What is Gravity? | Wondrium Perspectives

[Music] what goes up must come down right we've heard that old saying many times but as it turns out gravity isn't quite so simple hi i'm rich burnett for one dream in this episode of perspectives four experts discuss what we know about gravity as well as some of the things we're still trying to figure out we begin now with some of the earliest theories of gravity by a man often referred to as the first scientist aristotle enjoy [Music] in the 4th century bc aristotle tried to describe gravity as being the nature of matter his theory revolved around a geocentric model one in which the earth was in the center of the universe the natural location for all matter was the center hence when things fell it was explained simply as matter trying to get to its natural location other things had other natural locations fire for example was trying to reach the heavens which is why flames rise we're talking about gravity so let's stick with matter aristotle's theory of what we might call terrestrial gravity that is to say the behavior of gravity here on earth was simply an instance of objects trying to get to their natural place the sky was a different thing the sky was governed by the gods the sun the moon the planets comets and stars were each embedded in one of a series of concentric spheres centered on the earth the motion of the heavenly bodies was caused by the rotation of the spheres with the rotation made possible by the intervention of individual gods there is no concept of what we might call celestial gravity which is of course the gravity of the heavens so aristotle's ideas were developed in 400 bc give or take now did those evolve over time that's a long story beyond what we can describe here but involve some of the great minds of all times greek astronomer ptolemy postulated the earth was at the center of the universe but imagine something a little more complicated than aristotle's spheres in 1543 polish astronomer copernicus shook up the astronomical community when he postulated that the sun and not the earth was at the center of the universe the catholic church was not at all happy with this idea but it stood the test of time german astronomer johannes kepler made additional contributions when he used precise astronomical observations made by his mentor tycho brahi to determine that he could explain the data if the planets orbited the sun in elliptical orbits he also was able to work out the speed of the various planets and found that they moved at different speeds while they orbited it the penultimate hero in our study of gravity is italian astronomer and physicist galileo galilei he studied both the behavior of matter here on earth and also in the heavens legend has it that galileo dropped heavy and light objects from the leaning tower of pisa and demonstrated that they fell at the same rate this story is a probably apocryphal he probably drew his conclusions by rolling objects down ramps but either way he was right galileo also studied the heavens and used the newfangled telescope which he didn't invent but did lead the way in using it in astronomy he found that jupiter has moons which weighed heavily towards the idea of the sun and not the earth as being the center of the universe but again there was no unifying explanation between how things fell on earth and how objects moved in the sky it was british polymath sir isaac newton who made the truly crucial advance newton was one of the smartest and most scientifically influential people of all times the number of things he was able to explain is simply staggering however from the gravity point of view what he did was come up with a model that successfully demonstrated that celestial and terrestrial gravity were one and the same we call his gravity model newton's law of universal gravity for just that reason he worked out the idea that two objects would feel a mutual gravitational force governed by the mass of the two objects and the distance between them well the distance squared actually but newton's law as successful as it is is also quite mysterious let me emphasize it is very successful newton's law of gravity is all you need to get to the moon and back if you want to travel around the solar system you want to figure out how the planets move except for the tiny little exception of mercury which is closest to the sun where we need general relativity einstein's theory of gravity but for the rest of the solar system newton's theory of gravity is very good but it's mysterious and newton himself recognized this the reason why it's mysterious is how do the planets know how strong the gravitational force is supposed to be it's as if gravity is reaching out over empty space with nothing in between from the sun to the planets and saying here's the force that is acting on you newton was very bothered by this this is called action at a distance and newton himself didn't like it if you read his masterwork principia mathematica newton himself will say i don't know why there is this action at a distance involved with gravity i leave it to future generations to finally figure it out but in fact it didn't take very long not long after newton in the around the year 1800 a french mathematician and physicist named pierre simone laplace came up with a slightly different theory of gravity now laplace's theory of gravity is not one that you may have ever heard of because basically it's exactly the same as newton's theory in all of its experimental predictions once you have newton's inverse square law laplace is not giving you a different law he's giving you a different way to get the same law and what laplace says is instead of just thinking of a gravitational force think of a field pervading space think of a number at every point in space which laplace called the gravitational potential field and in laplace's version of gravity he replaces newton's equations with equations for this gravitational potential field and he says two things number one a big massive object warps the field by pushing on the field it changes its value and then number two the slope of the field the amount by which the field is changing gives you the force due to gravity so you can think of a hillside or something like that with a slope where the distance by which the field is changing the distance by which the the sharpness of the hill tells you the force that is acting on you now the answer you get for what the force is is exactly the same in laplace's view and in newton's view but the difference is that the plos does not have action at a distance instead the field is everywhere so the sun affects the field nearby and that affects the field near to that and that affects the field near to that all the way throughout the whole universe in laplace's version of gravity there's no action at a distance there's a field that is everywhere even though you don't see it and that is what gives rise to the force of gravity before einstein physicists thought of gravity simply as a force that attracts massive objects toward one another and in a sense this is correct gravity does pull us downward and toward the earth and gravity keeps the earth in its orbit by pulling it toward the sun but this view of gravity the newtonian view fails to recognize the greater significance of the phenomenon we call gravity what einstein had discovered is that gravity is not merely a force but is instead the very manifestation of the shape or geometry of space and time according to einstein the presence of mass and other energy changes the geometry of the surrounding space and time curving or warping it and this curving or warping causes objects to move through space differently than they would have otherwise when an object moves through space far away from any massive bodies and without being pulled or pushed by any forces it simply moves forward in a straight line well according to einstein when the earth moves in its orbit around the sun it too is moving in a straight line the presence of the sun has reshaped the geometry of the solar system bending space and transforming the earth's trajectory gravity isn't a force at all according to einstein it's geometry which is a consequence of mass and energy by explaining gravity in terms of geometry einstein overturned hundreds of years of established physics furthermore his theory was not only profoundly creative and mathematically elegant it is also right by this i mean that the predictions of this theory agree extremely well with any number of observations that have been made shortly after he completed his general theory of relativity in 1915 einstein reached the conclusion that moving objects could under certain circumstances create ripples or vibrations in the very fabric of space and time these rippling waves carry energy and they propagate through space at the speed of light at some level this isn't too surprising that einstein's field equations allow for the formation and propagation of gravitational waves as a point of comparison consider the equations that physicists use to describe the phenomena of electricity and magnetism these are known as maxwell's equations one of the solutions of maxwell's equations is a combination of an oscillating electric and an oscillating magnetic field together these time-varying electric and magnetic fields move through space at the speed of light in fact this combination of oscillating electric and magnetic fields is nothing more than the thing that we actually call light light is itself an electromagnetic wave in close analogy to this einstein's field equations have solutions which correspond to a propagating wave of oscillating curvature of space-time these gravitational waves travel through space at the same speed as light and they require no medium other than that of space and time itself initially most physicists thought that gravitational waves are probably real although although they knew that they'd be very difficult to detect or study but gradually attitudes toward gravitational waves began to shift towards skepticism and even disbelief for example in 1922 the famous physicist and astronomer arthur eddington wrote a paper in which he argued against the existence of gravitational waves arguing that the solutions to the field equations were mere mathematical artifacts and that they didn't have any self-consistent physical meaning when einstein made his first calculations involving gravitational waves back in 1916 he had used a common method of approximation known as perturbation theory in the 1930s he became interested once again in this kind of phenomena but this time he was determined to avoid the use of any approximations instead einstein wanted to find exact gravitational wave solutions to his field equations this led einstein to reach a very different conclusion than the one he had reached back in 1916. after taking all the details of the math into account einstein became convinced that nothing like a stable propagating gravitational wave could really exist instead he thought that gravitational waves were just a mathematical artifact that was somehow connected to the approximations that he had made in the earlier versions of his calculations he now thought that any gravitational waves would instantly collapse such a failed wave couldn't possibly carry any energy and it wouldn't have any measurable effects about all of this we now know that einstein was simply wrong in february of 2016 an international group of scientists announced one of the most exciting discoveries ever seen in astrophysics after months of analysis they had reached a remarkable conclusion five months earlier two of the world's most sensitive physics experiments in eastern washington state and southeast louisiana had detected gravitational waves both sites had picked up the fleeting but unmistakable signal of two black holes 29 and 36 times the mass of our sun colliding and merging 1.4 billion light years away the cataclysmic event had set off ripples in the fabric of space-time traveling at the speed of light and arriving at earth on the morning of september 14 2015. coincidentally the discovery came 100 years after the theory of general relativity was published when einstein himself predicted the phenomenon but also despaired of ever detecting it his theory had demonstrated that gravitational waves should exist but it also revealed that they would be absurdly minute seemingly imperceptible perturbations in the fabric of space-time that would surely be impossible to measure with any human-made machine a century later decades of effort from an enormous team of scientists and engineers would make the impossible happen since that first announcement gravitational wave astronomy has evolved from groundbreaking to almost commonplace new detections once closely guarded secrets are now announced on social media almost as soon as they happen we've found more pairs of black holes detected merging neutron stars and even begun to spot hints of other more exotic gravitational phenomena in just a few years we've already learned a great deal about gravitational waves and the colliding objects that produce them and we're facing a bright future in this young and exciting field to understand how these waves work i want you to imagine that you're holding a slinky between your hands if you look at that slinky there's two different ways that you could cause a wave one is the obvious way you could take the slinky lift one end and watch a curved wave propagate from one end to the other but this isn't quite how gravitational waves work gravitational waves are more like compression waves propagating through space-time if you instead hold the slinky and briefly bring one hand closer to the other you'll make a different type of wave this one squeezes and stretches the coils of the slinky as it travels and this is the best way to imagine a gravitational wave gravitational waves travel at the speed of light and squeeze and stretch space-time itself along with everything in it this includes our own planet as a gravitational wave passes through earth we'll be squeezed and stretched as well we don't understand how gravity works in the realm of the super small now this is kind of weird we know of four fundamental forces according to at least some ways of counting they are gravity electromagnetism and the strong and weak nuclear forces we know a great deal about the last three in the quantum world for those forces scientists have had a great degree of success envisioning them as subatomic matter particles exchanging a force-carrying particle for example in electromagnetism when two electrons bounce into one another the classical explanation is that both electrons generate an electric field and the electrons feel each other's fields this leads to the path of the two electrons being deflected in the quantum world of electromagnetism what happens is a little different as the two electrons approach one another one of the electrons emits a photon that electron recoils and then changes the direction it's moving the photon then travels to the other electron which absorbs it when the electron absorbs the photon the direction traveled by the second electron also changes in quantum mechanics you can't know even in principle which electron emitted the photon and which absorbed it and the recoil is due to ordinary momentum conservation that you might have learned in an introductory physics class if you never took physics don't worry about it momentum conservation is just a fancy way of describing that if you're standing in a boat in the middle of a lake and dive into the water the boat moves in a direction opposite to the direction you dove the most important thing to remember is that at the quantum level forces like electromagnetism and the strong and weak nuclear forces are best described by a matter particle emitting a force-carrying particle which is then absorbed by another matter particle the specifics of the matter particles and force-carrying particles change for the various forces but you don't need to sweat those details it's the exchange that matters that's how quantum forces work the same motivation that required the creation of ordinary quantum mechanics also means that there must be a solution of some sort to the answer of the problems of gravitational radiation in general relativity now does that mean that quantum gravity must be similar to ordinary quantum mechanics well no it doesn't in fact technically i guess you don't even need to call it quantum gravity but there are two things that are true the first is that there needs to be a solution to the problem of the stability of atoms to gravitational radiation and the second is that general relativity falls apart when it's applied to the world of the very small what we can prove is that at scales like the planck time length and energy the gravitational effects that we always ignore are no longer ignorable this has to be the size scale where quantum gravity absolutely must become important although of course it could become important at larger scales but it's mandatory at these scales furthermore while it might be possible to look at space at shorter distances it will require a completely new physical paradigm with new laws equations the whole enchilada okay so that's the deal about quantum gravity it is clear that general relativity breaks in the quantum world it predicts infinities and even if we somehow come up with a better theory of gravity at the plank length and time we have to take quantum effects into account what they will be we don't know but what we do know is that the future of quantum gravity will be truly fascinating hey thanks for watching if you'd like to learn more about the topics in this episode the full list of series that these clips came from is in the description below you can watch them all on one dream and don't forget to subscribe 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Length: 20min 13sec (1213 seconds)

Published: Sun Apr 17 2022