Essence of Gravity - Martin Rees

we're talking in Cambridge University and perhaps the best student we ever had here was Isaac Newton who is a student here in the 1860s and he is famous because he achieved the first unification in physics he realized that the force that makes the Apple fall and which holds us onto the ground is the same as the force which holds the moon in its orbit around us and holds the planets in their orbit around the Sun he showed that everything could be understood in our solar system if objects moved under the action of a force which depended on their mass and fell off as the inverse square of the distance if things are twice as far away the forces four times weaker and this was a wonderful insight and this allowed him to understand the motions of the planets and why we have eclipses etc Newton was actually lucky in that he seized on runs a few phenomena in nature which we can both understand and predict we can understand gravity we can understand the motion of the planets and we can see predict the motion of the planets many things in science we can't understand at all but even when we can understand something we often can't make predictions for instance whether we understand what causes the weather but we famously can't predict the weather more than a few days ahead which becomes chaotic but in the case of gravity and the planets we can understand it and we can predict it and since Newton's work we've understood that gravity applies not just in our solar system but further beyond stars are in equilibrium because they are held together between two forces the gravity which is crushing them and the pressure of their hot interiors is holding them up and galaxies like our Milky Way galaxy they are in equilibrium between the orbital motion of all the stars around a central hub of the galaxy and the gravity which is pulling them in so it's gravity which shows galaxies together and indeed on the even bigger scale clusters of galaxies Newton's theory of gravity is extremely precise and it works very well in explaining most stars and things in a solar system and also things within the galaxy but it's got limits first of all it doesn't explain why the law is an inverse square law wide falls off factor of 4 if you go twice as far away nor does it really explain very clearly why the force of gravity is the same on everything I mean why should it be that's the the feather and the LED ball in a vacuum formed the same speed it doesn't really explain that and also there are other limits to it which have been come more apparent in the last hundred years which is as it breaks down if things are moving very fast if things are moving at a speed confident speed of light and that's why the theory Einstein developed in 1915 just over a hundred years ago called general relativity was a huge advance on Newton's theory of gravity Einstein didn't prove Newton wrong it's often said that he did but that's unfair what Einstein did was he transcended and extended Newton he gave us a theory of gravity which agrees with Newton within Newton's domain of relevance but it applied for fast-moving objects and also applied when gravity was very strong and that became very important in astronomy when we discovered objects where gravity is indeed very strong in particular here in Cambridge in 1968 there was a discovery of objects called neutron stars these are objects which are as heavy as a star but heavier than a Sun in facts but they're only about ten kilometers across and if you squeeze the mass of a star they're very small then you get a very strong gravitational field I feel so strong that if you wanted to escape from a neutron star you'd have to fire a rocket at half the speed of light and if you were near to a neutron star then you'd find that light rays which are only very slightly bent by a star like the Sun would be bent a lot so you certainly need a theory beyond Newton's to understand the neutron star and Einstein's theory is important in that context even more extreme our object all black holes where not only has the material become very dense like a neutron star but it's gone on collapsing if you try to make a neutron star ten times as heavy as the Sun for instance you'd find that even the force of nuclei could not hold it up it would go on contracting and it would become what we now call a black hole a black hole is something which has collapsed so much that not even light can escape from it it's cut itself off from the rest of the universe leaving as it were a gravitational imprint frozen in a space it's left so it's a sort of dark domain in space which can suck things in but nothing comes out of it and we've known for the last forty years that these objects actually exist they're rather hard to detect of course because they are by definition black but many have been found indirectly the first ones were found by looking for objects in binary star systems when there is a small object orbiting around an ordinary star and its gravitational field is tugging material from the surface of the ordinary star and even though the black hole itself is invisible material that's pulled towards it and swirls down into it like a whirlpool gets very very hot and emits lots of radiation so astronomers observed objects are emitting very intense radiation from a small object orbiting around an ordinary star and they inferred that that small object was a black hole and the gas swirling into it gave sporadic variation often not just visible light but x-rays and gamma rays as well and so in that way we found that there were black holes black holes don't just exist as the endpoint of stars there's an even more dramatic way in which black holes form and that is in the Centers of galaxies a galaxy is a big swirling disk of stars around some central hub and the density of stars and the density of gas is higher towards the center and we now know that right in the middle of almost all galaxies there lurks a black hole weighing billions in some cases as much as the mass of the Sun these supermassive black holes are very important because if gas falls into them then you get something which is hugely bright far brighter on the galaxy and obvious called quasars which were discovered by astronomers in 1960s where something in the center of the galaxy outch on all the billions of stars in the galaxy by a factor of a hundred or so these are now understood as massive black holes in the Centers of galaxies which are capturing gas or even entire stars from their surroundings so black holes exist in our universe not just as stellar masses but as supermassive black holes now black holes are crucially important because they exemplify Einstein's theory in his most dramatic way Einstein's theory gave us a new way of looking at gravity he thought of space and time as being linked together so that near a large mass spaces were curved which means that light tries to follow the straightest path but that path is a curved path and near black hole spaces as it were falling in so Einstein's theory is really very counterintuitive because it tells us that we can't really think of space as being fixed and flat space is itself dynamic and the most dynamic manifestations of space occur when two black holes merge together if you imagine two black holes which are in orbit around each other then they will be in an orbit and they will as Einstein's theory predicts emit what's called gravitational radiation which is a ripple in space itself which moves outwards and that will take away energy and make these two black holes get closer and closer and then they'll eventually merge and these black holes will then form one big one and one of the most exciting developments recently in 2016 was the discovery of gravitational radiation from a merger of two black holes about a billion light years away what was observed was a tiny oscillation in space which was induced by this shattering event when two black holes merged and at this distance it's a tiny effect it was a one part in 10 to the power 21 and that's equivalent to moving by the thickness of a hair at the distance of Alpha Centauri the nearest star a tiny tiny effect but very very precise measurements actually revealed this effects just in 2016 and this is an amazing technical achievement but it is the most spectacular vindication we've had so far of Einstein's theory so Einstein's theory allows us to understand the extremes when gravity is overwhelmed all the other forces of nature to make a black and Einstein's theory also is crucial to understand the very beginning of our universe because we know a universe space and time were very different from what they are today and so Einstein's theory is crucially important and one of the challenges which awaits 21st century physicists is to produce the final grand unification unification between the force of gravity and Einstein's theory which effects they large objects like stars and the quantum theory which effects atoms and molecules now most of science gets by very well without a unification that's because quantum theory is important for small things like the atoms in a molecule and gravity is not very important between small things on the other hand astronomers need to worry about gravity when they think about the orbits of planets and of stars but the quantum fuzziness the quantum uncertainty is unimportant for things as big as the star or galaxy or even a planet so astronomers haven't had to worry about quantum theory when they talk about orbits but if we imagine the beginning of the universe when everything was squeezed together to very small dimensions then we need to worry about quantum effects and about gravity at the same time and so we need a theory which we don't yet have in order to understand the very beginning of the Big Bang and maybe whether our Big Bang was the only one and so that is a challenge for 21st century physics there's another thing about gravity which is very important and that is that it is those an important force for holding us on the ground and for astronomy it's in a sense a weak force in the following sense if you take two atoms then they have electrons in them and protons in them and the electrical forces between the electrons and the protons are stronger than the gravitational force between the electrons and the protons by a huge number almost forty powers of ten and that's why chemists don't need to worry about gravity but the difference between gravity and electric forces is that electric forces there are positive and negative charges and they always almost cancel out for any big object but gravity always as it were has the same charge it adds up and what this means is that on big objects gravity wins imagine you're building up solid objects let's imagine a sugar lump lump of rock then gravity is not important in those even an asteroid gravity is not important for something as big as a planet gravity is important and it then makes things round and if you make a planet as big as Jupiter then it starts to crush it and if you try to make a plan is heavier than Jupiter you'd find that it would get smaller not larger and eventually if you make something more than a hundred times as massive as Jupiter it turns into a star and so gravity wins for very big objects but because it's weak you have to pile together very many atoms in fact 10 to the power 57 atoms before you get something like a star and that's a good good thing because if gravity wasn't so weak then it would be possible for us to exist because we are able to exist because we are made of complexity and layer upon layer of structure and we could say many many atoms but we're not crushed by gravity so if there weren't these huge numbers of powers of 10 between the force of gravity on the microscopic scale and electric forces then our complex universe couldn't exist so gravity is a crucial force for molding the universe and allowing stars and galaxies to exist and allowing us to be held down on surface of the earth but the weaker it is the better so gravity although the weakest force in nature is crucial for its large-scale structure

Channel: Serious Science

Views: 14,048

Rating: 4.9312716 out of 5

Keywords: science, lecture, Serious Science, gravity, physics, Martin Rees, newton, Einstein, general relativity

Id: 312ZfjR7NCM

Length: 15min 25sec (925 seconds)

Published: Mon Dec 05 2016