The Fascinating Truth About Gravity | Jim Al-Khalili: Gravity and Me | Spark

[Music] would you like to lose a little bit of weight without doing any exercise or dieting would you like to age just a bit more slowly than your friends well you might be surprised to hear the laws of physics can help the keys to unlocking these everyday questions is gravity it sculpts the universe it warps space and time it's a fundamental force of nature but gravity's strange powers discovered by Albert Einstein also affect our daily lives in the most unexpected ways [Music] in this film we'll be using cutting-edge scientific techniques to investigate how gravity changes your weight it's gone up your height I really have shrunk and even your posture and with the help of thousands of volunteers I'll show you how gravity makes us all aged at different rates through the day I've just been logging on to the phone logging on to the app as a physicist gravity is central to my work and in exploring it I'll be challenged on how I understand this most mysterious force Wow okay I need to go write this one down and I'll have to tackle the very nature of reality itself gravity it binds together all the matter in the universe and it makes our existence here possible but in the end it all boils down to one simple question what happens if I drop an object [Music] gravity's many mysteries are all contained in this single action how an object falls here's the first puzzle why does a hammer fall faster than a feather you might think is because the hammer is heavier but that's not the real reason the answer is air resistance it's not the weight of the objects that matters is their shape and I can demonstrate this very easily with these two umbrellas they both have exactly the same weight but if I open one of them you can be pretty sure it'll drop more slowly than the other one in fact all objects would fall at the same rate if you could only remove the air the first person to realize this was the 16th century mathematician Galileo Galilei famously it said he worked it out by dropping objects of the Leaning Tower of Pisa and he was spectacularly proven right in an experiment carried out on the moon in 1971 my right hand a hammer and I'll drop the two up here and hopefully go hit the ground at the same time it worked perfectly [Applause] now Galileo was obsessed with a second question - when you drop an object it's actually quite hard to tell if it falls at a constant speed or picks up speed as it drops even in slow motion is pretty hard to tell but Galileo realized this first dropping objects a very short distance it lands with very little impact but of course drop it from higher up this time the ball easily breaks the tile which means it must have accelerates it gaining in speed and momentum as it dropped [Music] Galileo had identified something fundamental to all falling objects they accelerate he realized there might be a way to measure how much falling objects gain in speed what he devised was the first ever attempt to measure gravity itself he built a long wooden ramp rather like this that he had sloping at a shallow angle the idea was to roll balls down the ramp and measure their acceleration the crucial thing is that the ramp had to be at the shallow angle to reduce the effects of wind resistance it also meant that the balls would roll down slowly enough to give him time to measure their speed but the big problem was this how do you measure time accurately in an age when there were no accurate timepieces there alone stopwatches well Galileo came up with an ingenious idea involving the flow of water essentially measuring time from the amount of water collected in a cup so we're going to try and repeat Galileo's experiment I say we because I have a couple of willing volunteers Gavin and Johanna three two one go okay there's one now if you come down a quarter of the way down the ramp go okay so now half of the way down go okay and then three quarters of the way down go right turn the tap off okay so we have our four measurements and I can see a progression from fuller to emptier but what we need to do now is find the mathematical pattern by weighing carefully the water in each glass weighing the water should give us an idea of how long each roll took and in our experiment these were the results now there's one immediate thing you can tell the ball really sped up the longer it rolled in fact our results seem to show that the time it took to cover the first quarter of the ramp was about the same time it took to cover the next three quarters so we have a strong hint of a mathematical pattern now we'll see if we're right by placing bells along the ramp at intervals which are based on the results this arrangement looks a bit strange because the gap between the first two bells is much shorter than the gap between the third and fourth bells but that's okay because if we've got our calculations right the ball starts off slowly so it covers a shorter distance and as it picks up pace it'll cover longer and longer distances so we should hear the bells ringing at equal intervals in time go beautiful so what does this all mean what's the mathematical formula well this is something that Galileo worked out let's say from the star the ball covers a distance of one meter in the first second after two seconds it'll have covered four meters after three seconds nine meters after four seconds sixteen meters and so on if you recognize this progression you'll see that distance goes like the square of time Galileo had found the rate at which gravity speeds up objects and he'd found another fundamental principle you can measure the strength of gravity by how much it causes falling objects to accelerate detecting gravity has become exceptionally sophisticated these days but still uses exactly the same principle this is hearth mon-sol castle in Sussex and in its grounds lies the space geodesy facility here Vicki uses an astonishingly sensitive instrument to detect the exact strength of gravity on this one spot okay say they could tell me about this incredible gravity meter that you work with okay so this is the dropping chamber and a stripped down version so essentially what happens is you've got a cart that gets raised at the top and then the car accelerates them away from a mass in the middle and so this section here lifts off and as it drops it drops under freefall so this components in the middle as it drops is basically just Newton's apple falling to the ground yes so this is a stripped-down version but but that's the real thing this is the real thing how does it actually work in here it's a vacuum so there's no wind resistance inside a laser is used to measure exactly how fast the mass is accelerating this is the 21st century version of Galileo's ramp and the balls rolling down so we can we get it going of course if you just like to press the button on the laptop this one yep okay so it's not communicating with it oh here we go there we go so wait five seconds then takes a measurement of gravity and regains oh and enough you can see the or the results appearing now yeah each of those green dots is measurement of gravity with the actual number that it's seven for each one the unit Vicki uses has a familiar ring I see that the the number up at the top here so you've got this unit micro gal yes gal is essentially one centimeter per second squared the gal was named after Galileo so we've just taken the measurement of gravity here today and it's this highly accurate number nine eight one one two four zero seven micro [Music] the reading means that the Earth's gravity speeds up a falling object by around 9.81 meters per second for every second it drops Viki tells me something intriguing she takes a reading here every week and she's found that the strength of gravity changes by tiny amounts over time heavy rainfall for example can cause gravity to increase slightly [Music] presumably if gravity is changing here in one spot it'll have different values all around the world and so you can have a gravity map of the entire planet that's right yeah so what's the reason for these strange fluctuations that's what I want to investigate next so gravity changes as we move across the surface of the earth well this lies at the heart of a challenge that I've set to young volunteers I've given them a task to try and find the place in Britain where gravity is at its weakest so where objects will weigh the least and I've given them just three days to try and find it the volunteers are Astraeus Endre a PhD student I've been living in London for five six years and I'm originally from Seville in Spain and they're interested in taking part in this project because I would really like to know more about how these world works and poppy Begum a journalist who lives in London I did my degree in biomedical science and I did biology and chemistry for my a-levels but I have done any physics since I left school and fascinated to find out more about gravity and I actually enjoy a puzzle I like a challenge now the team can't just weigh themselves to see changes in gravity body weight fluctuates naturally by a couple of kilos over the course of a day whereas changes due to gravity as they travel around the country are going to be tiny in comparison a matter of a few grams so they're going to have to use sophisticated scientific methods if they want to measure gravity accurately and that's why the volunteers will be joined by three specialists in gravity science [Music] PhD student so knack Bose he'll be in charge of some very sensitive measuring apparatus from the National Physical Laboratory Shawn Hughes a geologist will be using a portable gravity meter and Andrew Ponson a cosmologists at University College London will help interpret the results we've taken a collective wait for the team before they set off its 380 kilograms so can they find the place in Britain where that'll decrease they're setting out in Snowdonia National Park in North Wales the railway climbs from here to the thousand meter summit of Snowden Sean takes his first gravity reading the inside is a mass on a beam and you turn this counter this dial until you get the beam central by Counting the number of turns of the dial Sean can calculate the downward pull of gravity acting on the mass inside the machine so neck has a simpler method so inside the box is a two kilogram mass and it's supposed to be sort of as perfectly two kilograms as as possible to get oh it's just coming under isn't it one nine nine eight point two grams two kilos in the laboratory but now here is the Bislett it's the first puzzle why does a two kilo mass tip the scales at just under two kilos and it's one which gets straight to the heart of what the challenge is really about maths is often confused with the related quantity weight the mass of these dumbbells is fixed it doesn't change it's a measure of how much stuff they contain weight is different it's a measure of the effect of gravity on these dumbbells the downward force pulling them to the ground in the same way that it's keeping my feet firmly stuck to the ground the crucial difference is this if I was holding these dumbbells on the moon they still have exactly the same mass but they would weigh six times less because the moon's gravity is so much weaker than the Earth's so that's why Sonax bringing along the 2 kilo mass if it changes weights then this should mean that gravity itself has changed ahead of them is the summit of the highest mountain in England and Wales famed for its stunning scenery oh it would be stunning if you could see it we came all the way up here for this amazing view at the top of Snowdon you wouldn't know it but honestly we are here so we now have near the summit of Snowden and I've set up the gravimeter again and we're gonna see what the difference in the reading is he has to turn the dial again and again to try and get a reading his clear gravity has changed but which way has it got stronger or weaker the team leaves Sean to work out his results and tries to position the scales as close as possible to the summit but the reading is all over the place it's fluctuating quite a lot too to the wind I have to say this is what science is always like isn't it it's never quite what you want it to be so they head inside to the cafe next to the summit the wind was being a bit naughty but hopefully one 98.2 down there one nine nine seven point eight we've got it that's point four for drama the mass weighs a tiny bit less it's lost about 1/5 thousandth of its weight and Sean's found that gravity itself has reduced at the top of the motor of the mountain we took the measurement and we discovered that the graph gravity had gone down it had gone down it equivalent of 206 turns of the dial that's equivalent to 219 milligrams so it's clear from the team's measurements gravity weakens as you go higher and you get a bit lighter it's just an excuse to say where are we like the light is who cares but is actually really interesting it's like an illustrative example I've seen how this is actually fluctuating depending on different factors yeah absolutely and that we could measure it and we could see it with our own eyes it actually makes you think about gravity in a very active way it's such a fundamental force phenomenon in nature but we don't know much about it but why does gravity change with altitude so understand that question you have to get to grips with the extraordinary discoveries of the next scientific giant in our story Isaac Newton born in England in the middle of the 17th century he spent his life wrestling with so many apparently separate questions from why things fall to the ground to why planets orbit the Sun it took the genius of mutant to realize that there was one single equation that could answer all these questions and here it is his famous law of gravity it might look complicated but this is one of the most important equations in the whole of science F here is the force now Newton said there's an attractive force between any two objects in the universe on this side of the equation G we call the gravitational constant now Newton knew it had to be there but he didn't know what its value was M 1 and M 2 represent the two objects and R is the distance between them now the equation tells us that the more massive the objects are the bigger m1 and m2 the greater the attractive force but the further apart they are the bigger the value of R here the weaker the gravitational force with Newton what was once mysterious now became clear Newton's equation describes why an object falls to the ground including his famous Apple but it's true genius is that it applies to any object anywhere in the universe so it's a very simple and elegant way of describing some of the seemingly most complicated phenomena in the cosmos his law of gravitation can still be used today to explain how orbits work to predict when a comet will return to describe why galaxies spin what a slingshot spacecraft around planets Newton tells us to look for the underlying simplicity and natural phenomena for instance how the moon orbits the earth if I let go of this Apple it'll fall straight down because of the pull of Earth's gravity but if I throw it to begin with it travels in a horizontal direction that's the direction of travel but Earth's gravity is still pulling it downwards so it ends up following a curved path now if I throw it harder it'll travel further before it hits the ground and in principle if I could throw it hard enough I could put it into orbit and that's exactly what's happening with the moon in orbit around the Earth it's a combination of wanting to travel in a straight line but also being pulled down by the Earth's gravity so it ends up constantly falling around the earth and constantly missing Newton's famous equation also explains the strange effects which the roadtrip team has discovered that objects get lighter as you gain in altitude when I weigh myself I'm represented by the first mass m1 the second mass m2 is the earth itself and the force pulling me down my weight depends on the distance between me and the center of the earth and that's the secret of the road trip if you want to find a place where you weigh the least then you have to get as far away as you can from the earth call [Music] so it's the afternoon of day 1 and the roadtrip team have to work out where to go next poppy and Austria have a good idea find somewhere higher than Mount Snowdon from the measurements that we that you guys did at Mount Snowdon altitude clearly plays an important part in gravity so with that in mind we've got to go to the highest point in the UK which is been nervous okay but there's just one thing that we haven't shown you so far we actually brought along an extra experiment so can we please show you this first before you make the final decision yeah so NACA actually has the other part of this experiments we always carry around some power tools as physicists always do so let's start off nice and gentle okay and then try and pick up some piece the point is that when something is spinning it kind of gets flung outwards and you can actually use that to make a nice flat piece of pizza and but this also applies to the earth the earth isn't it's perfectly round is what's known as an oblate spheroid it bulges at the equator where the spin is greatest we kind of got two competing effects now we're trying to get away from the center the actual core of the earth the point at the very center of this ball but now we can do it in two ways we can either kind of go up something tall or we can just go down towards the equator there's what we find when we're doing gravity surveys is that as you move south there tends to be in effect from latitude which is often usually larger than the effect from altitude so the closer to the equator you go the further you get from the Earth's core and the lighter you get so guys the sun's sitting just behind me here mm-hmm this is north mm-hmm from the conversations we've just had it sounds like we've got to go that way down south right yeah okay let's go let's go the team is starting to uncover the reasons why gravity changes as you cross the surface of the earth our planet is defined as shaped by the complicated forces which act upon it and detecting tiny fluctuations in its gravity field can give us important clues it can help us understand how our world is changing the space geodesy facility at Hearst also is one small part in an enormous global network which uses satellites to detect the tiniest of changes in the Earth's gravity field tell me what exactly your job is here what we're doing with this telescope is measuring very accurately the distances of satellites from here so we're using very short laser pulses which we direct towards the satellite on the satellite there are reflection cubes which return some of that light to us and we measure how long it takes the light to go to the satellite and back and how far away is the satellite image ranking now's at one of the Galileo satellite which is about 20,000 kilometers 20,000 kilometers away yes okay so we've got it aimed at the gallery satellite and you're gonna turn the laser on now yes oh wow and that laser beam that's being fired up towards the satellite yeah the time they'll take to get there and come back again it's a fraction of a second isn't it is there's about 150 thousands of a second hundred fifty milliseconds and we're sending what about 1,000 of those per second this strange-looking object is based on satellite readings it's the highly exaggerated representation of how Earth's gravity field varies over time fluctuations like these can give us important insights into climate change ice caps melting sea levels rising changes in groundwater all of these have an effect on the local strength of gravity so something as important as climate change in order to understand it and do something about it we need to know the distribution of the gravitational field of the earth very accurately absolutely yes and it's a global measure that we need [Music] for the road trippers is the start of day two and they're heading for the south coast they're stopping off in Herefordshire it's a good location as it's the same altitude as the base of Snowden but they've moved about 80 miles further south so if they find gravity changes here it must be due to latitude it's not a huge difference but it's noticeable our counter eating at the bottom of the mountain was 4840 yeah our counter reading here's 4717 alright so we do get to see a difference so we're actually at the same altitude as the base of Mount Snowdon but because we've traveled further down south overnight gravity's less here yeah [Music] they push on and by sunset they reach cydnus on the south coast Shaun takes the second gravity reading of the day and poppy improvises a map well sort of a map so I drew this map Scotland's a bit squashed Wales is quite high up but Cornwall is is there but you get the idea so Shaun we we've been traveling with you you've done quite a few gravity meter readings can you can you plot them on this not to scale badly drawn map please sure so if you remember we started off in Mount Snowdon about here and that was the zero measurement for our survey and then we've come all the way down here to the south coast the difference from the base of Snowdon is minus 212 really gals so the difference between going measuring gravity at the base of the mountain and the top of the mountain is about the same as appear at this latitude and down here at this latitude there quite clearly at sea level yet gravity here is roughly the same as it is at the top of Snowden that we're next we are here if we want to find out where we are the lightest why don't we travel all the way to the most southern new points in the UK which is here multitude can also help us so why not find a place in the country that is both low and latitude but also is high in altitude in the terms of height above sea level cuz that will get us somewhere that is really far away from the core of these while staying within the [Music] so the answer to the puzzle lies in a combination of two factors how much further south should they go and how much higher at the end of day two shores results show that the team weighs about 80 grams lighter in total than back at the base of Snowdon [Music] the way that weight changes is just one example of Newton's famous equation in action but Newton had left his masterpiece incomplete he didn't know the value of G the gravitational constant which sets the size of the force to harness the full power of the equation you need to know G and the vital clue came with an incredible experiment conducted in London at the end of the 18th century it was an attempt to work out the mass of the earth itself and it was carried out by an eccentric extravagantly rich aristocrats Henry Cavendish Cavendish was a chronically shy deeply solitary man living in total isolation in his house in Clapham the story goes that one day he accidentally bumped into a female servant on his staircase he was so traumatized by this event they had a new staircase built just for him so this horrible incident could never happen again Cavendish had inherited vast fortunes and was able to dedicate his life to devising pioneering experiments including one particularly extraordinary piece of equipment he set up something a bit like this it's called a torsion balance it involves four led spheres two large heavy ones which are held fixed in place and suspended by a very thin wire is a wooden rod six feet long with two smaller balls on either end now the crux of the experiment is the relationship between the large ball and the small ball now of course there's a gravitational pull downwards on both of the balls due to the Earth's gravity but Newton also tells us that there should be a very weak gravitational pull between the balls and this is effectively what Cavendish was trying to measure any slight movement of the small ball towards the large one should cause a twist in the torsion wire and that's what Cavendish was trying to detect of course this is all much easier said than done the experiment was incredibly sensitive the tiniest of vibrations a slightest breeze changes in temperature could all influence the measurements so Cavendish had to isolate the apparatus inside a box and the box within a shed he even realized that his mere presence next to the apparatus could influence things so he had to remove himself outside the shed Mauri then did was sit outside the shed and through a small hole in the shed wall looked through a telescope to detect the tiniest of twists in the wire it was an incredibly difficult process but after many months he finally felt confident enough that he had a reliable result [Music] Cavendish found that the small balls did move a tiny four millimeters he calculated his results by comparing the density of the balls with the density of water in the end the result of Cavendish's experiments and subsequent calculations was that the density of the earth was about five and a half times that of water or put another way the mass of the earth was five point nine trillion trillion kilograms what's most remarkable is that Cavendish got this number right to within an accuracy of one percent with Cavendish's astonishing results scientists were able to work out G then the equation could be used to determine the mass of any celestial body in orbit around another so astronomers were able to calculate the mass of the Sun and the planets and the moon and eventually even distant galaxies and of course back on earth we never escape gravity over the course of the day it actually squeezes your spine an effect you can see for yourself if you use a measuring rod okay so it's half past 7:00 in the morning I've just got up and I'm going to see how to land before gravity drags me down [Music] as a hundred and seventy-eight centimeters or just over five foot ten [Music] over the course of the day gravity compresses the fluids in your spine [Music] right it is just past 11 p.m. I've been standing up for most of the day so let's see if gravity has had an effect on my height that is 176 centimeters so I really have shrunk by just over half an inch over the course of today in the longer term gravity can affect your posture permanently but there are exercises you can do to counteract this effect part of my research has been looking at the effects of gravity on the human body so people might not be aware or they might not always think about the effect of gravity on our physical state on our health and particularly on our posture however because it's such a constant force gravity has a massive impact over the course of our lifetime as you get older you can develop a stoop which is damaging to your mobility doctor here's actually got a very good posture but I'd like you to just show not so good posture so when poor posture is really rounded shoulders and then loss of the the curve and the back as well I can't ask you to raise up your arms when you're in that posture so you know and then just come back down shoulders back in zhan and then raise your arms you can see the the effects of posture on function ironically the exercises which many gym goers do actually make your posture worse that's if you only exercise the frontal muscles like the chest and abdominals so it's recommended you exercise the back muscles just as much to straighten you out and counteract the effects of gravity [Music] meanwhile is the end of day two for the road trip and they've reached CID myth on the south coast looking for the place in Britain where they'll weigh the least they've worked out the answer lies in a combination of two factors the right mix of going south and being higher up and for the final leg of the journey I'm going to meet up with them I asked them to drive a short distance west to one of the most remote areas in mainland Britain Dartmoor National Park it's only 40 miles from the southernmost tip of Britain and it's very high very hilly territory Jim the team got to the south coast yesterday to find gravity at its weakest but we haven't quite figured out whether it's altitude or latitude do we go further south or do we go higher up you're right to ask do we go as far south as possible or as high as possible that's why I've brought you here to Dartmoor and we've charted the most important points on this map here so we are here two bridges these four dots represents these hills up there behind us which are at about 500 meters above sea level so that's what we want to check out these hills are close to the south coast and they're also the highest in the whole of the South of England so logic suggests there must be the right combination of latitude and altitude well there's another reason why this makes perfect sense one which we haven't looked at yet and that is the effect of the underlying rocks on gravity and I've got a map here that shows you're gonna Trump my map right on you here we are down here no these blue areas are the lowest areas according to the density of the rocks underneath the rocks around here are made of granite which will make gravity weaker still so they're helping as well as the altitude and the fact that we're further south yep it's also playing a part well we have a plausible theory but now we need to test it if I'm right then at the top our gravity reading should be by far the lowest reading of the trip of course there's another effect of gravity to deal with now it's knackering when you head uphill okay so I think this is pretty much the start of the hills we've located on the map so let's see if this is the lightest place Sean if you want to get the gravity meter out and we'll take another reading here yep okay Shawn sets up his equipment one more time but the news well the bottom of Mount Snowdon was our zero for this test we found we lost a certain amount by going up to the top of Mount Snowdon we found we lost a certain amount coming south to the South Coast not only have we beaten that we've smashed it brilliant we were minus 219 Milligan's lower at the top of Mount Snowdon here on Dartmoor we're minus 347 million so it is a combination of three things we're we're far south so that's the latitude we're at altitude with a high up and we're surrounded by all this granite rock which is low-density anyway I hope you all think it was worth to climb up here anyway there you go boom science now we already know that the altitude of these hills takes us much further from the Earth's core than anywhere else further south in Britain so gravity must be weakest here there's extra evidence to the British Geological Survey has compiled tens of thousands of gravity readings made in the UK and the lowest readings ever recorded were all taken around here on the high hills of Dartmoor what do we do to celebrate we weigh ourselves of course it's all them nutella pancakes for breakfast i can tell you that you should weigh something like 20 grams less than you did at the base of mass snowden guys i'm guessing something like 25 to 30 grams less so if you want to weigh as little as possible this is the place in britain to come but in any case it's such a tiny amount that it's gonna be wiped out entirely by whatever it was you had for breakfast this morning [Laughter] [Music] in Episode two I'll investigate how Albert Einstein's theory of gravity completely changed our understanding of space and time [Music] I'll travel to Louisiana in the USA to listen to the sound of black holes colliding biggest source of energy in the universe one of the biggest events you'd ever measure and we just barely saw it and with the help of thousands of volunteers and a smartphone app I'll explore how the science of gravity can help you age slower [Music]

Channel: Spark

Views: 1,517,265

Rating: 4.7600203 out of 5

Keywords: Spark, Science, Technology, Engineering, Learning, How To, education, documentary, factual, mind blown, construction, building, full documentary, 2017, 2016, 2015, full, space documentary, bbc documentary, Science documentary, inventions creativity, gadget geek, adventure culture, biologylife, documentaryphotography, scienceexplained, adventureisoutthere, technologysolutions, scienceandnature, science side of tumblr, creativeminds, astrophotography, technology friday, jim al-khalili, technology news

Id: 2_p2ELD7npw

Length: 44min 16sec (2656 seconds)

Published: Tue Mar 06 2018

Reddit Comments

[u/Oldamog]

Interesting documentary. I almost think I can grasp the concept of gravity.

When I looked it up on YouTube I found this one which shows as 1:48 but the full feature is only 1:30.