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00:00:00Would you like to lose a little bit of weight without doing any exercise or dieting?
00:00:05Would you like to age just a bit more slowly than your friends?
00:00:09Well, you might be surprised to hear the laws of physics can help.
00:00:15The key to unlocking these everyday questions is gravity.
00:00:20It sculpts the universe.
00:00:23It warps space and time.
00:00:27It's a fundamental force of nature.
00:00:32But gravity's strange powers, discovered by Albert Einstein,
00:00:36also affect our daily lives in the most unexpected ways.
00:00:44In this film, we'll be using cutting-edge scientific techniques
00:00:48to investigate how gravity changes your weight.
00:00:52It's got up!
00:00:54Your height.
00:00:55I really have shrunk.
00:00:57And even your posture.
00:00:59And with the help of thousands of volunteers, I'll show you how gravity makes us all age at different rates.
00:01:10I've just been logging onto the phone, logging onto the app.
00:01:13As a physicist, gravity is central to my work.
00:01:16Oh, wow.
00:01:18And in exploring it, I'll be challenged on how I understand this most mysterious force.
00:01:24Wow.
00:01:24OK.
00:01:25I need to go and write this one down.
00:01:27And I'll have to tackle the very nature of reality itself.
00:01:40gravity.
00:01:44It binds together all the matter in the universe, and it makes our existence here possible.
00:01:54But in the end, it all boils down to one simple question.
00:01:59What happens if I drop an object?
00:02:06Gravity's many mysteries are all contained in this single action, how an object falls.
00:02:14Here's the first puzzle.
00:02:16Why does a hammer fall faster than a feather?
00:02:20You might think it's because the hammer is heavier.
00:02:23But that's not the real reason.
00:02:27The answer is air resistance.
00:02:30It's not the weight of the objects that matters, it's their shape.
00:02:34And I can demonstrate this very easily with these two umbrellas.
00:02:37They both have exactly the same weight.
00:02:39But if I open one of them, you can be pretty sure it'll drop more slowly than the other one.
00:02:49In fact, all objects would fall at the same rate if you could only remove the air.
00:02:57The first person to realize this was the 16th century mathematician Galileo Galilei.
00:03:04Famously, it's said he worked it out by dropping objects off the leaning tower of Pisa.
00:03:13And he was spectacularly proven right in an experiment carried out on the moon in 1971.
00:03:22Well, in my left hand I have a feather, in my right hand a hammer.
00:03:27And I'll drop the two of them here and hopefully they'll hit the ground at the same time.
00:03:31It worked perfectly.
00:03:34How about that?
00:03:35How about that?
00:03:36It proves that Mr Galileo was correct in his findings.
00:03:43Now Galileo was obsessed with a second question too.
00:03:47When you drop an object, it's actually quite hard to tell if it falls at a constant speed or picks
00:03:54up speed as it drops.
00:03:58Even in slow motion, it's pretty hard to tell.
00:04:05But Galileo realized this.
00:04:09First, drop an object a very short distance.
00:04:13It lands with very little impact.
00:04:17But of course, drop it from higher up.
00:04:24This time, the ball easily breaks the tile, which means it must have accelerated, gaining in speed and momentum as
00:04:33it dropped.
00:04:36Galileo had identified something fundamental to all falling objects.
00:04:40They accelerate.
00:04:45He realized there might be a way to measure how much falling objects gain in speed.
00:04:51What he devised was the first ever attempt to measure gravity itself.
00:04:57He built a long wooden ramp, rather like this, that he had sloping at a shallow angle.
00:05:04The idea was to roll balls down the ramp and measure their acceleration.
00:05:09The crucial thing is that the ramp had to be at this shallow angle to reduce the effects of wind
00:05:14resistance.
00:05:15It also meant that the balls would roll down slowly enough to give him time to measure their speed.
00:05:21But the big problem was this.
00:05:23How do you measure time accurately in an age when there were no accurate time pieces, let alone stopwatches?
00:05:30Well, Galileo came up with an ingenious idea involving the flow of water.
00:05:34Essentially, measuring time from the amount of water collected in a cup.
00:05:39So, we're going to try and repeat Galileo's experiment.
00:05:43I say we because I have a couple of willing volunteers, Gavin and Joanna.
00:05:47Three, two, one, go.
00:05:56And stop.
00:05:57OK, there's one.
00:05:59Now, if you come down a quarter of the way down the ramp, go.
00:06:08Stop.
00:06:09OK, so now half of the way down, go.
00:06:15Stop.
00:06:17Just in time.
00:06:19OK, and then three quarters of the way down.
00:06:23Go.
00:06:25And stop.
00:06:28Right. Turn the tap off.
00:06:30OK, so we have our four measurements, and I can see a progression from fuller to emptier.
00:06:36But what we need to do now is find the mathematical pattern by weighing carefully the water in each glass.
00:06:43Weighing the water should give us an idea of how long each roll took.
00:06:47And in our experiment, these were the results.
00:06:51OK.
00:06:52Now, there's one immediate thing you can tell.
00:06:55The ball really sped up the longer it rolled.
00:07:01In fact, our results seem to show that the time it took to cover the first quarter of the ramp
00:07:07was about the same time it took to cover the next three quarters.
00:07:12Right.
00:07:13So we have a strong hint of a mathematical pattern.
00:07:18Now we'll see if we're right by placing bells along the ramp at intervals which are based on the results.
00:07:26OK.
00:07:27This arrangement looks a bit strange because the gap between the first two bells is much shorter than the gap
00:07:34between the third and fourth bells.
00:07:36But that's OK, because if we've got our calculations right, the ball starts off slowly, so it covers a shorter
00:07:42distance.
00:07:42And as it picks up pace, it'll cover longer and longer distances.
00:07:47So we should hear the bells ringing at equal intervals in time.
00:07:52Go.
00:08:00Beautiful.
00:08:04So what does this all mean?
00:08:06What's the mathematical formula?
00:08:07Well, this is something that Galileo worked out.
00:08:09Let's say, from the start, the ball covers a distance of one metre in the first second.
00:08:15After two seconds, it'll have covered four metres.
00:08:19After three seconds, nine metres.
00:08:21After four seconds, 16 metres.
00:08:23And so on.
00:08:25If you recognise this progression, you'll see that distance goes like the square of time.
00:08:33Galileo had found the rate at which gravity speeds up objects.
00:08:38And he'd found another fundamental principle.
00:08:41You can measure the strength of gravity by how much it causes falling objects to accelerate.
00:08:51Detecting gravity has become exceptionally sophisticated these days, but still uses exactly the same principle.
00:09:00This is Hurstmonceau Castle in Sussex.
00:09:03And in its grounds lies the Space Geodesy Facility.
00:09:09Here, Vicky uses an astonishingly sensitive instrument to detect the exact strength of gravity on this one spot.
00:09:18OK, so Vicky, tell me about this incredible gravity metre that you work with.
00:09:22OK, so this is the dropping chamber in a stripped down version.
00:09:26So essentially what happens is you've got a cart that gets raised to the top, and then the cart accelerates
00:09:31away from a mass in the middle.
00:09:33And so this section here lifts off, and as it drops, it drops under free fall.
00:09:38So this component in the middle, as it drops, is basically just Newton's apple falling to the ground.
00:09:43Yes.
00:09:44So this is a stripped down version, but that's the real thing.
00:09:47This is the real thing.
00:09:48How does it actually work?
00:09:49In here it's a vacuum.
00:09:51So there's no wind resistance as it falls.
00:09:53So there's no wind resistance.
00:09:55Inside, a laser is used to measure exactly how fast the mass is accelerating.
00:10:00This is the 21st century version of Galileo's ramp and the balls rolling down.
00:10:06So can we get it going?
00:10:07Of course, if you'd just like to press the button on the laptop.
00:10:09This one?
00:10:10Yep.
00:10:13OK.
00:10:13So it's now communicating with it.
00:10:15Oh, here we go.
00:10:15There we go.
00:10:16So it waits five seconds, then takes a measurement of gravity and repeats.
00:10:21Oh, and you can see the results appearing now.
00:10:27Yep.
00:10:27Each of those green dots is measurement of gravity with the actual number that it's getting for each one.
00:10:32The unit Vicky uses has a familiar ring.
00:10:35I see the number up at the top here.
00:10:39So you've got this unit, micro-gal.
00:10:42Yes, gal is essentially one centimetre per second squared.
00:10:46The gal was named after Galileo.
00:10:48So we've just taken the measurement of gravity here today,
00:10:51and it's this highly accurate number 981124007 micro-gal.
00:11:02The reading means that the Earth's gravity speeds up a falling object by around 9.81 metres per second for
00:11:11every second it drops.
00:11:15Vicky tells me something intriguing.
00:11:18She takes a reading here every week, and she's found that the strength of gravity changes by tiny amounts over
00:11:25time.
00:11:26Heavy rainfall, for example, can cause gravity to increase slightly.
00:11:33Presumably, if gravity is changing here in one spot, it'll have different values all around the world, and so you
00:11:40can have a gravity map of the entire planet.
00:11:43That's right, yeah.
00:11:45So what's the reason for these strange fluctuations?
00:11:49That's what I want to investigate next.
00:11:53So, gravity changes as we move across the surface of the Earth.
00:11:58Well, this lies at the heart of a challenge that I've set two young volunteers.
00:12:03I've given them a task to try and find the place in Britain where gravity is at its weakest, so
00:12:10where objects would weigh the least.
00:12:12And I've given them just three days to try and find it.
00:12:19The volunteers are Estrella Sendra, a PhD student.
00:12:24I've been living in London for five, six years, and I'm originally from Seville in Spain.
00:12:29I'm very interested in taking part in this project because I would really like to know more about how this
00:12:36world works.
00:12:37And Poppy Begum, a journalist who lives in London.
00:12:40I did my degree in biomedical science, and I did biology and chemistry for my A-levels, but I haven't
00:12:47done any physics since I left school.
00:12:50I'm fascinated to find out more about gravity, and I actually enjoy a puzzle. I like a challenge.
00:12:55Now, the team can't just weigh themselves to see changes in gravity.
00:13:00Body weight fluctuates naturally by a couple of kilos over the course of a day,
00:13:05whereas changes due to gravity as they travel around the country are going to be tiny in comparison, a matter
00:13:11of a few grams.
00:13:12So, they're going to have to use sophisticated scientific methods if they want to measure gravity accurately.
00:13:18And that's why the volunteers will be joined by three specialists in gravity science.
00:13:26PhD student Sonak Bose.
00:13:29He'll be in charge of some very sensitive measuring apparatus from the National Physical Laboratory.
00:13:36Sean Hughes, a geologist, who'll be using a portable gravity meter.
00:13:42And Andrew Ponson, a cosmologist at University College London, who'll help interpret the results.
00:13:49We've taken a collective weight for the team before they set off.
00:13:53It's 380 kilograms.
00:13:56So, can they find the place in Britain where that'll decrease?
00:14:02They're setting out in Snowdonia National Park in North Wales.
00:14:08The railway climbs from here to the thousand-metre summit of Snowdon.
00:14:13Sean takes his first gravity reading.
00:14:16The inside is a mass on a beam.
00:14:18And you turn this counter, this dial, until you get the beam central.
00:14:25By counting the number of turns of the dial, Sean can calculate the downward pull of gravity acting on the
00:14:32mass inside the machine.
00:14:36Sonak has a simpler method.
00:14:38So, inside the box is a two kilogram mass.
00:14:42And it's supposed to be sort of as perfectly two kilograms as it's possible to get.
00:14:47All right, and place it here.
00:14:52Oh, it's just coming under, isn't it?
00:14:55One nine nine eight point two grams.
00:14:57So, it was two kilos in the laboratory, but now here it's a bit less.
00:15:02It's the first puzzle.
00:15:04Why does a two kilo mass tip the scales at just under two kilos?
00:15:09And it's one which gets straight to the heart of what the challenge is really about.
00:15:17Mass is often confused with the related quantity, weight.
00:15:21The mass of these dumbbells is fixed. It doesn't change.
00:15:26It's a measure of how much stuff they contain.
00:15:30Weight is different. It's a measure of the effect of gravity on these dumbbells.
00:15:35It's a downward force pulling them to the ground in the same way that it's keeping my feet firmly stuck
00:15:41to the ground.
00:15:42The crucial difference is this.
00:15:44If I was holding these dumbbells on the moon, they'd still have exactly the same mass.
00:15:49But they would weigh six times less because the moon's gravity is so much weaker than the Earth's.
00:15:58So that's why Sonak's bringing along the two kilo mass.
00:16:02If it changes weight, then this should mean that gravity itself has changed.
00:16:10Ahead of them is the summit of the highest mountain in England and Wales, famed for its stunning scenery.
00:16:18Or it would be stunning if you could see it.
00:16:22And this is what we came all the way up here for, this amazing view at the top of Snowdon.
00:16:30You wouldn't know it, but honestly, we are here.
00:16:34So we're now near the summit of Snowdon and I've set up the gravimeter again.
00:16:38And we're going to see what the difference in the reading is.
00:16:44He has to turn the dial again and again to try and get a reading.
00:16:49It's clear gravity has changed, but which way? Has it got stronger or weaker?
00:16:55The team leaves Sean to work out his results and tries to position the scales as close as possible to
00:17:01the summit.
00:17:03But the reading is all over the place.
00:17:05Oh, it's gone up.
00:17:09It's fluctuating quite a lot due to the wind.
00:17:12I have to say, this is what science is always like, isn't it?
00:17:14It's never quite what you want it to be.
00:17:17So they head inside to the cafe next to the summit.
00:17:22The wind was being a bit naughty, but hopefully...
00:17:24I mean, now it's in zero-zero, so it should be all right.
00:17:27One nine nine eight point two down there.
00:17:31One nine nine seven point eight.
00:17:34We've got it.
00:17:35That's point four of a gram off.
00:17:38The mass weighs a tiny bit less.
00:17:41It's lost about one five thousandth of its weight.
00:17:45And Sean's found that gravity itself has reduced.
00:17:50At the top of the mountain we took the measurement.
00:17:53And we discovered that the pull of gravity had gone down.
00:17:58It had gone down an equivalent of two hundred and six turns of the dial.
00:18:01And we worked out that that's equivalent to two hundred and nineteen milligares.
00:18:09So it's clear from the team's measurements, gravity weakens as you go higher and you get a bit lighter.
00:18:18It's just an excuse to say, where are we like the lightest? Who cares?
00:18:22But in the sense that it's actually really interesting.
00:18:26It's like an illustrative example of seeing how this is actually fluctuating depending on different factors.
00:18:32Yeah, absolutely. And that we could measure it and we could see it with our own eyes.
00:18:36It actually makes you think about gravity in a very active way.
00:18:40It's such a fundamental force phenomenon in nature, but we don't know that much about it.
00:18:46But why does gravity change with altitude?
00:18:50To understand that question, you have to get to grips with the extraordinary discoveries of the next scientific giant in
00:18:57our story.
00:18:59Isaac Newton.
00:19:01Born in England in the middle of the 17th century, he spent his life wrestling with so many apparently separate
00:19:08questions from why things fall to the ground to why planets orbit the sun.
00:19:16It took the genius of Newton to realise that there was one single equation that could answer all these questions.
00:19:25And here it is, his famous law of gravity.
00:19:29It might look complicated, but this is one of the most important equations in the whole of science.
00:19:35F here is the force.
00:19:37Now Newton said there's an attractive force between any two objects in the universe.
00:19:42On this side of the equation, G we call the gravitational constant.
00:19:47Now Newton knew it had to be there, but he didn't know what its value was.
00:19:51M1 and M2 represent the two objects.
00:19:55And R is the distance between them.
00:19:58Now the equation tells us that the more massive the objects are, the bigger M1 and M2, the greater the
00:20:05attractive force.
00:20:05But the further apart they are, the bigger the value of R here, the weaker the gravitational force.
00:20:13With Newton, what was once mysterious now became clear.
00:20:19Newton's equation describes why an object falls to the ground, including his famous apple.
00:20:24But its true genius is that it applies to any object anywhere in the universe.
00:20:30So it's a very simple and elegant way of describing some of the seemingly most complicated phenomena in the cosmos.
00:20:43His law of gravitation can still be used today.
00:20:47To explain how orbits work.
00:20:50To predict when a comet will return.
00:20:54To describe why galaxies spin.
00:20:59Or to slingshot spacecraft around planets.
00:21:04Newton tells us to look for the underlying simplicity in natural phenomena.
00:21:08For instance, how the Moon orbits the Earth.
00:21:13If I let go of this apple, it'll fall straight down because of the pull of Earth's gravity.
00:21:19But if I throw it, to begin with it travels in a horizontal direction, that's the direction of travel.
00:21:25But Earth's gravity is still pulling it downwards.
00:21:27So it ends up following a curved path.
00:21:37Now if I throw it harder, it'll travel further before it hits the ground.
00:21:41And in principle, if I could throw it hard enough, I could put it into orbit.
00:21:45And that's exactly what's happening with the Moon in orbit around the Earth.
00:21:49It's a combination of wanting to travel in a straight line,
00:21:53but also being pulled down by the Earth's gravity.
00:21:55So it ends up constantly falling around the Earth and constantly missing.
00:22:03Newton's famous equation also explains the strange effects which the road trip team has discovered.
00:22:09That objects get lighter as you gain an altitude.
00:22:14When I weigh myself, I'm represented by the first mass, M1.
00:22:18The second mass, M2, is the Earth itself.
00:22:22And the force pulling me down, my weight, depends on the distance between me and the centre of the Earth.
00:22:29And that's the secret of the road trip.
00:22:31If you want to find the place where you weigh the least, then you have to get as far away
00:22:36as you can from the Earth's core.
00:22:46So it's the afternoon of day one, and the road trip team have to work out where to go next.
00:22:53Poppy and Estrella have a good idea.
00:22:56Find somewhere higher than Mount Snowdon.
00:22:59From the measurements that you guys did at Mount Snowdon, altitude clearly plays an important part in gravity.
00:23:06So with that in mind, we've got to go to the highest point in the UK, which is Ben Nevis.
00:23:10OK, but there's just one thing that we haven't shown you so far.
00:23:15We actually brought along an extra experiment.
00:23:18So can we please show you this first before you make the final decision?
00:23:22Sonak actually has the other part of this experiment.
00:23:24We always carry around some power tools, as physicists always do.
00:23:29So let's start off nice and gentle.
00:23:33OK.
00:23:34And then try and pick up some pace.
00:23:38Pizza.
00:23:40And...
00:23:41You've got some pizza there.
00:23:43Point proven.
00:23:44The point is that when something is spinning, it kind of gets flung outwards.
00:23:48And you can actually use that to make a nice flat piece of pizza.
00:23:52But this also applies to the Earth.
00:23:54The Earth isn't perfectly round.
00:23:57It's what's known as an oblate spheroid.
00:24:00It bulges at the equator where the spin is greatest.
00:24:05We've kind of got two competing effects now.
00:24:07We're trying to get away from the centre, the actual core of the Earth, the point at the very centre
00:24:13of this ball.
00:24:14But now we can do it in two ways.
00:24:16We can either kind of go up something tall, or we can just go down towards the equator.
00:24:22This is what we find when we're doing gravity surveys, is that as you move south, there tends to be
00:24:28an effect from latitude, which is often usually larger than the effect from altitude.
00:24:35So, the closer to the equator you go, the further you get from the Earth's core, and the lighter you
00:24:42get.
00:24:43So, guys, the sun's sitting just behind me here.
00:24:47Mm-hm.
00:24:47This is north.
00:24:48Mm-hm.
00:24:49From the conversations we've just had, it sounds like we've got to go that way down south.
00:24:54Is that right?
00:24:54Yep. Yep.
00:24:55Okay. Let's go.
00:24:55Let's go.
00:24:58The team is starting to uncover the reasons why gravity changes as you cross the surface of the Earth.
00:25:06Our planet is defined and shaped by the complicated forces which act upon it.
00:25:13And detecting tiny fluctuations in its gravity field can give us important clues.
00:25:19It can help us understand how our world is changing.
00:25:25The Space Geodesy Facility at Hurst-Monceau is one small part in an enormous global network, which uses satellites to
00:25:33detect the tiniest of changes in the Earth's gravity field.
00:25:37Tell me what exactly your job is here.
00:25:41What we're doing with this telescope is measuring very accurately the distances of satellites from here.
00:25:47So, we're using very short laser pulses which we direct towards the satellite.
00:25:51On the satellite there are reflecting cubes which return some of that light to us.
00:25:56And we measure how long it takes the light to go to the satellite and back.
00:26:00And how far away is the satellite typically?
00:26:02The one we're tracking now is at one of the Galileo satellites, which is about 20,000 kilometres.
00:26:0620,000 kilometres away?
00:26:08Yes.
00:26:09OK, so we've got it aimed at the Galileo satellite and you're going to turn the laser on now.
00:26:13Yes.
00:26:16Oh, wow.
00:26:18And that laser beam that's being fired up towards the satellite.
00:26:23Yeah.
00:26:24The time it'll take to get there and come back again.
00:26:26It's a fraction of a second, isn't it?
00:26:27It is. It's about 150 thousandths of a second, 150 milliseconds.
00:26:30And we're sending about 1,000 of those per second.
00:26:37This strange looking object is based on satellite readings.
00:26:41It's a highly exaggerated representation of how Earth's gravity field varies over time.
00:26:50Fluctuations like these can give us important insights into climate change.
00:26:54Ice caps melting.
00:26:58Sea levels rising.
00:27:00Changes in groundwater.
00:27:03All of these have an effect on the local strength of gravity.
00:27:07So something as important as climate change, in order to understand it and do something about it,
00:27:13we need to know the distribution of the gravitational field of the Earth very accurately.
00:27:19Absolutely, yes. And it's a global measure that we need.
00:27:29For the road trippers, it's the start of day two.
00:27:33And they're heading for the south coast.
00:27:37They're stopping off in Herefordshire.
00:27:40It's a good location, as it's the same altitude as the base of Snowdon.
00:27:45But they've moved about 80 miles further south.
00:27:48So if they find gravity changes here, it must be due to latitude.
00:27:53It's not a huge difference, but it's noticeable.
00:27:55Our counter reading at the bottom of the mountain was 4,840.
00:27:59Yeah.
00:28:00Our counter reading here is 4,717.
00:28:03All right, so we do get to see a difference.
00:28:06So we're actually at the same altitude as the base of Mount Snowdon,
00:28:09but because we've travelled further down south overnight, gravity's less here.
00:28:13Yeah.
00:28:17They push on.
00:28:23And by sunset, they reach Sidmouth on the south coast.
00:28:31Sean takes the second gravity reading of the day.
00:28:34And Poppy improvises a map.
00:28:37Well, sort of a map.
00:28:39Can we write not to scale at the top there?
00:28:42To scale.
00:28:45So I drew this map.
00:28:48Scotland's a bit squashed.
00:28:49Wales is quite high up, and Cornwall is there.
00:28:53But you get the idea.
00:28:55So, Sean, we've been travelling with you.
00:28:58You've done quite a few gravity metre readings.
00:29:00Can you plot them on this not to scale badly drawn map, please?
00:29:04Sure.
00:29:05So if you remember, we started off in Mount Snowdon, about here.
00:29:09And that was the zero measurement for our survey.
00:29:13And then we've come all the way down here to the south coast.
00:29:18The difference from the base of Snowdon is minus 212 milligals.
00:29:25Wow.
00:29:26So the difference between going, measuring gravity at the base of the mountain
00:29:31and the top of the mountain is about the same as here at this latitude
00:29:36and down here at this latitude.
00:29:39They're quite clearly at sea level.
00:29:41Yet gravity here is roughly the same as it is at the top of Snowdon.
00:29:46But where next?
00:29:48We are here.
00:29:50If we want to find out where we are the lightest,
00:29:54why don't we travel all the way to the most southerly point in the UK,
00:29:59which is here?
00:30:01But altitude can also help us.
00:30:03So why not find a place in the country that is both low in latitude
00:30:08but also is high in altitude in terms of height above sea level?
00:30:14Because that will get us somewhere that is really far away from the core of the earth
00:30:19while staying within the country.
00:30:27So the answer to the puzzle lies in a combination of two factors.
00:30:32How much further south should they go and how much higher?
00:30:38At the end of day two, Sean's results show that the team weighs about 80 grams lighter in total
00:30:44than back at the base of Snowdon.
00:30:57The way that weight changes is just one example of Newton's famous equation in action.
00:31:06But Newton had left his masterpiece incomplete.
00:31:09He didn't know the value of G, the gravitational constant, which sets the size of the force.
00:31:18To harness the full power of the equation, you need to know G.
00:31:22And the vital clue came with an incredible experiment conducted in London at the end of the 18th century.
00:31:32It was an attempt to work out the mass of the earth itself.
00:31:38And it was carried out by an eccentric, extravagantly rich aristocrat, Henry Cavendish.
00:31:46Cavendish was a chronically shy, deeply solitary man living in total isolation in his house in Clapham.
00:31:53The story goes that one day he accidentally bumped into a female servant on his staircase.
00:31:59He was so traumatized by this event that he had a new staircase built just for him,
00:32:04so this horrible incident could never happen again.
00:32:09Cavendish had inherited vast fortunes and was able to dedicate his life to devising pioneering experiments,
00:32:17including one particularly extraordinary piece of equipment.
00:32:25He set up something a bit like this. It's called a torsion balance.
00:32:29It involves four lead spheres, two large heavy ones which are held fixed in place,
00:32:35and suspended by a very thin wire is a wooden rod, six feet long, with two smaller balls on either
00:32:44end.
00:32:45Now, the crux of the experiment is the relationship between the large ball and the small ball.
00:32:50Now, of course, there's a gravitational pull downwards on both of the balls due to the Earth's gravity.
00:32:56But Newton also tells us that there should be a very weak gravitational pull between the balls.
00:33:02And this is effectively what Cavendish was trying to measure.
00:33:05Any slight movement of the small ball towards the large one should cause a twist in the torsion wire.
00:33:12And that's what Cavendish was trying to detect.
00:33:15Of course, this is all much easier said than done. The experiment was incredibly sensitive.
00:33:21The tiniest of vibrations, the slightest breeze, changes in temperature could all influence the measurements.
00:33:27So, Cavendish had to isolate the apparatus inside a box and the box within a shed.
00:33:34He even realised that his mere presence next to the apparatus could influence things.
00:33:39So, he had to remove himself outside the shed.
00:33:43What he then did was sit outside the shed and through a small hole in the shed wall,
00:33:48look through a telescope to detect the tiniest of twists in the wire.
00:33:53It was an incredibly difficult process, but after many months, he finally felt confident enough that he had a reliable
00:34:00result.
00:34:07Cavendish found that the small balls did move, a tiny four millimetres.
00:34:16He calculated his results by comparing the density of the balls with the density of water.
00:34:24In the end, the result of Cavendish's experiment and subsequent calculations
00:34:28was that the density of the earth was about five and a half times that of water.
00:34:34Or, put another way, the mass of the earth was 5.9 trillion trillion kilograms.
00:34:42What's most remarkable is that Cavendish got this number right to within an accuracy of 1%.
00:34:50With Cavendish's astonishing result, scientists were able to work out G.
00:34:57Then the equation could be used to determine the mass of any celestial body in orbit around another.
00:35:05So, astronomers were able to calculate the mass of the sun, and the planets, and the moon, and eventually even
00:35:14distant galaxies.
00:35:20And, of course, back on Earth, we never escape gravity.
00:35:25Over the course of the day, it actually squeezes your spine, an effect you can see for yourself if you
00:35:31use a measuring rod.
00:35:34OK, so it's half past seven in the morning.
00:35:36I've just gone up, and I'm going to see how tall I am before gravity drags me down.
00:35:50That's 178 centimetres, or just over 5 foot 10.
00:35:58Over the course of the day, gravity compresses the fluids in your spine.
00:36:05Right, it is just past 11pm.
00:36:08I've been standing up for most of the day, so let's see if gravity has had an effect on my
00:36:14height.
00:36:20That is 176 centimetres, so I really have shrunk by just over half an inch over the course of today.
00:36:34In the longer term, gravity can affect your posture permanently.
00:36:39But there are exercises you can do to counteract this effect.
00:36:44Part of my research has been looking at the effects of gravity on the human body.
00:36:48So, people might not be aware, or they might not always think about the effects of gravity on our physical
00:36:53state,
00:36:54on our health, and particularly on our posture.
00:36:56However, because it's such a constant force, gravity has a massive impact over the course of our lifetime.
00:37:03As you get older, you can develop a stoop, which is damaging to your mobility.
00:37:09The doctor here has actually got very good posture, but I'd like you to just show not so good posture.
00:37:15So, when poor posture is really rounded shoulders, and then loss of the curve on the back as well,
00:37:23I kind of just ask you to raise up your arms when you're in that posture.
00:37:26So, no, and then just come back down, shoulders back in normal, and then raise your arms.
00:37:32You can see the effects of posture on function.
00:37:37Ironically, the exercises which many gym-goers do actually make your posture worse.
00:37:42That's if you only exercise the frontal muscles, like the chest and abdominals.
00:37:49So, it's recommended you exercise the back muscles just as much, to straighten you out and counteract the effects of
00:37:56gravity.
00:38:05Meanwhile, it's the end of day two for the road trip, and they've reached Sidmouth on the south coast,
00:38:11looking for the place in Britain where they'll weigh the least.
00:38:15They've worked out the answer lies in a combination of two factors.
00:38:20The right mix of going south and being higher up.
00:38:26And for the final leg of the journey, I'm going to meet up with them.
00:38:31I asked them to drive a short distance west, to one of the most remote areas in mainland Britain.
00:38:38Dartmoor National Park.
00:38:41It's only 40 miles from the southernmost tip of Britain.
00:38:45Hello.
00:38:45Hi, Andrew. Nice to see you.
00:38:48And it's very high, very hilly territory.
00:38:52Jim, the team got to the south coast yesterday.
00:38:54Yeah.
00:38:55Where we defined gravity at its weakest.
00:38:58But we haven't quite figured out whether it's altitude or latitude.
00:39:02Do we go further south or do we go higher up?
00:39:04You're right to ask, do we go as far south as possible or as high as possible?
00:39:09That's why I've brought you here to Dartmoor.
00:39:12And we've charted the most important points on this map here.
00:39:17All right.
00:39:18Let's have a look.
00:39:19So we are here, two bridges.
00:39:22These four dots represent these hills up there behind us,
00:39:27which are at about 500 metres above sea level.
00:39:30So that's what we want to check out.
00:39:32These hills are close to the south coast
00:39:34and they're also the highest in the whole of the south of England.
00:39:40So logic suggests they must be the right combination of latitude and altitude.
00:39:46Well, there's another reason why this makes perfect sense,
00:39:48one which we haven't looked at yet,
00:39:50and that is the effect of the underlying rocks on gravity.
00:39:53And I've got a map here that shows...
00:39:55You're going to trump my map with yours, aren't you?
00:39:58Here we are down here.
00:39:59Now, these blue areas are the lowest areas
00:40:04according to the density of the rocks underneath.
00:40:07The rocks around here are made of granite,
00:40:10which will make gravity weaker still.
00:40:14So that's helping as well as the altitude
00:40:17and the fact that we are further south.
00:40:19Yep, it's also playing a part.
00:40:22Well, we have a plausible theory,
00:40:25but now we need to test it.
00:40:29If I'm right, then at the top,
00:40:31our gravity reading should be by far the lowest reading of the trip.
00:40:38Of course, there's another effect of gravity to deal with now.
00:40:41It's knackering when you head uphill.
00:40:45OK.
00:40:46So I think this is pretty much the start of the hills we've located on the map.
00:40:50So let's see if this is the lightest place.
00:40:53Sean, if you want to get the gravity meter out,
00:40:55and we'll take another reading here.
00:40:56Yep.
00:40:57OK.
00:41:01Sean sets up his equipment one more time.
00:41:05What's the news?
00:41:06Well, at the bottom of Mount Snowden was our zero for this test.
00:41:11We found we lost a certain amount by going up to the top of Mount Snowden.
00:41:15We found we lost a certain amount coming south to the south coast.
00:41:19Not only have we beaten that, we've smashed it.
00:41:21Brilliant.
00:41:22We were minus 219 milligals lower at the top of Mount Snowden.
00:41:28Here on Dartmoor, we're minus 347 milligals lower.
00:41:33Wow.
00:41:33Brilliant.
00:41:33So it is a combination of three things.
00:41:36We're far south, so that's the latitude.
00:41:38We're at altitude, look at high up.
00:41:40And we're surrounded by all this granite rock, which is low density anyway.
00:41:44I hope you all think it was worth the climb up here anyway.
00:41:46Absolutely.
00:41:47There you go.
00:41:48Boom.
00:41:49Science.
00:41:54Now, we already know that the altitude of these hills takes us much further from the Earth's
00:41:59core than anywhere else further south in Britain.
00:42:02So gravity must be weakest here.
00:42:05There's extra evidence too.
00:42:08The British Geological Survey has compiled tens of thousands of gravity readings made in the UK.
00:42:14And the lowest readings ever recorded were all taken around here, on the high hills of Dartmoor.
00:42:22What do we do to celebrate?
00:42:24We weigh ourselves, of course.
00:42:26The effects of weight, is that a match?
00:42:30It's all them Nutella pancakes for breakfast.
00:42:34I need to leave weight.
00:42:37I can tell you that you should weigh something like 20 grams less than you did at the base of
00:42:44Mount Snowdon.
00:42:45Guys, I'm guessing something like 25 to 30 grams less.
00:42:49So, if you want to weigh as little as possible, this is the place in Britain to come.
00:42:54But in any case, it's such a tiny amount that it's going to be wiped out entirely by whatever it
00:42:59was you had for breakfast this morning.
00:43:10Gravity.
00:43:11What goes up, must come down.
00:43:15All of our lives, we abide by its rules.
00:43:19It dominates our every action.
00:43:22But there's one select group of humans who know what it's like to live free of gravity.
00:43:28Two.
00:43:29One.
00:43:30Zero.
00:43:32Liftoff.
00:43:37Everybody's used to gravity.
00:43:39We're used to the oppression of it.
00:43:41Gravity is the ultimate oppressor.
00:43:44It grinds us under its heel, 24-7, with no release until you're in space and then suddenly you're free
00:43:53from gravity.
00:43:54You are, you're weightless in orbit.
00:43:58Canadian astronaut Chris Hadfield spent five months on board the International Space Station.
00:44:05You can pull your knees up to your chest and just tumble.
00:44:08Or if you take a wet cloth and you get it dripping wet,
00:44:13and everybody on Earth knows what'll happen when you wring it out,
00:44:16all the water will fall inevitably.
00:44:18If you do that in weightlessness, the water stays there,
00:44:22and it actually, because of the surface tension, starts crawling up your arms.
00:44:31It's a little bit mesmerizing and hypnotic to be in weightlessness.
00:44:36If you're weightless, you don't need a bed.
00:44:39You don't need a mattress.
00:44:40You don't need a pillow.
00:44:42Your body is floating completely suspended like magic.
00:44:50Movement becomes effortless.
00:44:52You can push off with one finger and fly and tumble.
00:44:56You don't need to hold yourself where you are with muscle.
00:45:00You can just, with a delicate fingertip pressure, you can stay where you are.
00:45:06Okay, separation confirmed.
00:45:08Timer is on.
00:45:09You're backing away at a rate of just a little over one-tenth of a meter per second.
00:45:13Re-entering gravity is a punishing experience.
00:45:17To come back to Earth is violent.
00:45:22It can be five times the force of gravity or eight times the force of gravity,
00:45:26crushing you down into the floor of the ship for quite a long time.
00:45:32Then, of course, you hit the ground and tumble and roll to a stop and now you're the victim of
00:45:39your past.
00:45:40You're the victim of your decision-making, lying there, trying to shake your head and get used to being in
00:45:47gravity again.
00:45:48I remarked at the time that I had forgotten that my lips have weight and my tongue has weight.
00:45:54You don't think about it.
00:45:55But if you try and talk articulately standing on your head, you'll notice that you have to sort of control
00:46:00your lips and your tongue a little differently
00:46:02just because gravity is pushing them the other way.
00:46:05And it's the same sort of thing.
00:46:06Raising your arm, holding your head up, turning your head when everything wants to tumble.
00:46:12Just keeping your balance.
00:46:14All of those things.
00:46:15It's a little bit like relearning to walk again like an infant.
00:46:25Gravity shapes our bodies and moulds our planet.
00:46:29Nothing happens on Earth without its power and influence.
00:46:35Sir Isaac Newton explained so many of its effects using one simple equation.
00:46:42And in the centuries that followed, his laws of physics led to breakthrough after breakthrough, spurring on the Industrial Revolution.
00:46:50But in the first decade of the 20th century, the next genius in our story challenged the very foundations of
00:46:58our understanding of gravity.
00:47:01A young German scientist called Albert Einstein was churning something over in his mind.
00:47:08He thought that something in Newton's laws didn't quite add up.
00:47:23Imagine I'm the sun and this tennis ball is the Earth in orbit around me.
00:47:28Newton's laws can describe very precisely the path the Earth takes around the sun
00:47:33in terms of the mutual gravitation of attraction between the two bodies.
00:47:37But what Newton can't explain is what connects them.
00:47:43In reality, of course, there is no invisible string between the Earth and the sun holding the two together.
00:47:48There's just empty space, a complete void.
00:47:51And yet, according to Newton, the Earth and sun pull on each other instantaneously across a vast distance.
00:47:59How can gravity act in this way when there's nothing to connect it or transmit it?
00:48:07After years puzzling over this, Einstein had a blinding flash of inspiration.
00:48:15Just like Galileo and his ramp, or Newton with his apple,
00:48:20Einstein's breakthrough came because he was thinking about one simple action.
00:48:28What happens when something falls?
00:48:36To explain, I'm visiting this 400-foot-high tower in Northampton, built to safety test lifts.
00:48:49One day in 1907, Einstein had what he called the happiest thought of his life.
00:48:59What if I was standing in a stationary lift, completely isolated from the outside world,
00:49:04not feeling anything apart from the pull of gravity on my feet?
00:49:09What if then the lift cable breaks and I start falling?
00:49:14What are the forces that I will feel as I'm plummeting to the ground?
00:49:25Well, I'm not going to try that.
00:49:31Fortunately, there's another way to test this without me having to plunge down a lift shaft.
00:49:36Sorry to disappoint you.
00:49:40This little device here that I've strapped to this plastic toy is an industrial accelerometer,
00:49:45so it measures acceleration.
00:49:47Now, I've got it connected to my laptop, and it's showing a measurement of 1g.
00:49:52Now, that's the downward acceleration due to the pull of Earth's gravity.
00:49:56So, basically, it works just like a gravity meter.
00:50:00But what happens if I were to drop it?
00:50:03Presumably, it'll carry on measuring 1g because it's falling in Earth's gravity.
00:50:07Okay, well, let's try that and see.
00:50:27So, you can see here, along this line at the bottom,
00:50:31that's when I was holding it still, and it's measuring an acceleration of 1g.
00:50:37These oscillations here is when I stood up, and there's a bit of disturbance,
00:50:41but this spike along here is the moment I released it.
00:50:45And this short duration along here is the time it was falling.
00:50:50And you see, while it was falling, it was registering an acceleration of 0.
00:50:55Now, if you think about it, this is really odd.
00:50:59The accelerometer is accelerating downwards.
00:51:02It's plummeting in the full grip of Earth's gravity,
00:51:05and yet it's measuring no acceleration at all.
00:51:09It's as though gravity has completely disappeared.
00:51:15Einstein's insight was that when something falls,
00:51:19it no longer feels the pull of gravity.
00:51:22In fact, falling is like floating in empty space.
00:51:27This is the essence of Einstein's happy thought,
00:51:31and what we now call his principle of equivalence.
00:51:36Einstein's point is that when the man in the lift falls,
00:51:39he doesn't just feel weightless, he is weightless.
00:51:44Einstein said the man feels no force pulling on him
00:51:48because there is no force pulling on him.
00:51:51Gravity doesn't act on him, it acts on the space and time around him,
00:51:55what we now call the geometry of space-time.
00:52:06This was a radical redefinition.
00:52:09Einstein says forget the idea of gravity as a force
00:52:12acting mysteriously between two objects.
00:52:16Now we have to think of it as the shape of space-time changing.
00:52:23You see, Newton saw space and time as independent, fixed and immutable.
00:52:29That three-dimensional space is the stage in which things happen.
00:52:33But time is separate.
00:52:35It ticks by at the same rate everywhere in the universe.
00:52:39According to Newton, an object would travel through space in a straight line,
00:52:43unless acted upon by a force like gravity
00:52:46that will cause it to deviate from that path.
00:52:50But Einstein said that space and time aren't fixed and immutable.
00:52:54They're interconnected,
00:52:55meshed together in what is known as space-time.
00:53:02And he said that space-time can be warped,
00:53:05that matter curves space and time around it.
00:53:14So, after Einstein, we no longer see gravity
00:53:18as an invisible string pulling objects together.
00:53:23Instead, a body like the Earth warps the structure of space and time around it.
00:53:30And an object in orbit follows a path
00:53:34which is as straight as possible through that space-time.
00:53:38It's a fundamental part of Einstein's vision of reality.
00:53:43Space and time can't be disentangled.
00:53:46You can't talk about space separately from time.
00:53:52So, matter warps time as well as space.
00:53:58It's known as gravitational time dilation.
00:54:02And it's possibly the strangest of all of Einstein's discoveries.
00:54:10I've got two identical clocks here.
00:54:12Now, because the clock lower down is closer to the centre of the Earth,
00:54:17it feels ever so slightly a stronger gravitational pull than the clock higher up.
00:54:23Einstein's theory says that the lower clock will tick by
00:54:27at a slightly slower rate than the higher clock.
00:54:31Basically, gravity slows time down.
00:54:37It's an extraordinary conception of reality that Einstein describes.
00:54:44Space is being curved and time is being distorted.
00:54:51So, why can't we perceive this in our everyday lives?
00:54:55Einstein had a rather nice way of explaining it.
00:55:00Most of us have had the experience as children of trying to work out
00:55:04what our parents do for a living.
00:55:05Well, imagine your father is Albert Einstein.
00:55:08When he was about 12 years old, young Edouard Einstein asked his father
00:55:12why he was so famous, what he'd discovered.
00:55:14Well, this put Einstein's senior on the spot,
00:55:17but he came up with a beautifully simple analogy.
00:55:23Einstein told his son,
00:55:25When a blind beetle crawls over the surface of a curved branch,
00:55:30it doesn't notice that the track it has covered is curved.
00:55:34I was lucky enough to notice what the beetle didn't notice.
00:55:39This is what Einstein meant.
00:55:41The beetle is free to move in any direction on the branch.
00:55:45It can move forwards, backwards, left and right,
00:55:47but it has no concept of a direction up off the branch.
00:55:51It's as though, for the beetle, the universe is missing the third dimension.
00:55:56The beetle may think it's moving in a straight line along the branch,
00:56:00but we can see that the surface it's walking on
00:56:02is itself curving and twisted.
00:56:09Einstein's point was that what we see as the twists and curves of the branch
00:56:13feel to the beetle like forces pushing and pulling it.
00:56:20OK, so consider this rather strange example.
00:56:23Imagine we have two beetles perched on this pumpkin,
00:56:26and for whatever reason, they want to walk up towards the top.
00:56:31Now, if they start at the equator, pointing due north,
00:56:36as they walk, they will begin by moving parallel to each other.
00:56:40That means their paths should never meet.
00:56:43But as they get closer to the top, their paths get closer together.
00:56:49Now, if they're clever beetles, they might try and figure out what's going on,
00:56:52and they could imagine that there's some mysterious force
00:56:55that's pulling them closer together.
00:56:58But for us, from our perspective, we can see there is no such force.
00:57:02All they're doing is following straight paths over a curved surface.
00:57:10Just as the beetles have no sense that the surface of the branch is curved,
00:57:16we completely fail to perceive the bizarre ways
00:57:20that gravity shapes the reality we live in.
00:57:26Einstein's problem was proving that he was right.
00:57:30After years more thought, he realised that there was a way.
00:57:35By looking far out into the solar system.
00:57:40Incredibly, here in the grounds of Hurst-Monceau Castle
00:57:43is housed one of the original telescopes
00:57:46that were used to prove Einstein was correct.
00:57:52In 1915, when Einstein developed his general theory of relativity,
00:57:57it was just that. It was a theory.
00:57:58It had no proof.
00:58:00In fact, many people found it completely outlandish.
00:58:03But then just four years later, in 1919,
00:58:06this telescope, and allow me to geek out a bit here,
00:58:10and I'll give it its correct name,
00:58:11this is the 13-inch astrographic refractor.
00:58:15This telescope proved that Einstein was in fact right,
00:58:19that gravity does curve space itself.
00:58:28Since then, observation after observation have confirmed
00:58:32that matter curves space and slows down time.
00:58:39So the simple question of why things fall the way they do
00:58:43has led us deeper and deeper into the very nature of space and time itself.
00:58:49Gravitational science shows us how galaxies, stars, and planets form.
00:58:55By measuring gravity, we've discovered the existence of dark matter,
00:58:59that 80% of the mass of our universe is invisible,
00:59:03and we don't know what it's made of.
00:59:07And we've detected exotic objects with extreme gravity,
00:59:12like neutron stars, which have more mass than our sun,
00:59:16yet are only 20 kilometers across.
00:59:21But it's another mysterious aspect of Einstein's universe
00:59:25that I want to explore in my next gravity project.
00:59:31Here at the University of Surrey,
00:59:33some colleagues and I have been working on it for months.
00:59:37What we're doing is devising a nationwide citizen science project.
00:59:42We're developing a smartphone app
00:59:44that uses the GPS contained on your phone
00:59:47to explore one of the strangest properties of gravity,
00:59:50how it affects the rate at which we age.
00:59:55I formulated the equations myself,
00:59:59and a small team of computer scientists and software developers
01:00:03is using them to devise the app.
01:00:11Einstein discovered that as gravity changes,
01:00:14so does the rate that time ticks.
01:00:18This means the strength of gravity you feel
01:00:21affects how quickly or slowly you age.
01:00:27The aim of my app is to demonstrate this effect.
01:00:32It works by using a phone's GPS data
01:00:35to estimate your local gravity.
01:00:39And it also calculates the average speed at which you move,
01:00:43because this, too, affects the rate at which you age.
01:00:48It then uses the equations I've written,
01:00:51which are based on Einstein's theory of relativity,
01:00:54to calculate overall how fast or slowly you're ageing.
01:01:02Once the app is ready, I tweet about it.
01:01:07Thousands of people download it,
01:01:09and we start to gather results from across the country.
01:01:14Some people send me videos giving me their results.
01:01:17How fast they're ageing compared with how time ticks
01:01:21out in space in zero gravity.
01:01:25Over the past day, I have aged less by about 172 microseconds.
01:01:30I have aged less by 10.02 milliseconds.
01:01:35So since downloading the app,
01:01:38I have aged less by 1.40 milliseconds.
01:01:41Since opening TimeWarp, I have aged less by 2.6 milliseconds.
01:01:50Our aim is to use their results to build up a map
01:01:53of how time flows because of gravity.
01:01:58My smartphone project provides just one insight
01:02:01into the space and time which Einstein's theories describe.
01:02:20gravity and its strange ways
01:02:23have given us astonishing insights
01:02:25into the dark secrets of our universe.
01:02:29Perhaps the weirdest objects in the universe are black holes,
01:02:33collapsed stars whose gravity is so strong
01:02:36that not even light can escape their grip.
01:02:41Now, for the first time ever,
01:02:43their effects have been felt on Earth,
01:02:45and they've been detected
01:02:46through the medium of gravity itself.
01:02:51It's a story that has revolutionised
01:02:53the study of modern cosmology.
01:02:591.3 billion years ago,
01:03:01in a galaxy far, far away,
01:03:04two black holes swirled around each other,
01:03:07drew closer and closer together,
01:03:09until they finally collided with incredible violence.
01:03:13In that final fraction of a second,
01:03:15at the precise moment that they merged,
01:03:18a disturbance was created
01:03:19that sent ripples out through the universe.
01:03:26Gravitational waves are a key prediction
01:03:28of Einstein's theory.
01:03:31matter doesn't just curve space-time,
01:03:34it can cause waves,
01:03:36ripples which expand outwards,
01:03:38exactly like a stone dropped in water.
01:03:42This particular wave was unimaginably large.
01:03:47The energy released was greater than all the light
01:03:51being given out by all the stars in the universe.
01:03:55The wave rippled through space at the speed of light.
01:03:59In 1.3 billion years,
01:04:02it covered a distance of over 10 billion trillion kilometres.
01:04:15until, on the morning of the 14th of September, 2015,
01:04:21it arrived here.
01:04:24The streets and cafes of New Orleans,
01:04:27in fact, everything in America and on Earth,
01:04:32expanded and contracted very, very slightly
01:04:35as the wave passed through.
01:04:38No one noticed,
01:04:40as by the time it arrived here,
01:04:42the distortion was phenomenally tiny.
01:04:49Except that one science laboratory did notice.
01:04:54And I'm going to see it.
01:05:00A thousand scientists across the world
01:05:03are collaborating on it.
01:05:07It's the culmination of over 50 years of effort
01:05:11and is one of the most sophisticated experiments
01:05:13ever devised by humanity.
01:05:18So I'm pretty excited to see it.
01:05:22It's a rather unusual setting.
01:05:24Here I am in the middle of rural Louisiana,
01:05:27about an hour's drive outside New Orleans.
01:05:29I don't expect to find such a multi-million dollar
01:05:33cutting-edge research facility as this.
01:05:35And yet, this is the place where recently
01:05:38one of the most important scientific discoveries
01:05:40in human history was made.
01:05:42This is LIGO.
01:05:46The Laser Interferometer Gravitational Wave Observatory
01:05:51is an enormous construction shaped like an L
01:05:56with a sophisticated laser system
01:05:58bouncing up and down the two arms.
01:06:02So we're standing on top of one of LIGO's two arms.
01:06:05This is the first LIGO arm.
01:06:07And in that tube, there's a laser beam
01:06:09that we bounce back and forth
01:06:11between a mirror in the end station
01:06:12and a mirror in this building.
01:06:14And the other bit goes that way, four kilometers,
01:06:17perpendicular to the arm we first saw.
01:06:19So this is the L shape.
01:06:19It's a big L on the ground.
01:06:21So the light bounces back and forth in that arm
01:06:23and bounces back and forth in this arm.
01:06:25And what we actually measure with LIGO
01:06:27is the length of this arm
01:06:29as measured by the light between the two mirrors
01:06:32and the length of that arm as measured
01:06:34by the light between two mirrors.
01:06:36And then the laser interferometer measures
01:06:37the difference between those two arm lengths.
01:06:41So as the gravitational wave passed through,
01:06:44the lasers picked it up.
01:06:46They detected that LIGO's two arms changed in length
01:06:50to a very, very tiny degree.
01:06:54The signal that we saw was just a few thousandth
01:06:58of the size of the atomic nucleus
01:07:02is the biggest the signal ever got.
01:07:04So far, far smaller than the size of a single atom.
01:07:08Oh, much, much smaller.
01:07:09Yeah, that one.
01:07:09And you need something this huge to pick that up.
01:07:12That's right.
01:07:13And so this is one of the most biggest,
01:07:15the biggest source of energy in the universe,
01:07:17one of the biggest events you'd ever measure.
01:07:19And we just barely saw it.
01:07:25The LIGO scientists
01:07:26turned the gravitational waves
01:07:28into sound waves.
01:07:30So what you're about to hear
01:07:32is, in a very real sense,
01:07:35the sound of two black holes colliding.
01:07:44It was the first observation of any kind
01:07:47of pairs of stellar mass black holes.
01:07:51Stellar mass means, you know,
01:07:52several or a bunch of suns in weight.
01:07:57And so we learned that they exist.
01:07:59We learned that there are enough of them
01:08:00that occasionally they run into each other
01:08:01and coalesce.
01:08:04And we also learned
01:08:06by comparing the waveform we observed
01:08:08with general relativity calculations
01:08:10that general relativity is,
01:08:12as far as we know, dead on right.
01:08:25The long concrete bunker to my left
01:08:28houses the beam line,
01:08:30one of the LIGO's laser arms.
01:08:33The detail and the effort
01:08:36that's gone into isolating the beam
01:08:38from the outside environment
01:08:40reminds me very much
01:08:41of Cavendish's famous experiment.
01:08:43He, too, had to worry about
01:08:44isolating his experiment
01:08:46from external disturbances.
01:08:48Only, of course, LIGO takes things
01:08:50to a far, far greater degree.
01:08:53Inside the arm is one of the largest
01:08:56and purest vacuums in the world.
01:08:59Atmospheric pressure in there
01:09:01has been reduced
01:09:02to one trillionth of the pressure outside.
01:09:05The mirrors inside are so reflective
01:09:08that they only absorb
01:09:09one in three million photons.
01:09:13And at the end of my little trip
01:09:15lies a British success story.
01:09:24Well, I made it all the way
01:09:26to the end of one of the LIGO arms.
01:09:28To be honest, it took me a bit longer
01:09:29than I thought,
01:09:30especially in that thing.
01:09:32But housed inside this building
01:09:34is one of the reflecting mirrors
01:09:36that bounces the laser beam
01:09:38all the way back
01:09:39down the four-kilometre arm
01:09:40to the main control centre.
01:09:42And the technology
01:09:43that went into developing
01:09:44these mirrors is quite remarkable.
01:09:46It was developed in the UK
01:09:48at the University of Glasgow.
01:09:57This is what the mirror looks like.
01:10:00Its surface is extraordinarily smooth.
01:10:03No bump bigger than a few billionths
01:10:06of a metre high.
01:10:09Equally amazing are these.
01:10:12Fused silica fibres,
01:10:14a few times the thickness
01:10:16of a human hair.
01:10:19Designed by the University of Glasgow
01:10:21in conjunction with scientists
01:10:23from other British universities.
01:10:26They isolate the mirror completely
01:10:29so it hangs perfectly still.
01:10:32You could say that in there
01:10:34is the quietest place on earth.
01:10:38Despite this,
01:10:39outside events do sometimes interfere
01:10:41with the work here
01:10:42as I witnessed for myself.
01:10:45I've wandered into the control room
01:10:47here at LIGO
01:10:48because I'm told something kicked off
01:10:50a few hours ago
01:10:51and they're all very busy.
01:10:53The image that's flickering up there
01:10:56is not meant to be like that.
01:10:58Essentially what they've picked up
01:11:00is a seismic disturbance,
01:11:01an earthquake.
01:11:02Now that's not an earthquake
01:11:04down the road.
01:11:05It started on the other side
01:11:07of the planet in Japan.
01:11:09So it just gives us a sense
01:11:11of the tremendous challenges
01:11:12faced by LIGO
01:11:14and the team here
01:11:15and the level of sensitivity needed
01:11:17that an earthquake
01:11:18on the other side of the earth
01:11:20can disrupt their measurements
01:11:22and they have to reset
01:11:23everything all over again.
01:11:28One of the scientists
01:11:29involved in developing
01:11:30this extraordinary place
01:11:32put it quite succinctly.
01:11:34Once we were blind,
01:11:36but now we can see.
01:11:40Throughout the entire history
01:11:41of astronomy,
01:11:42we've studied gravity
01:11:44and how it affects matter
01:11:45in the universe
01:11:46and how it warps space-time
01:11:48but only by looking at the light
01:11:51that enters our telescopes.
01:11:53Now for the first time,
01:11:55we can study the universe
01:11:56in a different way.
01:11:58The discovery of gravitational waves
01:11:59means we can see objects
01:12:01that cause extreme warping
01:12:03of space-time
01:12:03and its effect on gravity
01:12:05directly.
01:12:07This essentially opens up
01:12:09a new era in astronomy.
01:12:11It gives us a new way
01:12:12of looking out at the universe.
01:12:17Professor Sheila Rowan
01:12:19was one of the scientists
01:12:20who spearheaded
01:12:21the British effort for LIGO.
01:12:24For her and her colleagues,
01:12:26gravitational wave detection
01:12:27is just in its infancy.
01:12:30New instruments,
01:12:31even more sensitive than LIGO,
01:12:33are now being developed.
01:12:36There's so much that we don't understand
01:12:39about the universe that we live in
01:12:41and this has suddenly given us
01:12:42a new tool,
01:12:44a new way to probe
01:12:46the dark processes in the universe.
01:12:49Because every time we make
01:12:50the observatories more sensitive,
01:12:53we can sense gravitational wave signals
01:12:55from further away,
01:12:57from farther out in the universe,
01:13:00from further back in cosmic history.
01:13:02Things like supermassive black holes
01:13:04spiralling into collide,
01:13:06small black holes orbiting
01:13:08round supermassive black holes,
01:13:10tracing out the dense and space-time
01:13:13of those supermassive objects.
01:13:15A long-term goal
01:13:17is to probe back further
01:13:19towards what we think of
01:13:21as the Big Bang,
01:13:22the earliest moments
01:13:23that we understand
01:13:25of the universe as we know it.
01:13:42If you think about it,
01:13:44time and time again
01:13:45in the history of science,
01:13:46unlocking the mysteries of gravity
01:13:48have led to a deeper understanding
01:13:50of the universe.
01:13:52Galileo and his ramp,
01:13:53Newton and his apple,
01:13:55Einstein and the falling man
01:13:56and the lift.
01:13:57Each of these characters
01:13:58challenged the scientific consensus
01:14:01of the day.
01:14:03And even today,
01:14:05understanding the true nature of gravity
01:14:07remains one of the biggest challenges
01:14:09in science.
01:14:12Which brings me back
01:14:14to the smartphone app.
01:14:16And it's at this point
01:14:17that our story,
01:14:19for me at least,
01:14:20takes a completely unexpected turn.
01:14:24Unfortunately,
01:14:25it's all gone a bit pear-shaped.
01:14:28Okay, so here's what's happened.
01:14:31A couple of months ago,
01:14:32we launched the app
01:14:33and it was all going really well.
01:14:35Thousands of people downloaded it
01:14:37and have been sending us their results.
01:14:39We've been collecting the data
01:14:41to create this nationwide map
01:14:44to show how time flows
01:14:46at different rates
01:14:47for different people around the country.
01:14:50Unfortunately,
01:14:50I've just realised
01:14:52there's a big problem.
01:14:57You see,
01:14:58I was going over
01:14:59the scientific literature
01:15:00and I came across
01:15:02this subtle point
01:15:03about relativity
01:15:04which basically made me
01:15:06sit bolt upright.
01:15:07There was this horrible,
01:15:09dawning realisation
01:15:10that I made a mistake
01:15:12in the equations
01:15:13that get fed into the app.
01:15:16So what this means is
01:15:17all the results
01:15:18we've been gathering
01:15:19are wrong.
01:15:25The issue lies
01:15:27in the strange
01:15:27and subtle effects
01:15:29of Einstein's theories
01:15:30of relativity
01:15:30and it's fundamental
01:15:32to the way time flows
01:15:34across the surface
01:15:36of the globe.
01:15:37Now, what if I use
01:15:39my smartphone app
01:15:40where I live here
01:15:41on the south coast of England
01:15:43and then go and spend
01:15:44a few days
01:15:45down near the equator,
01:15:46say here on
01:15:47the west coast of Africa?
01:15:52Now, we know
01:15:53from the road trip
01:15:53that gravity is weaker
01:15:55by the equator.
01:15:58So that means
01:15:59time ticks faster there.
01:16:02But there's another
01:16:03important factor
01:16:04we have to take into account.
01:16:06Movement.
01:16:08You see,
01:16:09when I'm here
01:16:09near the equator,
01:16:10I'm moving more quickly
01:16:12than I was back in Britain
01:16:13because of the rotation
01:16:15of the earth.
01:16:16Einstein says
01:16:17movement slows down time
01:16:19so clocks will tick
01:16:21slower at the equator.
01:16:23This is where
01:16:24the error crept in.
01:16:25You see,
01:16:26I had taken into account
01:16:27these two effects,
01:16:28but I'd missed
01:16:29a crucial point.
01:16:30They cancel each other out
01:16:32exactly.
01:16:33In fact,
01:16:34the earth bulges out
01:16:36exactly the right amount
01:16:38for its rotational speed
01:16:40to make sure
01:16:41they cancel out.
01:16:42So all clocks
01:16:44on the surface of the earth
01:16:45at sea level
01:16:46tick at exactly
01:16:47the same rate.
01:16:49So now I'm having to go
01:16:51right back to square one
01:16:53and completely rewrite
01:16:54the equations for the app.
01:17:02And to test if it's working,
01:17:04I'm going to use it
01:17:05over the course
01:17:06of a normal working week.
01:17:08This is where I live.
01:17:10This is Portsmouth,
01:17:11which means I'm very close
01:17:13to sea level.
01:17:14And this is how I start
01:17:15most mornings,
01:17:16catching the train to work.
01:17:19The app records my speed
01:17:21as I'm on the train
01:17:24and calculates
01:17:25how this slows down
01:17:26my personal clock.
01:17:28I think the train journey
01:17:30should have slowed
01:17:31my time down
01:17:33by a tiny,
01:17:34a few trillionths
01:17:35of a second.
01:17:36I'm heading for the BBC's
01:17:38headquarters
01:17:39in central London
01:17:40and gravity should be
01:17:42a bit weaker here.
01:17:43I'm a few metres
01:17:44above sea level,
01:17:44I guess, here,
01:17:45and so there'll be
01:17:45a speed up of my time
01:17:47because of altitude.
01:17:49The app compares
01:17:51the way my time flows
01:17:52with a stationary clock
01:17:53at sea level.
01:17:55So what's my result?
01:17:57On an average day,
01:17:59my movement
01:17:59makes me age slower
01:18:01by a third of a nanosecond.
01:18:03That's a third
01:18:04of a billionth of a second.
01:18:07But the weaker gravity
01:18:09I'm in
01:18:09means I age faster.
01:18:12Overall,
01:18:13half a nanosecond faster.
01:18:16I've also given the app
01:18:17to some other volunteers
01:18:18to compare how they age
01:18:20over an average day.
01:18:23Nick flies cargo planes.
01:18:26He flies from Chicago
01:18:28to Germany.
01:18:34Tomorrow morning,
01:18:36we have to leave
01:18:37to go first to Milan
01:18:40and then on to Tokyo.
01:18:42His travel slows down
01:18:45his ageing.
01:18:46But much weaker gravity
01:18:48at high altitude
01:18:49speeds his clock up
01:18:51by just a bit more.
01:18:53Overall,
01:18:54he's ageing
01:18:54five nanoseconds
01:18:56faster than a stationary
01:18:57clock at sea level.
01:18:59Vanessa runs a pub
01:19:01in the Yorkshire Dales.
01:19:03I'm going to take you
01:19:04outside to see
01:19:04the weather conditions here.
01:19:06So here we are
01:19:07outside the Tanhill Inn.
01:19:08We live right in the middle
01:19:10of the National Park
01:19:10on the Moor.
01:19:12The Tanhill Inn
01:19:12is famous
01:19:13as Britain's highest
01:19:15altitude pub
01:19:16at over 500 metres
01:19:17above sea level.
01:19:19We don't have
01:19:19any neighbours.
01:19:20We just have sheep.
01:19:22Her altitude
01:19:23means she ages faster
01:19:25every day
01:19:25by around
01:19:26four nanoseconds
01:19:27compared to someone
01:19:29at sea level.
01:19:31There's Kevin,
01:19:32a mountaineer
01:19:33in the Highlands.
01:19:34I'm on a mountain
01:19:35in Glencoe
01:19:36called Skorna Hulia.
01:19:37I've been at an altitude
01:19:38generally of between
01:19:392,000 to 3,000 feet
01:19:41for a lot of the day.
01:19:42Throughout the day
01:19:43I've just been logging
01:19:44onto the phone,
01:19:44logging onto the app
01:19:45and just checking it out
01:19:47and having a look
01:19:47and I've been watching it
01:19:48get bigger,
01:19:49watching the value
01:19:50get bigger and bigger.
01:19:51So it's been
01:19:52quite a lot of fun.
01:19:54On an average day
01:19:55of climbing,
01:19:56Kevin's personal clock
01:19:57goes faster
01:19:58by one nanosecond.
01:20:03Gary works
01:20:04for a Scottish
01:20:04water retailer.
01:20:06My job takes me
01:20:07all over the UK
01:20:08dealing with
01:20:09energy consultants
01:20:10and energy brokers.
01:20:12As far up north
01:20:13as Inverness,
01:20:14as far down south
01:20:15as London.
01:20:16Approximately do
01:20:16about 1,000 miles
01:20:17a week,
01:20:18sometimes more
01:20:19depending on
01:20:19the number of meetings
01:20:20I have.
01:20:22Gary's car journeys
01:20:23do slow his time
01:20:24down a bit
01:20:25but being above sea level
01:20:27means he still
01:20:28ages faster
01:20:29by three quarters
01:20:30of a nanosecond.
01:20:32Our final volunteer
01:20:34is Walter.
01:20:35He lives close to sea level
01:20:37at the iconic
01:20:38John O'Groats.
01:20:40I run a tourism business
01:20:42and I started
01:20:43about 50 years ago
01:20:44so when people
01:20:45come here
01:20:46they can actually
01:20:47physically speak
01:20:47to someone
01:20:48who's been born
01:20:49in John O'Groats
01:20:49and if they ask
01:20:51questions I can tell
01:20:51them all sorts
01:20:52of useless information
01:20:53because I'm fully
01:20:54useless information.
01:20:56So our final results
01:20:58show that
01:20:58if you want to
01:20:59age more slowly
01:21:01try to live
01:21:02near sea level
01:21:03like Walter.
01:21:06Or there is
01:21:07another way
01:21:08to do it.
01:21:08Get a job
01:21:09on the
01:21:10International Space Station.
01:21:12Its 17,000 mile
01:21:14an hour orbit
01:21:15will give you
01:21:16a boost.
01:21:18We did the math
01:21:20for the astronauts.
01:21:21Every month
01:21:22you are about
01:21:23one millisecond
01:21:25younger.
01:21:26So you know
01:21:27a thousandth
01:21:28of a second.
01:21:28So after six months
01:21:29you are
01:21:31that much younger
01:21:32than people on Earth.
01:21:33So I'm younger
01:21:34than I should be.
01:21:35I hope I look it.
01:21:37Of course
01:21:38for us on Earth
01:21:39time dilation
01:21:41is so utterly
01:21:42minuscule
01:21:43a few billionths
01:21:44of a second
01:21:44between us
01:21:45you might think
01:21:46it's too frivolous
01:21:48to even bother about.
01:21:51And yet
01:21:52in the long
01:21:53and difficult
01:21:54process
01:21:55of designing
01:21:55the app
01:21:56I've come
01:21:57to an extraordinary
01:21:58conclusion.
01:22:00The different
01:22:00ways that
01:22:01time flows
01:22:02may not be
01:22:03some quirky
01:22:04byproduct
01:22:05of gravity.
01:22:06It may actually
01:22:08be gravity.
01:22:10It may be
01:22:11the cause
01:22:11of gravity.
01:22:13The reason
01:22:14why objects
01:22:15fall.
01:22:18One of the
01:22:19colleagues
01:22:19I've been
01:22:19consulting
01:22:20is Kip Thorne.
01:22:21He's one
01:22:22of the world's
01:22:22leading theoretical
01:22:23physicists
01:22:24and a driving
01:22:25force behind
01:22:25the creation
01:22:26of LIGO.
01:22:27While I was
01:22:28going back
01:22:28over some
01:22:29of the basic
01:22:30physics
01:22:30behind the
01:22:31app
01:22:31I came across
01:22:32an intriguing
01:22:33idea of his.
01:22:34It's a very
01:22:35interesting
01:22:36and different
01:22:36way of
01:22:37describing
01:22:38gravity.
01:22:41This is
01:22:42what Kip
01:22:43says.
01:22:45Everything
01:22:45likes to
01:22:46live
01:22:46where it'll
01:22:47age the
01:22:48most slowly
01:22:49and gravity
01:22:50pulls it
01:22:51there.
01:22:53Kip's
01:22:54based at
01:22:54Caltech
01:22:54in California
01:22:55and is
01:22:56one of the
01:22:57most respected
01:22:58theoretical
01:22:58physicists
01:22:59in the
01:22:59world.
01:23:00Firstly
01:23:01Kip,
01:23:01a serious
01:23:01thank you
01:23:02for helping
01:23:03out with
01:23:03the
01:23:04debacle
01:23:05over the
01:23:05app.
01:23:07Well
01:23:07I sympathize.
01:23:08I've made
01:23:09so many errors
01:23:09of my own
01:23:10over the
01:23:11years that
01:23:11I am
01:23:12totally
01:23:13sympathetic.
01:23:13One of
01:23:14the things
01:23:15that struck
01:23:16me thinking
01:23:18about this
01:23:18is something
01:23:19you wrote
01:23:19Kip.
01:23:20You said
01:23:20everything
01:23:21likes to
01:23:22live where
01:23:22it'll age
01:23:23the most
01:23:23slowly
01:23:24and gravity
01:23:25pulls it
01:23:26there.
01:23:27Was this
01:23:28a way of
01:23:28explaining
01:23:29something that
01:23:30you felt
01:23:30was a neat
01:23:32explanation or
01:23:32is there
01:23:33something
01:23:33deeply
01:23:34profound
01:23:34about that?
01:23:35I think
01:23:35there is
01:23:36something
01:23:36deeply
01:23:37profound
01:23:37in some
01:23:38sense.
01:23:39but it's
01:23:40a lovely
01:23:42description
01:23:43of
01:23:44Einstein's
01:23:45first major
01:23:46insight
01:23:47about gravity
01:23:48in 1912.
01:23:50He realized
01:23:51that gravity
01:23:52that we feel
01:23:54on Earth
01:23:54is due to
01:23:55a slowing
01:23:56of time
01:23:56on Earth.
01:23:57So time
01:23:58comes before
01:23:58gravity in that
01:23:59sense.
01:24:00On the Earth's
01:24:01surface time
01:24:01runs more
01:24:02slowly and
01:24:03that accounts
01:24:04for why
01:24:04gravity wants
01:24:05to keep us
01:24:05there.
01:24:06I think
01:24:06in a very
01:24:07deep sense
01:24:07this is true.
01:24:08Objects want
01:24:09to fall
01:24:10that the
01:24:11flow of time
01:24:12or the rate
01:24:12of flow
01:24:13of time
01:24:13is the thing
01:24:14that produces
01:24:15the gravity.
01:24:16It is the thing
01:24:17that is ultimately
01:24:18responsible
01:24:18for the fall.
01:24:20So somehow
01:24:22it's in the nature
01:24:23of all objects
01:24:24to move
01:24:24towards a region
01:24:25where time
01:24:27runs slower.
01:24:29Kipp's formulation
01:24:30works anywhere
01:24:31in the universe
01:24:31where the
01:24:32gravitational field
01:24:33is such as
01:24:34on Earth.
01:24:36The difference
01:24:37in the rate
01:24:37of flow
01:24:38of time
01:24:38is tiny.
01:24:40At high altitude
01:24:40and on the
01:24:41surface of the
01:24:42Earth
01:24:42the difference
01:24:43in the rate
01:24:43of flow
01:24:44of time
01:24:44is one second
01:24:45in 100 years.
01:24:48That's not
01:24:48very much
01:24:49but that is
01:24:51enough
01:24:51that is precisely
01:24:52the right amount
01:24:54to produce
01:24:55the gravitational
01:24:56pull that we
01:24:57feel
01:24:58and produce
01:24:58the accelerations
01:24:59we're talking
01:25:00about.
01:25:01Wow.
01:25:02Okay.
01:25:02I need to go
01:25:03and write this
01:25:03one down.
01:25:08So my
01:25:09investigation
01:25:10deep into
01:25:11the weird
01:25:11waves of gravity
01:25:12has finally
01:25:14left me
01:25:14face to face
01:25:15with one
01:25:16of the greatest
01:25:16mysteries
01:25:17in all of
01:25:18physics
01:25:18the nature
01:25:20of time
01:25:21itself.
01:25:22It sounds
01:25:23like such
01:25:24a simple
01:25:24question
01:25:25why does
01:25:26the apple
01:25:26fall
01:25:27and yet
01:25:28hundreds of
01:25:28years of
01:25:29scientific
01:25:29inquiry
01:25:30investigating
01:25:31this single
01:25:32action
01:25:33have led
01:25:33us to
01:25:34completely
01:25:34redefine
01:25:35the way
01:25:36we think
01:25:36about the
01:25:37very nature
01:25:37of space
01:25:38and time
01:25:40and now
01:25:41I've been
01:25:41presented
01:25:41with this
01:25:42extraordinary
01:25:42proposition
01:25:43that somehow
01:25:44in some
01:25:45profound way
01:25:46the apple
01:25:47falls because
01:25:49it's seeking
01:25:49out the place
01:25:50where time
01:25:51runs the
01:25:51slowest
01:25:52so does
01:25:54gravity
01:25:55dictate
01:25:55the flow
01:25:56of time
01:25:56or does
01:25:58time
01:25:58itself
01:25:58define
01:25:59gravity
01:26:00could this
01:26:02hint to
01:26:02fundamental
01:26:03new laws
01:26:04of physics
01:26:04as yet
01:26:05undiscovered
01:26:05I think
01:26:07I'm going to
01:26:07have to think
01:26:07about this
01:26:08a bit
01:26:08more
01:26:37news
01:26:47when
01:26:48you
01:26:48you
01:26:49you
01:26:49you
01:26:49you
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