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Would you like to lose some
weight without doing any exercise
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00:00:05,080 --> 00:00:06,160
or dieting?
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00:00:06,160 --> 00:00:09,440
Would you like to age just a bit
more slowly than your friends?
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00:00:09,440 --> 00:00:11,520
Well, you might
be surprised to hear,
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the laws of physics can help.
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00:00:15,680 --> 00:00:19,840
The key to unlocking these
everyday questions is gravity.
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It sculpts the universe.
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It warps space and time.
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It's a fundamental force of nature.
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But gravity's strange powers,
discovered by Albert Einstein,
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also affect our daily lives
in the most unexpected ways.
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00:00:45,120 --> 00:00:49,000
In this film, we'll be using
cutting edge scientific techniques
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to investigate how gravity
changes your weight...
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It's gone up.
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..your height...
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I really have shrunk.
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..and even your posture.
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And, with the help
of thousands of volunteers,
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I'll show you how gravity makes us
all age at different rates.
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Throughout the day I've just been
logging on the phone,
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logging on to the app.
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As a physicist,
gravity is central to my work.
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Oh, wow!
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And, in exploring it,
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I'll be challenged on how I
understand this mysterious force.
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Wow, OK. I need to go
and write this one down.
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And I'll have to tackle the
very nature of reality itself.
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Gravity.
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It binds together
all the matter in the universe
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and it makes our existence
here possible.
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But in the end, it all boils down
to one simple question.
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What happens if I drop an object?
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Gravity's many mysteries are all
contained in this single action.
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How an object falls.
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Here's the first puzzle.
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Why does a hammer fall faster
than a feather?
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You might think it's because
the hammer is heavier.
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But that's not the real reason.
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The answer is air resistance.
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It's not the weight of the objects
that matters, it's their shape.
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And I can demonstrate this very
easily with these two umbrellas.
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They both have exactly the same
weight, but if I open one of them,
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you can be pretty sure it will drop
more slowly than the other one.
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In fact, all objects would fall
at the same rate
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if you could only remove the air.
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The first person to realise this
was the 16th century mathematician,
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Galileo Galilei.
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Famously, it's said he worked it out
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by dropping objects
off the Leaning Tower of Pisa.
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And he was spectacularly proven
right
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in an experiment carried
out on the moon in 1971.
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In my left hand I have a feather.
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In my right hand, a hammer.
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I'll drop the two of them here,
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and hopefully they'll
hit the ground at the same time.
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It worked perfectly.
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How about that?
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It proves that Mr Galilei
was correct in his findings.
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Now, Galileo was obsessed
with a second question, too.
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When you drop an object,
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it's actually quite hard to tell
if it falls at a constant speed,
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or picks up speed as it drops.
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Even in slow motion,
it's pretty hard to tell.
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First, drop an object
a very short distance.
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It lands with very little impact.
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But, of course,
drop it from higher up...
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..this time, the ball
easily breaks the tile,
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which means it must have
accelerated,
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gaining in speed and momentum
as it dropped.
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Galileo had identified something
fundamental to all falling objects -
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they accelerate.
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He realised there might be a way
to measure
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how much falling objects
gain in speed.
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What he devised was the first-ever
attempt to measure gravity itself.
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He built a long wooden ramp,
rather like this,
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that he had sloping
at a shallow angle.
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The idea was to roll balls down the
ramp and measure their acceleration.
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The crucial thing is that the ramp
had to be at this shallow angle
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to reduce the effects
of wind resistance.
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It also meant the balls would roll
down slowly enough to give him time
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to measure their speed.
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But the big problem was this -
how do you measure time accurately
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in an age when there
were no accurate timepieces,
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let alone stopwatches?
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Well, Galileo came up with an
ingenious idea
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involving the flow of water -
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essentially, measuring time from the
amount of water collected in a cup.
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So, we're going to try and repeat
Galileo's experiment.
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I say we, because I have
a couple of willing volunteers,
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Gavin and Johanna.
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Three, two, one, go.
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And, stop.
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OK, there's one.
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Now, if you come down a quarter
of the way down the ramp.
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Go.
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Stop. OK.
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So, now half of the way down.
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Go.
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Stop.
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Just in time.
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OK, and then three-quarters
of the way down.
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Go.
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And, stop.
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Right, turn the tap off.
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OK, so we have our four
measurements.
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And I can see a progression
from fuller to emptier,
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but what we need to do now is find
the mathematical pattern
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by weighing carefully
the water in each glass.
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Weighing the water should give us
an idea of how long each roll took.
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And in our experiment,
these were the results.
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Now, there's one immediate thing
you can tell.
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The ball really sped up
the longer it rolled.
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00:07:01,760 --> 00:07:03,920
In fact, our results seem to show
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that the time it took to cover
the first quarter of the ramp
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was about the same time it took
to cover the next three-quarters.
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So, we have a strong hint
of a mathematical pattern.
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Now, we'll see if we're right,
by placing bells along the ramp,
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at intervals which
are based on the results.
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This arrangement looks a bit strange
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because the gap between the
first two bells is much shorter
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than the gap between the third
and fourth bells.
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But that's OK, because if we've
got our calculations right,
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the ball starts off slowly,
so it covers a shorter distance,
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and as it picks up pace, it'll cover
longer and longer distances.
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So, we should hear the bells ringing
at equal intervals in time.
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Go.
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BELLS RING
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Beautiful.
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00:08:04,160 --> 00:08:08,000
So, what does this all mean,
what's the mathematical formula?
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00:08:08,000 --> 00:08:10,800
Well, this is something
that Galileo worked out.
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00:08:10,800 --> 00:08:14,240
Let's say, from the start, the ball
covers a distance of one metre
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in the first second.
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After two seconds, it will have
covered four metres.
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After three seconds, nine metres.
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00:08:21,280 --> 00:08:25,640
After 4 seconds,
16 metres, and so on.
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If you recognise this progression,
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you'll see that distance goes like
the square of time.
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Galileo had found the rates at
which gravity speeds up objects.
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And he'd found another
fundamental principle -
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you can measure
the strength of gravity
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by how much it causes
falling objects to accelerate.
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Detecting gravity has become
exceptionally sophisticated
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these days, but still uses
exactly the same principle.
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This is Herstmonceux Castle
in Sussex,
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and in its grounds lies
the Space Geodesy Facility.
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Here, Vicky uses an astonishingly
sensitive instrument
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to detect the exact strength
of gravity on this one spot.
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Vicky, tell me about this incredible
gravity meter that you work with.
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OK, so this is the dropping chamber
in a stripped down version.
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Essentially what happens
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is you've got a cart that gets
raised to the top,
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and then the cart accelerates away
from a mass in the middle,
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and so this section lifts off and as
it drops, it drops under freefall.
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So, this component
in the middle as it drops
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is basically just Newton's apple
falling to the ground?
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Yes. So this is a stripped down
version, but that's the real thing?
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This is the real thing.
How does that actually work?
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In here, it's a vacuum. So there's
no wind resistance as it falls.
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There's no wind resistance.
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Inside, a laser is used
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to measure exactly how fast
the mass is accelerating.
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This is the 21st-century version
of Galileo's ramp
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and the balls rolling down.
So, can we get it going?
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Of course, if you'd just like
to press the button on the laptop.
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This one? Yep.
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00:10:12,760 --> 00:10:15,480
OK. So it's now communicating
with it.
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Oh, here we go. Here we go.
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It waits five seconds and then takes
the measurement of gravity.
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And again. Repeats.
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And you can see
the results appearing now.
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with the actual number that
it's getting for each one.
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The unit Vicky uses
has a familiar ring.
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I see that the number up
at the top here,
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you've got this unit, micro Gal?
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Yes, a Gal is essentially one
centimetre per second squared.
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The Gal was named after Galileo.
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So, we've just taken the measurement
of gravity here today
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and it's this highly
accurate number,
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981124007
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micro Gals.
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00:11:02,960 --> 00:11:07,280
The reading means that the Earth's
gravity speeds up a falling object
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by around 9.81 metres per second
for every second it drops.
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Vicky tells me something intriguing.
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She takes a reading here every week
and she's found that
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00:11:21,800 --> 00:11:26,200
the strength of gravity changes
by tiny amounts over time.
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00:11:27,240 --> 00:11:31,280
Heavy rainfall, for example, can
cause gravity to increase slightly.
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00:11:33,080 --> 00:11:36,720
Presumably, if gravity
is changing here in one spot,
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it'll have different values all
around the world
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and so you can have a gravity map
of the entire planet?
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That's right, yes.
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00:11:45,760 --> 00:11:49,320
So what's the reason
for these strange fluctuations?
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00:11:49,320 --> 00:11:52,280
That's what I want
to investigate next.
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So, gravity changes as we move
across the surface of the Earth.
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This is at the heart of a challenge
that I've set two young volunteers.
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00:12:03,440 --> 00:12:08,080
I've given them a task to try
and find the place in Britain
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where gravity is at its weakest.
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00:12:10,400 --> 00:12:13,800
So, where objects
would weigh the least.
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00:12:13,800 --> 00:12:16,360
I've given them just three days
to try and find it.
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00:12:18,320 --> 00:12:22,720
The volunteers are Astraya,
a PhD student.
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00:12:22,720 --> 00:12:24,960
I've been living in London
for five or six years,
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00:12:24,960 --> 00:12:27,800
and I'm originally
from Seville in Spain.
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00:12:27,800 --> 00:12:31,160
I'm very interested in taking part
in this project
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00:12:31,160 --> 00:12:35,120
because I would really like to know
more about how this world works.
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00:12:35,120 --> 00:12:38,880
And Poppy, a journalist
who lives in London.
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00:12:38,880 --> 00:12:42,040
I did my degree
in biomedical science.
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00:12:42,040 --> 00:12:45,560
And I did biology and chemistry
for my A-levels,
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00:12:45,560 --> 00:12:48,560
but I haven't done any physics
since I left school.
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00:12:48,560 --> 00:12:51,320
I'm fascinated to find out more
about gravity
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00:12:51,320 --> 00:12:54,560
and I actually enjoy a puzzle,
I like a challenge.
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00:12:54,560 --> 00:12:58,360
The team just can't weigh themselves
to see changes in gravity.
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00:12:58,360 --> 00:13:02,880
Body weight fluctuates by a couple
of kilos over the course of a day.
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00:13:02,880 --> 00:13:07,440
Whereas, changes due to gravity
as they travel around the country
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00:13:07,440 --> 00:13:11,000
are going to be tiny in comparison,
the matter of a few grams.
214
00:13:11,000 --> 00:13:14,600
So, they're going to have to use
sophisticated scientific methods
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00:13:14,600 --> 00:13:17,280
if they want to measure
gravity accurately.
216
00:13:17,280 --> 00:13:21,000
And that's why the volunteers will
be joined by three specialists
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00:13:21,000 --> 00:13:22,960
in gravity science.
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00:13:24,120 --> 00:13:27,560
PhD student Sonak.
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00:13:27,560 --> 00:13:31,240
He'll be in charge of some very
sensitive measuring apparatus
220
00:13:31,240 --> 00:13:33,840
from the
National Physical Laboratory.
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00:13:33,840 --> 00:13:38,880
Sean, a geologist, who will
be using a portable gravity meter.
222
00:13:40,840 --> 00:13:44,480
And Andrew, a cosmologist
at University College London,
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00:13:44,480 --> 00:13:46,560
who will help interpret the results.
224
00:13:48,120 --> 00:13:52,200
We've taken a collective weight
for the team before they set off.
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00:13:52,200 --> 00:13:54,720
It's 380 kilograms.
226
00:13:54,720 --> 00:13:59,520
So, can they find the place in
Britain where that will decrease?
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00:14:00,800 --> 00:14:05,240
They're setting out in Snowdonia
National Park in North Wales.
228
00:14:06,680 --> 00:14:11,200
The railway climbs from here to the
1,000 metre summit of Snowdon.
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00:14:11,200 --> 00:14:14,080
Sean takes
his first gravity reading.
230
00:14:14,080 --> 00:14:19,120
The inside is a mass on a beam
and you turn this counter,
231
00:14:19,120 --> 00:14:23,680
this dial, until you get
the beam central.
232
00:14:23,680 --> 00:14:26,720
By counting the
number of turns of the dial,
233
00:14:26,720 --> 00:14:30,160
Sean can calculate
the downward pull of gravity
234
00:14:30,160 --> 00:14:33,200
acting on the mass
inside the machine.
235
00:14:33,200 --> 00:14:36,560
Sonak has a simpler method.
236
00:14:36,560 --> 00:14:40,440
So, inside the box
is a two kilogram mass,
237
00:14:40,440 --> 00:14:43,200
and it's supposed to be sort of
as perfectly two kilograms
238
00:14:43,200 --> 00:14:44,880
as it's possible to get.
239
00:14:46,520 --> 00:14:49,320
All right. And place it here.
240
00:14:50,680 --> 00:14:52,920
Oh, it's just coming under,
isn't it?
241
00:14:52,920 --> 00:14:55,800
1998.2 grams.
242
00:14:55,800 --> 00:14:59,880
It was two kilos in the laboratory,
but now here it's a bit less.
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00:14:59,880 --> 00:15:02,160
It's the first puzzle.
244
00:15:02,160 --> 00:15:07,200
Why does a two kilo mass tip the
scales at just under two kilos?
245
00:15:07,440 --> 00:15:10,840
And it's one which
gets straight to the heart
246
00:15:10,840 --> 00:15:13,320
of what the challenge
is really about.
247
00:15:15,320 --> 00:15:19,800
Mass is often confused with
the related quantity, weight.
248
00:15:19,800 --> 00:15:24,840
The mass of these dumbbells
is fixed, it doesn't change.
249
00:15:24,920 --> 00:15:28,120
It's a measure of how much stuff
they contain.
250
00:15:28,120 --> 00:15:29,760
Weight is different.
251
00:15:29,760 --> 00:15:34,040
It's a measure of the effects
of gravity on these dumbbells.
252
00:15:34,040 --> 00:15:36,880
The downward force
pulling them to the ground
253
00:15:36,880 --> 00:15:40,440
in the same way that it's keeping
my feet firmly stuck to the ground.
254
00:15:40,440 --> 00:15:42,640
The crucial difference is this,
255
00:15:42,640 --> 00:15:45,280
if I was holding these dumbbells
on the moon,
256
00:15:45,280 --> 00:15:47,840
they'd still have
exactly the same mass,
257
00:15:47,840 --> 00:15:50,720
but they'd weigh six times less
258
00:15:50,720 --> 00:15:54,400
because the moon's gravity is
so much weaker than the Earth's.
259
00:15:56,840 --> 00:16:00,600
So that's why Sonak is bringing
along the two kilo mass.
260
00:16:00,600 --> 00:16:03,920
If it changes weight
then this should mean
261
00:16:03,920 --> 00:16:06,240
that gravity itself has changed.
262
00:16:08,160 --> 00:16:11,600
Ahead of them is the summit
of the highest mountain
263
00:16:11,600 --> 00:16:14,560
in England and Wales,
famed for its stunning scenery.
264
00:16:16,920 --> 00:16:19,680
Or it would be stunning
if you could see it.
265
00:16:20,760 --> 00:16:25,000
And this is what we came
all the way up here for,
266
00:16:25,000 --> 00:16:28,160
this amazing view
at the top of Snowdon.
267
00:16:28,160 --> 00:16:30,960
You wouldn't know it,
but honestly, we are here.
268
00:16:32,520 --> 00:16:36,400
We're now near the summit of Snowdon
and I've set up the gravimeter,
269
00:16:36,400 --> 00:16:39,560
and we're going to see what
the difference in the reading is.
270
00:16:42,680 --> 00:16:47,240
He has to turn the dial again and
again to try and get a reading.
271
00:16:47,240 --> 00:16:50,960
It's clear gravity has changed,
but which way?
272
00:16:50,960 --> 00:16:53,480
Has it got stronger, or weaker?
273
00:16:53,480 --> 00:16:56,720
The team leave Sean
to work out his results,
274
00:16:56,720 --> 00:17:00,680
and tries to position the scales
as close as possible to the summit.
275
00:17:02,000 --> 00:17:04,160
But the reading
is all over the place.
276
00:17:04,160 --> 00:17:07,360
Oh! It's gone up.
277
00:17:07,360 --> 00:17:09,560
It's fluctuating
quite a lot due to the wind.
278
00:17:09,560 --> 00:17:12,520
I have to say, this is what science
is always like, isn't it?
279
00:17:12,520 --> 00:17:15,120
It's never quite
what you want it to be.
280
00:17:15,120 --> 00:17:19,240
So, they head inside to the cafe
next to the summit.
281
00:17:20,840 --> 00:17:23,400
The wind was being a bit naughty,
but hopefully...
282
00:17:23,400 --> 00:17:25,680
Now it's in 00,
so it should be all right.
283
00:17:25,680 --> 00:17:29,160
1998.2 down there,
284
00:17:29,160 --> 00:17:31,840
1997.8!
285
00:17:31,840 --> 00:17:36,000
There you go. We've got it!
That's 0.4 of a gram off.
286
00:17:37,080 --> 00:17:39,720
The mass weighs a tiny bit less.
287
00:17:39,720 --> 00:17:44,480
It's lost about
one 5000th of its weight.
288
00:17:44,480 --> 00:17:49,040
And Sean has found that gravity
itself has reduced.
289
00:17:49,040 --> 00:17:51,520
At the top of the mountain
we took the measurement
290
00:17:51,520 --> 00:17:56,320
and we discovered that the
pull of gravity had gone down.
291
00:17:56,320 --> 00:17:59,960
It had gone down the equivalent
of 206 turns of the dial.
292
00:17:59,960 --> 00:18:04,960
And we worked out that that's
equivalent to 219 milligals.
293
00:18:07,200 --> 00:18:09,560
So it's clear from
the team's measurements,
294
00:18:09,560 --> 00:18:14,480
gravity weakens as you go higher,
and you get a bit lighter.
295
00:18:16,320 --> 00:18:19,840
It's just an excuse to say where
are we, like, the lightest.
296
00:18:19,840 --> 00:18:22,200
Who cares? Yes, who does care?
297
00:18:22,200 --> 00:18:26,040
It's actually really interestingly,
it's like an illustrative example
298
00:18:26,040 --> 00:18:29,320
of seeing how this
is actually fluctuating,
299
00:18:29,320 --> 00:18:31,840
depending on different factors.
Yeah, absolutely.
300
00:18:31,840 --> 00:18:34,640
And that we could measure it
and see it with our own eyes,
301
00:18:34,640 --> 00:18:38,240
it actually makes you think about
gravity in a very active way.
302
00:18:38,240 --> 00:18:41,240
It's such a fundamental
force phenomenon in nature,
303
00:18:41,240 --> 00:18:42,640
but we don't know much about it.
304
00:18:44,920 --> 00:18:48,000
But why does gravity change
with altitude?
305
00:18:48,960 --> 00:18:50,800
To understand that question,
306
00:18:50,800 --> 00:18:54,120
you've to get to grips with the
extraordinary discoveries
307
00:18:54,120 --> 00:18:56,760
of the next scientific giant
in our story -
308
00:18:56,760 --> 00:18:59,800
Isaac Newton.
309
00:18:59,800 --> 00:19:02,400
Born in England in the middle
of the 17th century,
310
00:19:02,400 --> 00:19:07,200
he spent his life wrestling with so
many apparently separate questions,
311
00:19:07,200 --> 00:19:11,800
from why things fall to the ground,
to why planets orbit the sun.
312
00:19:15,040 --> 00:19:17,640
It took the genius of Newton
to realise
313
00:19:17,640 --> 00:19:21,520
there was one single equation that
could answer all these questions.
314
00:19:24,160 --> 00:19:27,600
And here it is,
his famous law of gravity.
315
00:19:27,600 --> 00:19:29,240
It might look complicated,
316
00:19:29,240 --> 00:19:31,920
but this is one of the most
important equations
317
00:19:31,920 --> 00:19:33,440
in the whole of science.
318
00:19:33,440 --> 00:19:35,320
F here is the force.
319
00:19:35,320 --> 00:19:39,200
Newton said there's an attractive
force between any two objects
320
00:19:39,200 --> 00:19:41,080
in the universe.
321
00:19:41,080 --> 00:19:45,280
On this side of the equation, G, we
call the gravitational constant.
322
00:19:45,280 --> 00:19:49,080
Newton knew it had to be there, but
he didn't know what its value was.
323
00:19:49,080 --> 00:19:54,080
M1 and M2 represent the two objects,
and R is the distance between them.
324
00:19:55,640 --> 00:20:00,040
Now, the equation tells us that
the more massive the objects are,
325
00:20:00,040 --> 00:20:04,600
the bigger M1 and M2,
the greater the attractive force.
326
00:20:04,600 --> 00:20:08,600
But the further apart they are,
the bigger the value of R here,
327
00:20:08,600 --> 00:20:10,720
the weaker the gravitational force.
328
00:20:12,120 --> 00:20:16,160
With Newton, what was once
mysterious now became clear.
329
00:20:17,360 --> 00:20:21,520
Newton's equation describes
why an object falls to the ground,
330
00:20:21,520 --> 00:20:23,680
including his famous apple.
331
00:20:23,680 --> 00:20:27,120
But its true genius is
that it applies to any object,
332
00:20:27,120 --> 00:20:29,000
anywhere in the universe.
333
00:20:29,000 --> 00:20:32,720
So, it's a very simple
and elegant way of describing
334
00:20:32,720 --> 00:20:37,360
some of the seemingly most
complicated phenomena in the cosmos.
335
00:20:41,760 --> 00:20:46,040
His law of gravitation can still be
used today -
336
00:20:46,040 --> 00:20:49,120
to explain how orbits work,
337
00:20:49,120 --> 00:20:53,000
to predict when a comet will return,
338
00:20:53,000 --> 00:20:55,520
to describe why galaxies spin.
339
00:20:57,880 --> 00:21:00,640
Or to slingshot spacecraft
around planets.
340
00:21:02,680 --> 00:21:05,520
Newton tells us to look for the
underlying simplicity
341
00:21:05,520 --> 00:21:09,760
in natural phenomena. For instance,
how the moon orbits the Earth.
342
00:21:12,040 --> 00:21:13,520
If I let go of this apple,
343
00:21:13,520 --> 00:21:16,720
it'll fall straight down because
of the pull of Earth's gravity.
344
00:21:18,120 --> 00:21:20,000
But if I throw it, to begin with,
345
00:21:20,000 --> 00:21:22,160
it travels in
a horizontal direction,
346
00:21:22,160 --> 00:21:23,600
that's the direction of travel,
347
00:21:23,600 --> 00:21:26,120
but Earth's gravity
is still pulling it downwards,
348
00:21:26,120 --> 00:21:29,000
so it ends up following
a curved path.
349
00:21:35,560 --> 00:21:37,040
Now, if I throw it harder,
350
00:21:37,040 --> 00:21:40,760
it'll travel further before it hits
the ground and, in principle,
351
00:21:40,760 --> 00:21:44,320
if I could throw it hard enough,
I could put it into orbit.
352
00:21:44,320 --> 00:21:47,920
That's exactly what's happening with
the moon in orbit around the Earth.
353
00:21:47,920 --> 00:21:51,440
It's a combination of wanting
to travel in a straight line,
354
00:21:51,440 --> 00:21:54,240
but also being pulled down
by the Earth's gravity.
355
00:21:54,240 --> 00:21:56,400
So, it ends up constantly falling
356
00:21:56,400 --> 00:21:59,240
around the Earth
and constantly missing.
357
00:22:01,400 --> 00:22:02,960
Newton's famous equation
358
00:22:02,960 --> 00:22:05,120
also explains the strange effects
359
00:22:05,120 --> 00:22:07,880
which the road-trip team
has discovered.
360
00:22:07,880 --> 00:22:11,120
That objects get lighter
as you gain in altitude.
361
00:22:12,800 --> 00:22:16,840
When I weigh myself, I'm represented
by the first mass, M1.
362
00:22:16,840 --> 00:22:19,880
The second mass,
M2, is the Earth itself.
363
00:22:19,880 --> 00:22:23,440
And the force pulling me down,
my weight,
364
00:22:23,440 --> 00:22:27,800
depends on the distance between me
and the centre of the Earth.
365
00:22:27,800 --> 00:22:30,080
And that's the secret
of the road trip.
366
00:22:30,080 --> 00:22:33,000
If you want to find the place
where you weigh the least,
367
00:22:33,000 --> 00:22:36,840
then you have to get as far away as
you can from the Earth's core.
368
00:22:45,040 --> 00:22:47,320
So, it's the afternoon of day one,
369
00:22:47,320 --> 00:22:51,800
and the road-trip team have to work
out where to go next.
370
00:22:51,800 --> 00:22:54,560
Poppy and Astraya have a good idea,
371
00:22:54,560 --> 00:22:57,640
find somewhere higher
than Mount Snowdon.
372
00:22:57,640 --> 00:23:01,640
From the measurements that you guys
did at Mount Snowdon,
373
00:23:01,640 --> 00:23:04,680
altitude clearly plays
an important part in gravity.
374
00:23:04,680 --> 00:23:07,560
So, with that in mind, we've got to
go to the highest point in the UK,
375
00:23:07,560 --> 00:23:09,280
which is Ben Nevis.
376
00:23:09,280 --> 00:23:13,720
OK, BUT there's just one thing that
we haven't shown you so far.
377
00:23:13,720 --> 00:23:16,480
We actually brought
along an extra experiment,
378
00:23:16,480 --> 00:23:20,200
so can we please show you this first
before you make the final decision?
379
00:23:20,200 --> 00:23:23,520
Yes. Sonak actually has the other
part of this experiment.
380
00:23:23,520 --> 00:23:25,600
We always carry around...
381
00:23:25,600 --> 00:23:27,800
Some power tools,
as physicists always do.
382
00:23:27,800 --> 00:23:29,680
Let's start it off nice and gentle.
383
00:23:31,400 --> 00:23:32,800
OK.
384
00:23:32,800 --> 00:23:35,200
And then, try and pick up some pace.
385
00:23:36,440 --> 00:23:37,800
Pizza.
386
00:23:39,400 --> 00:23:42,520
You've got some pizza there.
OK. Point proven.
387
00:23:42,520 --> 00:23:45,280
The point is that
when something is spinning,
388
00:23:45,280 --> 00:23:48,760
it kind of gets flung outwards
and you can actually use that
389
00:23:48,760 --> 00:23:52,400
to make a nice, flat piece of pizza,
but this also applies to the Earth.
390
00:23:52,400 --> 00:23:55,720
The Earth isn't perfectly round.
391
00:23:55,720 --> 00:23:59,200
It's what's known
as an "oblate spheroid".
392
00:23:59,200 --> 00:24:01,240
It bulges at the equator
393
00:24:01,240 --> 00:24:03,560
where the spin is greatest.
394
00:24:03,560 --> 00:24:06,240
We've kind of got
two competing effects now.
395
00:24:06,240 --> 00:24:08,400
We're trying to get away from the
centre,
396
00:24:08,400 --> 00:24:10,120
the actual core of the Earth,
397
00:24:10,120 --> 00:24:13,280
the point at the very centre
of this ball.
398
00:24:13,280 --> 00:24:14,760
But now, we can do it in two ways.
399
00:24:14,760 --> 00:24:17,280
We can either go up something tall,
400
00:24:17,280 --> 00:24:20,920
or we can just
go down towards the equator.
401
00:24:20,920 --> 00:24:23,920
This is what we find
when we're doing gravity surveys,
402
00:24:23,920 --> 00:24:28,040
as you move south, there tends
to be an effect from latitude
403
00:24:28,040 --> 00:24:33,120
which is often usually larger
than the effect from altitude.
404
00:24:33,320 --> 00:24:36,440
So, the closer
to the equator you go,
405
00:24:36,440 --> 00:24:41,480
the further you get from the Earth's
core and the lighter you get.
406
00:24:41,680 --> 00:24:46,680
So, guys, the sun's setting just
behind me here. This is north.
407
00:24:49,200 --> 00:24:51,400
it sounds like we've
got to go that way,
408
00:24:51,400 --> 00:24:53,000
down south, is that right?
409
00:24:53,000 --> 00:24:54,400
Yes, OK. Let's go.
410
00:24:54,400 --> 00:24:56,720
THEY LAUGH
411
00:24:56,720 --> 00:25:01,160
The team is starting to uncover
the reasons why gravity changes
412
00:25:01,160 --> 00:25:03,200
as you cross the surface
of the Earth.
413
00:25:05,440 --> 00:25:07,520
Our planet is defined and shaped
414
00:25:07,520 --> 00:25:11,720
by the complicated forces
which act upon it.
415
00:25:11,720 --> 00:25:15,200
And detecting tiny fluctuations in
its gravity field
416
00:25:15,200 --> 00:25:18,560
can give us important clues.
417
00:25:18,560 --> 00:25:21,520
It can help us understand
how our world is changing.
418
00:25:23,480 --> 00:25:27,640
The Space Geodesy Facility at
Herstmonceux is one small part
419
00:25:27,640 --> 00:25:29,800
of an enormous global network
420
00:25:29,800 --> 00:25:34,160
which uses satellites to detect the
tiniest of changes
421
00:25:34,160 --> 00:25:36,400
in the Earth's gravity field.
422
00:25:36,400 --> 00:25:39,280
Tell me what exactly your job
is here?
423
00:25:39,280 --> 00:25:40,880
What we're doing
with this telescope
424
00:25:40,880 --> 00:25:42,400
is measuring very accurately
425
00:25:42,400 --> 00:25:45,520
the distances of satellites
from here,
426
00:25:45,520 --> 00:25:47,520
so we're using very short laser
pulses
427
00:25:47,520 --> 00:25:49,960
which we direct towards
the satellite.
428
00:25:49,960 --> 00:25:52,800
On the satellite,
there are reflecting cubes,
429
00:25:52,800 --> 00:25:55,280
which return some
of that light to us.
430
00:25:55,280 --> 00:25:56,800
We measure how long it takes
the light
431
00:25:56,800 --> 00:25:58,160
to go to the satellite and back.
432
00:25:58,160 --> 00:26:00,040
And how far away
is the satellite typically?
433
00:26:00,040 --> 00:26:02,520
The one we're tracking now
is one of the Galileo satellites,
434
00:26:02,520 --> 00:26:04,600
which is about 20,000 kilometres.
435
00:26:04,600 --> 00:26:07,720
20,000 kilometres away? Yes.
436
00:26:07,720 --> 00:26:09,840
OK, so, we've got it aimed at the
Galileo satellite
437
00:26:09,840 --> 00:26:13,000
and you're going to turn the laser
on now? Yes.
438
00:26:14,440 --> 00:26:17,120
Oh, wow!
439
00:26:17,120 --> 00:26:21,400
And that laser beam that's being
fired up towards the satellite,
440
00:26:21,400 --> 00:26:24,240
the time it'll take to get
there and come back again,
441
00:26:24,240 --> 00:26:26,920
it's a fraction of a second,
isn't it? It is.
442
00:26:26,920 --> 00:26:29,480
It's about 150 thousandths
of a second, 150 milliseconds.
443
00:26:29,480 --> 00:26:32,280
And we're sending about
1,000 of those per second.
444
00:26:36,440 --> 00:26:40,400
This strange-looking object
is based on satellite readings.
445
00:26:40,400 --> 00:26:43,240
It's a highly exaggerated
representation
446
00:26:43,240 --> 00:26:46,400
of how Earth's gravity field
varies over time.
447
00:26:48,520 --> 00:26:52,400
Fluctuations like these can give us
important insights
448
00:26:52,400 --> 00:26:53,880
into climate change,
449
00:26:53,880 --> 00:26:56,120
icecaps melting,
450
00:26:56,120 --> 00:26:59,080
sea levels rising,
451
00:26:59,080 --> 00:27:01,720
changes in ground water.
452
00:27:01,720 --> 00:27:03,720
All of these have an effect
453
00:27:03,720 --> 00:27:05,920
on the local strength of gravity.
454
00:27:05,920 --> 00:27:09,400
So, something as important
as climate change,
455
00:27:09,400 --> 00:27:11,680
in order to understand it
and do something about it,
456
00:27:11,680 --> 00:27:14,160
we need to know the distribution
457
00:27:14,160 --> 00:27:17,080
of the gravitational field
of the Earth very accurately?
458
00:27:17,080 --> 00:27:20,440
Absolutely, yes. And it's a global
measure that we need.
459
00:27:26,200 --> 00:27:29,120
For the road trippers,
it's the start of day two...
460
00:27:30,440 --> 00:27:32,960
..and they're heading
for the south coast.
461
00:27:34,360 --> 00:27:36,960
They're stopping off
in Herefordshire,
462
00:27:36,960 --> 00:27:39,920
it's a good location
as it's the same altitude
463
00:27:39,920 --> 00:27:41,440
as the base of Snowdon,
464
00:27:41,440 --> 00:27:44,840
but they've moved about 80 miles
further south.
465
00:27:44,840 --> 00:27:49,880
So, if they find gravity changes
here, it must be due to latitude.
466
00:27:50,080 --> 00:27:52,200
It's not a huge difference,
but it's noticeable.
467
00:27:52,200 --> 00:27:55,840
Our counter reading at the bottom
of the mountain was 4,840.
468
00:27:55,840 --> 00:27:59,720
Yes. Our counter reading
here's 4,717.
469
00:27:59,720 --> 00:28:02,560
Oh, right, so, we do get to see
a difference.
470
00:28:02,560 --> 00:28:05,400
So, we're at the same altitude
as the base of Mount Snowdon,
471
00:28:05,400 --> 00:28:08,520
but because we've travelled
further down south overnight,
472
00:28:08,520 --> 00:28:10,680
gravity's less here? Yes.
473
00:28:13,480 --> 00:28:14,680
They push on.
474
00:28:20,440 --> 00:28:24,960
And by sunset they reach
Sidmouth on the south coast.
475
00:28:27,240 --> 00:28:30,800
Sean takes the second gravity
reading of the day
476
00:28:30,800 --> 00:28:33,320
and Poppy improvises a map.
477
00:28:33,320 --> 00:28:36,080
Well, sort of a map.
478
00:28:36,080 --> 00:28:38,840
Can we write "not to scale"
at the top there.
479
00:28:38,840 --> 00:28:41,760
SHE MOUTHS, ALL LAUGH
480
00:28:41,760 --> 00:28:44,320
So, I drew this map.
481
00:28:44,320 --> 00:28:46,280
Scotland's a bit squashed.
482
00:28:46,280 --> 00:28:51,320
Wales is quite high up and Cornwall
is there, but you get the idea.
483
00:28:51,520 --> 00:28:54,400
Sean, we've been travelling
with you,
484
00:28:54,400 --> 00:28:57,560
you've done quite a few
gravity meter readings,
485
00:28:57,560 --> 00:28:59,680
can you plot them on this
not-to-scale,
486
00:28:59,680 --> 00:29:01,760
badly-drawn map, please? Sure.
487
00:29:01,760 --> 00:29:06,760
So, if you remember we started off
in Mount Snowdon, here,
488
00:29:06,760 --> 00:29:09,600
and that was the zero measurement
for our survey.
489
00:29:09,600 --> 00:29:12,640
Then we've come all the way
down here to the south coast.
490
00:29:15,000 --> 00:29:19,960
The difference from the base
of Snowdon is -212 milligals. Wow.
491
00:29:22,560 --> 00:29:26,040
So, the difference between going
and measuring gravity
492
00:29:26,040 --> 00:29:29,600
at the base of the mountain
and the top of the mountain
493
00:29:29,600 --> 00:29:32,240
is about the same as
here at this latitude
494
00:29:32,240 --> 00:29:35,800
and down here at this latitude.
495
00:29:35,800 --> 00:29:37,960
They're quite clearly at sea level,
496
00:29:37,960 --> 00:29:42,920
yet gravity here is roughly the same
as it is at the top of Snowdon.
497
00:29:43,000 --> 00:29:44,720
But where next?
498
00:29:44,720 --> 00:29:47,040
We are here.
499
00:29:47,040 --> 00:29:50,480
If we want to find out
where we are the lightest,
500
00:29:50,480 --> 00:29:55,480
why don't we travel all the way to
the most southerly point in the UK,
501
00:29:56,040 --> 00:29:59,720
which is here? But altitude
can also help us,
502
00:29:59,720 --> 00:30:02,680
so why not find a place
in the country
503
00:30:02,680 --> 00:30:07,360
that is both low in latitude but
also as high in altitude
504
00:30:07,360 --> 00:30:12,160
in terms of height above sea level,
because that will get us somewhere
505
00:30:12,160 --> 00:30:15,080
that is really far away
from the core of the Earth,
506
00:30:15,080 --> 00:30:17,400
whilst staying
within the country?
507
00:30:24,240 --> 00:30:28,840
So, the answer to the puzzle lies
in a combination of two factors.
508
00:30:28,840 --> 00:30:32,840
How much further south should
they go and how much higher?
509
00:30:35,000 --> 00:30:38,360
At the end of day two,
Sean's results show that the team
510
00:30:38,360 --> 00:30:41,040
weighs about 80 grams
lighter in total
511
00:30:41,040 --> 00:30:43,160
than back at the base of Snowdon.
512
00:30:54,440 --> 00:30:57,680
The way that weight changes
is just one example
513
00:30:57,680 --> 00:31:00,280
of Newton's famous
equation in action.
514
00:31:02,760 --> 00:31:06,000
But Newton had left
his masterpiece incomplete.
515
00:31:06,000 --> 00:31:08,200
He didn't know the value of G,
516
00:31:08,200 --> 00:31:10,520
the gravitational constant,
517
00:31:10,520 --> 00:31:14,680
which sets the size of the force.
518
00:31:14,680 --> 00:31:19,000
To harness the full power
of the equation, you need to know G.
519
00:31:19,000 --> 00:31:22,840
And the vital clue came
within an incredible experiment
520
00:31:22,840 --> 00:31:25,840
conducted in London at
the end of the 18th century.
521
00:31:29,400 --> 00:31:33,720
It was an attempt to work out
the mass of the Earth itself.
522
00:31:33,720 --> 00:31:37,040
And it was carried out
by an eccentric,
523
00:31:37,040 --> 00:31:39,680
extravagantly rich aristocrat,
524
00:31:39,680 --> 00:31:42,080
Henry Cavendish.
525
00:31:42,080 --> 00:31:45,520
Cavendish was a chronically shy,
526
00:31:45,520 --> 00:31:50,160
deeply solitary man living in total
isolation in his house in Clapham.
527
00:31:50,160 --> 00:31:52,080
The story goes that, one day,
528
00:31:52,080 --> 00:31:55,960
he accidentally bumped into a female
servant on his staircase.
529
00:31:55,960 --> 00:31:58,280
He was so traumatised by this event
530
00:31:58,280 --> 00:32:01,000
that he had a new
staircase built just for him
531
00:32:01,000 --> 00:32:04,280
so that this horrible incident
could never happen again.
532
00:32:05,720 --> 00:32:08,120
Cavendish had inherited
vast fortunes
533
00:32:08,120 --> 00:32:10,160
and was able to dedicate his life
534
00:32:10,160 --> 00:32:13,600
to devising pioneering experiments -
535
00:32:13,600 --> 00:32:17,240
including one particularly
extraordinary piece of equipment.
536
00:32:21,680 --> 00:32:23,760
He set up something a bit like this.
537
00:32:23,760 --> 00:32:26,120
It's called a "torsion balance".
538
00:32:26,120 --> 00:32:28,480
It involves four lead spheres,
539
00:32:28,480 --> 00:32:32,280
two large heavy ones which are held
fixed in place,
540
00:32:32,280 --> 00:32:36,760
and suspended by a very
thin wire is a wooden rod,
541
00:32:36,760 --> 00:32:41,120
six-feet-long, with two smaller
balls on either end.
542
00:32:41,120 --> 00:32:43,080
Now, the crux of the experiment
543
00:32:43,080 --> 00:32:46,720
is the relationship between
the large ball and the small ball.
544
00:32:46,720 --> 00:32:49,680
Now, of course, there's
a gravitational pull downwards
545
00:32:49,680 --> 00:32:52,360
on both of the balls due to the
Earth's gravity.
546
00:32:52,360 --> 00:32:53,840
But Newton also tells us
547
00:32:53,840 --> 00:32:58,360
that there should be a very weak
gravitational pull between the balls
548
00:32:58,360 --> 00:33:02,040
and this is effectively what
Cavendish was trying to measure.
549
00:33:02,040 --> 00:33:05,720
Any slight movement of the small
ball towards the large one
550
00:33:05,720 --> 00:33:08,720
should cause a twist
in the torsion wire
551
00:33:08,720 --> 00:33:11,880
and that's what Cavendish
was trying to detect.
552
00:33:11,880 --> 00:33:14,520
Of course, this is all
much easier said than done.
553
00:33:14,520 --> 00:33:17,200
The experiment
was incredibly sensitive.
554
00:33:17,200 --> 00:33:19,000
The tiniest of vibrations,
555
00:33:19,000 --> 00:33:21,880
the slightest breeze,
changes in temperature
556
00:33:21,880 --> 00:33:23,960
could all influence the
measurements.
557
00:33:23,960 --> 00:33:28,160
So, Cavendish had to isolate the
apparatus inside a box
558
00:33:28,160 --> 00:33:30,440
and the box within a shed.
559
00:33:30,440 --> 00:33:34,400
He even realised that his mere
presence next to the apparatus
560
00:33:34,400 --> 00:33:38,600
could influence things, so he had to
remove himself outside the shed.
561
00:33:39,880 --> 00:33:42,160
What he then did was sit outside
the shed,
562
00:33:42,160 --> 00:33:44,880
and through a small hole
in the shed wall,
563
00:33:44,880 --> 00:33:49,640
look through a telescope to detect
the tiniest of twists in the wire.
564
00:33:49,640 --> 00:33:52,960
It was an incredibly difficult
process, but after many months,
565
00:33:52,960 --> 00:33:56,800
he finally felt confident enough
that he had a reliable result.
566
00:34:03,960 --> 00:34:06,960
Cavendish found that the
small balls did move...
567
00:34:08,560 --> 00:34:10,440
..a tiny four millimetres.
568
00:34:13,440 --> 00:34:15,040
He calculated his results
569
00:34:15,040 --> 00:34:17,120
by comparing the density
of the balls
570
00:34:17,120 --> 00:34:18,920
with the density of water.
571
00:34:20,960 --> 00:34:23,360
In the end, the result of
Cavendish's experiment
572
00:34:23,360 --> 00:34:25,240
and subsequent calculations
573
00:34:25,240 --> 00:34:27,720
was that the density of the Earth
574
00:34:27,720 --> 00:34:31,000
was about five and a half times
that of water.
575
00:34:31,000 --> 00:34:32,840
Or, put another way,
576
00:34:32,840 --> 00:34:37,840
the mass of the Earth was
5.9 trillion trillion kilograms.
577
00:34:38,440 --> 00:34:42,560
What's most remarkable is that
Cavendish got this number right
578
00:34:42,560 --> 00:34:46,200
to within an accuracy of 1%.
579
00:34:46,200 --> 00:34:49,440
With Cavendish's astonishing result,
580
00:34:49,440 --> 00:34:52,080
scientists were able to work out G.
581
00:34:53,640 --> 00:34:55,520
Then the equation could be used
582
00:34:55,520 --> 00:34:57,960
to determine the mass of any
celestial body
583
00:34:57,960 --> 00:34:59,760
in orbit around another.
584
00:35:01,800 --> 00:35:05,680
So, astronomers were able to
calculate the mass of the sun
585
00:35:05,680 --> 00:35:08,520
and the planets, and the moon,
586
00:35:08,520 --> 00:35:12,640
and, eventually,
even distant galaxies.
587
00:35:17,600 --> 00:35:21,800
At the end of day two, the team were
in Sidmouth on the south coast,
588
00:35:21,800 --> 00:35:25,560
looking for the place in Britain
where they'll weigh the least.
589
00:35:25,560 --> 00:35:30,560
They've worked out the answer lies
in a combination of two factors -
590
00:35:30,560 --> 00:35:34,440
the right mix of going south
and being higher up.
591
00:35:36,720 --> 00:35:39,800
For the final leg of the journey,
I'm going to meet up with them.
592
00:35:42,080 --> 00:35:44,520
I asked them to drive a short
distance west
593
00:35:44,520 --> 00:35:48,600
to one of the most remote
areas in mainland Britain.
594
00:35:48,600 --> 00:35:50,360
Dartmoor National Park.
595
00:35:52,400 --> 00:35:55,760
'It's only 40 miles from the
southernmost tip of Britain.'
596
00:35:55,760 --> 00:35:57,200
Hello. Hi, Andrew.
597
00:35:57,200 --> 00:35:58,960
Good to see you. Nice to see you.
598
00:35:58,960 --> 00:36:02,440
'And it's very high,
very hilly territory.'
599
00:36:02,440 --> 00:36:05,120
Jim, the team got to the
south coast yesterday...
600
00:36:05,120 --> 00:36:08,440
Yes. ..to find gravity
at its weakest.
601
00:36:08,440 --> 00:36:12,480
But we haven't quite figured out
whether it's altitude or latitude.
602
00:36:12,480 --> 00:36:15,120
Do we go further south
or do we go higher up?
603
00:36:15,120 --> 00:36:18,800
You're right to ask, "Do we go as
far south as possible
604
00:36:18,800 --> 00:36:20,440
"or as high as possible?"
605
00:36:20,440 --> 00:36:23,480
That's why I've brought
you here to Dartmoor.
606
00:36:23,480 --> 00:36:27,920
We've charted the most important
points on this map here.
607
00:36:27,920 --> 00:36:29,480
Right. Let's have a look.
608
00:36:29,480 --> 00:36:32,520
So, we are here, Two Bridges.
609
00:36:32,520 --> 00:36:37,520
Yes. These four dots represent
these hills up there behind us,
610
00:36:38,160 --> 00:36:41,120
which are at about 500 metres
above sea level.
611
00:36:41,120 --> 00:36:42,880
That's what we want to check out.
612
00:36:42,880 --> 00:36:45,200
'These hills are close
to the south coast
613
00:36:45,200 --> 00:36:48,920
'and they're also the highest in
the whole of the south of England.
614
00:36:50,560 --> 00:36:54,240
'So, logic suggests they must be the
right combination
615
00:36:54,240 --> 00:36:56,160
'of latitude and altitude.'
616
00:36:56,160 --> 00:36:59,000
Well, there's another
reason why this makes perfect sense,
617
00:36:59,000 --> 00:37:00,680
one which we haven't looked at yet,
618
00:37:00,680 --> 00:37:03,960
and that's the effect of the
underlying rocks on gravity.
619
00:37:03,960 --> 00:37:06,320
And I've got a map here that
shows...
620
00:37:06,320 --> 00:37:08,920
You're going to trump my map
with yours, aren't you? I am!
621
00:37:08,920 --> 00:37:13,840
Here we are, down here, now these
blue areas are the lowest areas
622
00:37:13,840 --> 00:37:17,760
according to the density
of the rocks underneath.
623
00:37:17,760 --> 00:37:20,960
'The rocks around
here are made of granite,
624
00:37:20,960 --> 00:37:23,200
'which will make gravity
weaker still.'
625
00:37:24,600 --> 00:37:27,080
So, that's helping -
as well as the altitude
626
00:37:27,080 --> 00:37:29,120
and the fact that we're further
south.
627
00:37:29,120 --> 00:37:31,440
Yes, it's also playing a part.
628
00:37:33,360 --> 00:37:35,680
'Well, we have a plausible theory.
629
00:37:35,680 --> 00:37:37,640
'But now we need to test it.'
630
00:37:39,640 --> 00:37:42,880
'If I'm right, then, at the top,
our gravity reading
631
00:37:42,880 --> 00:37:45,880
'should be by far
the lowest reading of the trip.'
632
00:37:49,240 --> 00:37:52,160
'Of course, there's another effect
of gravity to deal with now -
633
00:37:52,160 --> 00:37:54,680
'it's knackering
when you head uphill.'
634
00:37:55,840 --> 00:37:59,200
OK, I think this is pretty much
the start of the hills
635
00:37:59,200 --> 00:38:02,960
we've located on the map. So, let's
see if this is the lightest place.
636
00:38:02,960 --> 00:38:05,400
Sean, if you want to get
the gravity meter out,
637
00:38:05,400 --> 00:38:08,560
and we'll take another
reading here. Yep. OK.
638
00:38:11,680 --> 00:38:14,920
'Sean sets up his equipment
one more time.'
639
00:38:14,920 --> 00:38:17,160
What's the news?
640
00:38:17,160 --> 00:38:21,800
Well, the bottom of Mount Snowdon
was our zero for this test.
641
00:38:21,800 --> 00:38:23,400
We found we lost a certain amount
642
00:38:23,400 --> 00:38:25,440
by going up to the top
of Mount Snowdon.
643
00:38:25,440 --> 00:38:29,520
We found we lost a certain amount
coming south to the south coast.
644
00:38:29,520 --> 00:38:32,520
Not only have we beaten that,
we've smashed it.
645
00:38:32,520 --> 00:38:37,120
Brilliant.
We were -219 milligals
646
00:38:37,120 --> 00:38:39,480
lower at the top of Mount Snowdon.
647
00:38:39,480 --> 00:38:41,200
Here on Dartmoor,
648
00:38:41,200 --> 00:38:44,960
we're -347 milligals lower.
Wow! Brilliant!
649
00:38:44,960 --> 00:38:47,080
So, it is a combination
of three things.
650
00:38:47,080 --> 00:38:50,160
We're far south, so it's the
latitude, we're at altitude,
651
00:38:50,160 --> 00:38:52,760
we're quite high up, and we're
surrounded by all this granite rock,
652
00:38:52,760 --> 00:38:54,800
which is low-density anyway.
653
00:38:54,800 --> 00:38:57,520
I hope you all think it was worth
the climb up here anyway?
654
00:38:57,520 --> 00:39:00,760
Yes, absolutely. There you go.
Boom, science!
655
00:39:00,760 --> 00:39:04,760
ALL LAUGH
656
00:39:04,760 --> 00:39:07,960
Now, we already know that the
altitude of these hills
657
00:39:07,960 --> 00:39:10,560
takes us much further
from the Earth's core
658
00:39:10,560 --> 00:39:13,280
than anywhere else
further south in Britain,
659
00:39:13,280 --> 00:39:16,720
so gravity must be weakest here.
660
00:39:16,720 --> 00:39:19,200
There's extra evidence, too.
661
00:39:19,200 --> 00:39:21,000
The British Geological Survey
662
00:39:21,000 --> 00:39:24,920
has compiled tens of thousands
of gravity readings made in the UK
663
00:39:24,920 --> 00:39:29,360
and the lowest readings ever
recorded were all taken around here
664
00:39:29,360 --> 00:39:31,960
on the high hills of Dartmoor.
665
00:39:33,000 --> 00:39:34,600
What do we do to celebrate?
666
00:39:34,600 --> 00:39:37,240
We weigh ourselves, of course.
667
00:39:37,240 --> 00:39:38,960
I bet you don't weigh that much.
668
00:39:38,960 --> 00:39:41,280
Whoa!
669
00:39:41,280 --> 00:39:44,160
It's all them Nutella pancakes
for breakfast!
670
00:39:44,160 --> 00:39:46,560
74, 75. I need to lose weight!
671
00:39:46,560 --> 00:39:47,640
LAUGHTER
672
00:39:47,640 --> 00:39:52,680
I can tell you that you should weigh
something like 20 grams less
673
00:39:52,920 --> 00:39:55,760
than you did
at the base of Mount Snowdon.
674
00:39:55,760 --> 00:39:59,560
Guys, I'm guessing something
like 25 to 30 grams less.
675
00:39:59,560 --> 00:40:02,480
So, if you want to weigh
as little as possible,
676
00:40:02,480 --> 00:40:04,720
this is the place
in Britain to come.
677
00:40:04,720 --> 00:40:06,920
But in any case,
it's such a tiny amount
678
00:40:06,920 --> 00:40:08,640
that it's going to be wiped out
entirely
679
00:40:08,640 --> 00:40:11,480
by whatever it was you had
for breakfast this morning.
680
00:40:11,480 --> 00:40:12,760
LAUGHTER
681
00:40:19,160 --> 00:40:22,360
Gravity.
What goes up must come down.
682
00:40:24,560 --> 00:40:27,520
All of our lives,
we abide by its rules.
683
00:40:28,520 --> 00:40:30,360
It dominates our every action.
684
00:40:31,960 --> 00:40:34,120
But there's one select group of
humans
685
00:40:34,120 --> 00:40:37,280
who know what it's like
to live free of gravity.
686
00:40:37,280 --> 00:40:39,320
'Two, one...
687
00:40:39,320 --> 00:40:40,400
'zero.
688
00:40:40,400 --> 00:40:42,800
'Lift-off!'
689
00:40:46,520 --> 00:40:48,800
Everybody's used to gravity.
690
00:40:48,800 --> 00:40:50,560
We're used to the oppression of it.
691
00:40:50,560 --> 00:40:53,080
Gravity is the ultimate oppressor.
692
00:40:53,080 --> 00:40:58,080
It grinds us under its heel 24/7
with no release,
693
00:40:58,680 --> 00:41:03,360
until you're in space and then,
suddenly, you're free from gravity.
694
00:41:03,360 --> 00:41:06,000
You're weightless in orbit.
695
00:41:07,440 --> 00:41:09,840
Canadian astronaut Chris Hadfield
696
00:41:09,840 --> 00:41:14,400
spent five months on board
the International Space Station.
697
00:41:14,400 --> 00:41:18,000
You can pull your knees up
to your chest and just tumble.
698
00:41:18,000 --> 00:41:20,120
Or, if you take a wet cloth,
699
00:41:20,120 --> 00:41:22,480
and you get it dripping wet,
700
00:41:22,480 --> 00:41:25,720
and everybody on Earth knows what'll
happen when you wring it out.
701
00:41:25,720 --> 00:41:27,680
All the water will fall, inevitably.
702
00:41:27,680 --> 00:41:29,640
If you do that in weightlessness,
703
00:41:29,640 --> 00:41:32,000
the water stays there
and it, actually,
704
00:41:32,000 --> 00:41:35,960
because of the surface tension,
starts crawling up your arms.
705
00:41:40,960 --> 00:41:44,560
It's a little bit mesmerising and
hypnotic to be in weightlessness.
706
00:41:46,200 --> 00:41:48,360
If you're weightless,
you don't need a bed,
707
00:41:48,360 --> 00:41:49,800
you don't need a mattress,
708
00:41:49,800 --> 00:41:51,880
you don't need a pillow.
709
00:41:51,880 --> 00:41:56,160
Your body is floating completely
suspended, like magic.
710
00:41:59,440 --> 00:42:01,320
Movement becomes effortless.
711
00:42:01,320 --> 00:42:05,480
You can push off with one finger
and fly, and it's humble.
712
00:42:05,480 --> 00:42:08,760
You don't need to hold yourself
where you are with muscle.
713
00:42:08,760 --> 00:42:12,440
You can just... With a delicate
fingertip pressure,
714
00:42:12,440 --> 00:42:14,320
you can stay where you are.
715
00:42:14,320 --> 00:42:17,560
But there is a price to pay.
716
00:42:17,560 --> 00:42:21,800
Astronauts' bones atrophy
and their muscles wither away.
717
00:42:24,320 --> 00:42:29,160
One of the things we do on board
a space station is exercise,
718
00:42:29,160 --> 00:42:31,160
purely to simulate gravity.
719
00:42:31,160 --> 00:42:34,520
If we don't do something,
then our heart will shrink,
720
00:42:34,520 --> 00:42:37,640
our ability to pump blood
to our head will diminish,
721
00:42:37,640 --> 00:42:40,720
our bones will start to dissolve
and our muscles will waste away.
722
00:42:44,760 --> 00:42:47,120
'OK. Separation confirmed.
Timer's on.'
723
00:42:47,120 --> 00:42:48,640
'Backing away at a rate
724
00:42:48,640 --> 00:42:52,640
'of just a little over one tenth
of a metre per second.'
725
00:42:52,640 --> 00:42:56,520
Re-entering gravity
is a punishing experience.
726
00:42:56,520 --> 00:42:59,120
To come back to Earth is violent.
727
00:43:01,280 --> 00:43:03,640
It can be five times
the force of gravity
728
00:43:03,640 --> 00:43:05,840
or eight times
the force of gravity,
729
00:43:05,840 --> 00:43:10,800
crushing you down into the floor
of the ship for quite a long time.
730
00:43:11,040 --> 00:43:13,480
Then, of course, you hit the ground
and tumble
731
00:43:13,480 --> 00:43:18,480
and roll to a stop and now
you are the victim of your past.
732
00:43:19,120 --> 00:43:23,000
You're the victim of your
decision-making, lying there,
733
00:43:23,000 --> 00:43:27,120
trying to shake your head and get
used to being in gravity again.
734
00:43:27,120 --> 00:43:31,280
I remarked, at the time, that I had
forgotten that my lips have weight
735
00:43:31,280 --> 00:43:33,000
and my tongue has weight.
736
00:43:33,000 --> 00:43:36,600
You don't think about it. But
if you try and talk articulately,
737
00:43:36,600 --> 00:43:39,640
standing on your head, you'll notice
that you have to sort of control
738
00:43:39,640 --> 00:43:41,800
your lips and your tongue
a little differently,
739
00:43:41,800 --> 00:43:44,040
just because gravity's pushing
them the other way.
740
00:43:44,040 --> 00:43:45,680
And it's the same sort of thing,
741
00:43:45,680 --> 00:43:47,720
raising your arm,
holding your head up,
742
00:43:47,720 --> 00:43:51,320
turning your head when everything
wants to tumble,
743
00:43:51,320 --> 00:43:54,720
just keeping your balance,
all of those things.
744
00:43:54,720 --> 00:43:59,480
It's a little bit like relearning
to walk again like an infant.
745
00:43:59,480 --> 00:44:01,920
REPORTERS CLAMOUR
746
00:44:04,080 --> 00:44:07,080
'Gravity on Earth
grinds us all down.
747
00:44:08,640 --> 00:44:12,480
'Over the course of the day,
it actually squeezes your spine,
748
00:44:12,480 --> 00:44:16,800
'an effect you can see for yourself
if you use a measuring rod.'
749
00:44:16,800 --> 00:44:19,000
OK, so it's 7:30 in the morning.
750
00:44:19,000 --> 00:44:22,200
I've just got up and I'm going to
see how tall I am
751
00:44:22,200 --> 00:44:24,360
before gravity drags me down.
752
00:44:32,600 --> 00:44:34,840
That's 178 centimetres
753
00:44:34,840 --> 00:44:37,480
or just over 5'10.
754
00:44:41,400 --> 00:44:45,680
Over the course of the day, gravity
compresses the fluids in your spine.
755
00:44:48,560 --> 00:44:51,600
Right, it is just past 11pm.
756
00:44:51,600 --> 00:44:54,040
I've been standing up
for most of the day
757
00:44:54,040 --> 00:44:57,440
so let's see if gravity
has had an effect on my height.
758
00:45:03,320 --> 00:45:07,840
That's 176 centimetres,
759
00:45:07,840 --> 00:45:11,240
so I really have shrunk by
just over half an inch
760
00:45:11,240 --> 00:45:13,680
over the course of today.
761
00:45:18,040 --> 00:45:22,240
In the longer term, gravity can
affect your posture permanently,
762
00:45:22,240 --> 00:45:27,160
but there are exercises you can do
to counteract this effect.
763
00:45:27,160 --> 00:45:30,040
Part of my research has been looking
at the effects of gravity
764
00:45:30,040 --> 00:45:32,560
on the human body. So people might
not be aware
765
00:45:32,560 --> 00:45:35,760
or they might not always think
about the effect of gravity
766
00:45:35,760 --> 00:45:37,200
on our physical state,
767
00:45:37,200 --> 00:45:40,000
on our health and, particularly,
on our posture.
768
00:45:40,000 --> 00:45:42,360
However, because it's such
a constant force,
769
00:45:42,360 --> 00:45:46,040
gravity has a massive impact
over the course of our lifetime.
770
00:45:46,040 --> 00:45:49,440
As you get older,
you can develop a stoop,
771
00:45:49,440 --> 00:45:52,720
which is damaging to your mobility.
772
00:45:52,720 --> 00:45:55,160
Gokun here has actually
got very good posture
773
00:45:55,160 --> 00:45:58,280
but I'd like you to just show
not so good posture.
774
00:45:58,280 --> 00:46:00,160
So when...
775
00:46:00,160 --> 00:46:03,160
Poor posture is really rounded
shoulders
776
00:46:03,160 --> 00:46:05,600
and then loss of the curve in the
back, as well.
777
00:46:05,600 --> 00:46:07,760
Can I just ask you to raise up
your arms
778
00:46:07,760 --> 00:46:10,760
when you're in that posture?
I can't go any higher.
779
00:46:10,760 --> 00:46:13,240
No, and then, just come back down,
shoulders back,
780
00:46:13,240 --> 00:46:15,480
and then raise your arms.
781
00:46:15,480 --> 00:46:18,440
You can see the effect
of posture on function.
782
00:46:19,680 --> 00:46:22,720
Ironically, the exercises
which many gym-goers do
783
00:46:22,720 --> 00:46:25,280
actually make your posture worse.
784
00:46:25,280 --> 00:46:28,560
That's if you only exercise
the frontal muscles,
785
00:46:28,560 --> 00:46:32,000
like the chest and abdominals.
786
00:46:32,000 --> 00:46:35,960
So, it's recommended you exercise
the back muscles just as much,
787
00:46:35,960 --> 00:46:39,680
to straighten you out and counteract
the effects of gravity.
788
00:46:50,760 --> 00:46:54,360
Gravity shapes our bodies
and moulds our planet.
789
00:46:54,360 --> 00:46:58,080
Nothing happens on Earth without its
power and influence.
790
00:47:00,240 --> 00:47:03,480
Sir Isaac Newton explained so many
of its effects
791
00:47:03,480 --> 00:47:05,680
using one simple equation.
792
00:47:07,080 --> 00:47:08,840
And, in the centuries that followed,
793
00:47:08,840 --> 00:47:12,280
his laws of physics led to
breakthrough after breakthrough,
794
00:47:12,280 --> 00:47:14,640
spurring on the
Industrial Revolution.
795
00:47:16,080 --> 00:47:18,760
But in the first decade
of the 20th century,
796
00:47:18,760 --> 00:47:20,520
the next genius in our story
797
00:47:20,520 --> 00:47:24,640
challenged the very foundations of
our understanding of gravity.
798
00:47:26,280 --> 00:47:29,280
A young German scientist
called Albert Einstein
799
00:47:29,280 --> 00:47:31,960
was churning something
over in his mind.
800
00:47:33,800 --> 00:47:37,760
He thought that something in
Newton's laws didn't quite add up.
801
00:47:49,760 --> 00:47:53,080
and this tennis ball is the Earth
in orbit around me.
802
00:47:53,080 --> 00:47:55,680
Newton's laws can describe,
very precisely,
803
00:47:55,680 --> 00:47:58,040
the path the Earth takes
around the sun
804
00:47:58,040 --> 00:48:03,040
in terms of the mutual gravitational
attraction between the two bodies.
805
00:48:03,680 --> 00:48:07,760
But what Newton can't explain
is what connects them.
806
00:48:07,760 --> 00:48:08,920
In reality, of course,
807
00:48:08,920 --> 00:48:11,800
there is no invisible string
between the Earth and the sun,
808
00:48:11,800 --> 00:48:13,400
holding the two together.
809
00:48:13,400 --> 00:48:16,640
There's just empty space,
a complete void.
810
00:48:16,640 --> 00:48:18,440
And yet, according to Newton,
811
00:48:18,440 --> 00:48:21,880
the Earth and sun pull
on each other instantaneously
812
00:48:21,880 --> 00:48:23,520
across a vast distance.
813
00:48:23,520 --> 00:48:25,800
How can gravity act in this way
814
00:48:25,800 --> 00:48:29,120
when there's nothing to connect
it or transmit it?
815
00:48:32,800 --> 00:48:35,080
After years puzzling over this,
816
00:48:35,080 --> 00:48:38,440
Einstein had a blinding flash
of inspiration.
817
00:48:40,240 --> 00:48:42,000
Just like Galileo and his ramp...
818
00:48:43,240 --> 00:48:45,200
..or Newton with his apple,
819
00:48:45,200 --> 00:48:47,120
Einstein's breakthrough came
820
00:48:47,120 --> 00:48:50,160
because he was thinking about one
simple action...
821
00:48:53,440 --> 00:48:56,000
..what happens when something falls.
822
00:49:01,000 --> 00:49:02,720
To explain, I'm visiting
823
00:49:02,720 --> 00:49:07,000
this 400-foot-high tower
in Northampton...
824
00:49:07,000 --> 00:49:09,240
built to safety-test lifts.
825
00:49:13,880 --> 00:49:15,400
One day in 1907,
826
00:49:15,400 --> 00:49:19,200
Einstein had what he called
the "happiest thought of his life".
827
00:49:23,880 --> 00:49:26,440
What if I were standing
in a stationary lift,
828
00:49:26,440 --> 00:49:28,840
completely isolated
from the outside world,
829
00:49:28,840 --> 00:49:33,840
not feeling anything apart from
the pull of gravity on my feet?
830
00:49:33,840 --> 00:49:36,840
What if, then, the lift cable breaks
831
00:49:36,840 --> 00:49:38,560
and I start falling?
832
00:49:38,560 --> 00:49:42,760
What are the forces that I will feel
as I'm plummeting to the ground?
833
00:49:48,400 --> 00:49:50,440
CRASHING
834
00:49:50,440 --> 00:49:52,680
Well, I'm not going to try that.
835
00:49:55,560 --> 00:49:58,080
Fortunately, there's another way
to test this
836
00:49:58,080 --> 00:50:00,760
without me having to plunge
down a lift shaft.
837
00:50:00,760 --> 00:50:02,360
Sorry to disappoint you!
838
00:50:04,680 --> 00:50:08,240
This little device here that I have
strapped to this plastic toy
839
00:50:08,240 --> 00:50:10,360
is an industrial accelerometer.
840
00:50:10,360 --> 00:50:12,440
So, it measures acceleration.
841
00:50:12,440 --> 00:50:14,280
Now, I've got it connected
to my laptop
842
00:50:14,280 --> 00:50:16,920
and it's showing a measurement
of 1G.
843
00:50:16,920 --> 00:50:19,200
Now, that's the
downward acceleration
844
00:50:19,200 --> 00:50:21,360
due to the pull of Earth's gravity.
845
00:50:21,360 --> 00:50:24,920
So, basically, it works
just like a gravity meter.
846
00:50:24,920 --> 00:50:27,800
But what happens
if I were to drop it?
847
00:50:27,800 --> 00:50:29,760
Presumably,
it'll carry on measuring 1G
848
00:50:29,760 --> 00:50:32,120
because it's falling
in Earth's gravity.
849
00:50:32,120 --> 00:50:34,200
OK, well, let's try that and see.
850
00:50:52,960 --> 00:50:56,280
So, you can see here,
along this line at the bottom,
851
00:50:56,280 --> 00:50:58,200
that's when I was holding it still
852
00:50:58,200 --> 00:51:00,880
and it's measuring an acceleration
of 1G.
853
00:51:00,880 --> 00:51:03,480
These oscillations here is when
I stood up
854
00:51:03,480 --> 00:51:06,000
and there's a bit of disturbance,
855
00:51:06,000 --> 00:51:10,320
but this spike along here
is the moment I released it.
856
00:51:10,320 --> 00:51:14,360
And this short duration along
here is the time it was falling.
857
00:51:14,360 --> 00:51:16,440
And you see, while it was falling,
858
00:51:16,440 --> 00:51:21,000
it was registering
an acceleration of zero.
859
00:51:21,000 --> 00:51:23,960
Now, if you think about it,
this is really odd.
860
00:51:23,960 --> 00:51:26,400
The accelerometer
is accelerating downwards.
861
00:51:26,400 --> 00:51:30,000
It's plummeting in the full grip of
Earth's gravity
862
00:51:30,000 --> 00:51:33,840
and yet it's measuring
no acceleration at all.
863
00:51:33,840 --> 00:51:37,760
It's as though gravity
has completely disappeared.
864
00:51:40,160 --> 00:51:43,440
Einstein's insight was
that when something falls,
865
00:51:43,440 --> 00:51:46,000
it no longer feels the pull of
gravity.
866
00:51:47,320 --> 00:51:50,880
In fact, falling is like floating
in empty space.
867
00:51:52,720 --> 00:51:56,040
This is the essence of Einstein's
"happy thought"
868
00:51:56,040 --> 00:51:59,560
and what we now call
his "principle of equivalence".
869
00:52:01,160 --> 00:52:04,560
Einstein's point is that,
when the man in the lift falls,
870
00:52:04,560 --> 00:52:09,600
he doesn't just feel weightless,
he is weightless.
871
00:52:09,600 --> 00:52:12,680
Einstein said the man feels no force
pulling on him
872
00:52:12,680 --> 00:52:15,360
because there is no
force pulling on him.
873
00:52:15,360 --> 00:52:17,560
Gravity doesn't act on him,
874
00:52:17,560 --> 00:52:20,560
it acts on the space
and time around him,
875
00:52:20,560 --> 00:52:24,080
what we now call
the "geometry of space-time".
876
00:52:30,920 --> 00:52:33,720
This was a radical redefinition.
877
00:52:33,720 --> 00:52:37,480
Einstein says to forget the idea
of gravity as a force,
878
00:52:37,480 --> 00:52:41,120
acting mysteriously
between two objects.
879
00:52:41,120 --> 00:52:46,200
Now we have to think of it as the
shape of space-time changing.
880
00:52:48,120 --> 00:52:52,120
You see, Newton saw space and time
as independent,
881
00:52:52,120 --> 00:52:53,920
fixed and immutable,
882
00:52:53,920 --> 00:52:58,320
that three-dimensional space is
the stage in which things happen,
883
00:52:58,320 --> 00:53:00,120
but time is separate,
884
00:53:00,120 --> 00:53:01,920
it ticks by at the same rate
885
00:53:01,920 --> 00:53:03,960
everywhere in the universe.
886
00:53:03,960 --> 00:53:07,360
According to Newton,
an object would travel through space
887
00:53:07,360 --> 00:53:11,000
in a straight line unless acted upon
by a force like gravity
888
00:53:11,000 --> 00:53:14,920
that would cause it to deviate
from that path.
889
00:53:14,920 --> 00:53:19,200
But Einstein said that space and
time aren't fixed and immutable,
890
00:53:19,200 --> 00:53:22,080
they're interconnected,
meshed together
891
00:53:22,080 --> 00:53:25,000
in what is known as space-time.
892
00:53:27,040 --> 00:53:30,160
And he said that space-time
can be warped -
893
00:53:30,160 --> 00:53:33,560
that matter curves
space and time around it.
894
00:53:39,800 --> 00:53:42,680
So, after Einstein,
we no longer see gravity
895
00:53:42,680 --> 00:53:46,320
as an invisible string
pulling objects together.
896
00:53:48,440 --> 00:53:50,600
Instead, a body like the Earth
897
00:53:50,600 --> 00:53:54,200
warps the structure of
space and time around it.
898
00:53:55,720 --> 00:53:57,400
And an object in orbit
899
00:53:57,400 --> 00:54:01,000
follows a path which
is as straight as possible
900
00:54:01,000 --> 00:54:04,000
through that space-time.
901
00:54:04,000 --> 00:54:08,440
It's a fundamental part of
Einstein's vision of reality.
902
00:54:08,440 --> 00:54:12,000
Space and time
can't be disentangled.
903
00:54:12,000 --> 00:54:15,560
You can't talk about space
separately from time.
904
00:54:17,640 --> 00:54:21,440
So, matter warps time
as well as space.
905
00:54:23,520 --> 00:54:27,960
It's known as
"gravitational time dilation",
906
00:54:27,960 --> 00:54:31,800
and it's possibly the strangest
of all of Einstein's discoveries.
907
00:54:35,040 --> 00:54:37,280
I've got two identical clocks here.
908
00:54:37,280 --> 00:54:39,760
Now, because the clock lower down
909
00:54:39,760 --> 00:54:42,400
is closer
to the centre of the Earth,
910
00:54:42,400 --> 00:54:46,320
it feels ever so slightly a stronger
gravitational pull
911
00:54:46,320 --> 00:54:48,240
than the clock higher up.
912
00:54:48,240 --> 00:54:52,360
Einstein's theory says that the
lower clock will tick by
913
00:54:52,360 --> 00:54:56,160
at a slightly slower rate
than the higher clock.
914
00:54:56,160 --> 00:55:00,360
Basically, gravity slows time down.
915
00:55:02,240 --> 00:55:07,240
It's an extraordinary conception
of reality that Einstein describes.
916
00:55:09,200 --> 00:55:13,400
Space is being curved
and time is being distorted.
917
00:55:15,920 --> 00:55:20,440
So, why can't we perceive this
in our everyday lives?
918
00:55:20,440 --> 00:55:23,240
Einstein had a rather nice way
of explaining it.
919
00:55:25,560 --> 00:55:27,560
Most of us have had the experience,
as children,
920
00:55:27,560 --> 00:55:30,520
of trying to work out
what our parents do for a living.
921
00:55:30,520 --> 00:55:33,360
Well, imagine your father
is Albert Einstein.
922
00:55:33,360 --> 00:55:34,640
When he was about 12 years old,
923
00:55:34,640 --> 00:55:38,160
young Eduard Einstein asked his
father why he was so famous,
924
00:55:38,160 --> 00:55:42,000
what he'd discovered. Well, this
put Einstein Sr on the spot,
925
00:55:42,000 --> 00:55:44,600
but he came up with
a beautifully simple analogy.
926
00:55:48,280 --> 00:55:50,560
Einstein told his son,
927
00:55:50,560 --> 00:55:54,920
"When a blind beetle crawls over
the surface of a curved branch,
928
00:55:54,920 --> 00:55:59,240
"it doesn't notice that the track
it has covered is curved.
929
00:55:59,240 --> 00:56:03,240
"I was lucky enough to notice
what the beetle didn't notice."
930
00:56:04,560 --> 00:56:06,400
This is what Einstein meant.
931
00:56:06,400 --> 00:56:09,920
The beetle is free to move
in any direction on the branch.
932
00:56:09,920 --> 00:56:12,480
It can move forwards,
backwards, left and right,
933
00:56:12,480 --> 00:56:16,160
but it has no concept of a direction
up off the branch.
934
00:56:16,160 --> 00:56:17,640
It's as though, for the beetle,
935
00:56:17,640 --> 00:56:21,000
the universe is missing
the third dimension.
936
00:56:21,000 --> 00:56:24,680
The beetle may think it's moving in
a straight line along the branch,
937
00:56:24,680 --> 00:56:27,400
but we can see that the surface
it's walking on
938
00:56:27,400 --> 00:56:30,000
is itself curving and twisted.
939
00:56:33,680 --> 00:56:37,480
Einstein's point was that what we
see as the twists and curves
940
00:56:37,480 --> 00:56:42,480
of the branch feel, to the beetle,
like forces pushing and pulling it.
941
00:56:45,320 --> 00:56:47,960
OK, so, consider this rather
strange example.
942
00:56:47,960 --> 00:56:52,000
Imagine we have two beetles perched
on this pumpkin and,
943
00:56:52,000 --> 00:56:56,040
for whatever reason, they want to
walk up towards the top.
944
00:56:56,040 --> 00:57:00,800
Now, if they start at the equator,
pointing due north,
945
00:57:00,800 --> 00:57:05,400
as they walk, they will begin
by moving parallel to each other.
946
00:57:05,400 --> 00:57:08,720
That means their paths should never
meet.
947
00:57:08,720 --> 00:57:13,640
But, as they get closer to the top,
their paths get closer together.
948
00:57:13,640 --> 00:57:15,080
Now, if they're clever beetles,
949
00:57:15,080 --> 00:57:17,360
they might try and figure out
what's going on,
950
00:57:17,360 --> 00:57:20,320
and they could imagine
that there's some mysterious force
951
00:57:20,320 --> 00:57:22,960
that's pulling
them closer together.
952
00:57:22,960 --> 00:57:24,800
But, for us, from our perspective,
953
00:57:24,800 --> 00:57:26,520
we can see there is no such force.
954
00:57:26,520 --> 00:57:27,720
All they're doing
955
00:57:27,720 --> 00:57:31,320
is following straight paths
over a curved surface.
956
00:57:35,040 --> 00:57:37,080
Just as the beetles have no sense
957
00:57:37,080 --> 00:57:39,520
that the surface of the branch
is curved,
958
00:57:39,520 --> 00:57:42,520
we completely fail to perceive
959
00:57:42,520 --> 00:57:45,520
the bizarre ways that gravity
960
00:57:45,520 --> 00:57:47,440
shapes the reality we live in.
961
00:57:50,920 --> 00:57:53,720
Einstein's problem was proving
that he was right.
962
00:57:55,720 --> 00:58:00,720
After years more thought,
he realised that there WAS a way...
963
00:58:05,200 --> 00:58:08,160
Incredibly, here in the grounds
of Herstmonceux Castle
964
00:58:08,160 --> 00:58:11,080
is housed one of
the original telescopes
965
00:58:11,080 --> 00:58:14,360
that were used
to prove Einstein was correct.
966
00:58:17,360 --> 00:58:21,720
In 1915, when Einstein developed
his general theory of relativity,
967
00:58:21,720 --> 00:58:24,840
it was just that - it was a theory,
it had no proof.
968
00:58:24,840 --> 00:58:27,840
In fact, many people found it
completely outlandish.
969
00:58:27,840 --> 00:58:30,520
But then, just four years later,
970
00:58:30,520 --> 00:58:33,960
in 1919, this telescope,
and allow me to geek out a bit here
971
00:58:33,960 --> 00:58:36,120
and I'll give it its correct name,
972
00:58:36,120 --> 00:58:40,080
this is the 13-inch
astrographic refractor,
973
00:58:40,080 --> 00:58:43,920
this telescope proved
that Einstein was, in fact, right.
974
00:58:43,920 --> 00:58:47,280
That gravity does curve
space itself.
975
00:58:54,080 --> 00:58:56,520
Marek Kukula is the public
astronomer
976
00:58:56,520 --> 00:58:58,920
at the Royal Observatory in London,
977
00:58:58,920 --> 00:59:00,960
and he's recently rediscovered
978
00:59:00,960 --> 00:59:04,480
a neglected treasure
in their archives.
979
00:59:04,480 --> 00:59:07,080
This is, perhaps,
one of the most important
980
00:59:07,080 --> 00:59:10,840
scientific artefacts we have in
the collection here in Greenwich
981
00:59:10,840 --> 00:59:12,960
and, for an astrophysicist like me,
982
00:59:12,960 --> 00:59:15,160
it's almost a holy relic.
983
00:59:16,280 --> 00:59:21,160
It's a glass plate photo of a solar
eclipse taken in 1919
984
00:59:21,160 --> 00:59:23,920
as part of a famous
scientific expedition.
985
00:59:26,520 --> 00:59:29,200
British astronomers had travelled
all the way to Brazil
986
00:59:29,200 --> 00:59:31,040
and the West Coast of Africa
987
00:59:31,040 --> 00:59:35,080
to take photographs which they
hoped would prove Einstein right.
988
00:59:36,280 --> 00:59:39,280
What we're seeing here
is the eclipse of 1919.
989
00:59:39,280 --> 00:59:43,200
You can see the black disc of the
moon silhouetted against the sun,
990
00:59:43,200 --> 00:59:46,160
blocking its light. Around it is
the solar corona,
991
00:59:46,160 --> 00:59:48,000
the sun's outer atmosphere,
992
00:59:48,000 --> 00:59:51,720
and this spectacular prominence
of gas leaping off the surface.
993
00:59:51,720 --> 00:59:54,560
But it's not the sun
that we're really interested in.
994
00:59:54,560 --> 00:59:56,680
The fundamental point that this
photo
995
00:59:56,680 --> 00:59:58,840
and others from the expedition show
996
00:59:58,840 --> 01:00:01,480
is that the positions,
the apparent positions,
997
01:00:01,480 --> 01:00:04,360
of the stars in the sky
are altered and shifted
998
01:00:04,360 --> 01:00:07,800
from where we would expect
them normally to be,
999
01:00:07,800 --> 01:00:10,480
and that proves this very strange
thing
1000
01:00:10,480 --> 01:00:13,040
that general relativity predicts -
1001
01:00:13,040 --> 01:00:14,880
that the mass of the sun
1002
01:00:14,880 --> 01:00:16,880
bends the space and time around it,
1003
01:00:16,880 --> 01:00:19,360
and that distortion is gravity.
1004
01:00:22,400 --> 01:00:25,400
This is a negative
of one of the photos.
1005
01:00:25,400 --> 01:00:28,320
It has markings showing
where the stars' positions
1006
01:00:28,320 --> 01:00:29,600
seem to have shifted.
1007
01:00:32,240 --> 01:00:35,000
Since then, observation after
observation
1008
01:00:35,000 --> 01:00:38,280
have confirmed that matter
curves space
1009
01:00:38,280 --> 01:00:39,880
and slows down time.
1010
01:00:42,720 --> 01:00:46,520
So, the simple question of why
things fall the way they do
1011
01:00:46,520 --> 01:00:48,360
has led us deeper and deeper
1012
01:00:48,360 --> 01:00:51,240
into the very nature of space and
time itself.
1013
01:00:52,760 --> 01:00:57,760
Gravitational science shows us how
galaxies, stars and planets form.
1014
01:00:58,560 --> 01:01:02,000
By measuring gravity, we've
discovered the existence
1015
01:01:02,000 --> 01:01:06,880
of dark matter, that 80% of the mass
of our universe is invisible
1016
01:01:06,880 --> 01:01:10,960
and we don't know what
it's made of.
1017
01:01:10,960 --> 01:01:14,680
And we've detected exotic objects
with extreme gravity...
1018
01:01:16,040 --> 01:01:17,760
..like neutron stars,
1019
01:01:17,760 --> 01:01:19,800
which have more mass than our sun
1020
01:01:19,800 --> 01:01:22,440
yet are only 20 kilometres across.
1021
01:01:25,400 --> 01:01:28,640
But it's another mysterious aspect
of Einstein's universe
1022
01:01:28,640 --> 01:01:31,880
that I want to explore in
my next gravity project.
1023
01:01:34,720 --> 01:01:36,400
Here at the University of Surrey,
1024
01:01:36,400 --> 01:01:39,480
some colleagues and I have been
working on it for months.
1025
01:01:40,640 --> 01:01:45,320
What we're doing is devising a
nationwide citizen science project.
1026
01:01:45,320 --> 01:01:47,760
We're developing a smartphone app
1027
01:01:47,760 --> 01:01:50,400
that uses the GPS
contained on your phone
1028
01:01:50,400 --> 01:01:53,840
to explore one of the strangest
properties of gravity -
1029
01:01:53,840 --> 01:01:56,680
how it affects the rate
at which we age.
1030
01:01:58,880 --> 01:02:01,280
'I formulated the equations
myself...
1031
01:02:03,280 --> 01:02:07,040
'..and a small team of computer
scientists and software developers
1032
01:02:07,040 --> 01:02:09,040
'is using them to devise the app.'
1033
01:02:12,720 --> 01:02:15,880
Einstein discovered that,
as gravity changes,
1034
01:02:15,880 --> 01:02:18,600
so does the rate that time ticks.
1035
01:02:20,440 --> 01:02:23,800
This means the strength
of gravity you feel
1036
01:02:23,800 --> 01:02:27,200
affects how quickly
or slowly you age.
1037
01:02:29,480 --> 01:02:33,040
The aim of my app
is to demonstrate this effect.
1038
01:02:33,040 --> 01:02:36,080
It works by using a phone's GPS data
1039
01:02:36,080 --> 01:02:39,120
to estimate your local gravity.
1040
01:02:41,080 --> 01:02:44,600
And it also calculates the average
speed at which you move
1041
01:02:44,600 --> 01:02:48,040
because this, too, affects
the rate at which you age.
1042
01:02:50,320 --> 01:02:52,760
It then uses the equations
I've written,
1043
01:02:52,760 --> 01:02:55,600
which are based on
Einstein's theory of relativity,
1044
01:02:55,600 --> 01:03:00,320
to calculate, overall, how
fast or slowly you're ageing.
1045
01:03:03,440 --> 01:03:06,240
Once the app is ready,
I tweet about it.
1046
01:03:08,520 --> 01:03:10,400
Thousands of people download it
1047
01:03:10,400 --> 01:03:13,840
and we start to gather results
from across the country.
1048
01:03:15,600 --> 01:03:19,760
Some people send me videos,
giving me their results,
1049
01:03:19,760 --> 01:03:21,560
how fast they are ageing
1050
01:03:21,560 --> 01:03:26,560
compared with how time ticks out
in space in zero gravity.
1051
01:03:26,640 --> 01:03:31,520
Over the past day, I have aged less
by about 172 microseconds.
1052
01:03:31,520 --> 01:03:36,560
I have aged less by 10.02
milliseconds.
1053
01:03:37,320 --> 01:03:42,360
So, since downloading the app, I
have aged less by 1.14 milliseconds.
1054
01:03:43,520 --> 01:03:46,520
Since opening Time Warper,
1055
01:03:46,520 --> 01:03:50,640
I have aged less
by 2.6 milliseconds.
1056
01:03:51,840 --> 01:03:54,240
Our aim is to use their results
1057
01:03:54,240 --> 01:03:56,560
to build up a map of how time flows
1058
01:03:56,560 --> 01:03:58,160
because of gravity.
1059
01:04:00,240 --> 01:04:05,040
My smartphone project provides just
one insight into the space and time
1060
01:04:05,040 --> 01:04:07,400
which Einstein's theories describe.
1061
01:04:22,640 --> 01:04:24,400
Gravity and its strange ways
1062
01:04:24,400 --> 01:04:26,640
have given us astonishing insights
1063
01:04:26,640 --> 01:04:28,960
into the dark secrets
of our universe.
1064
01:04:31,080 --> 01:04:35,240
Perhaps the weirdest objects
in the universe are black holes,
1065
01:04:35,240 --> 01:04:38,160
collapsed stars whose gravity
is so strong
1066
01:04:38,160 --> 01:04:41,200
that not even light can escape
their grip.
1067
01:04:42,480 --> 01:04:46,280
Now, for the first time ever, their
effects have been felt on Earth
1068
01:04:46,280 --> 01:04:50,360
and they've been detected through
the medium of gravity itself.
1069
01:04:52,760 --> 01:04:56,560
It's a story that has revolutionised
the study of modern cosmology.
1070
01:05:00,520 --> 01:05:02,720
1.3 billion years ago,
1071
01:05:02,720 --> 01:05:05,360
in a galaxy far, far away,
1072
01:05:05,360 --> 01:05:08,640
two black holes
swirled around each other,
1073
01:05:08,640 --> 01:05:11,080
drew closer and closer together,
1074
01:05:11,080 --> 01:05:14,920
until they finally collided
with incredible violence.
1075
01:05:14,920 --> 01:05:17,080
In that final fraction of a second,
1076
01:05:17,080 --> 01:05:19,400
at the precise moment that they
merged,
1077
01:05:19,400 --> 01:05:21,120
a disturbance was created
1078
01:05:21,120 --> 01:05:24,120
that sent ripples
out through the universe.
1079
01:05:27,240 --> 01:05:31,000
Gravitational waves are a key
prediction of Einstein's theory.
1080
01:05:32,760 --> 01:05:37,400
Matter doesn't just curve space
time, it can cause waves,
1081
01:05:37,400 --> 01:05:39,560
ripples which expand outwards,
1082
01:05:39,560 --> 01:05:42,080
exactly like a stone dropped
in water.
1083
01:05:44,360 --> 01:05:47,520
This particular wave
was unimaginably large.
1084
01:05:49,240 --> 01:05:54,040
The energy released was greater than
all the light being given out
1085
01:05:54,040 --> 01:05:56,000
by all the stars in the universe.
1086
01:05:57,760 --> 01:06:01,240
The wave rippled through space
at the speed of light.
1087
01:06:01,240 --> 01:06:03,640
In 1.3 billion years,
1088
01:06:03,640 --> 01:06:08,080
it covered a distance of over
10 billion trillion kilometres.
1089
01:06:17,440 --> 01:06:22,400
Until, on the morning of the 14th of
September, 2015,
1090
01:06:22,520 --> 01:06:24,200
it arrived here.
1091
01:06:26,000 --> 01:06:29,040
The streets and cafes
of New Orleans.
1092
01:06:29,040 --> 01:06:33,360
In fact, everything in America -
and on Earth -
1093
01:06:33,360 --> 01:06:36,960
expanded and contracted very, very
slightly
1094
01:06:36,960 --> 01:06:39,160
as the wave passed through.
1095
01:06:40,480 --> 01:06:43,560
No-one noticed as,
by the time it arrived here,
1096
01:06:43,560 --> 01:06:46,120
the distortion
was phenomenally tiny.
1097
01:06:50,560 --> 01:06:54,120
Except that one science
laboratory did notice...
1098
01:06:56,000 --> 01:06:57,520
..and I'm going to see it.
1099
01:07:02,280 --> 01:07:05,800
1,000 scientists across the world
are collaborating on it.
1100
01:07:09,320 --> 01:07:12,280
It's the culmination of over 50
years of effort
1101
01:07:12,280 --> 01:07:15,320
and is one of the most
sophisticated experiments
1102
01:07:15,320 --> 01:07:17,600
ever devised by humanity.
1103
01:07:19,920 --> 01:07:21,880
So, I'm pretty excited to see it.
1104
01:07:23,720 --> 01:07:25,720
It's a rather unusual setting.
1105
01:07:25,720 --> 01:07:28,280
Here I am, in the middle of
rural Louisiana,
1106
01:07:28,280 --> 01:07:31,040
about an hour's drive
outside New Orleans.
1107
01:07:31,040 --> 01:07:34,200
I don't expect to find such a
multi-million dollar,
1108
01:07:34,200 --> 01:07:36,800
cutting-edge research facility as
this,
1109
01:07:36,800 --> 01:07:39,360
and yet, this is the place
where, recently,
1110
01:07:39,360 --> 01:07:41,840
one of the most important scientific
discoveries
1111
01:07:41,840 --> 01:07:45,240
in human history was made.
This is LIGO.
1112
01:07:48,360 --> 01:07:52,600
The Laser Interferometer
Gravitational Wave Observatory
1113
01:07:52,600 --> 01:07:56,440
is an enormous construction
shaped like an L...
1114
01:07:57,600 --> 01:07:59,800
..with a sophisticated laser system
1115
01:07:59,800 --> 01:08:02,120
bouncing up and down the two arms.
1116
01:08:03,920 --> 01:08:06,720
So, we're standing on top of one of
LIGO's two arms.
1117
01:08:06,720 --> 01:08:08,400
This is the first LIGO arm.
1118
01:08:08,400 --> 01:08:12,400
And in that tube, there's a laser
beam that we bounce back and forth
1119
01:08:12,400 --> 01:08:15,400
between a mirror and the end station
and a mirror in this building.
1120
01:08:15,400 --> 01:08:18,000
And the other bit goes that
way four kilometres,
1121
01:08:18,000 --> 01:08:19,880
perpendicular
to the arm we first saw.
1122
01:08:19,880 --> 01:08:22,200
So, this is the L shape?
It's a big L on the ground.
1123
01:08:22,200 --> 01:08:23,960
So, the light bounces back and forth
1124
01:08:23,960 --> 01:08:26,680
in that arm and bounces back
and forth in this arm,
1125
01:08:26,680 --> 01:08:30,160
and what we actually measure with
LIGO is the length of this arm
1126
01:08:30,160 --> 01:08:32,960
as measured by the light between the
two mirrors,
1127
01:08:32,960 --> 01:08:35,760
and the length of that arm as
measured by the light
1128
01:08:35,760 --> 01:08:38,120
between two mirrors.
And then the laser interferometer
1129
01:08:38,120 --> 01:08:41,160
measures the difference
between those two arm lengths.
1130
01:08:42,920 --> 01:08:47,600
So, as the gravitational wave passed
through, the lasers picked it up.
1131
01:08:47,600 --> 01:08:51,520
They detected that LIGO's two arms
changed in length
1132
01:08:51,520 --> 01:08:53,680
to a very, very tiny degree.
1133
01:08:56,320 --> 01:08:58,360
The signal that we saw
1134
01:08:58,360 --> 01:09:02,520
was just a few thousandth
of the size of the atomic nucleus.
1135
01:09:02,520 --> 01:09:05,240
It's the biggest the signal
ever got.
1136
01:09:05,240 --> 01:09:09,200
So far, far smaller
than the size of a single atom?
1137
01:09:09,200 --> 01:09:10,920
Oh, much, much smaller, yeah.
1138
01:09:10,920 --> 01:09:13,800
And you need something this huge
to pick that up?
1139
01:09:13,800 --> 01:09:17,640
That's right. This is one
of the biggest sources of energy
1140
01:09:17,640 --> 01:09:20,640
in the universe, one of the biggest
events you'd ever measure,
1141
01:09:20,640 --> 01:09:22,560
and we just barely saw it.
1142
01:09:26,560 --> 01:09:30,440
The LIGO scientists turned
the gravitational waves
1143
01:09:30,440 --> 01:09:32,240
into sound waves,
1144
01:09:32,240 --> 01:09:36,440
so what you're about to hear is,
in a very real sense,
1145
01:09:36,440 --> 01:09:40,160
the sound of two black holes
colliding.
1146
01:09:40,160 --> 01:09:42,440
RHYTHMIC PULSES
1147
01:09:46,360 --> 01:09:49,440
It was the first observation
of any kind
1148
01:09:49,440 --> 01:09:51,960
of pairs of stellar mass
black holes.
1149
01:09:51,960 --> 01:09:54,240
"Stellar mass" means, you know,
1150
01:09:54,240 --> 01:09:57,280
several or a bunch of suns
in weight.
1151
01:09:57,280 --> 01:09:59,600
And so we learned that they exist,
1152
01:09:59,600 --> 01:10:02,440
we learned that there are enough
of them that, occasionally,
1153
01:10:02,440 --> 01:10:04,360
they run
into each other and coalesce.
1154
01:10:04,360 --> 01:10:06,360
And...
1155
01:10:06,360 --> 01:10:09,520
we also learned, by comparing
the waveform we observed
1156
01:10:09,520 --> 01:10:12,560
with the general relativity
calculations,
1157
01:10:12,560 --> 01:10:16,960
that general relativity is,
as far as we know, dead-on right.
1158
01:10:27,120 --> 01:10:31,120
The long concrete bunker to my left
houses the beam line,
1159
01:10:31,120 --> 01:10:33,480
one of the LIGO's laser arms.
1160
01:10:35,280 --> 01:10:39,600
The detail and the effort that's
gone into isolating the beam
1161
01:10:39,600 --> 01:10:41,320
from the outside environment
1162
01:10:41,320 --> 01:10:44,440
reminds me very much of Cavendish's
famous experiment.
1163
01:10:44,440 --> 01:10:47,920
He, too, had to worry about
isolating his experiment
1164
01:10:47,920 --> 01:10:49,880
from external disturbances.
1165
01:10:49,880 --> 01:10:53,480
Only, of course, LIGO takes things
to a far, far greater degree.
1166
01:10:55,200 --> 01:10:59,800
Inside the arm is one of the largest
and purest vacuums in the world.
1167
01:11:01,000 --> 01:11:03,200
Atmospheric pressure in there
1168
01:11:03,200 --> 01:11:07,160
has been reduced to one trillionth
of the pressure outside.
1169
01:11:07,160 --> 01:11:10,000
The mirrors inside are so reflective
1170
01:11:10,000 --> 01:11:14,560
that they only absorb
one in three million photons.
1171
01:11:14,560 --> 01:11:19,320
And at the end of my little trip,
lies a British success story.
1172
01:11:26,280 --> 01:11:29,840
Well, I made it all the way to the
end of one of the LIGO arms.
1173
01:11:29,840 --> 01:11:32,160
To be honest, it took me a bit
longer than I thought,
1174
01:11:32,160 --> 01:11:33,520
especially in that thing,
1175
01:11:33,520 --> 01:11:37,440
but housed inside this building is
one of the reflecting mirrors
1176
01:11:37,440 --> 01:11:39,240
that bounces the laser beam
1177
01:11:39,240 --> 01:11:41,720
all the way back down the
four kilometre arm
1178
01:11:41,720 --> 01:11:44,000
to the main control centre.
1179
01:11:44,000 --> 01:11:46,760
And the technology that went
into developing these mirrors
1180
01:11:46,760 --> 01:11:48,080
is quite remarkable.
1181
01:11:48,080 --> 01:11:51,600
It was developed in the UK
at the University of Glasgow.
1182
01:11:58,720 --> 01:12:00,560
This is what the mirror looks like.
1183
01:12:01,720 --> 01:12:05,040
Its surface
is extraordinarily smooth,
1184
01:12:05,040 --> 01:12:08,280
no bump bigger than a few billionths
of a metre high.
1185
01:12:11,040 --> 01:12:12,760
Equally amazing are these...
1186
01:12:14,040 --> 01:12:18,640
..fused silica fibres, a few times
the thickness of a human hair...
1187
01:12:21,080 --> 01:12:23,120
..designed by the University
of Glasgow
1188
01:12:23,120 --> 01:12:26,760
in conjunction with scientists
from other British universities.
1189
01:12:27,840 --> 01:12:30,880
They isolate the mirror completely
1190
01:12:30,880 --> 01:12:33,320
so it hangs perfectly still.
1191
01:12:33,320 --> 01:12:35,480
You could say that in there
1192
01:12:35,480 --> 01:12:38,600
is the quietest place on Earth.
1193
01:12:39,880 --> 01:12:43,200
Despite this, outside events do
sometimes interfere
1194
01:12:43,200 --> 01:12:45,800
with the work here,
as I witnessed for myself.
1195
01:12:47,280 --> 01:12:50,560
I've wandered into the control room
here at LIGO because I'm told
1196
01:12:50,560 --> 01:12:54,800
something kicked off a few hours ago
and they're all very busy.
1197
01:12:54,800 --> 01:12:57,640
The image that's flickering up there
1198
01:12:57,640 --> 01:12:59,800
is not meant to be like that.
1199
01:12:59,800 --> 01:13:01,760
Essentially, what they picked up
1200
01:13:01,760 --> 01:13:04,160
is a seismic disturbance,
an earthquake.
1201
01:13:04,160 --> 01:13:06,760
Now, that's not an earthquake
down the road.
1202
01:13:06,760 --> 01:13:10,600
It started on the other side
of the planet, in Japan.
1203
01:13:10,600 --> 01:13:12,400
So, it just gives us a sense
1204
01:13:12,400 --> 01:13:15,320
of the tremendous challenges
faced by LIGO
1205
01:13:15,320 --> 01:13:18,840
and the team here and the level of
sensitivity needed
1206
01:13:18,840 --> 01:13:21,720
that an earthquake on
the other side of the Earth
1207
01:13:21,720 --> 01:13:24,440
can disrupt their measurements
and they have
1208
01:13:24,440 --> 01:13:26,280
to reset everything all over again.
1209
01:13:29,960 --> 01:13:33,480
One of the scientists involved in
developing this extraordinary place
1210
01:13:33,480 --> 01:13:36,040
put it quite succinctly.
1211
01:13:36,040 --> 01:13:39,440
"Once we were blind,
but now we can see."
1212
01:13:41,520 --> 01:13:43,760
Throughout the entire history
of astronomy,
1213
01:13:43,760 --> 01:13:47,880
we've studied gravity and how it
affects matter in the universe
1214
01:13:47,880 --> 01:13:50,400
and how it warps space-time,
1215
01:13:50,400 --> 01:13:54,720
but only by looking at the light
that enters our telescopes,
1216
01:13:54,720 --> 01:13:56,480
now, for the first time,
1217
01:13:56,480 --> 01:13:59,600
we can study the universe
in a different way.
1218
01:13:59,600 --> 01:14:02,640
The discovery of gravitational waves
means we can see objects
1219
01:14:02,640 --> 01:14:05,400
that cause extreme warping of
space-time
1220
01:14:05,400 --> 01:14:09,040
and its effect on gravity directly.
1221
01:14:09,040 --> 01:14:12,400
This essentially opens up
a new era in astronomy,
1222
01:14:12,400 --> 01:14:15,640
it gives us a new way of looking out
at the universe.
1223
01:14:19,000 --> 01:14:21,480
Professor Sheila Rowan was one of
the scientists
1224
01:14:21,480 --> 01:14:25,080
who spearheaded the
British effort for LIGO.
1225
01:14:25,080 --> 01:14:27,440
For her and her colleagues,
1226
01:14:27,440 --> 01:14:30,880
gravitational wave detection
is just in its infancy.
1227
01:14:32,400 --> 01:14:35,120
New instruments - even more
sensitive than LIGO -
1228
01:14:35,120 --> 01:14:36,560
are now being developed.
1229
01:14:38,560 --> 01:14:40,640
There's so much
that we don't understand
1230
01:14:40,640 --> 01:14:43,240
about the universe that we live in,
1231
01:14:43,240 --> 01:14:46,680
and this has suddenly given us
a new tool, a new way,
1232
01:14:46,680 --> 01:14:50,200
to probe the dark processes
in the universe,
1233
01:14:50,200 --> 01:14:54,520
because every time we make the
observatories more sensitive,
1234
01:14:54,520 --> 01:14:59,360
we can sense gravitational wave
signals from further away,
1235
01:14:59,360 --> 01:15:03,800
from further out in the universe,
from further back in cosmic history.
1236
01:15:03,800 --> 01:15:07,680
Things like supermassive black holes
spiralling in to collide,
1237
01:15:07,680 --> 01:15:11,560
small black holes orbiting
round supermassive black holes,
1238
01:15:11,560 --> 01:15:16,600
tracing out the dents in space-time
of those supermassive objects.
1239
01:15:17,200 --> 01:15:20,320
A long-term goal
is to probe back further
1240
01:15:20,320 --> 01:15:23,240
towards what we think of
as the Big Bang,
1241
01:15:23,240 --> 01:15:26,120
the earliest moments
that we understand
1242
01:15:26,120 --> 01:15:28,720
of the universe as we know it.
1243
01:15:42,080 --> 01:15:45,840
If you think about it, time and time
again in the history of science,
1244
01:15:45,840 --> 01:15:47,840
unlocking the mysteries of gravity
1245
01:15:47,840 --> 01:15:51,480
have led to a deeper
understanding of the universe.
1246
01:15:51,480 --> 01:15:54,480
Galileo and his ramp,
Newton and his apple,
1247
01:15:54,480 --> 01:15:57,040
Einstein and the falling man
in the lift.
1248
01:15:57,040 --> 01:16:02,040
Each of these characters challenged
the scientific consensus of the day.
1249
01:16:02,440 --> 01:16:06,640
And even today, understanding the
true nature of gravity
1250
01:16:06,640 --> 01:16:09,800
remains one of the biggest
challenges in science.
1251
01:16:12,480 --> 01:16:15,920
Which brings me back
to the smartphone app.
1252
01:16:15,920 --> 01:16:19,600
And it's at this point that our
story, for me, at least,
1253
01:16:19,600 --> 01:16:21,960
takes a completely unexpected turn.
1254
01:16:24,000 --> 01:16:28,320
Unfortunately, it's all gone
a bit pear-shaped.
1255
01:16:28,320 --> 01:16:31,760
OK, so, here's what's happened.
A couple of months ago,
1256
01:16:31,760 --> 01:16:34,360
we launched the app and it was all
going really well.
1257
01:16:34,360 --> 01:16:36,160
Thousands of people downloaded it
1258
01:16:36,160 --> 01:16:39,040
and have been sending us their
results.
1259
01:16:39,040 --> 01:16:43,320
We've been collecting the data to
create this nationwide map
1260
01:16:43,320 --> 01:16:46,920
to show how time flows at different
rates for different people
1261
01:16:46,920 --> 01:16:49,280
around the country.
1262
01:16:49,280 --> 01:16:52,880
Unfortunately, I've just realised
there's a big problem.
1263
01:16:57,520 --> 01:17:00,360
You see, I was going over the
scientific literature
1264
01:17:00,360 --> 01:17:03,800
and I came across this
subtle point about relativity
1265
01:17:03,800 --> 01:17:06,680
which basically made me sit
bolt upright.
1266
01:17:06,680 --> 01:17:09,800
There was this horrible dawning
realisation
1267
01:17:09,800 --> 01:17:13,800
that I'd made a mistake in the
equations that get fed into the app.
1268
01:17:15,720 --> 01:17:20,480
What this means is all the results
we've been gathering are wrong.
1269
01:17:25,120 --> 01:17:27,920
The issue lies in the strange
and subtle effects
1270
01:17:27,920 --> 01:17:30,920
of Einstein's theories of
relativity,
1271
01:17:30,920 --> 01:17:34,280
and it's fundamental to
the way time flows
1272
01:17:34,280 --> 01:17:37,480
across the surface of the globe.
1273
01:17:37,480 --> 01:17:40,960
Now, what if I use my smartphone
app where I live here,
1274
01:17:40,960 --> 01:17:42,840
on the south coast of England
1275
01:17:42,840 --> 01:17:45,920
and then go and spend a few days
down near the equator?
1276
01:17:45,920 --> 01:17:48,600
So, here on the West Coast of
Africa.
1277
01:17:51,720 --> 01:17:55,600
Now, we know from the road trip that
gravity is weaker by the equator.
1278
01:17:57,640 --> 01:18:00,480
So, that means
time ticks faster there.
1279
01:18:01,920 --> 01:18:05,640
But there's another important factor
we have to take into account -
1280
01:18:05,640 --> 01:18:07,680
movement.
1281
01:18:07,680 --> 01:18:10,120
You see, when I'm here,
near the equator,
1282
01:18:10,120 --> 01:18:11,520
I'm moving more quickly
1283
01:18:11,520 --> 01:18:16,080
than I was back in Britain because
of the rotation of the Earth.
1284
01:18:16,080 --> 01:18:18,880
Einstein says movement
slows down time
1285
01:18:18,880 --> 01:18:21,880
so clocks will tick
slower at the equator.
1286
01:18:23,040 --> 01:18:25,120
This is where the error crept in.
1287
01:18:25,120 --> 01:18:28,080
You see, I had taken into account
these two effects,
1288
01:18:28,080 --> 01:18:29,960
but I'd missed a crucial point.
1289
01:18:29,960 --> 01:18:32,640
They cancel each other out exactly.
1290
01:18:32,640 --> 01:18:35,360
In fact, the Earth bulges out
1291
01:18:35,360 --> 01:18:39,520
exactly the right amount
for its rotational speed
1292
01:18:39,520 --> 01:18:42,120
to make sure they cancel out,
1293
01:18:42,120 --> 01:18:46,120
so all clocks on the surface
of the Earth, at sea level, tick
1294
01:18:46,120 --> 01:18:49,480
at exactly the same rate.
1295
01:18:49,480 --> 01:18:52,520
So, now I'm having to go right back
to square one
1296
01:18:52,520 --> 01:18:55,480
and completely rewrite
the equations for the app.
1297
01:19:02,120 --> 01:19:04,000
And, to test if it's working,
1298
01:19:04,000 --> 01:19:07,200
I'm going to use it over the course
of a normal working week.
1299
01:19:08,440 --> 01:19:10,840
This is where I live,
this is Portsmouth,
1300
01:19:10,840 --> 01:19:13,640
which means I'm very close
to sea level,
1301
01:19:13,640 --> 01:19:15,840
and this is how I start
most mornings,
1302
01:19:15,840 --> 01:19:18,520
catching the train to work.
1303
01:19:18,520 --> 01:19:23,600
The app records my speed
as I'm on the train
1304
01:19:23,600 --> 01:19:27,680
and calculates how this slows down
my personal clock.
1305
01:19:27,680 --> 01:19:30,120
I think the train journey
1306
01:19:30,120 --> 01:19:33,720
should have slowed my time down
by a tiny...
1307
01:19:33,720 --> 01:19:36,000
A few trillionths of second.
1308
01:19:36,000 --> 01:19:39,720
I'm heading for the BBC's
headquarters in Central London,
1309
01:19:39,720 --> 01:19:42,520
and gravity should be
a bit weaker here.
1310
01:19:42,520 --> 01:19:44,720
I'm a few metres above sea level,
I guess, here.
1311
01:19:44,720 --> 01:19:49,200
And so there will be a speed-up
of my time because of altitude.
1312
01:19:49,200 --> 01:19:52,200
The app compares
the way my time flows
1313
01:19:52,200 --> 01:19:55,240
with a stationary clock
at sea level.
1314
01:19:55,240 --> 01:19:57,320
So, what's my result?
1315
01:19:57,320 --> 01:20:01,600
On an average day, my movement makes
me age slower by a third
1316
01:20:01,600 --> 01:20:06,360
of a nanosecond. That's a third
of a billionth of a second.
1317
01:20:06,360 --> 01:20:09,080
But the weaker gravity I'm in
1318
01:20:09,080 --> 01:20:12,000
means I age faster - overall,
1319
01:20:12,000 --> 01:20:13,760
half a nanosecond faster.
1320
01:20:15,840 --> 01:20:18,480
I've also given the app to some
other volunteers
1321
01:20:18,480 --> 01:20:20,880
to compare how they age
over an average day.
1322
01:20:23,120 --> 01:20:25,600
Nick flies cargo planes.
1323
01:20:25,600 --> 01:20:28,600
He flies from Chicago to Germany.
1324
01:20:34,480 --> 01:20:36,400
Tomorrow morning,
1325
01:20:36,400 --> 01:20:41,440
we have to leave to go first to
Milan and then on to Tokyo.
1326
01:20:42,040 --> 01:20:45,960
His travel slows down his ageing,
1327
01:20:45,960 --> 01:20:49,120
but much weaker gravity
at high altitude
1328
01:20:49,120 --> 01:20:52,360
speeds his clock up
by just a bit more.
1329
01:20:52,360 --> 01:20:55,400
Overall, he's ageing five
nanoseconds faster
1330
01:20:55,400 --> 01:20:58,520
than a stationary clock
at sea level.
1331
01:20:58,520 --> 01:21:01,720
Vanessa runs a pub
in the Yorkshire Dales.
1332
01:21:01,720 --> 01:21:05,640
I'm going to take you outside to
see the weather conditions here.
1333
01:21:05,640 --> 01:21:08,240
So, here we are,
outside the Tan Hill Inn.
1334
01:21:08,240 --> 01:21:11,280
We live right in the middle of the
National Park on the moor.
1335
01:21:11,280 --> 01:21:15,720
The Tan Hill Inn is famous as
Britain's highest altitude pub
1336
01:21:15,720 --> 01:21:18,480
at over 500 metres above sea level.
1337
01:21:18,480 --> 01:21:22,040
We don't have any neighbours,
we just have sheep.
1338
01:21:22,040 --> 01:21:25,240
Her altitude means she ages faster
every day
1339
01:21:25,240 --> 01:21:27,320
by around four nanoseconds
1340
01:21:27,320 --> 01:21:29,240
compared to someone at sea level.
1341
01:21:30,520 --> 01:21:33,960
There's Kevin, a mountaineer
in the Highlands.
1342
01:21:33,960 --> 01:21:37,000
I'm on a mountain in Glencoe
called Sgor na h-Ulaidh.
1343
01:21:37,000 --> 01:21:40,200
I've been at an altitude generally
of between 2,000-3,000 feet
1344
01:21:40,200 --> 01:21:42,040
for a lot of the day.
Throughout the day,
1345
01:21:42,040 --> 01:21:44,880
I've just been logging on to the
phone, logging on to the app,
1346
01:21:44,880 --> 01:21:46,800
and just checking it out
and having a look,
1347
01:21:46,800 --> 01:21:48,680
and I've been watching it get bigger
1348
01:21:48,680 --> 01:21:50,880
and watching the value get bigger
and bigger.
1349
01:21:50,880 --> 01:21:52,640
So, it's been quite a lot of fun.
1350
01:21:53,880 --> 01:21:55,680
On an average day of climbing,
1351
01:21:55,680 --> 01:21:59,640
Kevin's personal clock
goes faster by one nanosecond.
1352
01:22:02,480 --> 01:22:04,840
Gary works for a Scottish
water retailer.
1353
01:22:06,120 --> 01:22:08,440
My job takes me all over the UK,
1354
01:22:08,440 --> 01:22:11,800
dealing with energy consultants
and energy brokers,
1355
01:22:11,800 --> 01:22:15,080
as far up north as Inverness,
as far down south as London.
1356
01:22:15,080 --> 01:22:18,280
I approximately do about
1,000 miles a week, sometimes more,
1357
01:22:18,280 --> 01:22:21,360
depending on the number
of meetings I have.
1358
01:22:21,360 --> 01:22:24,720
Gary's car journeys
do slow his time down a bit,
1359
01:22:24,720 --> 01:22:26,560
but being above sea level
1360
01:22:26,560 --> 01:22:31,080
means he still ages faster
by three quarters of a nanosecond.
1361
01:22:32,480 --> 01:22:35,280
Our final volunteer is Walter.
1362
01:22:35,280 --> 01:22:38,800
He lives close to sea level
at the iconic John O'Groats.
1363
01:22:40,040 --> 01:22:43,880
I run the tourism business
and I started about 50 years ago,
1364
01:22:43,880 --> 01:22:47,600
so when people come here,
they can actually speak to someone
1365
01:22:47,600 --> 01:22:50,640
who's been born in John O'Groats
and, if they ask questions,
1366
01:22:50,640 --> 01:22:53,120
I can tell them all sorts
of useless information
1367
01:22:53,120 --> 01:22:55,720
because I'm full of useless
information.
1368
01:22:55,720 --> 01:23:00,680
So our final results show that,
if you want to age more slowly,
1369
01:23:00,680 --> 01:23:03,360
try to live near sea level,
like Walter.
1370
01:23:05,760 --> 01:23:08,360
Or there is another way to do it -
1371
01:23:08,360 --> 01:23:12,160
get a job on the
International Space Station.
1372
01:23:12,160 --> 01:23:16,480
Its 17,000-mile-an-hour orbit
will give you a boost.
1373
01:23:18,560 --> 01:23:21,240
We did the maths for the astronauts.
1374
01:23:21,240 --> 01:23:25,920
Every month, you are about one
millisecond younger,
1375
01:23:25,920 --> 01:23:27,760
so one thousandth of a second.
1376
01:23:27,760 --> 01:23:29,360
So, after six months,
1377
01:23:29,360 --> 01:23:32,800
you're that much younger
than people on Earth.
1378
01:23:32,800 --> 01:23:34,520
So, I'm younger than I should be.
1379
01:23:34,520 --> 01:23:36,040
I hope I look it.
1380
01:23:37,280 --> 01:23:39,560
Of course, for us on Earth,
1381
01:23:39,560 --> 01:23:42,680
time dilation
is so utterly minuscule,
1382
01:23:42,680 --> 01:23:45,480
a few billionths of a second
between us,
1383
01:23:45,480 --> 01:23:48,840
you might think it's too frivolous
to even bother about.
1384
01:23:51,360 --> 01:23:55,520
And yet, in the long and difficult
process of designing the app,
1385
01:23:55,520 --> 01:23:59,480
I've come to an
extraordinary conclusion.
1386
01:23:59,480 --> 01:24:02,120
The different ways that time flows
1387
01:24:02,120 --> 01:24:06,560
may not be some quirky by-product
of gravity.
1388
01:24:06,560 --> 01:24:09,760
It may actually BE gravity.
1389
01:24:09,760 --> 01:24:12,640
It may be the CAUSE of gravity...
1390
01:24:12,640 --> 01:24:14,760
the reason why objects fall.
1391
01:24:17,880 --> 01:24:20,920
One of the colleagues I've been
consulting is Kip Thorne.
1392
01:24:20,920 --> 01:24:23,800
He's one of the world's leading
theoretical physicists
1393
01:24:23,800 --> 01:24:27,160
and a driving force
behind the creation of LIGO.
1394
01:24:27,160 --> 01:24:31,320
While I was going back over some of
the basic physics behind the app,
1395
01:24:31,320 --> 01:24:33,800
I came across an intriguing
idea of his.
1396
01:24:33,800 --> 01:24:36,400
It's a very interesting
and different way
1397
01:24:36,400 --> 01:24:38,280
of describing gravity.
1398
01:24:41,520 --> 01:24:42,920
This is what Kip says.
1399
01:24:44,280 --> 01:24:49,120
"Everything likes to live
where it'll age the most slowly,
1400
01:24:49,120 --> 01:24:51,080
"and gravity pulls it there."
1401
01:24:52,720 --> 01:24:55,200
Kip's based at Caltech in California
1402
01:24:55,200 --> 01:24:59,960
and is one of the most respected
theoretical physicists in the world.
1403
01:24:59,960 --> 01:25:02,320
Firstly, Kip, a serious thank you
1404
01:25:02,320 --> 01:25:05,960
for helping out
with the debacle over the app!
1405
01:25:05,960 --> 01:25:07,920
Well, I sympathise.
1406
01:25:07,920 --> 01:25:11,080
I've made so many errors of my own
over the years
1407
01:25:11,080 --> 01:25:13,480
that I am totally sympathetic.
1408
01:25:13,480 --> 01:25:16,040
One of the things that struck me,
1409
01:25:16,040 --> 01:25:19,040
thinking about this,
is something you wrote, Kip.
1410
01:25:19,040 --> 01:25:23,840
You said, "Everything likes to live
where it'll age the most slowly,
1411
01:25:23,840 --> 01:25:26,880
"and gravity pulls it there."
1412
01:25:26,880 --> 01:25:29,440
Was this a way of explaining
something
1413
01:25:29,440 --> 01:25:31,880
that you felt was a neat explanation
1414
01:25:31,880 --> 01:25:34,560
or is there something
deeply profound about that?
1415
01:25:34,560 --> 01:25:38,840
I think there is something deeply
profound, in some sense,
1416
01:25:38,840 --> 01:25:43,240
but it's a lovely description
1417
01:25:43,240 --> 01:25:48,240
of Einstein's first major
insight about gravity.
1418
01:25:48,720 --> 01:25:51,960
In 1912, he realised that gravity
1419
01:25:51,960 --> 01:25:56,240
that we feel on Earth is due to a
slowing of time on Earth.
1420
01:25:56,240 --> 01:25:59,680
So, time comes before gravity,
in that sense?
1421
01:25:59,680 --> 01:26:02,320
On the Earth's surface,
time runs more slowly
1422
01:26:02,320 --> 01:26:05,400
and that accounts for why gravity
wants to keep us there?
1423
01:26:05,400 --> 01:26:08,080
Well, I think, in a very deep sense,
this is true.
1424
01:26:08,080 --> 01:26:09,920
Objects WANT to fall.
1425
01:26:09,920 --> 01:26:12,720
The flow of time, or the rate
of flow of the time,
1426
01:26:12,720 --> 01:26:15,960
is the thing that produces
the gravity,
1427
01:26:15,960 --> 01:26:20,440
it is the thing that is ultimately
responsible for the fall.
1428
01:26:20,440 --> 01:26:23,720
So, somehow, it's in the nature
of all objects
1429
01:26:23,720 --> 01:26:27,720
to move towards a region
where time runs slower.
1430
01:26:27,720 --> 01:26:30,760
Kip's formulation works anywhere in
the universe
1431
01:26:30,760 --> 01:26:34,360
where the gravitational field is
such as on Earth.
1432
01:26:35,680 --> 01:26:39,000
The difference in the rate of flow
of time is tiny.
1433
01:26:39,000 --> 01:26:42,160
At high altitude
and on the surface of the Earth,
1434
01:26:42,160 --> 01:26:46,800
the difference in the rate of flow
of time is one second in 100 years.
1435
01:26:46,800 --> 01:26:49,040
That's not very much!
1436
01:26:49,040 --> 01:26:53,760
But that is enough that it's
precisely the right amount
1437
01:26:53,760 --> 01:26:56,560
to produce the gravitational pull
that we feel
1438
01:26:56,560 --> 01:27:00,240
and produce the accelerations
we're talking about.
1439
01:27:00,240 --> 01:27:04,000
Wow, OK. I need to go
and write this one down!
1440
01:27:04,000 --> 01:27:06,000
THEY LAUGH
1441
01:27:08,280 --> 01:27:12,240
So, my investigation deep into the
weird ways of gravity
1442
01:27:12,240 --> 01:27:14,840
has finally left me face-to-face
1443
01:27:14,840 --> 01:27:18,880
with one of the greatest mysteries
in all of physics,
1444
01:27:18,880 --> 01:27:22,280
the nature of time itself.
1445
01:27:22,280 --> 01:27:24,600
It sounds like such a simple
question.
1446
01:27:24,600 --> 01:27:27,080
Why does the apple fall?
1447
01:27:27,080 --> 01:27:30,080
And yet, hundreds of years of
scientific enquiry
1448
01:27:30,080 --> 01:27:32,320
investigating this single action
1449
01:27:32,320 --> 01:27:34,560
have led us to completely redefine
1450
01:27:34,560 --> 01:27:37,920
the way we think about the very
nature of space and time.
1451
01:27:39,840 --> 01:27:42,960
And now I've been presented with
this extraordinary proposition,
1452
01:27:42,960 --> 01:27:46,480
that somehow, in some profound way,
1453
01:27:46,480 --> 01:27:49,760
the apple falls because it's seeking
out the place
1454
01:27:49,760 --> 01:27:52,960
where time runs the slowest.
1455
01:27:52,960 --> 01:27:56,680
So, does gravity dictate the flow
of time?
1456
01:27:56,680 --> 01:28:00,800
Or does time itself define gravity?
1457
01:28:00,800 --> 01:28:04,400
Could this hint to fundamental new
laws of physics,
1458
01:28:04,400 --> 01:28:06,040
as yet undiscovered?
1459
01:28:06,040 --> 01:28:08,480
I think I'm going to have to think
about this a bit more.
123640
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