What is General Relativity?

[Music] welcome back to science click today general relativity what is gravitation gravitation or gravity is a fundamental phenomenon in the universe which tends to bring objects closer together if the Apple falls to the ground the moon orbits the earth and the Earth orbits the Sun that is all because of gravity which operates between these objects and prevents them from moving away one from the other at first glance we might think that gravitation is a force that acts by attracting objects according to their mass the more mass of an object the more it pull other objects in the universe in its direction this representation of gravity is a force enables us to understand the behavior of objects when they fall or the way in which planets go around the Sun however in reality certain subtle indicators show that gravitation is not really a force for instance when we observe a satellite falling towards the Earth if gravity was a force it should fall straight towards the center of the planet but when we carry out this experiment we see that its trajectory is slightly curved following the direction of the rotation of the earth another indicator is the fact that Mercury's orbit changes orientation with a certain speed as time goes by if gravity was a force then we would be able to accurately predict the speed with which this orbit should change however what we observe is that the orbit changes with a slightly different speed [Music] if we take into account these subtle indicators we come to the conclusion that gravitation is by no means of force capturing its nature requires us to entirely review our understanding of the world of the entire universe as well as of space and time this is what we call general relativity for its elaboration general relativity based itself almost entirely on the equivalence principle the equivalence principle states that all objects fall in the same way it says that free fall is not the action of a force it is not an accelerated movement of the object falling rather free fall is the natural movement for all entities when objects fall on earth this is equivalent to these objects actually being at rest while it is in reality the ground which accelerates towards them [Music] contrary to our naive intuition it is the act of falling which constitutes a natural movement a rest position in sorts while actually being motionless on the surface of the earth comes down to being in constant resistance to free fall and constantly exerting an upward acceleration it is for this reason that we actually only feel our weight once we have touched the ground during freefall our body follows its natural movement and it is only when touching the ground that we feel our weight the action of our feet putting a stop to the fall in general relativity gravitation is thus no longer a force we can almost say that gravitation does not exist that it is a sort of illusion if entities give off the impression that they are falling and that they attract each other this is because they are moving naturally being at rest but in a universe whose geometry is curved objects do not attract each other they merely move about in straight lines but they also curved space-time around them as a result of their mass this is what influences the trajectories of other surrounding objects the most important idea in general relativity is that all objects that possess a mass distort space-time around them to understand if we conceptualize our universe is an immense web a grid of space and time then a massive object will indeed have the effect of distorting this space-time around it the notions of distance of direction or even of the flow of time are thus distorted altered by the presence of the object naturally an object will have a tendency to move about in a straight line for example if we throw an apple in the middle of intergalactic emptiness it would continue its trajectory in a straight line always with the same speed but what if we were to throw this same Apple next to a massive object like Earth since it is so massive earth bend space-time around itself straight lines become distorted curved and intertwined in the direction of the planet as such when we release the Apple it goes in a straight line but since straight lines have been distorted by the mass of the earth the Apple gradually comes closer to the ground this effect is precisely what we call gravitation when an object is very massive it distorts the very structure of the universe round it curving straight lines in its direction and consequently all the objects around it will have a tendency to follow these curved lines and hence get closer to the object to better understand let us imagine simultaneously the following two scenarios on the one hand two observers are moving on a straight upward line on a flat plane which is not a curved surface on the other the two observers are moving in a straight line towards North but on the surface of the earth which is a sphere so this time a curved surface in the first scenario for the flat surface the two observers will travel in parallel straight lines in this way they will never get closer one to the other however in the second situation on the surface of the earth by walking straight our two observers will oddly end up meeting when they reach the North Pole although each of them has been walking in a perfect straight line due to the curvature of the earth and consequently the fact that straight lines are bent on its surface these two people will feel like a mysterious attraction force is making them progressively get closer one to the other with this example we understand better how gravitation can emerge from the curvature of space-time objects seem to be attracted to each other and their trajectory seemed to progressively meet up but in reality this phenomenon is due to the curvature of straight lines along which objects move to illustrate this peculiar phenomena which appear in general relativity let's focus on one fascinating object of our universe the black hole a black hole is a very massive object whose matter is mostly contained in one single point we call this central point a gravitational singularity at this precise spot in the universe the fabric of space-time is distorting to infinity and modern science is not able to understand fully what happens there close to the singularity the curvature of space-time is so great that all straight lines point in this direction here no object can hope escaping the black hole even for light rays or possible paths lead towards the singularity the black hole emits no light and neither matter nor radiation can escape it for this reason we see it as an entirely black sphere outside this capture zone beyond this limit that we call its horizon the black hole behaves similarly to any other object when we are close to the horizon it is impossible to stay in stable orbit around the black hole the distortion of space-time is too great which renders the trajectories of objects chaotic and difficult to predict at this distance however as soon as we find ourselves sufficiently far from the horizon as is the case for a planet or a star it is possible to gravitate in a stable manner around the black hole an object passing nearby will be slightly deviated but will still be able to escape its attraction in particular light rays coming from distant stars change direction when they get close to the black hole for this reason if we observe a black hole face-on we can see a distorted image of faraway stars which are actually behind it this is what we call gravitational lensing another interesting phenomenon appears when the black hole rotates on itself in the case of a black hole in quick rotation this will slightly pull the fabric of space-time into the rotation an object falling towards the black hole traveling on a straight line will thus be deviated according to the rotation of the black hole this very strange effect is also applicable to the earth whose rotation on itself leads to a slight deviation in the full of object finally one of the most noteworthy phenomena is what we call gravitational time dilation when we find ourselves close to massive objects like a black hole the straight lines that we follow are distorted not only in space but also in mostly in time the flow of time will therefore be altered according to the distance to which we are from the massive object at a great distance from the black hole where the curvature of space-time is almost imperceptible time flows more or less like elsewhere however the closer we get to the black hole the more we notice that the flow of time slows down for our clock close to the horizon of black hole space-time is so distorted that it is possible that for each second that goes by for us a whole year would go by for a distant observer in general the closer we are to a source of gravitation in other words a massive object like the Sun of the earth the slower the flow of our proper time at the surface of the earth for example if we compare the internal clocks of two observers one being at the base and the other at the top of the Eiffel Tower we would see that the internal clock of the person at the bottom is slightly slower since this person is closer to the center of the earth typically for each year that goes by the person at the bottom of the tower ages a microsecond less than the person at the top this time dilation effect is very subtle but extremely important on earth it is crucial to take it into account when we design GPS satellites such that we can ensure the synchronization of their internal clocks to sum up general relativity gives a comprehensive framework to grasp the phenomenon of gravitation where a massive object lies in space-time it distorts the geometry around it curving straight lines in time and in space which in turn leads to the deviation of surrounding objects and modifies their perception of time apart from gravitation general relativity also brings about a number of other surprising consequences one of these is the fact that space-time is not distorted instantaneously just like a fabric that we might poke if an object appears out from nowhere its gravitational influence would take some time to spread progressively through space and time curving the geometry of the universe as it progresses always at the speed of light this particularity leads to the existence of gravitational waves phenomena of distortion of space and time which propagate through the universe at the speed of light these gravitational waves are generated light-years away from the earth by cataclysmic events such as the fusion of two black holes they pass through us regularly slightly distorting our planet and everything on it as they go by finally this idea that our universe might have a distorted geometry begs the question of its shape at a larger scale and fuels numerous imaginations such as the possible existence of geometric shortcuts through space and time still very speculative at the moment which we call wormholes [Music] you

Channel: ScienceClic English

Views: 293,191

Keywords: general relativity, einstein, spacetime

Id: DYq774z4dws

Length: 13min 43sec (823 seconds)

Published: Wed Feb 19 2020