,600
and all we can really say
is that something dramatic
happened 13.7 billion years ago.

400
00:31:42,800 --> 00:31:46,205
'Neil's theory is controversial.

401
00:31:46,240 --> 00:31:50,740
'Rooted in string theory
and requiring additional

402
00:31:50,775 --> 00:31:55,240
'unseen dimensions of space,
it's challenging stuff.

403
00:31:55,275 --> 00:31:59,920
'But if it's true,
time has always existed. '

404
00:32:09,400 --> 00:32:14,160
Whether time began at the Big Bang
or time is eternal,

405
00:32:14,195 --> 00:32:17,525
I want to know
what time actually is.

406
00:32:17,560 --> 00:32:22,205
From decades to centuries,
we feel the need to divide time.

407
00:32:22,240 --> 00:32:27,680
But if we start to break it down
to hours and minutes and seconds,

408
00:32:27,715 --> 00:32:31,080
is there a point where we
can't divide it any further?

409
00:32:31,115 --> 00:32:34,097
Is there a smallest unit of time?

410
00:32:34,132 --> 00:32:37,045
When you look out into the universe,

411
00:32:37,080 --> 00:32:42,045
you see things happening
on time scales of millions
or even billions of years.

412
00:32:42,080 --> 00:32:47,440
Here on earth, we're used to
thinking of things in days or
minutes, or even seconds.

413
00:32:47,475 --> 00:32:50,840
But actually, a lot of things
can happen in a second.

414
00:32:52,480 --> 00:32:56,440
Just as we need a microscope
to see small things,

415
00:32:56,475 --> 00:33:00,200
we need specialist kit
to see smaller times.

416
00:33:02,360 --> 00:33:07,360
I've come to use the very
latest high-speed video camera.

417
00:33:07,395 --> 00:33:08,560
And... action!

418
00:33:13,040 --> 00:33:15,965
By filming very short time
intervals,

419
00:33:16,000 --> 00:33:20,040
this camera can then play back
action in super-slow motion.

420
00:33:20,075 --> 00:33:22,360
I'm hoping for when I'm about 60,
I can go...

421
00:33:22,395 --> 00:33:23,925
it's an absolute outrage!

422
00:33:23,960 --> 00:33:28,045
With the camera capturing
the world in milliseconds,

423
00:33:28,080 --> 00:33:32,560
this is life 40 times slower
than we're used to seeing it.

424
00:33:32,595 --> 00:33:34,600
He looks like some sort of fish!

425
00:33:34,635 --> 00:33:36,000
Action!

426
00:33:39,920 --> 00:33:42,285
What does that make
you think about your face?

427
00:33:42,320 --> 00:33:45,520
It's rather more elastic
than I had hitherto suspected.

428
00:33:45,555 --> 00:33:48,640
Print that one and
let's get another one.

429
00:33:54,600 --> 00:33:57,525
As we continue to divide time,

430
00:33:57,560 --> 00:34:01,080
we start to see previously
hidden details.

431
00:34:06,920 --> 00:34:09,205
With the action 80 times slower,

432
00:34:09,240 --> 00:34:12,920
each millisecond reveals
the world in a new light.

433
00:34:17,280 --> 00:34:18,920
Oh! Oh!

434
00:34:31,920 --> 00:34:33,280
Cut!

435
00:34:35,960 --> 00:34:38,565
The world looks
completely different

436
00:34:38,600 --> 00:34:42,280
when we start to see time
divided into smaller chunks,

437
00:34:42,315 --> 00:34:46,320
but just how far
can we keep on dividing time?

438
00:34:46,355 --> 00:34:49,085
There's nothing special about
a second.

439
00:34:49,120 --> 00:34:52,320
You can break it down, you can
break it down into milliseconds

440
00:34:52,355 --> 00:34:56,440
or microseconds, nanoseconds,
picoseconds, attoseconds.

441
00:34:58,800 --> 00:35:04,120
These tiny fractions of time,
million billionths of a second

442
00:35:04,155 --> 00:35:06,840
are impossible for our eyes
to perceive.

443
00:35:10,760 --> 00:35:13,765
But within an atom,
a second is an eternity

444
00:35:13,800 --> 00:35:17,485
filled with billions and billions
of interactions.

445
00:35:17,520 --> 00:35:22,680
This is a world where it seems that
time can be divided again and again,

446
00:35:22,715 --> 00:35:24,645
but can it?

447
00:35:24,680 --> 00:35:28,220
The smallest unit of time that
has any sort of significance in

448
00:35:28,255 --> 00:35:31,760
the universe as we understand it
is called the plank time,

449
00:35:31,795 --> 00:35:34,245
which is ten to the -43 seconds.

450
00:35:34,280 --> 00:35:38,320
That's a million million million
million million million millionth

451
00:35:38,355 --> 00:35:41,120
and a little bit more of a second.

452
00:35:42,560 --> 00:35:46,440
This is the time it takes
light to travel the shortest

453
00:35:46,475 --> 00:35:50,320
possible distance that our
current theories can handle.

454
00:35:50,355 --> 00:35:54,160
Beyond this point,
our understanding of time...

455
00:35:54,195 --> 00:35:56,120
.. stops.

456
00:36:00,400 --> 00:36:06,320
To tell the time, we used to look
up at the sun and stars but the
earth doesn't keep good time.

457
00:36:06,355 --> 00:36:09,845
With the advent of atomic clocks,

458
00:36:09,880 --> 00:36:15,365
we can now track time
with far greater accuracy
and by looking back in time,

459
00:36:15,400 --> 00:36:22,960
we can work out the moment
when time itself began, at least
in the universe we see around us.

460
00:36:22,995 --> 00:36:26,760
If we want to know,
"What time is it?", we're
certainly making progress.

461
00:36:30,080 --> 00:36:33,520
But there's one last profound
question I want to answer.

462
00:36:33,555 --> 00:36:37,405
It really encapsulates
our experience of life.

463
00:36:37,440 --> 00:36:42,440
It cuts to the very heart of
what it feels like to be human.

464
00:36:44,080 --> 00:36:49,085
It's how we define
the time line of our lives.

465
00:36:49,120 --> 00:36:54,040
Why does time seem to tick along
in the way that it does?

466
00:36:54,840 --> 00:36:56,765
Have you ever wondered

467
00:36:56,800 --> 00:37:01,240
why you have to go into the
future at the rate that you do,

468
00:37:01,275 --> 00:37:03,285
at the speed that you do?

469
00:37:03,320 --> 00:37:06,800
Do you get up and look at
yourself in the mirror and say,

470
00:37:06,835 --> 00:37:08,325
"Why am I getting older?"

471
00:37:08,360 --> 00:37:10,365
"Why do I have to
move into the future?"

472
00:37:10,400 --> 00:37:12,860
"Why am I not allowed to
go back into the past?"

473
00:37:12,895 --> 00:37:15,285
"Why am I not allowed
to stand still in time?"

474
00:37:15,320 --> 00:37:19,245
And so if you want to understand
why, you know, why existence

475
00:37:19,280 --> 00:37:24,280
feels like this, then you need
to know what time is and why it
passes in the way that it does.

476
00:37:33,520 --> 00:37:37,560
A hundred years ago,
it was Albert Einstein

477
00:37:37,595 --> 00:37:41,097
who started tackling
these profound questions.

478
00:37:41,132 --> 00:37:44,565
As part of his radical
new theory of nature,

479
00:37:44,600 --> 00:37:50,405
he fundamentally altered
the way we understand time.

480
00:37:50,440 --> 00:37:56,685
Einstein's picture is that
time is a dimension like space,

481
00:37:56,720 --> 00:38:01,565
and you move through it and so
that's how you feel it's passing.

482
00:38:01,600 --> 00:38:07,920
Really what you're doing is you're
moving through it just in the same
way that you can move through space.

483
00:38:10,280 --> 00:38:16,400
Einstein was the first to link
space and time in this unique way.

484
00:38:18,960 --> 00:38:23,140
In Einstein's universe,
when you sit still in space,

485
00:38:23,175 --> 00:38:27,320
you travel through time,
you travel into the future,

486
00:38:27,355 --> 00:38:30,445
and you travel through time
at a fixed speed.

487
00:38:30,480 --> 00:38:36,880
It sounds really weird, how you
travel at a speed, but you do and
that speed is the speed of light.

488
00:38:36,915 --> 00:38:38,845
That just doesn't sound right.

489
00:38:38,880 --> 00:38:43,960
It does sound strange because it's
one of the laws of the universe

490
00:38:43,995 --> 00:38:47,445
that you can't travel
at the speed of light, right?

491
00:38:47,480 --> 00:38:52,240
Actually, you can't travel through
space at the speed of light,
that's what you can't do.

492
00:38:52,275 --> 00:38:56,880
You can and do travel through time
at the speed of light.

493
00:39:01,960 --> 00:39:07,000
And it's this that creates our
sense of moving into the future.

494
00:39:09,720 --> 00:39:15,645
So it's a "one, two, three", the
time ticking on your watch, that is,

495
00:39:15,680 --> 00:39:22,920
in Einstein's theory, you flying
through the time dimension, into
the future, at the speed of light.

496
00:39:22,955 --> 00:39:24,760
It's quite, it's quite a...

497
00:39:26,880 --> 00:39:28,640
.. counter-intuitive picture.

498
00:39:28,675 --> 00:39:30,680
You're not kidding.

499
00:39:32,880 --> 00:39:39,480
We experience time passing
because we are all travelling
along the time dimension.

500
00:39:39,515 --> 00:39:46,240
But strangely, Einstein also said we
don't all experience the same time.

501
00:39:47,920 --> 00:39:54,880
For Einstein, space and time
are not the separate things
that we feel them to be.

502
00:39:54,915 --> 00:39:57,445
They're in many ways the same,

503
00:39:57,480 --> 00:40:00,120
and in fact,
they're merged together

504
00:40:00,155 --> 00:40:02,760
into a single entity
called space time.

505
00:40:06,280 --> 00:40:10,085
Space time can be pictured
as a sort of fabric

506
00:40:10,120 --> 00:40:14,885
where time and space are
inextricably woven together.

507
00:40:14,920 --> 00:40:20,365
As a result, the dimensions of
space and time can get mixed up.

508
00:40:20,400 --> 00:40:25,680
Although we are all
travelling through space time
at the speed of light,

509
00:40:25,715 --> 00:40:28,365
it's the mixing of time and space

510
00:40:28,400 --> 00:40:34,000
that Einstein said causes time to
tick differently for each of us.

511
00:40:36,440 --> 00:40:37,885
For Einstein,

512
00:40:37,920 --> 00:40:41,485
time wasn't like a...

513
00:40:41,520 --> 00:40:46,240
a metronome that just ticks
the same for everybody.

514
00:40:46,275 --> 00:40:48,685
It's different for you and me,

515
00:40:48,720 --> 00:40:54,520
and everywhere in the universe, the
metronomes tick at different rates.

516
00:40:57,280 --> 00:41:01,285
Einstein said that two people
will only ever agree on

517
00:41:01,320 --> 00:41:05,600
the speed time ticks if they're
standing next to each other.

518
00:41:09,880 --> 00:41:17,480
If I were to fly past you
incredibly fast, I would see your
time tick much slower than mine.

519
00:41:19,080 --> 00:41:25,040
This idea lies at the heart of
Einstein's theory of relativity.

520
00:41:25,075 --> 00:41:29,640
No-one has a right to claim
that their time is the time,

521
00:41:29,675 --> 00:41:31,325
the absolute time.

522
00:41:31,360 --> 00:41:35,480
It just depends on who's
moving relative to who.

523
00:41:35,515 --> 00:41:39,525
Because of the mixing of
space and time,

524
00:41:39,560 --> 00:41:44,240
time ticks differently for you
relative to other people,

525
00:41:44,275 --> 00:41:47,760
depending on how fast
everyone is moving.

526
00:41:47,795 --> 00:41:50,765
When someone moves relative to me,

527
00:41:50,800 --> 00:41:55,360
they use some of their speed of
light through space time
to move through space.

528
00:41:55,395 --> 00:41:58,405
So they haven't got as much left
to move through time,

529
00:41:58,440 --> 00:42:02,400
and that means that their speed
through time is a bit slower.

530
00:42:02,435 --> 00:42:06,325
They've sort of, in a very
real sense, used a bit of it up.

531
00:42:06,360 --> 00:42:11,800
It's a really profound
way of understanding
Einstein's theory of space time.

532
00:42:14,160 --> 00:42:17,845
And the strange nature of time
doesn't stop there.

533
00:42:17,880 --> 00:42:25,160
It's not just how fast you're
moving but what you're next to
that also affects time.

534
00:42:26,800 --> 00:42:28,765
According to Einstein,

535
00:42:28,800 --> 00:42:35,240
you should see the time
tick slower at my feet than
at the top of my head.

536
00:42:35,275 --> 00:42:40,080
This is because the nearer you are
to a big object like the earth,

537
00:42:40,115 --> 00:42:43,760
the more bent and warped
is the space time,

538
00:42:43,795 --> 00:42:46,245
and the slower time ticks.

539
00:42:46,280 --> 00:42:51,480
On our planet,
the effect is miniscule
but out there in the universe,

540
00:42:51,515 --> 00:42:54,417
the vast mass of the stars
and galaxies

541
00:42:54,452 --> 00:42:57,466
bend and warp the space time
so much

542
00:42:57,501 --> 00:43:00,480
that time ticks all over the place.

543
00:43:12,880 --> 00:43:20,760
In the 1960s, Einstein's strange
notion of time was put to the test.

544
00:43:20,795 --> 00:43:24,565
Using the large Haystack radio
antenna just north of Boston,

545
00:43:24,600 --> 00:43:30,600
an experiment was devised
to see if the sun causes
time to tick differently

546
00:43:30,635 --> 00:43:33,600
to how it ticks here on earth.

547
00:43:36,000 --> 00:43:39,200
Astronomer Irwin Shapiro
was at the helm.

548
00:43:42,240 --> 00:43:49,240
What we wanted to see was how long
it would take a light signal

549
00:43:49,275 --> 00:43:51,005
to get from the earth to Mercury,

550
00:43:51,040 --> 00:43:52,940
and its echo to get
back to the earth,

551
00:43:52,975 --> 00:43:54,805
and that's what we were looking for,

552
00:43:54,840 --> 00:43:58,000
a measurement of time
to very high accuracy.

553
00:44:00,800 --> 00:44:07,440
But even using one of the most
powerful radio antennas in the
world, this wasn't going to be easy.

554
00:44:10,600 --> 00:44:16,320
Any echo coming back from Mercury
would be incredibly weak.

555
00:44:16,355 --> 00:44:20,525
The echo has very little power.

556
00:44:20,560 --> 00:44:25,040
I can describe it as the
power put out

557
00:44:25,075 --> 00:44:28,857
by an ordinary housefly
crawling up a wall

558
00:44:28,892 --> 00:44:32,640
at the rate of a millimetre
per millennium,

559
00:44:32,675 --> 00:44:34,737
that's per thousand years.

560
00:44:34,772 --> 00:44:36,765
That's how little comes back.

561
00:44:36,800 --> 00:44:42,360
By knowing the distance to Mercury
and the speed that light travels,

562
00:44:42,395 --> 00:44:45,577
Irwin knew exactly
how long it should take

563
00:44:45,612 --> 00:44:48,760
for a signal to go out
and come back again.

564
00:44:48,795 --> 00:44:51,005
But applying Einstein's theory,

565
00:44:51,040 --> 00:44:55,000
the presence of the sun would have
an effect on the result.

566
00:44:55,035 --> 00:44:56,885
We see something strange.

567
00:44:56,920 --> 00:45:00,725
It's like a spike appears
in the orbit

568
00:45:00,760 --> 00:45:05,365
whenever the planet goes behind
the sun as seen from the earth.

569
00:45:05,400 --> 00:45:09,760
So it looks like it's drifted
a bit further away as it goes
round the back of the sun.

570
00:45:09,795 --> 00:45:13,360
Right, it looks like
it deviated from its orbit.

571
00:45:13,395 --> 00:45:15,645
Instead of going in
a nice smooth orbit,

572
00:45:15,680 --> 00:45:20,360
it had a spike in the orbit...
Which of course it doesn't have.

573
00:45:20,395 --> 00:45:24,537
It's just a manifestation
of the fact that light took longer

574
00:45:24,572 --> 00:45:28,680
to get to the planet and back
when the light went near the sun.

575
00:45:31,120 --> 00:45:36,920
The apparent blip in Mercury's orbit
is all down to the bending of time.

576
00:45:38,800 --> 00:45:43,320
The huge mass of the sun
bends and curves the space time,

577
00:45:43,355 --> 00:45:45,560
the fabric of the universe.

578
00:45:47,960 --> 00:45:51,045
As Irwin's radar beam
passed by the sun,

579
00:45:51,080 --> 00:45:55,720
the warping of space time meant
that time for the radar pulse

580
00:45:55,755 --> 00:45:59,557
got stretched
relative to time on earth.

581
00:45:59,592 --> 00:46:03,360
Time was slowed down
by the mass of the sun.

582
00:46:05,000 --> 00:46:08,725
This effect came to be known
as the Shapiro time delay.

583
00:46:08,760 --> 00:46:13,840
There are very few physicists
actually that have an effect
or anything named after them.

584
00:46:13,875 --> 00:46:16,165
Well, it's sort of embarrassing,

585
00:46:16,200 --> 00:46:20,560
but on the other hand I must say
I take a secret pleasure in it!

586
00:46:20,595 --> 00:46:22,720
Maybe not so secret!

587
00:46:24,600 --> 00:46:29,405
Irwin's measurement agreed perfectly
with Einstein's prediction.

588
00:46:29,440 --> 00:46:34,760
We really do see time slow down near
the biggest objects in the universe.

589
00:46:34,795 --> 00:46:40,520
But this observation leads to
an unsettling conclusion.

590
00:46:40,555 --> 00:46:43,805
If time can get slowed down,

591
00:46:43,840 --> 00:46:47,165
near the sun, for example,
relative to me,

592
00:46:47,200 --> 00:46:52,960
is there somewhere you can go in
the universe where time would slow
down so much that it would stop?

593
00:46:56,040 --> 00:46:59,400
There are objects
out there in the cosmos

594
00:46:59,435 --> 00:47:02,725
that bend and warp space time
so much

595
00:47:02,760 --> 00:47:07,200
that something very peculiar
happens to the passing of time.

596
00:47:09,520 --> 00:47:13,600
These fearsome phenomena
are black holes.

597
00:47:17,160 --> 00:47:21,920
Black holes have fascinated
MIT cosmologist Max Tegmark
for some time.

598
00:47:23,320 --> 00:47:26,560
So, the further down
I get in the gravitational field,

599
00:47:26,595 --> 00:47:30,485
the slo-o-ower and slo-o-ower
time goes.

600
00:47:30,520 --> 00:47:35,845
And if I were to then be orbiting
around there and picking up

601
00:47:35,880 --> 00:47:41,120
radio signals from earth, it would
look like life back here was
going in fast forward...

602
00:47:42,800 --> 00:47:45,860
.. We'll notice that
it's going too fast.

603
00:47:45,895 --> 00:47:48,885
So the voices would literally
do that.

604
00:47:48,920 --> 00:47:52,120
Yeah, because time is running
at different paces, right?

605
00:47:52,155 --> 00:47:54,280
They're going to sound like
Donald Duck.

606
00:47:57,160 --> 00:48:02,085
Closer still to a black hole,
and the passing of time

607
00:48:02,120 --> 00:48:07,080
relative to someone out in space
does something very weird.

608
00:48:07,115 --> 00:48:12,005
If I were to throw you into
a black hole and watch what

609
00:48:12,040 --> 00:48:18,760
happened to the time ticking along
on your wristwatch, I'd see it tick
slower and slower and slower.

610
00:48:18,795 --> 00:48:24,525
Arghhhhhhhhhh!

611
00:48:24,560 --> 00:48:30,880
And it would look to you like
my time had slowed down and I
eventually got frozen on the horizon.

612
00:48:30,915 --> 00:48:34,565
I would look very red in the face,
not just because my blood pressure

613
00:48:34,600 --> 00:48:40,480
had gone up but because
actually, even the frequency of
the light waves had slowed down.

614
00:48:40,515 --> 00:48:44,405
At some point, I would
actually see your watch stop.

615
00:48:44,440 --> 00:48:49,400
As far as I was concerned,
time for you would have stopped.

616
00:48:54,800 --> 00:48:59,360
Whether you're standing
on the edge of a black hole
or standing on earth,

617
00:48:59,395 --> 00:49:03,517
Einstein showed that
your time is unique to you.

618
00:49:03,552 --> 00:49:07,640
So the answer to the question
"What time is it?"

619
00:49:07,675 --> 00:49:10,157
is that it depends
what you're doing.

620
00:49:10,192 --> 00:49:12,605
Two people with identical
atomic clocks,

621
00:49:12,640 --> 00:49:15,845
moving around in different ways
or sitting closer

622
00:49:15,880 --> 00:49:19,440
or further away from a planet
will give you different answers,

623
00:49:19,475 --> 00:49:20,845
and they're both right.

624
00:49:20,880 --> 00:49:24,520
There's no single answer to
the question "What time is it?"

625
00:49:28,120 --> 00:49:32,680
Time is something we all have
a very close affinity with.

626
00:49:34,360 --> 00:49:36,960
We celebrate special moments
in time,

627
00:49:36,995 --> 00:49:40,160
yet often despair at its passing.

628
00:49:41,720 --> 00:49:44,160
If you want to know what's
special about time,

629
00:49:44,195 --> 00:49:46,445
you just have to
look at our language.

630
00:49:46,480 --> 00:49:50,560
The whole language that
describes the human condition

631
00:49:50,595 --> 00:49:53,925
is a language...
it's a temporal language.

632
00:49:53,960 --> 00:49:57,680
I had a great time yesterday.
I'm looking forward to tomorrow.

633
00:49:57,715 --> 00:50:00,680
It's the thing by which
we label regrets and aspirations

634
00:50:00,715 --> 00:50:01,885
and hopes and fears.

635
00:50:01,920 --> 00:50:04,080
They're all labelled
in terms of time.

636
00:50:04,115 --> 00:50:06,565
And it's quite...

637
00:50:06,600 --> 00:50:13,840
unbelievable almost that
it's probably the thing we
know least about, even now.

638
00:50:17,600 --> 00:50:22,445
Our intuition about time
is that the past has happened

639
00:50:22,480 --> 00:50:28,800
and only exists in our memories,
and the future is yet to come.

640
00:50:28,835 --> 00:50:32,725
But if Einstein is taken
at face value,

641
00:50:32,760 --> 00:50:35,605
that time is just like
the dimensions of space,

642
00:50:35,640 --> 00:50:39,600
then we're hit with something
completely counter-intuitive...

643
00:50:41,160 --> 00:50:45,645
.. all of time has always existed.

644
00:50:45,680 --> 00:50:49,725
Think about it. Space is here,
and there are three dimensions.

645
00:50:49,760 --> 00:50:56,880
I could walk down over there to
the ocean and you wouldn't say that
the ocean hadn't happened yet.

646
00:50:56,915 --> 00:50:59,045
It's there and I can walk to it.

647
00:50:59,080 --> 00:51:03,720
Well, there's no
difference really between time
and space in Einstein's theory.

648
00:51:03,755 --> 00:51:05,885
So you decide time's a dimension,

649
00:51:05,920 --> 00:51:09,840
then it's there in the same way
that the ocean's over there.

650
00:51:09,875 --> 00:51:16,080
So all moments in time
already exist.

651
00:51:16,115 --> 00:51:19,245
It's like moving along this road.

652
00:51:19,280 --> 00:51:25,640
The past - we've experienced it
but it's still there, and we're
moving forward into the future,

653
00:51:25,675 --> 00:51:30,160
but the future's there just in the
same way that this road is there.

654
00:51:32,120 --> 00:51:33,645
According to Einstein,

655
00:51:33,680 --> 00:51:39,280
the past, present
and future all exist.

656
00:51:41,160 --> 00:51:46,680
So, the feeling we have of our
future unfolding is misleading.

657
00:51:49,760 --> 00:51:53,640
All of space and
all of time are there,

658
00:51:53,675 --> 00:51:58,045
and your life would be just a line,

659
00:51:58,080 --> 00:52:01,045
like a piece of string,
a route through space time.

660
00:52:01,080 --> 00:52:06,720
So your birth would be here and your
death would be there, and there's
your life, and it just sits there.

661
00:52:06,755 --> 00:52:09,560
So at face value, that means
that time doesn't pass.

662
00:52:09,595 --> 00:52:11,845
There's no such thing
as the passing of time.

663
00:52:11,880 --> 00:52:17,840
There's just a line, a path
through space time that you took.

664
00:52:21,000 --> 00:52:24,420
This is an astounding proposal.

665
00:52:24,455 --> 00:52:27,840
According to Einstein, my birth...

666
00:52:27,875 --> 00:52:29,725
my first day at school...

667
00:52:29,760 --> 00:52:36,520
my graduation - all these events
still exist somewhere in space time.

668
00:52:36,555 --> 00:52:38,885
But there's more than that.

669
00:52:38,920 --> 00:52:44,880
If you take Einstein at face value,
the future is as real as the past.

670
00:52:44,915 --> 00:52:50,240
A week next Tuesday,
my 70th birthday, even my death -

671
00:52:50,275 --> 00:52:54,640
all these future moments in time
already exist.

672
00:52:59,920 --> 00:53:03,045
This picture that Einstein has
of time being this dimension

673
00:53:03,080 --> 00:53:07,120
and the future being all mapped out
and you just career headlong into it

674
00:53:07,155 --> 00:53:10,085
is not very satisfactory,
it doesn't feel right.

675
00:53:10,120 --> 00:53:16,920
And actually, in physics it's not
right, because Einstein's theory is
what's called a classical theory.

676
00:53:16,955 --> 00:53:20,320
It ignores the quantum world,
the world of the small.

677
00:53:23,200 --> 00:53:26,445
Quantum mechanics
is what I do for a living.

678
00:53:26,480 --> 00:53:32,640
In the quantum world,
the world of sub-atomic particles,
nothing is certain.

679
00:53:32,675 --> 00:53:35,085
It's a world of probabilities,

680
00:53:35,120 --> 00:53:39,640
one in which the future
is certainly not predetermined.

681
00:53:41,920 --> 00:53:45,485
The test we have now at
the cutting edge of physics is to

682
00:53:45,520 --> 00:53:51,720
find a way of getting Einstein's
space time to work with our modern
picture of quantum mechanics.

683
00:53:53,880 --> 00:54:00,000
This challenge is
what theoretical physicist
Fay Dowker has been working on.

684
00:54:00,035 --> 00:54:06,120
She's not throwing Einstein away
but trying to bring him
into the 21st century.

685
00:54:07,200 --> 00:54:11,560
Our understanding of space time
that we get from Einstein's theory

686
00:54:11,595 --> 00:54:14,405
is not the final answer,
it's not the final story.

687
00:54:14,440 --> 00:54:21,840
It's not compatible with our other
best theory, physical theory, that
we have, which is quantum mechanics.

688
00:54:23,680 --> 00:54:28,820
Fay's theory assumes that at
small scales, at the quantum level,

689
00:54:28,855 --> 00:54:33,960
space time isn't the smooth
structure that Einstein suggested

690
00:54:33,995 --> 00:54:36,560
but is made up of tiny bits.

691
00:54:37,960 --> 00:54:43,800
Space time is in fact bitty,
or granular, and if that's correct,

692
00:54:43,835 --> 00:54:47,960
then what we experience as, say,
the interval of a second

693
00:54:47,995 --> 00:54:50,325
would in fact not be continuous

694
00:54:50,360 --> 00:54:54,925
but it would be made up of
individual... individual events,

695
00:54:54,960 --> 00:55:01,000
grains of time, if you like,
that accumulate one after the other.

696
00:55:01,035 --> 00:55:06,000
It's almost like there are
grains of time,

697
00:55:06,035 --> 00:55:08,765
like grains of sand, right?

698
00:55:08,800 --> 00:55:13,645
So it's like every event
is a grain.

699
00:55:13,680 --> 00:55:17,860
As one event happens, one grain of
sand, then another one can happen

700
00:55:17,895 --> 00:55:22,040
on top of it, and another one on top
of it and another one on top of it,

701
00:55:22,075 --> 00:55:25,405
and you build up the future,
if you want.

702
00:55:25,440 --> 00:55:31,420
You build up the universe as layers
and layers of these grains.

703
00:55:31,455 --> 00:55:37,365
How does that help us then
understand what time actually is?

704
00:55:37,400 --> 00:55:42,000
One thing that space time could do
if it was grainy would be to grow

705
00:55:42,035 --> 00:55:43,765
grain by grain.

706
00:55:43,800 --> 00:55:47,720
And that's a possibility that
a continuous space time doesn't have.

707
00:55:47,755 --> 00:55:49,285
It can't...

708
00:55:49,320 --> 00:55:52,125
can't... come into being in that way,

709
00:55:52,160 --> 00:55:55,800
piece by piece, because there are
no pieces because it's a continuum.

710
00:55:55,835 --> 00:55:58,125
The past and the future
just there?

711
00:55:58,160 --> 00:56:03,160
Yes, so a grainy reality could...
It could grow.

712
00:56:03,195 --> 00:56:06,965
So it could come into being,

713
00:56:07,000 --> 00:56:09,685
element by element, grain by grain,

714
00:56:09,720 --> 00:56:13,640
and that growth itself
could be the passage of time.

715
00:56:16,080 --> 00:56:23,800
In Fay's universe,
our reality is built up by
tiny grains of space time.

716
00:56:23,835 --> 00:56:27,960
This means that the future
isn't set in stone.

717
00:56:27,995 --> 00:56:30,600
It grows out of the past.

718
00:56:35,040 --> 00:56:39,560
This picture of space and time
is still pretty strange,

719
00:56:39,595 --> 00:56:42,197
but at least it feels
more intuitive -

720
00:56:42,232 --> 00:56:44,765
you know,
the future's not yet happened.

721
00:56:44,800 --> 00:56:50,320
So I think, in this attempt to merge
Einstein with quantum mechanics,

722
00:56:50,355 --> 00:56:52,765
we're improving on Einstein.

723
00:56:52,800 --> 00:56:58,000
We're getting a better picture of
what time is and why time flows.

724
00:57:06,360 --> 00:57:09,525
"What time is it?"
is a profound question,

725
00:57:09,560 --> 00:57:15,005
because it needs you to think
about where time began.

726
00:57:15,040 --> 00:57:19,260
It needs you to think about how much
time has passed since time began.

727
00:57:19,295 --> 00:57:23,480
It needs you to think about how
you chop time up, how you count it.

728
00:57:23,515 --> 00:57:25,200
How do you count moments?

729
00:57:27,040 --> 00:57:34,240
From the journey of the sun
to atomic clocks, we can accurately
track the passing of time.

730
00:57:35,040 --> 00:57:37,525
But what is time?

731
00:57:37,560 --> 00:57:43,085
Did time have a beginning?
Or has it always been?

732
00:57:43,120 --> 00:57:46,480
Do we create our own future?
Or is it already written?

733
00:57:46,515 --> 00:57:50,325
Time has confounded us
for millennia.

734
00:57:50,360 --> 00:57:53,885
Irwin, what time is it?
Well, that depends.

735
00:57:53,920 --> 00:57:57,320
Are you talking about
universal time or eastern time?

736
00:57:57,355 --> 00:58:00,125
It's 13.7 billion years.

737
00:58:00,160 --> 00:58:05,160
That's what time it is. I would say
it's 4:45:54 but I'm 12 seconds off.

738
00:58:05,195 --> 00:58:07,600
It's not an easy question to answer.

739
00:58:07,635 --> 00:58:09,857
A great time to be a physicist.

740
00:58:09,892 --> 00:58:12,045
Time to go home and have dinner.

741
00:58:12,080 --> 00:58:15,200
It's a great question,
although some great questions

742
00:58:15,235 --> 00:58:17,720
actually turn out to be
trick ones, right?

743
00:58:17,755 --> 00:58:19,645
The time today...

744
00:58:19,680 --> 00:58:22,445
is something we have no idea about.

745
00:58:22,480 --> 00:58:28,000
We might not be in a position
at this moment in time,

746
00:58:28,035 --> 00:58:31,285
with our current
understanding of nature,

747
00:58:31,320 --> 00:58:36,400
to even understand
what it is that we're asking.

748
00:58:36,435 --> 00:58:39,960
MUSIC: "Just In Time"
by Frank Sinatra

749
00:58:44,080 --> 00:58:46,085
# Just in time

750
00:58:46,120 --> 00:58:49,725
# I found you just in time

751
00:58:49,760 --> 00:58:56,280
# Before you came,
my time was runnin' low... #

752
00:58:56,315 --> 00:58:58,597
Subtitles by Red Bee Media Ltd

753
00:58:58,632 --> 00:59:00,880
E- mail subtitling@bbc. co. uk