Gravity and Me: The Force That Shapes Our Lives

0:00:02 > 0:00:04Would you like to lose some weight without doing any exercise

0:00:04 > 0:00:05or dieting?

0:00:05 > 0:00:09Would you like to age just a bit more slowly than your friends?

0:00:09 > 0:00:11Well, you might be surprised to hear,

0:00:11 > 0:00:12the laws of physics can help.

0:00:15 > 0:00:19The key to unlocking these everyday questions is gravity.

0:00:21 > 0:00:23It sculpts the universe.

0:00:24 > 0:00:26It warps space and time.

0:00:27 > 0:00:29It's a fundamental force of nature.

0:00:32 > 0:00:36But gravity's strange powers, discovered by Albert Einstein,

0:00:36 > 0:00:41also affect our daily lives in the most unexpected ways.

0:00:44 > 0:00:48In this film, we'll be using cutting edge scientific techniques

0:00:48 > 0:00:52to investigate how gravity changes your weight...

0:00:52 > 0:00:54It's gone up.

0:00:54 > 0:00:55..your height...

0:00:55 > 0:00:57I really have shrunk.

0:00:57 > 0:00:58..and even your posture.

0:01:00 > 0:01:02And, with the help of thousands of volunteers,

0:01:02 > 0:01:07I'll show you how gravity makes us all age at different rates.

0:01:09 > 0:01:11Throughout the day I've just been logging on the phone,

0:01:11 > 0:01:13logging on to the app.

0:01:13 > 0:01:16As a physicist, gravity is central to my work.

0:01:16 > 0:01:18Oh, wow!

0:01:18 > 0:01:19And, in exploring it,

0:01:19 > 0:01:23I'll be challenged on how I understand this mysterious force.

0:01:23 > 0:01:26Wow, OK. I need to go and write this one down.

0:01:28 > 0:01:31And I'll have to tackle the very nature of reality itself.

0:01:40 > 0:01:41Gravity.

0:01:44 > 0:01:48It binds together all the matter in the universe

0:01:48 > 0:01:51and it makes our existence here possible.

0:01:54 > 0:01:58But in the end, it all boils down to one simple question.

0:02:00 > 0:02:02What happens if I drop an object?

0:02:06 > 0:02:11Gravity's many mysteries are all contained in this single action.

0:02:11 > 0:02:13How an object falls.

0:02:15 > 0:02:16Here's the first puzzle.

0:02:16 > 0:02:20Why does a hammer fall faster than a feather?

0:02:20 > 0:02:23You might think it's because the hammer is heavier.

0:02:23 > 0:02:26But that's not the real reason.

0:02:27 > 0:02:29The answer is air resistance.

0:02:30 > 0:02:33It's not the weight of the objects that matters, it's their shape.

0:02:33 > 0:02:37And I can demonstrate this very easily with these two umbrellas.

0:02:37 > 0:02:42They both have exactly the same weight, but if I open one of them,

0:02:42 > 0:02:46you can be pretty sure it will drop more slowly than the other one.

0:02:49 > 0:02:52In fact, all objects would fall at the same rate

0:02:52 > 0:02:55if you could only remove the air.

0:02:57 > 0:03:01The first person to realise this was the 16th century mathematician,

0:03:01 > 0:03:03Galileo Galilei.

0:03:04 > 0:03:06Famously, it's said he worked it out

0:03:06 > 0:03:09by dropping objects off the Leaning Tower of Pisa.

0:03:13 > 0:03:16And he was spectacularly proven right

0:03:16 > 0:03:21in an experiment carried out on the moon in 1971.

0:03:21 > 0:03:24In my left hand I have a feather.

0:03:24 > 0:03:27In my right hand, a hammer.

0:03:27 > 0:03:28I'll drop the two of them here,

0:03:28 > 0:03:31and hopefully they'll hit the ground at the same time.

0:03:31 > 0:03:33It worked perfectly.

0:03:33 > 0:03:35How about that?

0:03:35 > 0:03:39It proves that Mr Galilei was correct in his findings.

0:03:43 > 0:03:47Now, Galileo was obsessed with a second question, too.

0:03:47 > 0:03:49When you drop an object,

0:03:49 > 0:03:53it's actually quite hard to tell if it falls at a constant speed,

0:03:53 > 0:03:55or picks up speed as it drops.

0:03:58 > 0:04:01Even in slow motion, it's pretty hard to tell.

0:04:06 > 0:04:09But Galileo realised this.

0:04:09 > 0:04:12First, drop an object a very short distance.

0:04:14 > 0:04:16It lands with very little impact.

0:04:17 > 0:04:20But, of course, drop it from higher up...

0:04:24 > 0:04:27..this time, the ball easily breaks the tile,

0:04:27 > 0:04:30which means it must have accelerated,

0:04:30 > 0:04:34gaining in speed and momentum as it dropped.

0:04:36 > 0:04:41Galileo had identified something fundamental to all falling objects -

0:04:41 > 0:04:42they accelerate.

0:04:45 > 0:04:48He realised there might be a way to measure

0:04:48 > 0:04:51how much falling objects gain in speed.

0:04:51 > 0:04:57What he devised was the first-ever attempt to measure gravity itself.

0:04:57 > 0:05:00He built a long wooden ramp, rather like this,

0:05:00 > 0:05:04that he had sloping at a shallow angle.

0:05:04 > 0:05:09The idea was to roll balls down the ramp and measure their acceleration.

0:05:09 > 0:05:12The crucial thing is that the ramp had to be at this shallow angle

0:05:12 > 0:05:15to reduce the effects of wind resistance.

0:05:15 > 0:05:19It also meant the balls would roll down slowly enough to give him time

0:05:19 > 0:05:21to measure their speed.

0:05:21 > 0:05:25But the big problem was this - how do you measure time accurately

0:05:25 > 0:05:28in an age when there were no accurate timepieces,

0:05:28 > 0:05:30let alone stopwatches?

0:05:30 > 0:05:33Well, Galileo came up with an ingenious idea

0:05:33 > 0:05:34involving the flow of water -

0:05:34 > 0:05:39essentially, measuring time from the amount of water collected in a cup.

0:05:39 > 0:05:42So, we're going to try and repeat Galileo's experiment.

0:05:42 > 0:05:45I say we, because I have a couple of willing volunteers,

0:05:45 > 0:05:47Gavin and Johanna.

0:05:47 > 0:05:50Three, two, one, go.

0:05:56 > 0:05:57And, stop.

0:05:57 > 0:05:59OK, there's one.

0:05:59 > 0:06:03Now, if you come down a quarter of the way down the ramp.

0:06:03 > 0:06:04Go.

0:06:08 > 0:06:09Stop. OK.

0:06:09 > 0:06:11So, now half of the way down.

0:06:11 > 0:06:12Go.

0:06:15 > 0:06:16Stop.

0:06:16 > 0:06:17Just in time.

0:06:20 > 0:06:23OK, and then three-quarters of the way down.

0:06:23 > 0:06:24Go.

0:06:25 > 0:06:27And, stop.

0:06:27 > 0:06:29Right, turn the tap off.

0:06:29 > 0:06:32OK, so we have our four measurements.

0:06:32 > 0:06:35And I can see a progression from fuller to emptier,

0:06:35 > 0:06:38but what we need to do now is find the mathematical pattern

0:06:38 > 0:06:42by weighing carefully the water in each glass.

0:06:43 > 0:06:47Weighing the water should give us an idea of how long each roll took.

0:06:47 > 0:06:51And in our experiment, these were the results.

0:06:52 > 0:06:54Now, there's one immediate thing you can tell.

0:06:55 > 0:06:59The ball really sped up the longer it rolled.

0:07:01 > 0:07:03In fact, our results seem to show

0:07:03 > 0:07:07that the time it took to cover the first quarter of the ramp

0:07:07 > 0:07:11was about the same time it took to cover the next three-quarters.

0:07:13 > 0:07:17So, we have a strong hint of a mathematical pattern.

0:07:18 > 0:07:22Now, we'll see if we're right, by placing bells along the ramp,

0:07:22 > 0:07:26at intervals which are based on the results.

0:07:27 > 0:07:29This arrangement looks a bit strange

0:07:29 > 0:07:33because the gap between the first two bells is much shorter

0:07:33 > 0:07:36than the gap between the third and fourth bells.

0:07:36 > 0:07:39But that's OK, because if we've got our calculations right,

0:07:39 > 0:07:43the ball starts off slowly, so it covers a shorter distance,

0:07:43 > 0:07:47and as it picks up pace, it'll cover longer and longer distances.

0:07:47 > 0:07:52So, we should hear the bells ringing at equal intervals in time.

0:07:52 > 0:07:53Go.

0:07:53 > 0:07:59BELLS RING

0:07:59 > 0:08:01Beautiful.

0:08:03 > 0:08:07So, what does this all mean, what's the mathematical formula?

0:08:07 > 0:08:10Well, this is something that Galileo worked out.

0:08:10 > 0:08:13Let's say, from the start, the ball covers a distance of one metre

0:08:13 > 0:08:15in the first second.

0:08:15 > 0:08:18After two seconds, it will have covered four metres.

0:08:18 > 0:08:20After three seconds, nine metres.

0:08:20 > 0:08:25After 4 seconds, 16 metres, and so on.

0:08:25 > 0:08:27If you recognise this progression,

0:08:27 > 0:08:31you'll see that distance goes like the square of time.

0:08:32 > 0:08:37Galileo had found the rates at which gravity speeds up objects.

0:08:38 > 0:08:41And he'd found another fundamental principle -

0:08:41 > 0:08:44you can measure the strength of gravity

0:08:44 > 0:08:47by how much it causes falling objects to accelerate.

0:08:50 > 0:08:54Detecting gravity has become exceptionally sophisticated

0:08:54 > 0:08:58these days, but still uses exactly the same principle.

0:09:00 > 0:09:03This is Herstmonceux Castle in Sussex,

0:09:03 > 0:09:07and in its grounds lies the Space Geodesy Facility.

0:09:09 > 0:09:13Here, Vicky uses an astonishingly sensitive instrument

0:09:13 > 0:09:18to detect the exact strength of gravity on this one spot.

0:09:18 > 0:09:22Vicky, tell me about this incredible gravity meter that you work with.

0:09:22 > 0:09:26OK, so this is the dropping chamber in a stripped down version.

0:09:26 > 0:09:27Essentially what happens

0:09:27 > 0:09:30is you've got a cart that gets raised to the top,

0:09:30 > 0:09:33and then the cart accelerates away from a mass in the middle,

0:09:33 > 0:09:37and so this section lifts off and as it drops, it drops under freefall.

0:09:37 > 0:09:40So, this component in the middle as it drops

0:09:40 > 0:09:43is basically just Newton's apple falling to the ground?

0:09:43 > 0:09:46- Yes.- So this is a stripped down version, but that's the real thing?

0:09:46 > 0:09:49- This is the real thing. - How does that actually work?

0:09:49 > 0:09:52- In here, it's a vacuum.- So there's no wind resistance as it falls.

0:09:52 > 0:09:54There's no wind resistance.

0:09:54 > 0:09:56Inside, a laser is used

0:09:56 > 0:10:00to measure exactly how fast the mass is accelerating.

0:10:00 > 0:10:04This is the 21st-century version of Galileo's ramp

0:10:04 > 0:10:07and the balls rolling down. So, can we get it going?

0:10:07 > 0:10:09Of course, if you'd just like to press the button on the laptop.

0:10:09 > 0:10:11- This one?- Yep.

0:10:12 > 0:10:15- OK.- So it's now communicating with it.

0:10:15 > 0:10:16- Oh, here we go.- Here we go.

0:10:16 > 0:10:19It waits five seconds and then takes the measurement of gravity.

0:10:19 > 0:10:21- And again.- Repeats.

0:10:21 > 0:10:26And you can see the results appearing now.

0:10:26 > 0:10:29Yup, each of those green dots is a measurement of gravity

0:10:29 > 0:10:32with the actual number that it's getting for each one.

0:10:32 > 0:10:35The unit Vicky uses has a familiar ring.

0:10:35 > 0:10:38I see that the number up at the top here,

0:10:38 > 0:10:42you've got this unit, micro Gal?

0:10:42 > 0:10:46Yes, a Gal is essentially one centimetre per second squared.

0:10:46 > 0:10:48The Gal was named after Galileo.

0:10:48 > 0:10:51So, we've just taken the measurement of gravity here today

0:10:51 > 0:10:55and it's this highly accurate number,

0:10:55 > 0:11:00981124007

0:11:00 > 0:11:02micro Gals.

0:11:02 > 0:11:06The reading means that the Earth's gravity speeds up a falling object

0:11:06 > 0:11:12by around 9.81 metres per second for every second it drops.

0:11:15 > 0:11:17Vicky tells me something intriguing.

0:11:17 > 0:11:21She takes a reading here every week and she's found that

0:11:21 > 0:11:25the strength of gravity changes by tiny amounts over time.

0:11:26 > 0:11:30Heavy rainfall, for example, can cause gravity to increase slightly.

0:11:32 > 0:11:36Presumably, if gravity is changing here in one spot,

0:11:36 > 0:11:40it'll have different values all around the world

0:11:40 > 0:11:42and so you can have a gravity map of the entire planet?

0:11:42 > 0:11:44That's right, yes.

0:11:45 > 0:11:48So what's the reason for these strange fluctuations?

0:11:48 > 0:11:52That's what I want to investigate next.

0:11:53 > 0:11:57So, gravity changes as we move across the surface of the Earth.

0:11:57 > 0:12:03This is at the heart of a challenge that I've set two young volunteers.

0:12:03 > 0:12:07I've given them a task to try and find the place in Britain

0:12:07 > 0:12:10where gravity is at its weakest.

0:12:10 > 0:12:13So, where objects would weigh the least.

0:12:13 > 0:12:16I've given them just three days to try and find it.

0:12:17 > 0:12:22The volunteers are Astraya, a PhD student.

0:12:22 > 0:12:24I've been living in London for five or six years,

0:12:24 > 0:12:27and I'm originally from Seville in Spain.

0:12:27 > 0:12:30I'm very interested in taking part in this project

0:12:30 > 0:12:34because I would really like to know more about how this world works.

0:12:34 > 0:12:38And Poppy, a journalist who lives in London.

0:12:38 > 0:12:41I did my degree in biomedical science.

0:12:41 > 0:12:45And I did biology and chemistry for my A-levels,

0:12:45 > 0:12:48but I haven't done any physics since I left school.

0:12:48 > 0:12:50I'm fascinated to find out more about gravity

0:12:50 > 0:12:54and I actually enjoy a puzzle, I like a challenge.

0:12:54 > 0:12:57The team just can't weigh themselves to see changes in gravity.

0:12:57 > 0:13:02Body weight fluctuates by a couple of kilos over the course of a day.

0:13:02 > 0:13:07Whereas, changes due to gravity as they travel around the country

0:13:07 > 0:13:10are going to be tiny in comparison, the matter of a few grams.

0:13:10 > 0:13:14So, they're going to have to use sophisticated scientific methods

0:13:14 > 0:13:16if they want to measure gravity accurately.

0:13:16 > 0:13:20And that's why the volunteers will be joined by three specialists

0:13:20 > 0:13:22in gravity science.

0:13:23 > 0:13:27PhD student Sonak.

0:13:27 > 0:13:30He'll be in charge of some very sensitive measuring apparatus

0:13:30 > 0:13:33from the National Physical Laboratory.

0:13:33 > 0:13:40Sean, a geologist, who will be using a portable gravity meter.

0:13:40 > 0:13:44And Andrew, a cosmologist at University College London,

0:13:44 > 0:13:46who will help interpret the results.

0:13:47 > 0:13:51We've taken a collective weight for the team before they set off.

0:13:51 > 0:13:54It's 380 kilograms.

0:13:54 > 0:13:59So, can they find the place in Britain where that will decrease?

0:14:00 > 0:14:04They're setting out in Snowdonia National Park in North Wales.

0:14:06 > 0:14:10The railway climbs from here to the 1,000 metre summit of Snowdon.

0:14:10 > 0:14:13Sean takes his first gravity reading.

0:14:13 > 0:14:18The inside is a mass on a beam and you turn this counter,

0:14:18 > 0:14:23this dial, until you get the beam central.

0:14:23 > 0:14:26By counting the number of turns of the dial,

0:14:26 > 0:14:29Sean can calculate the downward pull of gravity

0:14:29 > 0:14:32acting on the mass inside the machine.

0:14:32 > 0:14:36Sonak has a simpler method.

0:14:36 > 0:14:40So, inside the box is a two kilogram mass,

0:14:40 > 0:14:42and it's supposed to be sort of as perfectly two kilograms

0:14:42 > 0:14:44as it's possible to get.

0:14:46 > 0:14:49All right. And place it here.

0:14:50 > 0:14:52Oh, it's just coming under, isn't it?

0:14:52 > 0:14:551998.2 grams.

0:14:55 > 0:14:59It was two kilos in the laboratory, but now here it's a bit less.

0:14:59 > 0:15:01It's the first puzzle.

0:15:01 > 0:15:07Why does a two kilo mass tip the scales at just under two kilos?

0:15:07 > 0:15:10And it's one which gets straight to the heart

0:15:10 > 0:15:13of what the challenge is really about.

0:15:14 > 0:15:19Mass is often confused with the related quantity, weight.

0:15:19 > 0:15:24The mass of these dumbbells is fixed, it doesn't change.

0:15:24 > 0:15:27It's a measure of how much stuff they contain.

0:15:27 > 0:15:29Weight is different.

0:15:29 > 0:15:33It's a measure of the effects of gravity on these dumbbells.

0:15:33 > 0:15:36The downward force pulling them to the ground

0:15:36 > 0:15:40in the same way that it's keeping my feet firmly stuck to the ground.

0:15:40 > 0:15:42The crucial difference is this,

0:15:42 > 0:15:44if I was holding these dumbbells on the moon,

0:15:44 > 0:15:47they'd still have exactly the same mass,

0:15:47 > 0:15:50but they'd weigh six times less

0:15:50 > 0:15:54because the moon's gravity is so much weaker than the Earth's.

0:15:56 > 0:16:00So that's why Sonak is bringing along the two kilo mass.

0:16:00 > 0:16:03If it changes weight then this should mean

0:16:03 > 0:16:05that gravity itself has changed.

0:16:07 > 0:16:11Ahead of them is the summit of the highest mountain

0:16:11 > 0:16:14in England and Wales, famed for its stunning scenery.

0:16:16 > 0:16:19Or it would be stunning if you could see it.

0:16:20 > 0:16:24And this is what we came all the way up here for,

0:16:24 > 0:16:27this amazing view at the top of Snowdon.

0:16:27 > 0:16:30You wouldn't know it, but honestly, we are here.

0:16:32 > 0:16:36We're now near the summit of Snowdon and I've set up the gravimeter,

0:16:36 > 0:16:39and we're going to see what the difference in the reading is.

0:16:42 > 0:16:46He has to turn the dial again and again to try and get a reading.

0:16:46 > 0:16:50It's clear gravity has changed, but which way?

0:16:50 > 0:16:53Has it got stronger, or weaker?

0:16:53 > 0:16:56The team leave Sean to work out his results,

0:16:56 > 0:17:00and tries to position the scales as close as possible to the summit.

0:17:01 > 0:17:03But the reading is all over the place.

0:17:03 > 0:17:07- Oh!- It's gone up.

0:17:07 > 0:17:09It's fluctuating quite a lot due to the wind.

0:17:09 > 0:17:12I have to say, this is what science is always like, isn't it?

0:17:12 > 0:17:14It's never quite what you want it to be.

0:17:14 > 0:17:18So, they head inside to the cafe next to the summit.

0:17:20 > 0:17:23The wind was being a bit naughty, but hopefully...

0:17:23 > 0:17:25Now it's in 00, so it should be all right.

0:17:25 > 0:17:281998.2 down there,

0:17:28 > 0:17:311997.8!

0:17:31 > 0:17:35- There you go.- We've got it! That's 0.4 of a gram off.

0:17:36 > 0:17:39The mass weighs a tiny bit less.

0:17:39 > 0:17:44It's lost about one 5000th of its weight.

0:17:44 > 0:17:48And Sean has found that gravity itself has reduced.

0:17:48 > 0:17:51At the top of the mountain we took the measurement

0:17:51 > 0:17:56and we discovered that the pull of gravity had gone down.

0:17:56 > 0:17:59It had gone down the equivalent of 206 turns of the dial.

0:17:59 > 0:18:04And we worked out that that's equivalent to 219 milligals.

0:18:06 > 0:18:09So it's clear from the team's measurements,

0:18:09 > 0:18:14gravity weakens as you go higher, and you get a bit lighter.

0:18:16 > 0:18:19It's just an excuse to say where are we, like, the lightest.

0:18:19 > 0:18:21- Who cares?- Yes, who does care?

0:18:21 > 0:18:25It's actually really interestingly, it's like an illustrative example

0:18:25 > 0:18:28of seeing how this is actually fluctuating,

0:18:28 > 0:18:31- depending on different factors. - Yeah, absolutely.

0:18:31 > 0:18:34And that we could measure it and see it with our own eyes,

0:18:34 > 0:18:37it actually makes you think about gravity in a very active way.

0:18:37 > 0:18:40It's such a fundamental force phenomenon in nature,

0:18:40 > 0:18:42but we don't know much about it.

0:18:44 > 0:18:47But why does gravity change with altitude?

0:18:48 > 0:18:50To understand that question,

0:18:50 > 0:18:53you've to get to grips with the extraordinary discoveries

0:18:53 > 0:18:56of the next scientific giant in our story -

0:18:56 > 0:18:59Isaac Newton.

0:18:59 > 0:19:02Born in England in the middle of the 17th century,

0:19:02 > 0:19:06he spent his life wrestling with so many apparently separate questions,

0:19:06 > 0:19:11from why things fall to the ground, to why planets orbit the sun.

0:19:14 > 0:19:17It took the genius of Newton to realise

0:19:17 > 0:19:21there was one single equation that could answer all these questions.

0:19:23 > 0:19:27And here it is, his famous law of gravity.

0:19:27 > 0:19:28It might look complicated,

0:19:28 > 0:19:31but this is one of the most important equations

0:19:31 > 0:19:33in the whole of science.

0:19:33 > 0:19:34F here is the force.

0:19:34 > 0:19:38Newton said there's an attractive force between any two objects

0:19:38 > 0:19:40in the universe.

0:19:40 > 0:19:44On this side of the equation, G, we call the gravitational constant.

0:19:44 > 0:19:48Newton knew it had to be there, but he didn't know what its value was.

0:19:48 > 0:19:55M1 and M2 represent the two objects, and R is the distance between them.

0:19:55 > 0:19:59Now, the equation tells us that the more massive the objects are,

0:19:59 > 0:20:04the bigger M1 and M2, the greater the attractive force.

0:20:04 > 0:20:08But the further apart they are, the bigger the value of R here,

0:20:08 > 0:20:10the weaker the gravitational force.

0:20:11 > 0:20:15With Newton, what was once mysterious now became clear.

0:20:16 > 0:20:21Newton's equation describes why an object falls to the ground,

0:20:21 > 0:20:23including his famous apple.

0:20:23 > 0:20:26But its true genius is that it applies to any object,

0:20:26 > 0:20:28anywhere in the universe.

0:20:28 > 0:20:32So, it's a very simple and elegant way of describing

0:20:32 > 0:20:37some of the seemingly most complicated phenomena in the cosmos.

0:20:41 > 0:20:45His law of gravitation can still be used today -

0:20:45 > 0:20:48to explain how orbits work,

0:20:48 > 0:20:52to predict when a comet will return,

0:20:52 > 0:20:55to describe why galaxies spin.

0:20:57 > 0:21:00Or to slingshot spacecraft around planets.

0:21:02 > 0:21:05Newton tells us to look for the underlying simplicity

0:21:05 > 0:21:09in natural phenomena. For instance, how the moon orbits the Earth.

0:21:11 > 0:21:13If I let go of this apple,

0:21:13 > 0:21:16it'll fall straight down because of the pull of Earth's gravity.

0:21:17 > 0:21:19But if I throw it, to begin with,

0:21:19 > 0:21:21it travels in a horizontal direction,

0:21:21 > 0:21:23that's the direction of travel,

0:21:23 > 0:21:25but Earth's gravity is still pulling it downwards,

0:21:25 > 0:21:28so it ends up following a curved path.

0:21:35 > 0:21:36Now, if I throw it harder,

0:21:36 > 0:21:40it'll travel further before it hits the ground and, in principle,

0:21:40 > 0:21:43if I could throw it hard enough, I could put it into orbit.

0:21:43 > 0:21:47That's exactly what's happening with the moon in orbit around the Earth.

0:21:47 > 0:21:51It's a combination of wanting to travel in a straight line,

0:21:51 > 0:21:53but also being pulled down by the Earth's gravity.

0:21:53 > 0:21:56So, it ends up constantly falling

0:21:56 > 0:21:58around the Earth and constantly missing.

0:22:01 > 0:22:02Newton's famous equation

0:22:02 > 0:22:04also explains the strange effects

0:22:04 > 0:22:07which the road-trip team has discovered.

0:22:07 > 0:22:10That objects get lighter as you gain in altitude.

0:22:12 > 0:22:16When I weigh myself, I'm represented by the first mass, M1.

0:22:16 > 0:22:19The second mass, M2, is the Earth itself.

0:22:19 > 0:22:23And the force pulling me down, my weight,

0:22:23 > 0:22:27depends on the distance between me and the centre of the Earth.

0:22:27 > 0:22:29And that's the secret of the road trip.

0:22:29 > 0:22:32If you want to find the place where you weigh the least,

0:22:32 > 0:22:36then you have to get as far away as you can from the Earth's core.

0:22:44 > 0:22:47So, it's the afternoon of day one,

0:22:47 > 0:22:51and the road-trip team have to work out where to go next.

0:22:51 > 0:22:54Poppy and Astraya have a good idea,

0:22:54 > 0:22:57find somewhere higher than Mount Snowdon.

0:22:57 > 0:23:01From the measurements that you guys did at Mount Snowdon,

0:23:01 > 0:23:04altitude clearly plays an important part in gravity.

0:23:04 > 0:23:07So, with that in mind, we've got to go to the highest point in the UK,

0:23:07 > 0:23:08which is Ben Nevis.

0:23:08 > 0:23:13OK, BUT there's just one thing that we haven't shown you so far.

0:23:13 > 0:23:16We actually brought along an extra experiment,

0:23:16 > 0:23:19so can we please show you this first before you make the final decision?

0:23:19 > 0:23:23- Yes.- Sonak actually has the other part of this experiment.

0:23:23 > 0:23:25We always carry around...

0:23:25 > 0:23:27Some power tools, as physicists always do.

0:23:27 > 0:23:29Let's start it off nice and gentle.

0:23:31 > 0:23:32OK.

0:23:32 > 0:23:34And then, try and pick up some pace.

0:23:36 > 0:23:37Pizza.

0:23:39 > 0:23:42- You've got some pizza there. - OK. Point proven.

0:23:42 > 0:23:44The point is that when something is spinning,

0:23:44 > 0:23:48it kind of gets flung outwards and you can actually use that

0:23:48 > 0:23:52to make a nice, flat piece of pizza, but this also applies to the Earth.

0:23:52 > 0:23:55The Earth isn't perfectly round.

0:23:55 > 0:23:58It's what's known as an "oblate spheroid".

0:23:58 > 0:24:00It bulges at the equator

0:24:00 > 0:24:03where the spin is greatest.

0:24:03 > 0:24:05We've kind of got two competing effects now.

0:24:05 > 0:24:08We're trying to get away from the centre,

0:24:08 > 0:24:09the actual core of the Earth,

0:24:09 > 0:24:12the point at the very centre of this ball.

0:24:12 > 0:24:14But now, we can do it in two ways.

0:24:14 > 0:24:16We can either go up something tall,

0:24:16 > 0:24:20or we can just go down towards the equator.

0:24:20 > 0:24:23This is what we find when we're doing gravity surveys,

0:24:23 > 0:24:27as you move south, there tends to be an effect from latitude

0:24:27 > 0:24:32which is often usually larger than the effect from altitude.

0:24:32 > 0:24:36So, the closer to the equator you go,

0:24:36 > 0:24:41the further you get from the Earth's core and the lighter you get.

0:24:41 > 0:24:46So, guys, the sun's setting just behind me here. This is north.

0:24:46 > 0:24:48From the conversations we've just had,

0:24:48 > 0:24:51it sounds like we've got to go that way,

0:24:51 > 0:24:52down south, is that right?

0:24:52 > 0:24:54- Yes, OK.- Let's go.

0:24:54 > 0:24:56THEY LAUGH

0:24:56 > 0:25:00The team is starting to uncover the reasons why gravity changes

0:25:00 > 0:25:02as you cross the surface of the Earth.

0:25:05 > 0:25:07Our planet is defined and shaped

0:25:07 > 0:25:11by the complicated forces which act upon it.

0:25:11 > 0:25:14And detecting tiny fluctuations in its gravity field

0:25:14 > 0:25:18can give us important clues.

0:25:18 > 0:25:21It can help us understand how our world is changing.

0:25:23 > 0:25:27The Space Geodesy Facility at Herstmonceux is one small part

0:25:27 > 0:25:29of an enormous global network

0:25:29 > 0:25:33which uses satellites to detect the tiniest of changes

0:25:33 > 0:25:36in the Earth's gravity field.

0:25:36 > 0:25:38Tell me what exactly your job is here?

0:25:38 > 0:25:40What we're doing with this telescope

0:25:40 > 0:25:42is measuring very accurately

0:25:42 > 0:25:45the distances of satellites from here,

0:25:45 > 0:25:47so we're using very short laser pulses

0:25:47 > 0:25:49which we direct towards the satellite.

0:25:49 > 0:25:52On the satellite, there are reflecting cubes,

0:25:52 > 0:25:54which return some of that light to us.

0:25:54 > 0:25:56We measure how long it takes the light

0:25:56 > 0:25:57to go to the satellite and back.

0:25:57 > 0:25:59And how far away is the satellite typically?

0:25:59 > 0:26:02The one we're tracking now is one of the Galileo satellites,

0:26:02 > 0:26:04which is about 20,000 kilometres.

0:26:04 > 0:26:07- 20,000 kilometres away?- Yes.

0:26:07 > 0:26:09OK, so, we've got it aimed at the Galileo satellite

0:26:09 > 0:26:12- and you're going to turn the laser on now?- Yes.

0:26:14 > 0:26:16Oh, wow!

0:26:16 > 0:26:21And that laser beam that's being fired up towards the satellite,

0:26:21 > 0:26:23the time it'll take to get there and come back again,

0:26:23 > 0:26:26- it's a fraction of a second, isn't it?- It is.

0:26:26 > 0:26:29It's about 150 thousandths of a second, 150 milliseconds.

0:26:29 > 0:26:32And we're sending about 1,000 of those per second.

0:26:36 > 0:26:40This strange-looking object is based on satellite readings.

0:26:40 > 0:26:42It's a highly exaggerated representation

0:26:42 > 0:26:46of how Earth's gravity field varies over time.

0:26:48 > 0:26:52Fluctuations like these can give us important insights

0:26:52 > 0:26:53into climate change,

0:26:53 > 0:26:55icecaps melting,

0:26:55 > 0:26:58sea levels rising,

0:26:58 > 0:27:01changes in ground water.

0:27:01 > 0:27:03All of these have an effect

0:27:03 > 0:27:05on the local strength of gravity.

0:27:05 > 0:27:09So, something as important as climate change,

0:27:09 > 0:27:11in order to understand it and do something about it,

0:27:11 > 0:27:13we need to know the distribution

0:27:13 > 0:27:16of the gravitational field of the Earth very accurately?

0:27:16 > 0:27:20Absolutely, yes. And it's a global measure that we need.

0:27:25 > 0:27:28For the road trippers, it's the start of day two...

0:27:30 > 0:27:32..and they're heading for the south coast.

0:27:34 > 0:27:36They're stopping off in Herefordshire,

0:27:36 > 0:27:39it's a good location as it's the same altitude

0:27:39 > 0:27:41as the base of Snowdon,

0:27:41 > 0:27:44but they've moved about 80 miles further south.

0:27:44 > 0:27:49So, if they find gravity changes here, it must be due to latitude.

0:27:49 > 0:27:51It's not a huge difference, but it's noticeable.

0:27:51 > 0:27:55Our counter reading at the bottom of the mountain was 4,840.

0:27:55 > 0:27:59- Yes.- Our counter reading here's 4,717.

0:27:59 > 0:28:02Oh, right, so, we do get to see a difference.

0:28:02 > 0:28:05So, we're at the same altitude as the base of Mount Snowdon,

0:28:05 > 0:28:08but because we've travelled further down south overnight,

0:28:08 > 0:28:10- gravity's less here?- Yes.

0:28:13 > 0:28:14They push on.

0:28:20 > 0:28:24And by sunset they reach Sidmouth on the south coast.

0:28:26 > 0:28:30Sean takes the second gravity reading of the day

0:28:30 > 0:28:33and Poppy improvises a map.

0:28:33 > 0:28:35Well, sort of a map.

0:28:35 > 0:28:38Can we write "not to scale" at the top there.

0:28:38 > 0:28:41SHE MOUTHS, ALL LAUGH

0:28:41 > 0:28:44So, I drew this map.

0:28:44 > 0:28:45Scotland's a bit squashed.

0:28:45 > 0:28:51Wales is quite high up and Cornwall is there, but you get the idea.

0:28:51 > 0:28:54Sean, we've been travelling with you,

0:28:54 > 0:28:57you've done quite a few gravity meter readings,

0:28:57 > 0:28:59can you plot them on this not-to-scale,

0:28:59 > 0:29:01- badly-drawn map, please?- Sure.

0:29:01 > 0:29:06So, if you remember we started off in Mount Snowdon, here,

0:29:06 > 0:29:09and that was the zero measurement for our survey.

0:29:09 > 0:29:12Then we've come all the way down here to the south coast.

0:29:14 > 0:29:22- The difference from the base of Snowdon is -212 milligals.- Wow.

0:29:22 > 0:29:25So, the difference between going and measuring gravity

0:29:25 > 0:29:29at the base of the mountain and the top of the mountain

0:29:29 > 0:29:31is about the same as here at this latitude

0:29:31 > 0:29:35and down here at this latitude.

0:29:35 > 0:29:37They're quite clearly at sea level,

0:29:37 > 0:29:42yet gravity here is roughly the same as it is at the top of Snowdon.

0:29:42 > 0:29:44But where next?

0:29:44 > 0:29:46We are here.

0:29:46 > 0:29:50If we want to find out where we are the lightest,

0:29:50 > 0:29:55why don't we travel all the way to the most southerly point in the UK,

0:29:55 > 0:29:59- which is here?- But altitude can also help us,

0:29:59 > 0:30:02so why not find a place in the country

0:30:02 > 0:30:07that is both low in latitude but also as high in altitude

0:30:07 > 0:30:11in terms of height above sea level, because that will get us somewhere

0:30:11 > 0:30:14that is really far away from the core of the Earth,

0:30:14 > 0:30:17whilst staying within the country?

0:30:23 > 0:30:28So, the answer to the puzzle lies in a combination of two factors.

0:30:28 > 0:30:32How much further south should they go and how much higher?

0:30:34 > 0:30:38At the end of day two, Sean's results show that the team

0:30:38 > 0:30:40weighs about 80 grams lighter in total

0:30:40 > 0:30:42than back at the base of Snowdon.

0:30:54 > 0:30:57The way that weight changes is just one example

0:30:57 > 0:31:00of Newton's famous equation in action.

0:31:02 > 0:31:05But Newton had left his masterpiece incomplete.

0:31:05 > 0:31:07He didn't know the value of G,

0:31:07 > 0:31:10the gravitational constant,

0:31:10 > 0:31:14which sets the size of the force.

0:31:14 > 0:31:18To harness the full power of the equation, you need to know G.

0:31:18 > 0:31:22And the vital clue came within an incredible experiment

0:31:22 > 0:31:25conducted in London at the end of the 18th century.

0:31:29 > 0:31:33It was an attempt to work out the mass of the Earth itself.

0:31:33 > 0:31:36And it was carried out by an eccentric,

0:31:36 > 0:31:39extravagantly rich aristocrat,

0:31:39 > 0:31:41Henry Cavendish.

0:31:41 > 0:31:45Cavendish was a chronically shy,

0:31:45 > 0:31:49deeply solitary man living in total isolation in his house in Clapham.

0:31:49 > 0:31:51The story goes that, one day,

0:31:51 > 0:31:55he accidentally bumped into a female servant on his staircase.

0:31:55 > 0:31:57He was so traumatised by this event

0:31:57 > 0:32:00that he had a new staircase built just for him

0:32:00 > 0:32:03so that this horrible incident could never happen again.

0:32:05 > 0:32:07Cavendish had inherited vast fortunes

0:32:07 > 0:32:09and was able to dedicate his life

0:32:09 > 0:32:13to devising pioneering experiments -

0:32:13 > 0:32:16including one particularly extraordinary piece of equipment.

0:32:21 > 0:32:23He set up something a bit like this.

0:32:23 > 0:32:25It's called a "torsion balance".

0:32:25 > 0:32:28It involves four lead spheres,

0:32:28 > 0:32:31two large heavy ones which are held fixed in place,

0:32:31 > 0:32:36and suspended by a very thin wire is a wooden rod,

0:32:36 > 0:32:40six-feet-long, with two smaller balls on either end.

0:32:40 > 0:32:42Now, the crux of the experiment

0:32:42 > 0:32:46is the relationship between the large ball and the small ball.

0:32:46 > 0:32:49Now, of course, there's a gravitational pull downwards

0:32:49 > 0:32:52on both of the balls due to the Earth's gravity.

0:32:52 > 0:32:53But Newton also tells us

0:32:53 > 0:32:58that there should be a very weak gravitational pull between the balls

0:32:58 > 0:33:01and this is effectively what Cavendish was trying to measure.

0:33:01 > 0:33:05Any slight movement of the small ball towards the large one

0:33:05 > 0:33:08should cause a twist in the torsion wire

0:33:08 > 0:33:11and that's what Cavendish was trying to detect.

0:33:11 > 0:33:14Of course, this is all much easier said than done.

0:33:14 > 0:33:16The experiment was incredibly sensitive.

0:33:16 > 0:33:18The tiniest of vibrations,

0:33:18 > 0:33:21the slightest breeze, changes in temperature

0:33:21 > 0:33:23could all influence the measurements.

0:33:23 > 0:33:27So, Cavendish had to isolate the apparatus inside a box

0:33:27 > 0:33:30and the box within a shed.

0:33:30 > 0:33:34He even realised that his mere presence next to the apparatus

0:33:34 > 0:33:38could influence things, so he had to remove himself outside the shed.

0:33:39 > 0:33:41What he then did was sit outside the shed,

0:33:41 > 0:33:44and through a small hole in the shed wall,

0:33:44 > 0:33:49look through a telescope to detect the tiniest of twists in the wire.

0:33:49 > 0:33:52It was an incredibly difficult process, but after many months,

0:33:52 > 0:33:56he finally felt confident enough that he had a reliable result.

0:34:03 > 0:34:06Cavendish found that the small balls did move...

0:34:08 > 0:34:10..a tiny four millimetres.

0:34:13 > 0:34:14He calculated his results

0:34:14 > 0:34:16by comparing the density of the balls

0:34:16 > 0:34:18with the density of water.

0:34:20 > 0:34:23In the end, the result of Cavendish's experiment

0:34:23 > 0:34:24and subsequent calculations

0:34:24 > 0:34:27was that the density of the Earth

0:34:27 > 0:34:30was about five and a half times that of water.

0:34:30 > 0:34:32Or, put another way,

0:34:32 > 0:34:38the mass of the Earth was 5.9 trillion trillion kilograms.

0:34:38 > 0:34:42What's most remarkable is that Cavendish got this number right

0:34:42 > 0:34:45to within an accuracy of 1%.

0:34:45 > 0:34:49With Cavendish's astonishing result,

0:34:49 > 0:34:51scientists were able to work out G.

0:34:53 > 0:34:55Then the equation could be used

0:34:55 > 0:34:57to determine the mass of any celestial body

0:34:57 > 0:34:59in orbit around another.

0:35:01 > 0:35:05So, astronomers were able to calculate the mass of the sun

0:35:05 > 0:35:08and the planets, and the moon,

0:35:08 > 0:35:12and, eventually, even distant galaxies.

0:35:17 > 0:35:21At the end of day two, the team were in Sidmouth on the south coast,

0:35:21 > 0:35:25looking for the place in Britain where they'll weigh the least.

0:35:25 > 0:35:30They've worked out the answer lies in a combination of two factors -

0:35:30 > 0:35:34the right mix of going south and being higher up.

0:35:36 > 0:35:39For the final leg of the journey, I'm going to meet up with them.

0:35:41 > 0:35:44I asked them to drive a short distance west

0:35:44 > 0:35:48to one of the most remote areas in mainland Britain.

0:35:48 > 0:35:50Dartmoor National Park.

0:35:52 > 0:35:55'It's only 40 miles from the southernmost tip of Britain.'

0:35:55 > 0:35:56Hello. Hi, Andrew.

0:35:56 > 0:35:58- Good to see you.- Nice to see you.

0:35:58 > 0:36:02'And it's very high, very hilly territory.'

0:36:02 > 0:36:04Jim, the team got to the south coast yesterday...

0:36:04 > 0:36:08- Yes.- ..to find gravity at its weakest.

0:36:08 > 0:36:12But we haven't quite figured out whether it's altitude or latitude.

0:36:12 > 0:36:14Do we go further south or do we go higher up?

0:36:14 > 0:36:18You're right to ask, "Do we go as far south as possible

0:36:18 > 0:36:20"or as high as possible?"

0:36:20 > 0:36:23That's why I've brought you here to Dartmoor.

0:36:23 > 0:36:27We've charted the most important points on this map here.

0:36:27 > 0:36:29- Right.- Let's have a look.

0:36:29 > 0:36:32So, we are here, Two Bridges.

0:36:32 > 0:36:37- Yes.- These four dots represent these hills up there behind us,

0:36:37 > 0:36:40which are at about 500 metres above sea level.

0:36:40 > 0:36:42That's what we want to check out.

0:36:42 > 0:36:44'These hills are close to the south coast

0:36:44 > 0:36:48'and they're also the highest in the whole of the south of England.

0:36:50 > 0:36:53'So, logic suggests they must be the right combination

0:36:53 > 0:36:55'of latitude and altitude.'

0:36:55 > 0:36:58Well, there's another reason why this makes perfect sense,

0:36:58 > 0:37:00one which we haven't looked at yet,

0:37:00 > 0:37:03and that's the effect of the underlying rocks on gravity.

0:37:03 > 0:37:05And I've got a map here that shows...

0:37:05 > 0:37:08- You're going to trump my map with yours, aren't you?- I am!

0:37:08 > 0:37:13Here we are, down here, now these blue areas are the lowest areas

0:37:13 > 0:37:17according to the density of the rocks underneath.

0:37:17 > 0:37:20'The rocks around here are made of granite,

0:37:20 > 0:37:22'which will make gravity weaker still.'

0:37:24 > 0:37:26So, that's helping - as well as the altitude

0:37:26 > 0:37:28and the fact that we're further south.

0:37:28 > 0:37:31Yes, it's also playing a part.

0:37:33 > 0:37:35'Well, we have a plausible theory.

0:37:35 > 0:37:37'But now we need to test it.'

0:37:39 > 0:37:42'If I'm right, then, at the top, our gravity reading

0:37:42 > 0:37:45'should be by far the lowest reading of the trip.'

0:37:48 > 0:37:51'Of course, there's another effect of gravity to deal with now -

0:37:51 > 0:37:54'it's knackering when you head uphill.'

0:37:55 > 0:37:58OK, I think this is pretty much the start of the hills

0:37:58 > 0:38:02we've located on the map. So, let's see if this is the lightest place.

0:38:02 > 0:38:05Sean, if you want to get the gravity meter out,

0:38:05 > 0:38:08- and we'll take another reading here.- Yep.- OK.

0:38:11 > 0:38:14'Sean sets up his equipment one more time.'

0:38:14 > 0:38:16What's the news?

0:38:16 > 0:38:21Well, the bottom of Mount Snowdon was our zero for this test.

0:38:21 > 0:38:23We found we lost a certain amount

0:38:23 > 0:38:25by going up to the top of Mount Snowdon.

0:38:25 > 0:38:29We found we lost a certain amount coming south to the south coast.

0:38:29 > 0:38:32Not only have we beaten that, we've smashed it.

0:38:32 > 0:38:36- Brilliant. - We were -219 milligals

0:38:36 > 0:38:39lower at the top of Mount Snowdon.

0:38:39 > 0:38:40Here on Dartmoor,

0:38:40 > 0:38:44- we're -347 milligals lower. - Wow!- Brilliant!

0:38:44 > 0:38:46So, it is a combination of three things.

0:38:46 > 0:38:49We're far south, so it's the latitude, we're at altitude,

0:38:49 > 0:38:52we're quite high up, and we're surrounded by all this granite rock,

0:38:52 > 0:38:54which is low-density anyway.

0:38:54 > 0:38:57I hope you all think it was worth the climb up here anyway?

0:38:57 > 0:39:00- Yes, absolutely.- There you go. Boom, science!

0:39:00 > 0:39:04ALL LAUGH

0:39:04 > 0:39:07Now, we already know that the altitude of these hills

0:39:07 > 0:39:10takes us much further from the Earth's core

0:39:10 > 0:39:12than anywhere else further south in Britain,

0:39:12 > 0:39:16so gravity must be weakest here.

0:39:16 > 0:39:18There's extra evidence, too.

0:39:18 > 0:39:20The British Geological Survey

0:39:20 > 0:39:24has compiled tens of thousands of gravity readings made in the UK

0:39:24 > 0:39:29and the lowest readings ever recorded were all taken around here

0:39:29 > 0:39:31on the high hills of Dartmoor.

0:39:32 > 0:39:34What do we do to celebrate?

0:39:34 > 0:39:36We weigh ourselves, of course.

0:39:36 > 0:39:38I bet you don't weigh that much.

0:39:38 > 0:39:40Whoa!

0:39:40 > 0:39:43It's all them Nutella pancakes for breakfast!

0:39:43 > 0:39:46- 74, 75.- I need to lose weight!

0:39:46 > 0:39:47LAUGHTER

0:39:47 > 0:39:52I can tell you that you should weigh something like 20 grams less

0:39:52 > 0:39:55than you did at the base of Mount Snowdon.

0:39:55 > 0:39:59Guys, I'm guessing something like 25 to 30 grams less.

0:39:59 > 0:40:02So, if you want to weigh as little as possible,

0:40:02 > 0:40:04this is the place in Britain to come.

0:40:04 > 0:40:06But in any case, it's such a tiny amount

0:40:06 > 0:40:08that it's going to be wiped out entirely

0:40:08 > 0:40:11by whatever it was you had for breakfast this morning.

0:40:11 > 0:40:12LAUGHTER

0:40:18 > 0:40:22Gravity. What goes up must come down.

0:40:24 > 0:40:27All of our lives, we abide by its rules.

0:40:28 > 0:40:30It dominates our every action.

0:40:31 > 0:40:33But there's one select group of humans

0:40:33 > 0:40:37who know what it's like to live free of gravity.

0:40:37 > 0:40:39'Two, one...

0:40:39 > 0:40:40'zero.

0:40:40 > 0:40:42'Lift-off!'

0:40:46 > 0:40:48Everybody's used to gravity.

0:40:48 > 0:40:50We're used to the oppression of it.

0:40:50 > 0:40:52Gravity is the ultimate oppressor.

0:40:52 > 0:40:58It grinds us under its heel 24/7 with no release,

0:40:58 > 0:41:03until you're in space and then, suddenly, you're free from gravity.

0:41:03 > 0:41:05You're weightless in orbit.

0:41:07 > 0:41:09Canadian astronaut Chris Hadfield

0:41:09 > 0:41:14spent five months on board the International Space Station.

0:41:14 > 0:41:17You can pull your knees up to your chest and just tumble.

0:41:17 > 0:41:19Or, if you take a wet cloth,

0:41:19 > 0:41:22and you get it dripping wet,

0:41:22 > 0:41:25and everybody on Earth knows what'll happen when you wring it out.

0:41:25 > 0:41:27All the water will fall, inevitably.

0:41:27 > 0:41:29If you do that in weightlessness,

0:41:29 > 0:41:31the water stays there and it, actually,

0:41:31 > 0:41:35because of the surface tension, starts crawling up your arms.

0:41:40 > 0:41:44It's a little bit mesmerising and hypnotic to be in weightlessness.

0:41:45 > 0:41:48If you're weightless, you don't need a bed,

0:41:48 > 0:41:49you don't need a mattress,

0:41:49 > 0:41:51you don't need a pillow.

0:41:51 > 0:41:55Your body is floating completely suspended, like magic.

0:41:59 > 0:42:00Movement becomes effortless.

0:42:00 > 0:42:05You can push off with one finger and fly, and it's humble.

0:42:05 > 0:42:08You don't need to hold yourself where you are with muscle.

0:42:08 > 0:42:12You can just... With a delicate fingertip pressure,

0:42:12 > 0:42:14you can stay where you are.

0:42:14 > 0:42:17But there is a price to pay.

0:42:17 > 0:42:21Astronauts' bones atrophy and their muscles wither away.

0:42:24 > 0:42:28One of the things we do on board a space station is exercise,

0:42:28 > 0:42:30purely to simulate gravity.

0:42:30 > 0:42:34If we don't do something, then our heart will shrink,

0:42:34 > 0:42:37our ability to pump blood to our head will diminish,

0:42:37 > 0:42:40our bones will start to dissolve and our muscles will waste away.

0:42:44 > 0:42:46'OK. Separation confirmed. Timer's on.'

0:42:46 > 0:42:48'Backing away at a rate

0:42:48 > 0:42:52'of just a little over one tenth of a metre per second.'

0:42:52 > 0:42:56Re-entering gravity is a punishing experience.

0:42:56 > 0:42:58To come back to Earth is violent.

0:43:00 > 0:43:03It can be five times the force of gravity

0:43:03 > 0:43:05or eight times the force of gravity,

0:43:05 > 0:43:10crushing you down into the floor of the ship for quite a long time.

0:43:10 > 0:43:13Then, of course, you hit the ground and tumble

0:43:13 > 0:43:18and roll to a stop and now you are the victim of your past.

0:43:18 > 0:43:22You're the victim of your decision-making, lying there,

0:43:22 > 0:43:26trying to shake your head and get used to being in gravity again.

0:43:26 > 0:43:31I remarked, at the time, that I had forgotten that my lips have weight

0:43:31 > 0:43:32and my tongue has weight.

0:43:32 > 0:43:36You don't think about it. But if you try and talk articulately,

0:43:36 > 0:43:39standing on your head, you'll notice that you have to sort of control

0:43:39 > 0:43:41your lips and your tongue a little differently,

0:43:41 > 0:43:43just because gravity's pushing them the other way.

0:43:43 > 0:43:45And it's the same sort of thing,

0:43:45 > 0:43:47raising your arm, holding your head up,

0:43:47 > 0:43:50turning your head when everything wants to tumble,

0:43:50 > 0:43:54just keeping your balance, all of those things.

0:43:54 > 0:43:59It's a little bit like relearning to walk again like an infant.

0:43:59 > 0:44:01REPORTERS CLAMOUR

0:44:03 > 0:44:06'Gravity on Earth grinds us all down.

0:44:08 > 0:44:12'Over the course of the day, it actually squeezes your spine,

0:44:12 > 0:44:16'an effect you can see for yourself if you use a measuring rod.'

0:44:16 > 0:44:18OK, so it's 7:30 in the morning.

0:44:18 > 0:44:21I've just got up and I'm going to see how tall I am

0:44:21 > 0:44:24before gravity drags me down.

0:44:32 > 0:44:34That's 178 centimetres

0:44:34 > 0:44:37or just over 5'10.

0:44:41 > 0:44:45Over the course of the day, gravity compresses the fluids in your spine.

0:44:48 > 0:44:51Right, it is just past 11pm.

0:44:51 > 0:44:53I've been standing up for most of the day

0:44:53 > 0:44:57so let's see if gravity has had an effect on my height.

0:45:03 > 0:45:07That's 176 centimetres,

0:45:07 > 0:45:10so I really have shrunk by just over half an inch

0:45:10 > 0:45:13over the course of today.

0:45:17 > 0:45:21In the longer term, gravity can affect your posture permanently,

0:45:21 > 0:45:26but there are exercises you can do to counteract this effect.

0:45:26 > 0:45:29Part of my research has been looking at the effects of gravity

0:45:29 > 0:45:32on the human body. So people might not be aware

0:45:32 > 0:45:35or they might not always think about the effect of gravity

0:45:35 > 0:45:36on our physical state,

0:45:36 > 0:45:39on our health and, particularly, on our posture.

0:45:39 > 0:45:42However, because it's such a constant force,

0:45:42 > 0:45:45gravity has a massive impact over the course of our lifetime.

0:45:45 > 0:45:49As you get older, you can develop a stoop,

0:45:49 > 0:45:52which is damaging to your mobility.

0:45:52 > 0:45:54Gokun here has actually got very good posture

0:45:54 > 0:45:57but I'd like you to just show not so good posture.

0:45:57 > 0:45:59So when...

0:45:59 > 0:46:02Poor posture is really rounded shoulders

0:46:02 > 0:46:05and then loss of the curve in the back, as well.

0:46:05 > 0:46:07Can I just ask you to raise up your arms

0:46:07 > 0:46:10- when you're in that posture? - I can't go any higher.

0:46:10 > 0:46:12No, and then, just come back down, shoulders back,

0:46:12 > 0:46:15and then raise your arms.

0:46:15 > 0:46:18You can see the effect of posture on function.

0:46:19 > 0:46:22Ironically, the exercises which many gym-goers do

0:46:22 > 0:46:24actually make your posture worse.

0:46:24 > 0:46:28That's if you only exercise the frontal muscles,

0:46:28 > 0:46:31like the chest and abdominals.

0:46:31 > 0:46:35So, it's recommended you exercise the back muscles just as much,

0:46:35 > 0:46:39to straighten you out and counteract the effects of gravity.

0:46:50 > 0:46:54Gravity shapes our bodies and moulds our planet.

0:46:54 > 0:46:57Nothing happens on Earth without its power and influence.

0:46:59 > 0:47:03Sir Isaac Newton explained so many of its effects

0:47:03 > 0:47:05using one simple equation.

0:47:06 > 0:47:08And, in the centuries that followed,

0:47:08 > 0:47:11his laws of physics led to breakthrough after breakthrough,

0:47:11 > 0:47:14spurring on the Industrial Revolution.

0:47:15 > 0:47:18But in the first decade of the 20th century,

0:47:18 > 0:47:20the next genius in our story

0:47:20 > 0:47:24challenged the very foundations of our understanding of gravity.

0:47:25 > 0:47:28A young German scientist called Albert Einstein

0:47:28 > 0:47:31was churning something over in his mind.

0:47:33 > 0:47:37He thought that something in Newton's laws didn't quite add up.

0:47:47 > 0:47:49Imagine I'm the sun

0:47:49 > 0:47:52and this tennis ball is the Earth in orbit around me.

0:47:52 > 0:47:55Newton's laws can describe, very precisely,

0:47:55 > 0:47:57the path the Earth takes around the sun

0:47:57 > 0:48:03in terms of the mutual gravitational attraction between the two bodies.

0:48:03 > 0:48:07But what Newton can't explain is what connects them.

0:48:07 > 0:48:08In reality, of course,

0:48:08 > 0:48:11there is no invisible string between the Earth and the sun,

0:48:11 > 0:48:13holding the two together.

0:48:13 > 0:48:16There's just empty space, a complete void.

0:48:16 > 0:48:18And yet, according to Newton,

0:48:18 > 0:48:21the Earth and sun pull on each other instantaneously

0:48:21 > 0:48:23across a vast distance.

0:48:23 > 0:48:25How can gravity act in this way

0:48:25 > 0:48:28when there's nothing to connect it or transmit it?

0:48:32 > 0:48:34After years puzzling over this,

0:48:34 > 0:48:38Einstein had a blinding flash of inspiration.

0:48:39 > 0:48:41Just like Galileo and his ramp...

0:48:42 > 0:48:44..or Newton with his apple,

0:48:44 > 0:48:46Einstein's breakthrough came

0:48:46 > 0:48:49because he was thinking about one simple action...

0:48:53 > 0:48:55..what happens when something falls.

0:49:00 > 0:49:02To explain, I'm visiting

0:49:02 > 0:49:06this 400-foot-high tower in Northampton...

0:49:06 > 0:49:08built to safety-test lifts.

0:49:13 > 0:49:15One day in 1907,

0:49:15 > 0:49:18Einstein had what he called the "happiest thought of his life".

0:49:23 > 0:49:26What if I were standing in a stationary lift,

0:49:26 > 0:49:28completely isolated from the outside world,

0:49:28 > 0:49:33not feeling anything apart from the pull of gravity on my feet?

0:49:33 > 0:49:36What if, then, the lift cable breaks

0:49:36 > 0:49:38and I start falling?

0:49:38 > 0:49:42What are the forces that I will feel as I'm plummeting to the ground?

0:49:48 > 0:49:50CRASHING

0:49:50 > 0:49:52Well, I'm not going to try that.

0:49:55 > 0:49:57Fortunately, there's another way to test this

0:49:57 > 0:50:00without me having to plunge down a lift shaft.

0:50:00 > 0:50:02Sorry to disappoint you!

0:50:04 > 0:50:08This little device here that I have strapped to this plastic toy

0:50:08 > 0:50:10is an industrial accelerometer.

0:50:10 > 0:50:12So, it measures acceleration.

0:50:12 > 0:50:13Now, I've got it connected to my laptop

0:50:13 > 0:50:16and it's showing a measurement of 1G.

0:50:16 > 0:50:18Now, that's the downward acceleration

0:50:18 > 0:50:21due to the pull of Earth's gravity.

0:50:21 > 0:50:24So, basically, it works just like a gravity meter.

0:50:24 > 0:50:27But what happens if I were to drop it?

0:50:27 > 0:50:29Presumably, it'll carry on measuring 1G

0:50:29 > 0:50:31because it's falling in Earth's gravity.

0:50:31 > 0:50:33OK, well, let's try that and see.

0:50:52 > 0:50:55So, you can see here, along this line at the bottom,

0:50:55 > 0:50:57that's when I was holding it still

0:50:57 > 0:51:00and it's measuring an acceleration of 1G.

0:51:00 > 0:51:03These oscillations here is when I stood up

0:51:03 > 0:51:05and there's a bit of disturbance,

0:51:05 > 0:51:09but this spike along here is the moment I released it.

0:51:09 > 0:51:14And this short duration along here is the time it was falling.

0:51:14 > 0:51:16And you see, while it was falling,

0:51:16 > 0:51:20it was registering an acceleration of zero.

0:51:20 > 0:51:23Now, if you think about it, this is really odd.

0:51:23 > 0:51:26The accelerometer is accelerating downwards.

0:51:26 > 0:51:29It's plummeting in the full grip of Earth's gravity

0:51:29 > 0:51:33and yet it's measuring no acceleration at all.

0:51:33 > 0:51:37It's as though gravity has completely disappeared.

0:51:39 > 0:51:43Einstein's insight was that when something falls,

0:51:43 > 0:51:45it no longer feels the pull of gravity.

0:51:46 > 0:51:50In fact, falling is like floating in empty space.

0:51:52 > 0:51:55This is the essence of Einstein's "happy thought"

0:51:55 > 0:51:59and what we now call his "principle of equivalence".

0:52:00 > 0:52:04Einstein's point is that, when the man in the lift falls,

0:52:04 > 0:52:09he doesn't just feel weightless, he is weightless.

0:52:09 > 0:52:12Einstein said the man feels no force pulling on him

0:52:12 > 0:52:15because there is no force pulling on him.

0:52:15 > 0:52:17Gravity doesn't act on him,

0:52:17 > 0:52:20it acts on the space and time around him,

0:52:20 > 0:52:23what we now call the "geometry of space-time".

0:52:30 > 0:52:33This was a radical redefinition.

0:52:33 > 0:52:37Einstein says to forget the idea of gravity as a force,

0:52:37 > 0:52:40acting mysteriously between two objects.

0:52:40 > 0:52:46Now we have to think of it as the shape of space-time changing.

0:52:47 > 0:52:51You see, Newton saw space and time as independent,

0:52:51 > 0:52:53fixed and immutable,

0:52:53 > 0:52:58that three-dimensional space is the stage in which things happen,

0:52:58 > 0:52:59but time is separate,

0:52:59 > 0:53:01it ticks by at the same rate

0:53:01 > 0:53:03everywhere in the universe.

0:53:03 > 0:53:07According to Newton, an object would travel through space

0:53:07 > 0:53:10in a straight line unless acted upon by a force like gravity

0:53:10 > 0:53:14that would cause it to deviate from that path.

0:53:14 > 0:53:18But Einstein said that space and time aren't fixed and immutable,

0:53:18 > 0:53:21they're interconnected, meshed together

0:53:21 > 0:53:24in what is known as space-time.

0:53:26 > 0:53:29And he said that space-time can be warped -

0:53:29 > 0:53:33that matter curves space and time around it.

0:53:39 > 0:53:42So, after Einstein, we no longer see gravity

0:53:42 > 0:53:46as an invisible string pulling objects together.

0:53:48 > 0:53:50Instead, a body like the Earth

0:53:50 > 0:53:53warps the structure of space and time around it.

0:53:55 > 0:53:57And an object in orbit

0:53:57 > 0:54:00follows a path which is as straight as possible

0:54:00 > 0:54:03through that space-time.

0:54:03 > 0:54:08It's a fundamental part of Einstein's vision of reality.

0:54:08 > 0:54:11Space and time can't be disentangled.

0:54:11 > 0:54:15You can't talk about space separately from time.

0:54:17 > 0:54:21So, matter warps time as well as space.

0:54:23 > 0:54:27It's known as "gravitational time dilation",

0:54:27 > 0:54:31and it's possibly the strangest of all of Einstein's discoveries.

0:54:34 > 0:54:36I've got two identical clocks here.

0:54:36 > 0:54:39Now, because the clock lower down

0:54:39 > 0:54:42is closer to the centre of the Earth,

0:54:42 > 0:54:46it feels ever so slightly a stronger gravitational pull

0:54:46 > 0:54:47than the clock higher up.

0:54:47 > 0:54:51Einstein's theory says that the lower clock will tick by

0:54:51 > 0:54:55at a slightly slower rate than the higher clock.

0:54:55 > 0:55:00Basically, gravity slows time down.

0:55:01 > 0:55:07It's an extraordinary conception of reality that Einstein describes.

0:55:08 > 0:55:13Space is being curved and time is being distorted.

0:55:15 > 0:55:20So, why can't we perceive this in our everyday lives?

0:55:20 > 0:55:22Einstein had a rather nice way of explaining it.

0:55:25 > 0:55:27Most of us have had the experience, as children,

0:55:27 > 0:55:30of trying to work out what our parents do for a living.

0:55:30 > 0:55:33Well, imagine your father is Albert Einstein.

0:55:33 > 0:55:34When he was about 12 years old,

0:55:34 > 0:55:37young Eduard Einstein asked his father why he was so famous,

0:55:37 > 0:55:41what he'd discovered. Well, this put Einstein Sr on the spot,

0:55:41 > 0:55:44but he came up with a beautifully simple analogy.

0:55:48 > 0:55:50Einstein told his son,

0:55:50 > 0:55:54"When a blind beetle crawls over the surface of a curved branch,

0:55:54 > 0:55:58"it doesn't notice that the track it has covered is curved.

0:55:58 > 0:56:02"I was lucky enough to notice what the beetle didn't notice."

0:56:04 > 0:56:06This is what Einstein meant.

0:56:06 > 0:56:09The beetle is free to move in any direction on the branch.

0:56:09 > 0:56:12It can move forwards, backwards, left and right,

0:56:12 > 0:56:15but it has no concept of a direction up off the branch.

0:56:15 > 0:56:17It's as though, for the beetle,

0:56:17 > 0:56:20the universe is missing the third dimension.

0:56:20 > 0:56:24The beetle may think it's moving in a straight line along the branch,

0:56:24 > 0:56:27but we can see that the surface it's walking on

0:56:27 > 0:56:29is itself curving and twisted.

0:56:33 > 0:56:37Einstein's point was that what we see as the twists and curves

0:56:37 > 0:56:42of the branch feel, to the beetle, like forces pushing and pulling it.

0:56:45 > 0:56:47OK, so, consider this rather strange example.

0:56:47 > 0:56:51Imagine we have two beetles perched on this pumpkin and,

0:56:51 > 0:56:55for whatever reason, they want to walk up towards the top.

0:56:55 > 0:57:00Now, if they start at the equator, pointing due north,

0:57:00 > 0:57:05as they walk, they will begin by moving parallel to each other.

0:57:05 > 0:57:08That means their paths should never meet.

0:57:08 > 0:57:13But, as they get closer to the top, their paths get closer together.

0:57:13 > 0:57:14Now, if they're clever beetles,

0:57:14 > 0:57:17they might try and figure out what's going on,

0:57:17 > 0:57:19and they could imagine that there's some mysterious force

0:57:19 > 0:57:22that's pulling them closer together.

0:57:22 > 0:57:24But, for us, from our perspective,

0:57:24 > 0:57:26we can see there is no such force.

0:57:26 > 0:57:27All they're doing

0:57:27 > 0:57:31is following straight paths over a curved surface.

0:57:34 > 0:57:36Just as the beetles have no sense

0:57:36 > 0:57:39that the surface of the branch is curved,

0:57:39 > 0:57:42we completely fail to perceive

0:57:42 > 0:57:45the bizarre ways that gravity

0:57:45 > 0:57:47shapes the reality we live in.

0:57:50 > 0:57:53Einstein's problem was proving that he was right.

0:57:55 > 0:58:00After years more thought, he realised that there WAS a way...

0:58:00 > 0:58:03by looking far out into the solar system.

0:58:04 > 0:58:07Incredibly, here in the grounds of Herstmonceux Castle

0:58:07 > 0:58:10is housed one of the original telescopes

0:58:10 > 0:58:14that were used to prove Einstein was correct.

0:58:17 > 0:58:21In 1915, when Einstein developed his general theory of relativity,

0:58:21 > 0:58:24it was just that - it was a theory, it had no proof.

0:58:24 > 0:58:27In fact, many people found it completely outlandish.

0:58:27 > 0:58:30But then, just four years later,

0:58:30 > 0:58:33in 1919, this telescope, and allow me to geek out a bit here

0:58:33 > 0:58:35and I'll give it its correct name,

0:58:35 > 0:58:39this is the 13-inch astrographic refractor,

0:58:39 > 0:58:43this telescope proved that Einstein was, in fact, right.

0:58:43 > 0:58:47That gravity does curve space itself.

0:58:53 > 0:58:56Marek Kukula is the public astronomer

0:58:56 > 0:58:58at the Royal Observatory in London,

0:58:58 > 0:59:00and he's recently rediscovered

0:59:00 > 0:59:04a neglected treasure in their archives.

0:59:04 > 0:59:06This is, perhaps, one of the most important

0:59:06 > 0:59:10scientific artefacts we have in the collection here in Greenwich

0:59:10 > 0:59:12and, for an astrophysicist like me,

0:59:12 > 0:59:14it's almost a holy relic.

0:59:15 > 0:59:20It's a glass plate photo of a solar eclipse taken in 1919

0:59:20 > 0:59:23as part of a famous scientific expedition.

0:59:26 > 0:59:28British astronomers had travelled all the way to Brazil

0:59:28 > 0:59:30and the West Coast of Africa

0:59:30 > 0:59:34to take photographs which they hoped would prove Einstein right.

0:59:35 > 0:59:38What we're seeing here is the eclipse of 1919.

0:59:38 > 0:59:42You can see the black disc of the moon silhouetted against the sun,

0:59:42 > 0:59:45blocking its light. Around it is the solar corona,

0:59:45 > 0:59:47the sun's outer atmosphere,

0:59:47 > 0:59:51and this spectacular prominence of gas leaping off the surface.

0:59:51 > 0:59:54But it's not the sun that we're really interested in.

0:59:54 > 0:59:56The fundamental point that this photo

0:59:56 > 0:59:58and others from the expedition show

0:59:58 > 1:00:01is that the positions, the apparent positions,

1:00:01 > 1:00:04of the stars in the sky are altered and shifted

1:00:04 > 1:00:07from where we would expect them normally to be,

1:00:07 > 1:00:10and that proves this very strange thing

1:00:10 > 1:00:12that general relativity predicts -

1:00:12 > 1:00:14that the mass of the sun

1:00:14 > 1:00:16bends the space and time around it,

1:00:16 > 1:00:19and that distortion is gravity.

1:00:22 > 1:00:25This is a negative of one of the photos.

1:00:25 > 1:00:28It has markings showing where the stars' positions

1:00:28 > 1:00:29seem to have shifted.

1:00:31 > 1:00:34Since then, observation after observation

1:00:34 > 1:00:37have confirmed that matter curves space

1:00:37 > 1:00:39and slows down time.

1:00:42 > 1:00:46So, the simple question of why things fall the way they do

1:00:46 > 1:00:47has led us deeper and deeper

1:00:47 > 1:00:50into the very nature of space and time itself.

1:00:52 > 1:00:58Gravitational science shows us how galaxies, stars and planets form.

1:00:58 > 1:01:01By measuring gravity, we've discovered the existence

1:01:01 > 1:01:06of dark matter, that 80% of the mass of our universe is invisible

1:01:06 > 1:01:10and we don't know what it's made of.

1:01:10 > 1:01:14And we've detected exotic objects with extreme gravity...

1:01:15 > 1:01:17..like neutron stars,

1:01:17 > 1:01:19which have more mass than our sun

1:01:19 > 1:01:22yet are only 20 kilometres across.

1:01:25 > 1:01:28But it's another mysterious aspect of Einstein's universe

1:01:28 > 1:01:31that I want to explore in my next gravity project.

1:01:34 > 1:01:36Here at the University of Surrey,

1:01:36 > 1:01:39some colleagues and I have been working on it for months.

1:01:40 > 1:01:45What we're doing is devising a nationwide citizen science project.

1:01:45 > 1:01:47We're developing a smartphone app

1:01:47 > 1:01:50that uses the GPS contained on your phone

1:01:50 > 1:01:53to explore one of the strangest properties of gravity -

1:01:53 > 1:01:56how it affects the rate at which we age.

1:01:58 > 1:02:01'I formulated the equations myself...

1:02:02 > 1:02:06'..and a small team of computer scientists and software developers

1:02:06 > 1:02:08'is using them to devise the app.'

1:02:12 > 1:02:15Einstein discovered that, as gravity changes,

1:02:15 > 1:02:18so does the rate that time ticks.

1:02:20 > 1:02:23This means the strength of gravity you feel

1:02:23 > 1:02:26affects how quickly or slowly you age.

1:02:29 > 1:02:32The aim of my app is to demonstrate this effect.

1:02:32 > 1:02:35It works by using a phone's GPS data

1:02:35 > 1:02:38to estimate your local gravity.

1:02:40 > 1:02:44And it also calculates the average speed at which you move

1:02:44 > 1:02:47because this, too, affects the rate at which you age.

1:02:50 > 1:02:52It then uses the equations I've written,

1:02:52 > 1:02:55which are based on Einstein's theory of relativity,

1:02:55 > 1:03:00to calculate, overall, how fast or slowly you're ageing.

1:03:03 > 1:03:05Once the app is ready, I tweet about it.

1:03:08 > 1:03:10Thousands of people download it

1:03:10 > 1:03:13and we start to gather results from across the country.

1:03:15 > 1:03:19Some people send me videos, giving me their results,

1:03:19 > 1:03:21how fast they are ageing

1:03:21 > 1:03:26compared with how time ticks out in space in zero gravity.

1:03:26 > 1:03:31Over the past day, I have aged less by about 172 microseconds.

1:03:31 > 1:03:37I have aged less by 10.02 milliseconds.

1:03:37 > 1:03:43So, since downloading the app, I have aged less by 1.14 milliseconds.

1:03:43 > 1:03:46Since opening Time Warper,

1:03:46 > 1:03:50I have aged less by 2.6 milliseconds.

1:03:51 > 1:03:53Our aim is to use their results

1:03:53 > 1:03:56to build up a map of how time flows

1:03:56 > 1:03:57because of gravity.

1:03:59 > 1:04:04My smartphone project provides just one insight into the space and time

1:04:04 > 1:04:07which Einstein's theories describe.

1:04:22 > 1:04:24Gravity and its strange ways

1:04:24 > 1:04:26have given us astonishing insights

1:04:26 > 1:04:28into the dark secrets of our universe.

1:04:30 > 1:04:34Perhaps the weirdest objects in the universe are black holes,

1:04:34 > 1:04:37collapsed stars whose gravity is so strong

1:04:37 > 1:04:40that not even light can escape their grip.

1:04:42 > 1:04:45Now, for the first time ever, their effects have been felt on Earth

1:04:45 > 1:04:50and they've been detected through the medium of gravity itself.

1:04:52 > 1:04:56It's a story that has revolutionised the study of modern cosmology.

1:05:00 > 1:05:021.3 billion years ago,

1:05:02 > 1:05:04in a galaxy far, far away,

1:05:04 > 1:05:08two black holes swirled around each other,

1:05:08 > 1:05:10drew closer and closer together,

1:05:10 > 1:05:14until they finally collided with incredible violence.

1:05:14 > 1:05:16In that final fraction of a second,

1:05:16 > 1:05:19at the precise moment that they merged,

1:05:19 > 1:05:20a disturbance was created

1:05:20 > 1:05:23that sent ripples out through the universe.

1:05:26 > 1:05:30Gravitational waves are a key prediction of Einstein's theory.

1:05:32 > 1:05:37Matter doesn't just curve space time, it can cause waves,

1:05:37 > 1:05:39ripples which expand outwards,

1:05:39 > 1:05:41exactly like a stone dropped in water.

1:05:44 > 1:05:47This particular wave was unimaginably large.

1:05:48 > 1:05:53The energy released was greater than all the light being given out

1:05:53 > 1:05:55by all the stars in the universe.

1:05:57 > 1:06:00The wave rippled through space at the speed of light.

1:06:00 > 1:06:03In 1.3 billion years,

1:06:03 > 1:06:07it covered a distance of over 10 billion trillion kilometres.

1:06:17 > 1:06:22Until, on the morning of the 14th of September, 2015,

1:06:22 > 1:06:23it arrived here.

1:06:25 > 1:06:28The streets and cafes of New Orleans.

1:06:28 > 1:06:33In fact, everything in America - and on Earth -

1:06:33 > 1:06:36expanded and contracted very, very slightly

1:06:36 > 1:06:38as the wave passed through.

1:06:40 > 1:06:43No-one noticed as, by the time it arrived here,

1:06:43 > 1:06:45the distortion was phenomenally tiny.

1:06:50 > 1:06:53Except that one science laboratory did notice...

1:06:55 > 1:06:57..and I'm going to see it.

1:07:01 > 1:07:051,000 scientists across the world are collaborating on it.

1:07:08 > 1:07:11It's the culmination of over 50 years of effort

1:07:11 > 1:07:15and is one of the most sophisticated experiments

1:07:15 > 1:07:17ever devised by humanity.

1:07:19 > 1:07:21So, I'm pretty excited to see it.

1:07:23 > 1:07:25It's a rather unusual setting.

1:07:25 > 1:07:27Here I am, in the middle of rural Louisiana,

1:07:27 > 1:07:30about an hour's drive outside New Orleans.

1:07:30 > 1:07:33I don't expect to find such a multi-million dollar,

1:07:33 > 1:07:36cutting-edge research facility as this,

1:07:36 > 1:07:39and yet, this is the place where, recently,

1:07:39 > 1:07:41one of the most important scientific discoveries

1:07:41 > 1:07:44in human history was made. This is LIGO.

1:07:48 > 1:07:52The Laser Interferometer Gravitational Wave Observatory

1:07:52 > 1:07:56is an enormous construction shaped like an L...

1:07:57 > 1:07:59..with a sophisticated laser system

1:07:59 > 1:08:01bouncing up and down the two arms.

1:08:03 > 1:08:06So, we're standing on top of one of LIGO's two arms.

1:08:06 > 1:08:08This is the first LIGO arm.

1:08:08 > 1:08:12And in that tube, there's a laser beam that we bounce back and forth

1:08:12 > 1:08:15between a mirror and the end station and a mirror in this building.

1:08:15 > 1:08:17And the other bit goes that way four kilometres,

1:08:17 > 1:08:19perpendicular to the arm we first saw.

1:08:19 > 1:08:21- So, this is the L shape? - It's a big L on the ground.

1:08:21 > 1:08:23So, the light bounces back and forth

1:08:23 > 1:08:26in that arm and bounces back and forth in this arm,

1:08:26 > 1:08:29and what we actually measure with LIGO is the length of this arm

1:08:29 > 1:08:32as measured by the light between the two mirrors,

1:08:32 > 1:08:35and the length of that arm as measured by the light

1:08:35 > 1:08:37between two mirrors. And then the laser interferometer

1:08:37 > 1:08:40measures the difference between those two arm lengths.

1:08:42 > 1:08:47So, as the gravitational wave passed through, the lasers picked it up.

1:08:47 > 1:08:51They detected that LIGO's two arms changed in length

1:08:51 > 1:08:53to a very, very tiny degree.

1:08:56 > 1:08:58The signal that we saw

1:08:58 > 1:09:02was just a few thousandth of the size of the atomic nucleus.

1:09:02 > 1:09:04It's the biggest the signal ever got.

1:09:04 > 1:09:08So far, far smaller than the size of a single atom?

1:09:08 > 1:09:10Oh, much, much smaller, yeah.

1:09:10 > 1:09:13And you need something this huge to pick that up?

1:09:13 > 1:09:17That's right. This is one of the biggest sources of energy

1:09:17 > 1:09:20in the universe, one of the biggest events you'd ever measure,

1:09:20 > 1:09:22and we just barely saw it.

1:09:26 > 1:09:30The LIGO scientists turned the gravitational waves

1:09:30 > 1:09:31into sound waves,

1:09:31 > 1:09:36so what you're about to hear is, in a very real sense,

1:09:36 > 1:09:39the sound of two black holes colliding.

1:09:39 > 1:09:42RHYTHMIC PULSES

1:09:46 > 1:09:49It was the first observation of any kind

1:09:49 > 1:09:51of pairs of stellar mass black holes.

1:09:51 > 1:09:53"Stellar mass" means, you know,

1:09:53 > 1:09:57several or a bunch of suns in weight.

1:09:57 > 1:09:59And so we learned that they exist,

1:09:59 > 1:10:02we learned that there are enough of them that, occasionally,

1:10:02 > 1:10:04they run into each other and coalesce.

1:10:04 > 1:10:06And...

1:10:06 > 1:10:09we also learned, by comparing the waveform we observed

1:10:09 > 1:10:12with the general relativity calculations,

1:10:12 > 1:10:16that general relativity is, as far as we know, dead-on right.

1:10:26 > 1:10:30The long concrete bunker to my left houses the beam line,

1:10:30 > 1:10:33one of the LIGO's laser arms.

1:10:34 > 1:10:39The detail and the effort that's gone into isolating the beam

1:10:39 > 1:10:40from the outside environment

1:10:40 > 1:10:44reminds me very much of Cavendish's famous experiment.

1:10:44 > 1:10:47He, too, had to worry about isolating his experiment

1:10:47 > 1:10:49from external disturbances.

1:10:49 > 1:10:53Only, of course, LIGO takes things to a far, far greater degree.

1:10:54 > 1:10:59Inside the arm is one of the largest and purest vacuums in the world.

1:11:00 > 1:11:02Atmospheric pressure in there

1:11:02 > 1:11:06has been reduced to one trillionth of the pressure outside.

1:11:06 > 1:11:09The mirrors inside are so reflective

1:11:09 > 1:11:14that they only absorb one in three million photons.

1:11:14 > 1:11:19And at the end of my little trip, lies a British success story.

1:11:25 > 1:11:29Well, I made it all the way to the end of one of the LIGO arms.

1:11:29 > 1:11:31To be honest, it took me a bit longer than I thought,

1:11:31 > 1:11:33especially in that thing,

1:11:33 > 1:11:37but housed inside this building is one of the reflecting mirrors

1:11:37 > 1:11:38that bounces the laser beam

1:11:38 > 1:11:41all the way back down the four kilometre arm

1:11:41 > 1:11:43to the main control centre.

1:11:43 > 1:11:46And the technology that went into developing these mirrors

1:11:46 > 1:11:47is quite remarkable.

1:11:47 > 1:11:51It was developed in the UK at the University of Glasgow.

1:11:58 > 1:12:00This is what the mirror looks like.

1:12:01 > 1:12:04Its surface is extraordinarily smooth,

1:12:04 > 1:12:08no bump bigger than a few billionths of a metre high.

1:12:10 > 1:12:12Equally amazing are these...

1:12:13 > 1:12:18..fused silica fibres, a few times the thickness of a human hair...

1:12:20 > 1:12:22..designed by the University of Glasgow

1:12:22 > 1:12:26in conjunction with scientists from other British universities.

1:12:27 > 1:12:30They isolate the mirror completely

1:12:30 > 1:12:33so it hangs perfectly still.

1:12:33 > 1:12:35You could say that in there

1:12:35 > 1:12:38is the quietest place on Earth.

1:12:39 > 1:12:42Despite this, outside events do sometimes interfere

1:12:42 > 1:12:45with the work here, as I witnessed for myself.

1:12:46 > 1:12:50I've wandered into the control room here at LIGO because I'm told

1:12:50 > 1:12:54something kicked off a few hours ago and they're all very busy.

1:12:54 > 1:12:57The image that's flickering up there

1:12:57 > 1:12:59is not meant to be like that.

1:12:59 > 1:13:01Essentially, what they picked up

1:13:01 > 1:13:03is a seismic disturbance, an earthquake.

1:13:03 > 1:13:06Now, that's not an earthquake down the road.

1:13:06 > 1:13:10It started on the other side of the planet, in Japan.

1:13:10 > 1:13:12So, it just gives us a sense

1:13:12 > 1:13:14of the tremendous challenges faced by LIGO

1:13:14 > 1:13:18and the team here and the level of sensitivity needed

1:13:18 > 1:13:21that an earthquake on the other side of the Earth

1:13:21 > 1:13:24can disrupt their measurements and they have

1:13:24 > 1:13:26to reset everything all over again.

1:13:29 > 1:13:33One of the scientists involved in developing this extraordinary place

1:13:33 > 1:13:35put it quite succinctly.

1:13:35 > 1:13:39"Once we were blind, but now we can see."

1:13:41 > 1:13:43Throughout the entire history of astronomy,

1:13:43 > 1:13:47we've studied gravity and how it affects matter in the universe

1:13:47 > 1:13:50and how it warps space-time,

1:13:50 > 1:13:54but only by looking at the light that enters our telescopes,

1:13:54 > 1:13:56now, for the first time,

1:13:56 > 1:13:59we can study the universe in a different way.

1:13:59 > 1:14:02The discovery of gravitational waves means we can see objects

1:14:02 > 1:14:05that cause extreme warping of space-time

1:14:05 > 1:14:08and its effect on gravity directly.

1:14:08 > 1:14:12This essentially opens up a new era in astronomy,

1:14:12 > 1:14:15it gives us a new way of looking out at the universe.

1:14:18 > 1:14:21Professor Sheila Rowan was one of the scientists

1:14:21 > 1:14:24who spearheaded the British effort for LIGO.

1:14:24 > 1:14:27For her and her colleagues,

1:14:27 > 1:14:30gravitational wave detection is just in its infancy.

1:14:32 > 1:14:34New instruments - even more sensitive than LIGO -

1:14:34 > 1:14:36are now being developed.

1:14:38 > 1:14:40There's so much that we don't understand

1:14:40 > 1:14:42about the universe that we live in,

1:14:42 > 1:14:46and this has suddenly given us a new tool, a new way,

1:14:46 > 1:14:49to probe the dark processes in the universe,

1:14:49 > 1:14:54because every time we make the observatories more sensitive,

1:14:54 > 1:14:59we can sense gravitational wave signals from further away,

1:14:59 > 1:15:03from further out in the universe, from further back in cosmic history.

1:15:03 > 1:15:07Things like supermassive black holes spiralling in to collide,

1:15:07 > 1:15:11small black holes orbiting round supermassive black holes,

1:15:11 > 1:15:16tracing out the dents in space-time of those supermassive objects.

1:15:16 > 1:15:20A long-term goal is to probe back further

1:15:20 > 1:15:22towards what we think of as the Big Bang,

1:15:22 > 1:15:25the earliest moments that we understand

1:15:25 > 1:15:28of the universe as we know it.

1:15:41 > 1:15:45If you think about it, time and time again in the history of science,

1:15:45 > 1:15:47unlocking the mysteries of gravity

1:15:47 > 1:15:51have led to a deeper understanding of the universe.

1:15:51 > 1:15:54Galileo and his ramp, Newton and his apple,

1:15:54 > 1:15:56Einstein and the falling man in the lift.

1:15:56 > 1:16:02Each of these characters challenged the scientific consensus of the day.

1:16:02 > 1:16:06And even today, understanding the true nature of gravity

1:16:06 > 1:16:09remains one of the biggest challenges in science.

1:16:12 > 1:16:15Which brings me back to the smartphone app.

1:16:15 > 1:16:19And it's at this point that our story, for me, at least,

1:16:19 > 1:16:21takes a completely unexpected turn.

1:16:23 > 1:16:27Unfortunately, it's all gone a bit pear-shaped.

1:16:27 > 1:16:31OK, so, here's what's happened. A couple of months ago,

1:16:31 > 1:16:34we launched the app and it was all going really well.

1:16:34 > 1:16:35Thousands of people downloaded it

1:16:35 > 1:16:38and have been sending us their results.

1:16:38 > 1:16:43We've been collecting the data to create this nationwide map

1:16:43 > 1:16:46to show how time flows at different rates for different people

1:16:46 > 1:16:48around the country.

1:16:48 > 1:16:52Unfortunately, I've just realised there's a big problem.

1:16:57 > 1:16:59You see, I was going over the scientific literature

1:16:59 > 1:17:03and I came across this subtle point about relativity

1:17:03 > 1:17:06which basically made me sit bolt upright.

1:17:06 > 1:17:09There was this horrible dawning realisation

1:17:09 > 1:17:13that I'd made a mistake in the equations that get fed into the app.

1:17:15 > 1:17:20What this means is all the results we've been gathering are wrong.

1:17:24 > 1:17:27The issue lies in the strange and subtle effects

1:17:27 > 1:17:30of Einstein's theories of relativity,

1:17:30 > 1:17:33and it's fundamental to the way time flows

1:17:33 > 1:17:37across the surface of the globe.

1:17:37 > 1:17:40Now, what if I use my smartphone app where I live here,

1:17:40 > 1:17:42on the south coast of England

1:17:42 > 1:17:45and then go and spend a few days down near the equator?

1:17:45 > 1:17:48So, here on the West Coast of Africa.

1:17:51 > 1:17:55Now, we know from the road trip that gravity is weaker by the equator.

1:17:57 > 1:18:00So, that means time ticks faster there.

1:18:01 > 1:18:05But there's another important factor we have to take into account -

1:18:05 > 1:18:07movement.

1:18:07 > 1:18:09You see, when I'm here, near the equator,

1:18:09 > 1:18:11I'm moving more quickly

1:18:11 > 1:18:15than I was back in Britain because of the rotation of the Earth.

1:18:15 > 1:18:18Einstein says movement slows down time

1:18:18 > 1:18:21so clocks will tick slower at the equator.

1:18:22 > 1:18:24This is where the error crept in.

1:18:24 > 1:18:27You see, I had taken into account these two effects,

1:18:27 > 1:18:29but I'd missed a crucial point.

1:18:29 > 1:18:32They cancel each other out exactly.

1:18:32 > 1:18:34In fact, the Earth bulges out

1:18:34 > 1:18:39exactly the right amount for its rotational speed

1:18:39 > 1:18:41to make sure they cancel out,

1:18:41 > 1:18:45so all clocks on the surface of the Earth, at sea level, tick

1:18:45 > 1:18:49at exactly the same rate.

1:18:49 > 1:18:52So, now I'm having to go right back to square one

1:18:52 > 1:18:55and completely rewrite the equations for the app.

1:19:01 > 1:19:03And, to test if it's working,

1:19:03 > 1:19:06I'm going to use it over the course of a normal working week.

1:19:08 > 1:19:10This is where I live, this is Portsmouth,

1:19:10 > 1:19:13which means I'm very close to sea level,

1:19:13 > 1:19:15and this is how I start most mornings,

1:19:15 > 1:19:18catching the train to work.

1:19:18 > 1:19:23The app records my speed as I'm on the train

1:19:23 > 1:19:27and calculates how this slows down my personal clock.

1:19:27 > 1:19:29I think the train journey

1:19:29 > 1:19:33should have slowed my time down by a tiny...

1:19:33 > 1:19:35A few trillionths of second.

1:19:35 > 1:19:39I'm heading for the BBC's headquarters in Central London,

1:19:39 > 1:19:42and gravity should be a bit weaker here.

1:19:42 > 1:19:44I'm a few metres above sea level, I guess, here.

1:19:44 > 1:19:48And so there will be a speed-up of my time because of altitude.

1:19:48 > 1:19:51The app compares the way my time flows

1:19:51 > 1:19:54with a stationary clock at sea level.

1:19:54 > 1:19:56So, what's my result?

1:19:56 > 1:20:01On an average day, my movement makes me age slower by a third

1:20:01 > 1:20:06of a nanosecond. That's a third of a billionth of a second.

1:20:06 > 1:20:08But the weaker gravity I'm in

1:20:08 > 1:20:11means I age faster - overall,

1:20:11 > 1:20:13half a nanosecond faster.

1:20:15 > 1:20:18I've also given the app to some other volunteers

1:20:18 > 1:20:20to compare how they age over an average day.

1:20:22 > 1:20:25Nick flies cargo planes.

1:20:25 > 1:20:28He flies from Chicago to Germany.

1:20:34 > 1:20:36Tomorrow morning,

1:20:36 > 1:20:41we have to leave to go first to Milan and then on to Tokyo.

1:20:41 > 1:20:45His travel slows down his ageing,

1:20:45 > 1:20:48but much weaker gravity at high altitude

1:20:48 > 1:20:52speeds his clock up by just a bit more.

1:20:52 > 1:20:55Overall, he's ageing five nanoseconds faster

1:20:55 > 1:20:58than a stationary clock at sea level.

1:20:58 > 1:21:01Vanessa runs a pub in the Yorkshire Dales.

1:21:01 > 1:21:05I'm going to take you outside to see the weather conditions here.

1:21:05 > 1:21:07So, here we are, outside the Tan Hill Inn.

1:21:07 > 1:21:10We live right in the middle of the National Park on the moor.

1:21:10 > 1:21:15The Tan Hill Inn is famous as Britain's highest altitude pub

1:21:15 > 1:21:18at over 500 metres above sea level.

1:21:18 > 1:21:21We don't have any neighbours, we just have sheep.

1:21:21 > 1:21:24Her altitude means she ages faster every day

1:21:24 > 1:21:27by around four nanoseconds

1:21:27 > 1:21:29compared to someone at sea level.

1:21:30 > 1:21:33There's Kevin, a mountaineer in the Highlands.

1:21:33 > 1:21:36I'm on a mountain in Glencoe called Sgor na h-Ulaidh.

1:21:36 > 1:21:39I've been at an altitude generally of between 2,000-3,000 feet

1:21:39 > 1:21:41for a lot of the day. Throughout the day,

1:21:41 > 1:21:44I've just been logging on to the phone, logging on to the app,

1:21:44 > 1:21:46and just checking it out and having a look,

1:21:46 > 1:21:48and I've been watching it get bigger

1:21:48 > 1:21:50and watching the value get bigger and bigger.

1:21:50 > 1:21:52So, it's been quite a lot of fun.

1:21:53 > 1:21:55On an average day of climbing,

1:21:55 > 1:21:59Kevin's personal clock goes faster by one nanosecond.

1:22:02 > 1:22:04Gary works for a Scottish water retailer.

1:22:05 > 1:22:08My job takes me all over the UK,

1:22:08 > 1:22:11dealing with energy consultants and energy brokers,

1:22:11 > 1:22:14as far up north as Inverness, as far down south as London.

1:22:14 > 1:22:17I approximately do about 1,000 miles a week, sometimes more,

1:22:17 > 1:22:21depending on the number of meetings I have.

1:22:21 > 1:22:24Gary's car journeys do slow his time down a bit,

1:22:24 > 1:22:26but being above sea level

1:22:26 > 1:22:30means he still ages faster by three quarters of a nanosecond.

1:22:32 > 1:22:34Our final volunteer is Walter.

1:22:34 > 1:22:38He lives close to sea level at the iconic John O'Groats.

1:22:39 > 1:22:43I run the tourism business and I started about 50 years ago,

1:22:43 > 1:22:47so when people come here, they can actually speak to someone

1:22:47 > 1:22:50who's been born in John O'Groats and, if they ask questions,

1:22:50 > 1:22:52I can tell them all sorts of useless information

1:22:52 > 1:22:55because I'm full of useless information.

1:22:55 > 1:23:00So our final results show that, if you want to age more slowly,

1:23:00 > 1:23:03try to live near sea level, like Walter.

1:23:05 > 1:23:08Or there is another way to do it -

1:23:08 > 1:23:11get a job on the International Space Station.

1:23:11 > 1:23:16Its 17,000-mile-an-hour orbit will give you a boost.

1:23:18 > 1:23:20We did the maths for the astronauts.

1:23:20 > 1:23:25Every month, you are about one millisecond younger,

1:23:25 > 1:23:27so one thousandth of a second.

1:23:27 > 1:23:29So, after six months,

1:23:29 > 1:23:32you're that much younger than people on Earth.

1:23:32 > 1:23:34So, I'm younger than I should be.

1:23:34 > 1:23:35I hope I look it.

1:23:37 > 1:23:39Of course, for us on Earth,

1:23:39 > 1:23:42time dilation is so utterly minuscule,

1:23:42 > 1:23:45a few billionths of a second between us,

1:23:45 > 1:23:48you might think it's too frivolous to even bother about.

1:23:51 > 1:23:55And yet, in the long and difficult process of designing the app,

1:23:55 > 1:23:59I've come to an extraordinary conclusion.

1:23:59 > 1:24:01The different ways that time flows

1:24:01 > 1:24:06may not be some quirky by-product of gravity.

1:24:06 > 1:24:09It may actually BE gravity.

1:24:09 > 1:24:12It may be the CAUSE of gravity...

1:24:12 > 1:24:14the reason why objects fall.

1:24:17 > 1:24:20One of the colleagues I've been consulting is Kip Thorne.

1:24:20 > 1:24:23He's one of the world's leading theoretical physicists

1:24:23 > 1:24:26and a driving force behind the creation of LIGO.

1:24:26 > 1:24:30While I was going back over some of the basic physics behind the app,

1:24:30 > 1:24:33I came across an intriguing idea of his.

1:24:33 > 1:24:36It's a very interesting and different way

1:24:36 > 1:24:37of describing gravity.

1:24:41 > 1:24:42This is what Kip says.

1:24:43 > 1:24:48"Everything likes to live where it'll age the most slowly,

1:24:48 > 1:24:50"and gravity pulls it there."

1:24:52 > 1:24:54Kip's based at Caltech in California

1:24:54 > 1:24:59and is one of the most respected theoretical physicists in the world.

1:24:59 > 1:25:01Firstly, Kip, a serious thank you

1:25:01 > 1:25:05for helping out with the debacle over the app!

1:25:05 > 1:25:07Well, I sympathise.

1:25:07 > 1:25:10I've made so many errors of my own over the years

1:25:10 > 1:25:13that I am totally sympathetic.

1:25:13 > 1:25:15One of the things that struck me,

1:25:15 > 1:25:18thinking about this, is something you wrote, Kip.

1:25:18 > 1:25:23You said, "Everything likes to live where it'll age the most slowly,

1:25:23 > 1:25:26"and gravity pulls it there."

1:25:26 > 1:25:29Was this a way of explaining something

1:25:29 > 1:25:31that you felt was a neat explanation

1:25:31 > 1:25:34or is there something deeply profound about that?

1:25:34 > 1:25:38I think there is something deeply profound, in some sense,

1:25:38 > 1:25:42but it's a lovely description

1:25:42 > 1:25:48of Einstein's first major insight about gravity.

1:25:48 > 1:25:51In 1912, he realised that gravity

1:25:51 > 1:25:55that we feel on Earth is due to a slowing of time on Earth.

1:25:55 > 1:25:59So, time comes before gravity, in that sense?

1:25:59 > 1:26:01On the Earth's surface, time runs more slowly

1:26:01 > 1:26:05and that accounts for why gravity wants to keep us there?

1:26:05 > 1:26:07Well, I think, in a very deep sense, this is true.

1:26:07 > 1:26:09Objects WANT to fall.

1:26:09 > 1:26:12The flow of time, or the rate of flow of the time,

1:26:12 > 1:26:15is the thing that produces the gravity,

1:26:15 > 1:26:20it is the thing that is ultimately responsible for the fall.

1:26:20 > 1:26:23So, somehow, it's in the nature of all objects

1:26:23 > 1:26:27to move towards a region where time runs slower.

1:26:27 > 1:26:30Kip's formulation works anywhere in the universe

1:26:30 > 1:26:34where the gravitational field is such as on Earth.

1:26:35 > 1:26:38The difference in the rate of flow of time is tiny.

1:26:38 > 1:26:41At high altitude and on the surface of the Earth,

1:26:41 > 1:26:46the difference in the rate of flow of time is one second in 100 years.

1:26:46 > 1:26:48That's not very much!

1:26:48 > 1:26:53But that is enough that it's precisely the right amount

1:26:53 > 1:26:56to produce the gravitational pull that we feel

1:26:56 > 1:26:59and produce the accelerations we're talking about.

1:26:59 > 1:27:03Wow, OK. I need to go and write this one down!

1:27:03 > 1:27:05THEY LAUGH

1:27:07 > 1:27:11So, my investigation deep into the weird ways of gravity

1:27:11 > 1:27:14has finally left me face-to-face

1:27:14 > 1:27:18with one of the greatest mysteries in all of physics,

1:27:18 > 1:27:21the nature of time itself.

1:27:21 > 1:27:24It sounds like such a simple question.

1:27:24 > 1:27:26Why does the apple fall?

1:27:26 > 1:27:29And yet, hundreds of years of scientific enquiry

1:27:29 > 1:27:32investigating this single action

1:27:32 > 1:27:34have led us to completely redefine

1:27:34 > 1:27:37the way we think about the very nature of space and time.

1:27:39 > 1:27:42And now I've been presented with this extraordinary proposition,

1:27:42 > 1:27:46that somehow, in some profound way,

1:27:46 > 1:27:49the apple falls because it's seeking out the place

1:27:49 > 1:27:52where time runs the slowest.

1:27:52 > 1:27:56So, does gravity dictate the flow of time?

1:27:56 > 1:28:00Or does time itself define gravity?

1:28:00 > 1:28:04Could this hint to fundamental new laws of physics,

1:28:04 > 1:28:05as yet undiscovered?

1:28:05 > 1:28:08I think I'm going to have to think about this a bit more.