- 1 week ago
Dark matter, which makes up about 85% of the matter in the universe, is invisible because it does not interact with light. At the frontiers of quantum science, physicists propose that this "missing" matter isn't new space debris, but rather undiscovered subatomic particles.....
已知夸克构成的独特暗物质——产生于大爆炸期间...
已知夸克构成的独特暗物质——产生于大爆炸期间...
Category
📚
LearningTranscript
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.
00:43:32One.
00:43:32One.
00:43:37One.
00:43:38Everybody'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 seven with no release until you're in space.
00:43:52and then suddenly you're free from gravity you are you're weightless in orbit canadian astronaut
00:43:59chris hadfield spent five months on board the international space station you can pull your
00:44:06knees up to your chest and just tumble or if you take a wet cloth and you get a dripping
00:44:13red and
00:44:13everybody on earth knows what'll happen when you wring it out all the water will fall inevitably
00:44:18if you do that in weightlessness the water stays there and it actually because of the surface
00:44:24tension starts crawling up your arms it's a little bit mesmerizing and hypnotic to be in weightlessness
00:44:36if you're weightless you don't need a bed you don't need a mattress you don't need a pillow
00:44:42your body is floating completely suspended like magic
00:44:50movement becomes effortless you can push off with one finger and and fly and tumble you don't need
00:44:57to hold yourself where you are with with muscle you can just with the delicate fingertip pressure
00:45:03you can stay where you are okay separation confirmed timer is on backing away at a rate of just a
00:45:10little over one-tenth of a meter per second re-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 crushing you down into
00:45:28the
00:45:28floor of the ship for quite a long time then of course you hit the ground and tumble and roll
00:45:34to
00:45:34a stop and and now you're the victim of your past you're the the victim of your decision making lying
00:45:43there trying to shake your head and get used to being in gravity again i remarked at the time that
00:45:49i
00:45:49had forgotten that my lips have weight and my tongue has weight you don't think about it but if you
00:45:56try and
00:45:56talk articulately standing on your head you'll notice that you have to sort of control your lips and
00:46:01your tongue a little differently just because gravity's pushing them the other way and it's the
00:46:05same sort of thing raising your arm holding your head up about turning your head when everything wants
00:46:10to tumble uh just keeping your balance all of those things it's a little bit like like relearning to walk
00:46:19again like like an infant gravity shapes our bodies and molds our planet nothing happens on earth without
00:46:31its power and influence sir 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
00:46:47spurring on the industrial revolution but in the first decade of the 20th century the next genius
00:46:55in our story challenged the very foundations of our understanding of gravity a young german scientist
00:47:03called albert einstein was churning something over in his mind he thought that something in newton's
00:47:11laws didn't quite add up
00:47:23imagine i'm the sun and this tennis ball is the earth in orbit around me newton's laws can describe
00:47:30very precisely the path the earth takes around the sun in terms of the mutual gravitational attraction
00:47:36between the two bodies but what newton can't explain is what connects them in reality of course there is
00:47:44no invisible string between the earth and the sun holding the two together there's just empty space
00:47:50a complete void and yet according to newton the earth and sun pull on each other instantaneously
00:47:57across a vast distance how can gravity act in this way when there's nothing to connect it or transmit it
00:48:08after years puzzling over this einstein had a blinding flash of inspiration
00:48:15just like galileo and his ramp or newton with his apple einstein's breakthrough came because he was
00:48:23thinking about one simple action what 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
00:48:58what 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
00:49:08what if then the lift cable breaks and i start falling what are the forces that i will feel as
00:49:16i'm
00:49:16plummeting to the ground
00:49:25well i'm not going to try that
00:49:30fortunately there's another way to test this without me having to plunge down a lift shaft
00:49:36sorry to disappoint you this little device here that i've strapped to this plastic toy is an
00:49:44industrial accelerometer so it measures acceleration now i've got it connected to my laptop and it's
00:49:50showing a measurement of 1g now that's the downward acceleration due to the pull of earth's gravity so
00:49:57basically it works just like a gravity meter but what happens if i were to drop it presumably it'll carry
00:50:04on measuring 1g because it's falling in earth's gravity okay well let's try that and see
00:50:27so you can see here along this line at the bottom that's when i was holding it still and it's
00:50:34measuring an acceleration of 1g
00:50:36these oscillations here is when i stood up and a bit of disturbance but this spike along here is
00:50:43the moment i released it and this short duration along here is the time it was falling and you see
00:50:50while it was falling it was registering an acceleration of zero now if you think about it this is really
00:50:58odd the accelerometer is accelerating downwards it's plummeting in the full grip of earth's gravity
00:51:05and yet it's measuring no acceleration at all it's as though gravity has completely disappeared
00:51:15einstein's insight was that when something falls it 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 what we now call his principle of equivalence
00:51:36einstein's point is that when the man in the lift falls he doesn't just feel weightless he is weightless
00:51:44einstein said the man feels no force pulling on him because there is no force pulling on him gravity
00:51:51doesn't act on him it acts on the space and time around him what we now call the geometry of
00:51:58space-time
00:52:05this was a radical redefinition einstein says forget the idea of gravity as a force acting mysteriously
00:52:14between two objects now 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 the three-dimensional space is
00:52:31the stage in which things happen but time is separate it ticks by at the same rate everywhere in the
00:52:38universe
00:52:39according to newton an object would travel through space in a straight line unless acted upon by a force
00:52:45like gravity that will cause it to deviate from that path but einstein said that space and time
00:52:52aren't fixed and immutable they're interconnected mesh together in what is known as space-time
00:53:01and he said that space-time can be warped that matter curves space and time around it
00:53:14so after einstein we no longer see gravity as 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 which is as straight as possible through that space-time
00:53:39it's a fundamental part of einstein's vision of reality space and time can't be disentangled
00:53:46you can't talk about space separately from time so matter warps time as well as space
00:53:58it's known as gravitational time dilation and it's possibly the strangest of all of einstein's discoveries
00:54:10i've got two identical clocks here now because the clock lower down is closer to the center of the earth
00:54:17it feels ever so slightly a stronger gravitational pull than the clock higher up einstein's theory says
00:54:25that the lower clock will tick by at a slightly slower rate than the higher clock basically gravity slows time
00:54:34down
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 einstein had a rather nice way of explaining it
00:55:00most of us have had the experience as children of trying to work out what our parents do for a
00:55:05living
00:55:05well imagine your father is elbert einstein when he was about 12 years old young edward einstein asked
00:55:11his father why he was so famous what he discovered well this put einstein's senior on the spot but he
00:55:17came up with a beautifully simple analogy einstein told his son when a blind beetle crawls over the
00:55:28surface of a curved branch it doesn't notice that the track it has covered is curved i was lucky enough
00:55:35to
00:55:35notice what the beetle didn't notice this is what einstein meant the beetle is free to move in any
00:55:43direction on the branch it can move forwards backwards left and right but it has no concept of a direction
00:55:49up off
00:55:50the branch it'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
00:55:59but we can see that the surface it's walking on is itself curving and twisted
00:56:09einstein's point was that what we see as the twists and curves of the branch feel to the beetle like
00:56:16forces pushing and pulling it okay so consider this rather strange example imagine we have two beetles
00:56:25perched on this pumpkin and for whatever reason they want to walk up towards the top
00:56:30now if they start at the equator pointing due north as they walk they will begin by moving parallel to
00:56:39each other that means their paths should never meet but as they get closer to the top their paths get
00:56:47closer together now if they're clever beetles they might try and figure out what's going on and they
00:56:52could imagine that there's some mysterious force that's pulling them closer together but for us from
00:56:59our perspective we can see there is no such force all they're doing is following straight paths over a
00:57:05curved surface just as the beetles have no sense that the surface of the branch is curved
00:57:16we completely failed to perceive the bizarre ways that 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 realized that there was a way by looking far out into the solar system
00:57:40incredibly here in the grounds of hurst monceau castle is 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 it was just that it was a theory it had
00:57:59no proof in fact many people found it completely outlandish but then just four years later in 1919
00:58:06this telescope and allow me to geek out a bit here and i'll give it its correct name this is
00:58:12the 13
00:58:12inch astrographic refractor this telescope proved that einstein was in fact right that gravity does curve
00:58:21space itself
00:58:29since then observation after observation have confirmed that matter curves space
00:58:34and slows down time
00:58:39so the simple question of why things fall the way they do has led us deeper and deeper into the
00:58:45very nature of space and time itself gravitational science shows us how galaxies stars and planets
00:58:54form by measuring gravity we've discovered the existence of dark matter that eighty percent of the
00:59:01mass of our universe is invisible and 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 yet are only 20 kilometers across
00:59:21but it's another mysterious aspect of einstein's universe that i want to explore in my next gravity project
00:59:31here at the university of surrey some colleagues and i have been working on it for months
00:59:37what we're doing is devising a nationwide citizen science project we're developing a smartphone app
00:59:44that uses the gps contained on your phone to explore one of the strangest properties of gravity
00:59:50how it affects the rate at which we age
00:59:53i've gained through speed i formulated the equations myself
00:59:59and a small team of computer scientists and software developers is using them to devise the app
01:00:11einstein discovered that as gravity changes so does the rate that time ticks
01:00:18this means the strength of gravity you feel affects how quickly or slowly you age
01:00:28the aim of my app is to demonstrate this effect
01:00:32it works by using a phone's gps data to 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 which are based on einstein's theory of relativity
01:00:54to calculate overall how fast or slowly you're aging
01:01:01once the app is ready i tweet about it
01:01:07thousands of people download it and we start to gather results from across the country
01:01:13some people send me videos giving me their results
01:01:18how fast they're aging compared with how time ticks out in space in zero gravity
01:01:25over the past day i have aged less by about 172 microseconds i have aged less by 10 10.02
01:01:34milliseconds
01:01:35uh so since downloading the app i have aged less by 1.14 milliseconds since opening time warp i have
01:01:45aged less by 2.6 milliseconds
01:01:50our aim is to use their results to build up a map of how time flows because of gravity
01:01:58my smartphone project provides just one insight into the space and time which heinstein's theories describe
01:02:20gravity and its strange ways have given us astonishing insights into the dark secrets of our universe
01:02:29perhaps the weirdest objects in the universe are black holes collapsed stars whose gravity is so strong
01:02:36that not even light can escape their grip now for the first time ever their effects have been felt on
01:02:44earth
01:02:45and they've been detected through the medium of gravity itself
01:02:51it's a story that has revolutionized the study of modern cosmology
01:02:591.3 billion years ago in a galaxy far far away two black holes swirled around each other drew closer
01:03:08and
01:03:08closer together until they finally collided with incredible violence in that final fraction of a
01:03:15second at the precise moment that they merged a disturbance was created that sent ripples out through the universe
01:03:25gravitational waves are a key prediction of einstein's theory
01:03:31matter matter doesn't just curve space-time it can cause waves ripples which expand outwards exactly like a stone dropped
01:03:40in water
01:03:43this particular wave was unimaginably large
01:03:47the energy released was greater than all the light being given out by all the stars in the universe
01:03:55the wave rippled through space at the speed of light in 1.3 billion years it covered a distance of
01:04:03over 10
01:04:04billion trillion kilometers
01:04:15until on the morning of the 14th of september 2015 it arrived here
01:04:24the streets and cafes of new orleans in fact everything in america and on earth
01:04:32expanded and contracted very very slightly as the wave passed through
01:04:39no one noticed as by the time it arrived here the distortion was phenomenally tiny
01:04:49except that one science laboratory did notice and i'm going to see it
01:05:00a thousand scientists across the world are collaborating on it
01:05:07it's the culmination of over 50 years of effort and is one of the most sophisticated experiments ever devised by
01:05:15humanity
01:05:18so i'm pretty excited to see it
01:05:22it's a rather unusual setting here i am in the middle of rural louisiana about an hour's drive outside new
01:05:28orleans
01:05:29i don't expect to find such a multi-million dollar cutting-edge research facility as this
01:05:35and yet this is the place where recently one of the most important scientific discoveries in human history was made
01:05:42this is ligo
01:05:46the laser interferometer gravitational wave observatory is an enormous construction shaped like an l
01:05:56of the world
01:05:56with a sophisticated laser system bouncing up and down the two arms
01:06:02so we're standing on top of one of ligo's two arms this is the first ligo arm and in that
01:06:07tube we
01:06:08there's a laser beam that we bounce back and forth between a mirror in the end station and a mirror
01:06:13in this building
01:06:14and the other bit goes that way four kilometers perpendicular to the arm we first saw
01:06:18this is the l shape it's a big l on the ground so the light bounces back and forth in
01:06:22that arm
01:06:23and bounces back and forth in this arm and what we actually measure with ligo is the length of this
01:06:28arm
01:06:29as measured by the light between the two mirrors
01:06:32and the length of that arm as measured by the light between two mirrors
01:06:35and then the laser interferometer measures the difference between those two arm lengths
01:06:41so as the gravitational wave passed through the lasers picked it up they detected that ligo's two arms
01:06:48changed in length to a very very tiny degree
01:06:54the signal that we saw was just a few thousandth of the size of the um of the atomic nucleus
01:07:02is the
01:07:02biggest the signal ever got so far far smaller than the size of a single atom oh much much smaller
01:07:09yeah
01:07:09that one and you need something this huge to pick that up that's right and so this is one of
01:07:14the most
01:07:14biggest biggest source of energy in the universe one of the biggest events you'd ever measure
01:07:18and we just barely saw it
01:07:25the ligo scientists turned the gravitational waves into sound waves so what you're about to hear
01:07:32is in a very real sense the sound of two black holes colliding
01:07:44it was the first observation of any kind of pairs of stellar mass black holes stellar mass means you
01:07:52know several or a bunch of of sons in weight um and and so we learned that they exist we
01:07:59learned that
01:07:59there are enough of them that occasionally they run into each other and coalesce um and um and we also
01:08:06learned by comparing the waveform we observed with general relativity calculations that general
01:08:11relativity is is as far as we know dead on right
01:08:25the long concrete bunker to my left houses the beam line one of the ligo's laser arms
01:08:33the detail and and the effort that's gone into isolating the beam from the outside environment
01:08:40reminds me very much of cavendish's famous experiment he too had to worry about isolating his his
01:08:46experiment from external disturbances only of course ligo takes things to a far far greater degree
01:08:53inside the arm is one of the largest and purest vacuums in the world
01:08:59atmospheric pressure in there has been reduced to one trillionth of the pressure outside
01:09:06the mirrors inside are so reflective that they only absorb one in three million photons
01:09:13and at the end of my little trip lies a british success story
01:09:24well i made it all the way to the end of one of the ligo arms to be honest it
01:09:29took me a bit longer
01:09:29than i thought especially in that thing but housed inside this building is one of the reflecting mirrors
01:09:36that bounces the laser beam all the way back down the four kilometer arm to the main control center
01:09:42and the technology that went into developing these mirrors is quite remarkable it was developed in the uk
01:09:48at the university of glasgow
01:09:57this is what the mirror looks like its surface is extraordinarily smooth no bump bigger than a few
01:10:05billionths of a meter high equally amazing are these fused silica fibers a few times the thickness of a human
01:10:16hair
01:10:19designed by the university of glasgow in conjunction with scientists from other british universities
01:10:26they isolate the mirror completely so it hangs perfectly still you could say that in there is the quietest place
01:10:36on earth
01:10:38despite this outside events do sometimes interfere with the work here as i witnessed for myself
01:10:45i've wandered into the control room here at ligo because i'm told something kicked off a few hours ago
01:10:51and they're all very busy the image that's flickering up there is not meant to be like that essentially
01:10:59what they've picked up is a seismic disturbance an earthquake now that's not an earthquake down the road
01:11:05it started on the other side of the planet in japan so it just gives us a sense of the
01:11:11tremendous
01:11:12challenges faced by ligo and the team here and the level of sensitivity needed that an earthquake on the
01:11:19other side of the earth can disrupt their measurements and they have to reset everything all over again
01:11:28one of the scientists involved in developing this extraordinary place put it quite succinctly
01:11:34once we were blind but now we can see
01:11:40throughout the entire history of astronomy we've studied gravity and how it affects matter in the
01:11:46universe and how it warps space-time but only by looking at the light that enters our telescopes
01:11:53now for the first time we can study the universe in a different way the discovery of gravitational waves
01:11:59means we can see objects that cause extreme warping of space-time and its effect on gravity directly
01:12:07this essentially opens up a new era in astronomy it gives us a new way of looking out at the
01:12:13universe
01:12:17on google
01:12:18professor sheila rowan was one of the scientists who spearheaded the british effort for ligo
01:12:22q
01:12:24for her and her colleagues gravitational wave detection is just in its infancy
01:12:30new instruments even more sensitive than ligo are now being developed
01:12:36there's so much that we don't understand about the universe that we live in
01:12:41And this has suddenly given us a new tool, a new way to probe the dark processes in the universe.
01:12:49Because every time we make the observatories more sensitive,
01:12:53we can sense gravitational wave signals from further away,
01:12:58from farther out in the universe, from further back in cosmic history.
01:13:02Things like supermassive black holes spiralling into collide,
01:13:06small black holes orbiting round supermassive black holes, tracing out the dense and space-time of those supermassive objects.
01:13:15A long-term goal is to probe back further towards what we think of as the Big Bang,
01:13:22the earliest moments that we understand of the universe as we know it.
01:13:42If you think about it, time and time again in the history of science,
01:13:46unlocking the mysteries of gravity have led to a deeper understanding of the universe.
01:13:52Galileo and his ramp, Newton and his apple, Einstein and the falling man in the lift.
01:13:57Each of these characters challenged the scientific consensus of the day.
01:14:02And even today, understanding the true nature of gravity remains one of the biggest challenges in science.
01:14:12Which brings me back to the smartphone app.
01:14:16And it's at this point that our story, for me at least, takes a completely unexpected turn.
01:14:24Unfortunately, it's all gone a bit pear-shaped.
01:14:28OK, so here's what's happened.
01:14:31A couple of months ago, we launched the app.
01:14:33And it was all going really well.
01:14:35Thousands of people downloaded it and have been sending us their results.
01:14:39We've been collecting the data to create this nationwide map
01:14:44to show how time flows at different rates for different people around the country.
01:14:50Unfortunately, I've just realised there's a big problem.
01:14:57You see, I was going over the scientific literature
01:15:00and I came across this subtle point about relativity
01:15:04which basically made me sit bolt upright.
01:15:07There was this horrible, dawning realisation
01:15:10that I made a mistake in the equations that get fed into the app.
01:15:16So what this means is
01:15:17all the results we've been gathering are wrong.
01:15:25The issue lies in the strange and subtle effects
01:15:29of Einstein's theories of relativity
01:15:30and it's fundamental to the way time flows across the surface of the globe.
01:15:37Now, what if I use my smartphone app
01:15:40where I live here on the south coast of England
01:15:43and then go and spend a few days down near the equator,
01:15:46say here on the west coast of Africa?
01:15:52Now, we know from the road trip that gravity is weaker by the equator.
01:15:58So that means time ticks faster there.
01:16:02But there's another important factor we have to take into account.
01:16:06Movement.
01:16:08You see, when I'm here near the equator,
01:16:10I'm moving more quickly than I was back in Britain
01:16:13because of the rotation of the Earth.
01:16:16Einstein says movement slows down time
01:16:19so clocks will tick slower at the equator.
01:16:23This is where the error crept in.
01:16:25You see, I had taken into account these two effects,
01:16:28but I'd missed a crucial point.
01:16:30They cancel each other out exactly.
01:16:32In fact, the Earth bulges out exactly the right amount
01:16:38for its rotational speed to make sure they cancel out.
01:16:42So all clocks on the surface of the Earth at sea level
01:16:46tick at exactly the same rate.
01:16:49So now I'm having to go right back to square one
01:16:53and completely rewrite the equations for the app.
01:17:02And to test if it's working,
01:17:04I'm going to use it over the course of a normal working week.
01:17:09This is where I live.
01:17:10This is Portsmouth, which means I'm very close to sea level.
01:17:14And this is how I start most mornings, catching the train to work.
01:17:19The app records my speed as I'm on the train
01:17:24and calculates how this slows down my personal clock.
01:17:28I think the train journey should have slowed my time down
01:17:33by a tiny few trillionths of a second.
01:17:36I'm heading for the BBC's headquarters in central London
01:17:40and gravity should be a bit weaker here.
01:17:43I'm a few metres above sea level, I guess, here,
01:17:45and so there'll be a speed up of my time because of altitude.
01:17:49The app compares the way my time flows
01:17:52with a stationary clock at sea level.
01:17:55So what's my result?
01:17:57On an average day, my movement makes me age slower
01:18:01by a third of a nanosecond.
01:18:03That's a third of a billionth of a second.
01:18:07But the weaker gravity I'm in means I age faster.
01:18:12Overall, half a nanosecond faster.
01:18:16I've also given the app to some other volunteers
01:18:18to compare how they age over an average day.
01:18:23Nick flies cargo planes.
01:18:26He flies from Chicago to Germany.
01:18:34Tomorrow morning, we have to leave to go first to Milan
01:18:40and then on to Tokyo.
01:18:43His travel slows down his ageing.
01:18:46But much weaker gravity at high altitude
01:18:49speeds his clock up by just a bit more.
01:18:53Overall, he's ageing five nanoseconds faster
01:18:56than a stationary clock at sea level.
01:18:59Vanessa runs a pub in the Yorkshire Dales.
01:19:03I'm going to take you outside
01:19:04to see the weather conditions here.
01:19:05So here we are outside the Tanhill Inn.
01:19:08We live right in the middle of the National Park
01:19:10on the moor.
01:19:12The Tanhill Inn is famous
01:19:13as Britain's highest altitude pub
01:19:16at over 500 metres above sea level.
01:19:19We don't have any neighbours.
01:19:20We just have sheep.
01:19:22Her altitude means she ages faster every day
01:19:25by around four nanoseconds
01:19:27compared to someone at sea level.
01:19:31There's Kevin, a mountaineer in the highlands.
01:19:34I'm on a mountain in Glencoe called Skorna Hulia.
01:19:37I've been at an altitude generally
01:19:39of between 2,000 to 3,000 feet
01:19:41for a lot of the day.
01:19:42Throughout the day,
01:19:43I've just been logging on to the phone,
01:19:44logging on to the app
01:19:45and just checking it out
01:19:47and having a look
01:19:47and I've been watching it get bigger,
01:19:49watching the value get bigger and bigger.
01:19:51So it's been quite a lot of fun.
01:19:54On an average day of climbing,
01:19:56Kevin's personal clock
01:19:57goes faster by one nanosecond.
01:20:03Gary works for a Scottish water retailer.
01:20:06My job takes me all over the UK
01:20:08dealing with energy consultants
01:20:10and energy brokers.
01:20:12As far up north as Inverness,
01:20:14as far down south as London.
01:20:16Approximately do about 1,000 miles a week,
01:20:18sometimes more,
01:20:19depending on the number of meetings I have.
01:20:22Gary's car journeys
01:20:23do slow his time down a bit,
01:20:25but being above sea level
01:20:27means he still ages faster
01:20:29by three quarters of a nanosecond.
01:20:32Our final volunteer is Walter.
01:20:35He lives close to sea level
01:20:37at the iconic John O'Groats.
01:20:40I run a tourism business
01:20:42and I started about 50 years ago.
01:20:44So when people come here,
01:20:46they can actually physically speak
01:20:47to someone who's been born in John O'Groats
01:20:49and if they ask questions,
01:20:51I can tell them all sorts of useless information
01:20:53because I'm fully useless information.
01:20:56So our final results show that
01:20:58if you want to age more slowly,
01:21:01try to live near sea level,
01:21:03like Walter.
01:21:06or there is another way to do it.
01:21:08Get a job on the International Space Station.
01:21:12Its 17,000 mile an hour orbit
01:21:15will give you a boost.
01:21:19We did the math for the astronauts.
01:21:21Every month,
01:21:22you are about one millisecond younger.
01:21:26So, you know,
01:21:27a thousandth of a second.
01:21:28So after six months,
01:21:29you are that much younger
01:21:32than people on Earth.
01:21:33So I'm younger than I should be.
01:21:35I hope I look it.
01:21:37Of course, for us on Earth,
01:21:40time dilation is so utterly minuscule,
01:21:43a few billionths of a second between us,
01:21:45you might think it's too frivolous
01:21:48to even bother about.
01:21:51And yet,
01:21:53in the long and difficult process
01:21:55of designing the app,
01:21:56I've come to an extraordinary conclusion.
01:22:00The different ways that time flows
01:22:02may not be some quirky by-product of gravity.
01:22:06It may actually be gravity.
01:22:09It may be the cause of gravity,
01:22:13the reason why objects fall.
01:22:18One of the colleagues I've been consulting
01:22:20is Kip Thorne.
01:22:21He's one of the world's leading theoretical physicists
01:22:24and a driving force
01:22:25behind the creation of LIGO.
01:22:27While I was going back over
01:22:29some of the basic physics behind the app,
01:22:31I came across an intriguing idea of his.
01:22:34It's a very interesting
01:22:36and different way
01:22:37describing gravity.
01:22:41This is what Kip says.
01:22:44Everything likes to live
01:22:46where it'll age the most slowly.
01:22:49And gravity pulls it there.
01:22:53Kip's based at Caltech in California
01:22:55and is one of the most respected
01:22:58theoretical physicists in the world.
01:23:00Firstly, Kip,
01:23:01a serious thank you
01:23:02for helping out
01:23:03with the debacle over the app.
01:23:07Well, I sympathize.
01:23:08I've made so many errors
01:23:09of my own over the years
01:23:11that I am totally sympathetic.
01:23:14One of the things
01:23:15that struck me
01:23:17thinking about this
01:23:18is something you wrote, Kip.
01:23:20You said,
01:23:21everything likes to live
01:23:22where it'll age the most slowly
01:23:24and gravity pulls it there.
01:23:27Was this a way of explaining
01:23:29something that you felt
01:23:30was a neat explanation
01:23:32or is there something
01:23:33deeply profound about that?
01:23:35I think there is something
01:23:36deeply profound in some sense.
01:23:39But it's a lovely description
01:23:44of Einstein's first major insight
01:23:47about gravity in 1912.
01:23:50He realized that gravity
01:23:52that we feel on Earth
01:23:54is due to a slowing of time on Earth.
01:23:57So time comes before gravity
01:23:59in that sense.
01:24:00On the Earth's surface,
01:24:01time runs more slowly
01:24:03and that accounts for why gravity
01:24:05wants to keep us there.
01:24:06Well, I think in a very deep sense
01:24:07this is true.
01:24:08Objects want to fall
01:24:10that the flow of time
01:24:12or the rate of flow of time
01:24:13is the thing that produces the gravity.
01:24:16It is the thing
01:24:17that is ultimately responsible
01:24:18for the fall.
01:24:20So somehow,
01:24:22it's in the nature of all objects
01:24:24to move towards a region
01:24:25where time runs slower.
01:24:29Kip's formulation works
01:24:30anywhere in the universe
01:24:31where the gravitational field
01:24:33is such as on Earth.
01:24:36The difference in the rate of flow of time
01:24:38is tiny.
01:24:40At high altitude
01:24:40and on the surface of the Earth,
01:24:42the difference in the rate of flow of time
01:24:44is one second in 100 years.
01:24:48That's not very much.
01:24:50But that is enough.
01:24:51That is precisely the right amount
01:24:53to produce the gravitational pull
01:24:57that we feel
01:24:58and produce the accelerations
01:24:59we're talking about.
01:25:01Wow.
01:25:02OK.
01:25:02I need to go and write this one down.
01:25:08So my investigation
01:25:10deep into the weird ways of gravity
01:25:12has finally left me face to face
01:25:15with one of the greatest mysteries
01:25:17in all of physics.
01:25:19The nature of time itself.
01:25:22It sounds like such a simple question.
01:25:25Why does the apple fall?
01:25:27And yet,
01:25:28hundreds of years
01:25:29of scientific inquiry
01:25:30investigating this single action
01:25:33have led us
01:25:34to completely redefine
01:25:35the way we think
01:25:36about the very nature
01:25:37of space and time.
01:25:40And now I've been presented
01:25:41with this extraordinary proposition
01:25:43that somehow,
01:25:45in some profound way,
01:25:47the apple falls
01:25:48because it's seeking out
01:25:49the place
01:25:50where time runs the slowest.
01:25:53So,
01:25:54does gravity dictate
01:25:55the flow of time
01:25:56or does time itself
01:25:59define gravity?
01:26:01Could this hint
01:26:02to fundamental new laws of physics
01:26:04as yet undiscovered?
01:26:06I think I'm going to have to think
01:26:07about this a bit more.
01:26:19like,
01:26:42"'That wayZekyllXX''
01:26:43I've had to wait
01:26:43To know
01:26:43as he yelled
01:26:51Transcription by CastingWords
Comments