Feynman's Lectures on Physics - The Law of Gravitation

Transcript Search in video 0:24 [Music] 0:42 so [Music] 0:55 [Music] 1:10 so [Music] 1:25 ladies and gentlemen it's my privilege to introduce the messenger lecturer professor richard p feynman of the 1:32 california institute of technology professor feinman is a distinguished 1:38 theoretical physicist and he's done much to bring order out of the confusion which has marked much of the spectacular 1:45 development in physics during the post-war period among his honors and awards i will 1:52 mention only the albert einstein award in 1954 this is an award which is made every 1:58 third year and which includes a gold medal and a substantial sum of money 2:04 professor feinman did his undergraduate work at mit and his graduate work at princeton he 2:10 worked on the manhattan project at princeton and later at los alamos he was appointed an assistant professor 2:17 here at cornell in 1944 although he did not assume residents until the end of the war 2:25 i thought it might be interesting to see what was said about him when he was appointed at cornell so i searched the minutes of our board 2:32 of trustees and there's absolutely no record of his appointment 2:42 there are however some 20 references uh to leaves of absence salary 2:48 and promotions one reference interested me especially on july 31 2:56 1945 the chairman of the physics department wrote the dean of the arts college stating that 3:02 dr fineman is an outstanding teacher and investigator the equal of whom develops infrequently 3:09 [Music] 3:16 the the chairman suggested that an annual 3:22 salary of three thousand dollars was a bit too low for a distinguished faculty member 3:29 and recommended that professor feinman's salary be increased nine hundred dollars [Music] 3:36 the dean in an act of unusual generosity and 3:44 with complete disregard for the solvency of the university crossed out the 900 and made it an even 3:51 thousand you can see that we thought highly of professor feinman even then 4:02 feynman took up residence here at the end of 1945 and spent five highly productive years on our faculty 4:09 he left cornell in 1950 and went to cal tech where he has been ever since before i 4:15 let him talk i want to tell you just a little bit more about him three or four years ago he started 4:20 teaching a beginning physics course at cal tech and the result has added a new 4:26 dimension to his fame his lectures are now published in two volumes 4:32 and they represent a refreshing approach to the subject in the preface of the published lectures 4:38 there's a picture of feynman performing happily on the bongo drums 4:46 my caltech friends tell me that he sometimes drops in on the los angeles night spots and takes over the work of 4:52 the drummer but professor feinman tells me that that's not so 4:58 another of his specialties is safe cracking 5:05 one legend says that he once opened a locked safe in a secret establishment removed the 5:11 secret document and left the note saying guess who [Music] 5:18 i could tell you about the time that he learned spanish before he went to give a series of lectures in brazil 5:25 but i won't [Music] 5:34 this this gives me enough this gives you enough background i think so let me say that i'm delighted 5:40 to welcome professor feynman back to cornell his general topic 5:46 is the nature of physical law and his topic for tonight is the law of gravitation an example of 5:53 physical law professor finan 6:11 [Applause] it's odd but in the infrequent occasions when i've 6:17 been called upon in a formal place to play the bongo drums the introducer never seems to find it 6:23 necessary to mention that i also do theoretical physics [Music] 6:32 i believe that's probably uh that we respect the arts more than the sciences 6:38 the artist of the renaissance said that man's main concern should be for man 6:46 and yet uh there are some other things of interest in the world even the artists appreciate sunsets and 6:52 the ocean waves and the march of the stars across the heavens 6:58 and there is some reason then to talk of other things sometimes 7:05 as we look into these things we get an acidic pleasure from directly on observation 7:11 but there's also a rhythm and a pattern between the phenomena of nature 7:17 which isn't apparent to the eye but only to the eye of uh analysis and it's these rhythms and 7:24 patterns which we call physical laws what i want to talk about in a series of 7:29 lectures is the general characteristics of these physical laws that's even another level 7:34 if you will of higher generality over the laws themselves and it's uh really i'm talking about is 7:42 nature as seen as a result of detailed analysis but only the most overall 7:49 general qualities of nature is what i mainly wish to speak about now such a topic has a tendency to 7:55 become too philosophical because it becomes so general that a person talks in such generalities 8:00 that everybody can understand him and it's considered to be some deep philosophy if you 8:06 however i would like to be very rather more special and i would like to be understood in an 8:12 honest way rather than in a vague way to some extent and so if you don't mind i'm going to 8:17 try to give instead of only the generalities 8:23 in this first lecture an example of physical law so that you have at least one example of 8:28 the things about which i am speaking generally in this way uh i can use this example 8:34 again and again to give an instance to make a reality out of something which will otherwise be too abstract 8:40 now i've chosen for my special example of physical law to tell you about the theory of gravitation or the phenomena of gravity 8:49 why i chose gravity i don't know whatever i chose you would ask the same question 8:56 actually it's uh was one of the first great laws to be discovered and that has 9:02 an interesting history you might say yes but then it's all hard i would like to hear something about 9:08 science more modern science more recent perhaps but not more modern modern science is exactly in the same 9:15 tradition as the discoveries of the law of gravitation it is only more recent discoveries that we would be talking about 9:21 and so i have no i do not feel at all bad about telling you of the law of gravitation because i am in describing 9:27 its history and the methods the character of its discovery and its quality talking about modern science 9:33 completely modern this law has been called the greatest 9:39 generalization achieved by the human mind and you can get already from the by 9:45 introduction i'm more interested not so much in the human mind as in the 9:50 marvel of nature who can obey such an elegant and simple law as this law of gravitation so our main 9:57 concentration will not be on how clever we are to have found it all out but on how 10:03 clever she is to pay attention to it 10:09 now uh what is this law of gravitation that they're going to talk about the law is that uh 10:17 two bodies or bodies exert a force upon each other which is inversely as the square of the 10:23 distance between them and varies directly as the product of their masses and the mathematician mathematically we 10:29 can write that great law down and formula some kind of a constant times a product of the two masses 10:36 divided by the square of the distance now if i add the remark that a body reacts to a force 10:44 by accelerating or by changing its velocity every second to an extent inversely as its mass it 10:51 reacts uh changes velocity more if the mass is lower and so on inversely as the mass 10:57 then i have said everything about the law of gravitation that needs to be said everything else is a consequence a 11:04 mathematical consequence of those two things that i said that's a remarkable enough phenomenon 11:09 itself that the next lecture will consider this in more detail now i know you're 11:15 not all uh here i know some of you are but you're not all mathematicians and so you cannot all immediately see all of the 11:20 consequences of these two remarks and so what i would like to do in this lecture is to briefly tell you 11:27 the story of the discovery tell you what some of the consequences are what the effect 11:32 of this discovery had on the history of science what kinds of mysteries such a law entails 11:38 something about the refinements made by einstein and uh possibly the relation to other 11:43 laws of physics the history of the thing briefly is this that the ancients first observed the way the planets seemed to move about in the 11:50 sky and concluded that they all went around where along with the earth went around the sun 11:56 this discovery was later made independently by copernicus if they had forgotten that 12:01 people had forgotten that had already been made now the next thing question that came up 12:09 in to study was exactly how do they go around the sun that is exactly what kind of motion do 12:17 they go with the sun at the center of a circle or do they go in some other kind of a curve how fast they move and so on and this 12:23 discovery took for longer to make the 12:30 times after copernicus were times in which there were great debates about 12:35 whether the planets in fact went around the sun along with the earth or whether the earth was at the center of the 12:40 universe and so on and there were considerable arguments about this when a man named tycho bray got an idea 12:49 of a way of answering the question he thought that it might perhaps be a good idea to look very very carefully and to 12:56 record where the planets actually appear in the sky and then the alternative theories might 13:02 be distinguished from one another this is the key of modern science and is the beginning 13:07 of the true understanding of nature this idea that to look at the thing to record the 13:14 details and to hope that in the information thus obtained may lie a clue to one or another of a 13:19 possible theoretical interpretation so tycho who is a rich man and owned i 13:25 believe an island near copenhagen outfitted his island with great brass 13:31 circles and special observing positions uh situation chairs that you could look 13:37 through little holes and recorded night after night the position of the planets it's only 13:43 through such hard work that we can find out anything 13:51 when these all these data were collected they came into the hands of kepler 13:57 who then tried to analyze what kinds of motions the the planets made around the sun 14:04 and he did this by a method of trial and error at one stage he thought he had it he assumed he figured out that they went 14:11 around the sun in circles with the sun off center and noticed that one planet i think it was mars but i 14:17 don't know uh was eight minutes of arc off and he decided this was too big for tycho bray 14:22 to have made an error and that this was not the right answer so because of the precision of experiments he was able to proceed 14:29 and find that to go on to another trial and found in fact ultimately this three things first that 14:36 the planets went in ellipses around the sun with the sun of the focus an ellipse is a curve 14:42 all artists know about because it's a foreshortened circle when children know about because somebody told them that if you take a 14:48 string and tie it to two tacks and put a pencil in there it'll make an ellipse 14:54 these two tacks are the foci and if the sun is here the shape of the orbit of a planet around the sun 15:00 is one of these curves the next question is and going around 15:05 the ellipse how does it go does it go faster when it's near the sun slower when it's further from the sun and so on we take away the other focus 15:12 we have the sun then and the planet going around and kepler found the answer to this too 15:20 he found this that if you put the position of the planet down in two at two times separated by 15:27 some definite time let's say uh three weeks and then in another place in the orbit 15:32 put the positions of the planets again separated by three weeks and draw lines from the sun to the 15:38 planet technically called radius radius vectors anyway lines from the 15:44 sun to the planet then the area that's enclosed in the orbit of the 15:50 planet and the two lines that are separated by the planets positioned three weeks apart is the same no matter what part of the 15:56 orbit the thing is on so that it has to go faster when it's closer in order to get the same area as it goes 16:02 slower when it's further away and in this precise manner some several years later he found the 16:10 third rule and uh that had not to do with the exactly 16:15 emotion of a single planet around the sun but related the various planets to each other and it said 16:21 that the times that it took the planet to go all the way around was related to the size of the orbit 16:29 and that the times went as the square root of the cube of the size of the orbit and for the size of the orbit is the 16:36 diameter all the way across the biggest distance on the ellipse so uh he has these three laws which are 16:43 summarized by saying it's an ellipse and that equal areas are swept in equal 16:48 times and that the time to go around varies as a three-half power 16:55 of the size the square root of the two of the side so there's three laws of kepler 17:01 which is a very complete description of the motion of the planets around the sun 17:10 the next question was what makes them go around well how can we understand this in more detail or is there anything else to say 17:18 in the meantime galileo was investigating the laws of motion 17:23 incidentally at the time of uh kepler the problem of what drove the planets around the sun was answered in 17:30 some by some people by saying that there were angels behind here beating their wings and 17:36 pushing the planet along iran orbit as we'll see that that answer is not very far from the truth 17:42 the only difference is that the angels sit in a different direction and were wings boner 17:55 but the point that the angels sit in a different direction is the one that i must now come to galileo in studying the laws of motion 18:02 and doing a number of experiments to see how balls roll down inclined planes and pendulus swung and so on 18:08 discovered a idealization a great principle called the principle of inertia which is this that if a thing has 18:15 nothing acting on it for object there's nothing acting on and it's going along in a certain velocity in a straight line it will go at the same velocity at 18:22 exactly the same straight line forever unbelievable though that may sound to anybody who has tried to 18:28 make a ball roll forever the idealization did is correct and that 18:34 there were no influences acting such as the friction on the floor and so on the thing would go at a uniform speed 18:40 forever the next point was made by newton 18:45 who discussed the next question which is when it doesn't go in a straight line then what 18:52 and the answer this way that a force is needed to change the velocity in any manner 18:58 first for instance if you're pushing it in a direction that it moves it will speed up 19:04 if you find that it changes direction then the force has must have been sideways 19:09 and that the force can be measured by the product of two effects first how much does the velocity change 19:17 in a small interval of time how fast is the velocity changing how much is it accelerating 19:22 in this direction or how much is the velocity changing when it changes direction that's called the acceleration and when 19:28 that's multiplied by a coefficient called the mass of an object or it's inertia coefficient then that together is a force one can 19:36 measure the for instance if one has a stone on the end of a string and swings it in the circle over his head 19:42 then one can measure everyone finds one has to pull the reason is that the speed of this the velocity the speed is not changing 19:49 as it goes around the circle but it's changing its direction so there must be perpetually an inpulling force 19:56 and this uh is proportional to the mass so that if we were to take two different objects first swing one 20:02 and then swing another one at the same speed around the head and measure the force in the second one that second one uh the new force is 20:09 bigger than the other force in the proportion that the masses are different this is a way of measuring the masses by how much how hard it is to change the speed 20:20 now then newton saw from this that for instance to take a 20:27 simple example if a planet is going in a circle around the sun no force is needed to make it go 20:34 sideways tangentially if there were no force at all on it it would have just keep coasting this way 20:40 but actually the planet doesn't keep coasting this way but finds itself later not out here where it 20:46 would go if there were no force at all but further down towards the sun 20:54 in other words its velocity its motion has been deflected toward the sun so what the angels have 21:00 to do is to beat their wings in toward the sun all the time that the motion to keep it going in a 21:07 straight line has no known reason the reason why things coast forever has never been 21:14 found out the law of inertia is no known origin so the angels don't exist but the 21:21 continuation of the motion does but in order to obtain the falling operation 21:26 we do need a force so it became apparent that the origin of that the force 21:32 was toward the sun as a matter of fact newton was able to demonstrate that the 21:38 statement that equal areas are swept in equal times was a direct consequence of the simple 21:43 idea that all of the changes in velocity are directed exactly to the sun even in the elliptical case 21:49 and maybe i'll have time next time to show you how that works in detail 21:57 so from this law he would confirm the idea that the forces toward the sun and from knowing how the periods of the 22:03 different planets vary with the distance away from the sun it's possible to determine how that force must weaken at different distances 22:10 and he was able to determine that the force must vary inversely as a square of the distance 22:17 now so far he hasn't said anything yes because he only said two things which 22:24 kepler said in a different language one is exactly equivalent to the statement that the 22:29 forces toward the sun and the other is exactly equivalent to the state that the law is inversely as the square of the distance but 22:36 people had seen in telescopes the jupiter's satellites going around jupiter and it looked like a little solar system 22:41 so the satellites were attracted to jupiter and the moon is attracted to the earth and this goes around the earth is attracted the same way 22:47 so it looks like everything's attracted to everything else and so the next statement was to generalize this and to say that every object attracts every 22:53 other object if so the earth must be pulling on the moon just as the sun pulls on the planet 23:01 but it's unknown that the earth pulls on things because you're all sitting tightly in your seats in spite of your 23:06 desires to float out of the hall at this time the pull up for objects on the earth was 23:14 well known in the phenomenon of gravitation and it was newton's idea then 23:19 that maybe the gravitation which held the moon in the orbit also applied was the same 23:25 gravitation that pulled the objects toward the earth now it is easy 23:30 to figure out how far the moon falls in one second 23:35 because if it went in a straight line you know the size of the orbit you though it takes a month to go around 23:40 and if you figure out how far it goes in one second you can figure out how far the circle of the moon's orbit has 23:46 fallen below the straight line that it would have been in if it didn't go the way it does go 23:52 and this distance is one twentieth of an inch now the 24:00 moon is sixty times as far away from the earth's center than we are we're 4 000 24:05 miles away from the center and the moon is 240 000 miles away from the center so if the law of inverse square is right 24:12 an object that the earth's surface should fall in one second by one twentieth of an inch times 24:18 thirty six hundred being the square of sixty because the force has been weakened by sixty times sixty for the 24:24 inverse square law in getting out there to the moon and if you multiply a twentieth of an inch by 24:29 thirty six hundred you get about 16 feet and low it is known already from galileo's measurements that things 24:36 fell in one second on the earth's surface by 16 feet so this meant you see that he was on the 24:43 right track there was no going back now because a new fact that was completely 24:48 independent previously which is the period of the moon's orbit and its distance from the earth was connected to another fact which is 24:54 how long it takes something to fall in one second so this was a dramatic test that 24:59 everything's all right further he had a lot of other predictions he was able to calculate what the shape of the 25:05 orbit should be if the law were the inverse square and found indeed that it was an ellipse so he got three for two 25:10 as it were in addition a number of new phenomena had their obvious explanations one was 25:18 the tides the tides were due to the pull of the moon on the earth 25:24 this had sometimes been thought of before with the difficulty that if it's the pull of the moon 25:29 on the earth the earth being here the waters being pulled up to the moon 25:35 then there would only be one tide a day where that bump of water is under the moon but actually you know there are tides 25:41 every 12 hours roughly and that's two tides a day but you must there was also another school of thought 25:47 that had a different conclusion their theory was that it was the earth that was pulled by the moon away from 25:52 the water 26:01 so actually newton was the first one to realize what actually was going on that the force of the moon on the earth 26:08 and on the water is the same at the same distance and that the water here is closer to the 26:14 moon and the water here is further from the moon than the earth then the rigid earth so that the water 26:19 is pulled more toward the moon here and here is less toward the moon than the earth so there's a combination of 26:25 those two pictures that makes it double tide actually the earth does the same trick 26:30 as the moon it goes around a circle really i mean the force of the moon on the earth is 26:36 balanced but by what by the fact that just like the moon goes in a circle to balance the earth's force the earth 26:42 is also going in a circle actually the center of the circle is somewhere inside the earth it's also going in a circle 26:48 uh to balance the moon so the two of them go around the common center here and if you wish this 26:53 water is thrown off by centrifugal force more than the earth is and this water is attracted more than 26:58 this average of the earth at any rate the tides were then explained in it 27:04 and the fact that they were two a day a lot of other things became quite clear why the earth 27:09 is round because everything gets pulled in and why it isn't round because it's spinning so that the 27:16 outside gets thrown out a little bit and it balances and why the sun and moon around and so 27:21 on now as the science developed 27:26 and measurements were made ever more accurately the tests of newton's law became much more stringent and the first 27:32 careful tests involved the moons of jupiter by careful observations of the way they went around 27:38 over a long period of time one could be very careful to check that everything was according to newton uh turned out not to be the case 27:45 the moons of jupiter appeared to be first uh get sometimes to eight minutes ahead of time and sometimes eight minutes 27:52 behind ty's schedule where schedule is the calculated values according to newton's laws 27:57 it was noticed that they were ahead of schedule when they were close when jupiter was close to the earth and behind schedule when it was far away 28:03 rather odd circumstance and mr rumor having confidence in the law of gravitation 28:09 came to an interesting conclusion that it takes light some time to travel from the moons to the earth 28:14 and that what we're looking at when we see the moons are not how they are now but how they were the time ago that it took the light to 28:21 get here now when jupiter is near us it takes less time for the light to come and when jupiter is further it takes 28:26 longer time so he had to correct the observations for the differences in time and by the fact that they were this much 28:33 too early or that much too late was able to determine the velocity of light this was the first demonstration that light was not an instantaneously 28:40 propagating material i bring this particular matter to your attention because it illustrates something that when a law 28:47 is right it can be used to find another one that 28:52 by having confidence in this law if something is the matter it suggests perhaps some other phenomenon and if we had not known the 28:59 law of gravitation we would have taken much longer to find the speed of light because we would not have known what to 29:05 expect of jupiter's satellites this process has developed into an 29:10 avalanche of discoveries each new discovery permits the tools for much more discovery and 29:15 this uh begin it's the beginning of that avalanche which has gone on now for 400 years in a continuous 29:22 process and we're still avalanching along at high speed at this time 29:45 another problem came up the planets shouldn't really go in ellipses because according to newton's laws they're not 29:51 attracted only by the sun but also they pull on each other a little bit only a little bit but a little bit there's something 29:58 and we'll alter the motion a little bit so jupiter saturn and uranus were big 30:04 planets that were known and the calculations were made as to how slightly different than the perfect ellipses of kepler the planets ought to 30:10 be going jupiter saturn and uranus by the pull of one on each other and when they were finished the 30:16 calculations i mean and the observations it was noticed that jupiter and saturn went according to the calculations 30:22 but that uranus was doing something funny another opportunity for newton's laws to be found wanting but 30:28 courage two uh men both who made these 30:34 calculations adams and la verriere independently and almost exactly the same time proposed that the motions of 30:40 uranus were due to an unseen as yet new planet and they wrote letters to 30:46 their respective observatories telling them to look turn your telescope and look there and you'll find a planet 30:52 how absurd said one of the observatories that some guy sitting with pieces of paper and pencils 30:57 can tell us where it would look to find something new planet and the other observatory was more uh 31:04 less well the administration was different and uh 31:14 they found the neptune [Music] 31:20 more recently in the beginning of the 20th century it became apparent that the motion of the planet mercury was not 31:25 exactly right and this caused a lot of trouble and had no explanation 31:31 until a modification of newton's this did show ultimately that newton's laws were slightly off 31:36 and that they had to be modified i will not discuss the modification in detail it was made by einstein now the question is 31:44 how far does this law extend does it extend outside the solar system 31:51 and so i show on the first slide evidence that the law of gravitation is on a wider scale than just the 31:57 solar system here is a series of three pictures of a so-called double star there's a 32:05 third star fortunately in the picture so you can see that they're really turning around and that nobody just simply turned the frames of the pictures 32:11 around which is easy to do on astronomical pictures but the stars are actually going around and by watching these things and 32:18 plotting the orbit you see the orbit that they make on the next slide it's it's evident that they're 32:24 attracting each other and that they're going around in the lips according to the way expected these are succession of pictures 32:30 uh going for all these different periods of time i think yes it goes around this way 32:35 and they didn't see it well when it was too close and here it is in 1905 my slide is very old it's gone around 32:41 maybe once more since and you'll be happy except when you notice if you have noticed already that this center is 32:48 not and a focus of the ellipse but it's quite a bit off so something's the matter with the law no 32:55 if god hasn't presented us with this orbit face on it's tilted at a funny angle and if you 33:00 take an ellipse and mark its focus and then hold the paper in an odd angle and look at it in projection this the focus doesn't have to be at the 33:06 focus of the projected image so it's uh because it's orbit is tilted in space that it looks 33:12 that way it looks like it's not the right pattern but it's all right and you can figure everything out satisfactorily for that 33:18 how about a diff a bigger distance there's forces between the stars does it go any further than these 33:24 distances which are not more than two or three times the solar system's diameter here's something in the next slide 33:30 that's a hundred thousand times as big as a solar system in diameter and this is a large number of stars 33:39 tremendous number of stars this white spot is not a solid white spot it's just because of the failure of our 33:44 instruments to resolve it but our very very tiny dots just like the other stars well separated from one another not 33:50 hitting each other each one falling through and back and forth through this great globular cluster 33:56 it's one of the most beautiful things in the sky as good as sea waves and sunsets and the distribution of this material 34:04 it's perfectly clear that the thing that holds this together is the gravitational attraction of the stars for each other and the 34:11 distribution of the material in the sense of how the stars peter out as you go out in distance permits one to find out roughly 34:18 how what the law is a force between the stars and of course it comes out right that it is roughly 34:23 the inverse square the accuracy of these calculations and measurements is not anywhere near as 34:29 careful as in the solar system onward because gravity extends still further 34:35 this is a little pinpoint inside of a big galaxy and the next slide shows a typical galaxy 34:41 and it's clear that this thing again is held together somehow and the only candidate that's reasonable 34:47 is gravitation but when we get to this con this size we have in any way any longer to check the inverse square law 34:54 but there seems to be no doubt that these great agglomerations of stars and so these galaxies which are 50 to 100 000 35:02 light years across the solar system is well from the earth to the sun is only eight white minutes 35:09 this is a hundred thousand light years that the gravity is extending even over these distances and 35:15 in the next slide as evidence extends even further here is what is called a cluster of 35:20 galaxies there's a galaxy here and here and here galaxies here they're 35:26 all in one lump of galaxies analogous to the cluster of stars but this time what's clustered are those 35:33 big babies that i showed you in this previous slide 35:38 now we uh this is as far as about one tenth or well a 35:44 hundredth maybe of the size of the universe and as far as we have any direct evidence that gravitational forces extend 35:52 so the earth's gravitation if we take the view 35:57 has no edge as you may read in the newspapers when the planet gets outside the field or the gravitation 36:03 it keeps on going ever weaker and weaker inversely as the square of the distance dividing by four each time you're twice 36:10 as far away until it mingles with the strong fields and gets lost in the confusion of the strong fields of other stars 36:16 but all together with the stars in its neighborhood pulls the other stars to form the galaxy 36:22 and all together they pull on other galaxies to make a pattern a cluster of galaxies 36:27 so the earth's gravitational field never ends but peters out very slowly in a precise 36:33 and careful law probably to the edges of the 36:38 universe the law of gravitation is different than 36:44 many of the others is very important in the 36:50 economy or in the machinery of the universe there are many places where gravity has its practical 36:56 applications as far as the universe is concerned but atypically among all the other laws of 37:01 physics gravitation has relatively few practical applications i mean the new knowledge of the lord has 37:07 a lot of application it keeps people in their seats it's on but it has few that knowledge of the law has few practical applications 37:14 relatively speaking compared to the other laws this is one case in which i picked an atypical example it is impossible by the way by picking 37:20 one example of anything to avoid picking one which is atypical in some sense that's the wonder of the world the only 37:28 application i could think of were first in some geophysical prospecting in predicting the tides 37:34 nowadays more modernly in working out the motions of the satellites and 37:41 the and the planet probes and so on that we send up and also modernly to calculate the predictions of the 37:46 planet's position which have great utility for astrologers to publish their predictions and horoscopes 37:53 in the magazines that's the strange world we live in that 37:59 all the advances in understanding are used only to continue the nonsense 38:05 which has existed for two thousand years [Applause] 38:10 now that that shows that gravitation extends to the great distances but newton said 38:17 that everything attracted everything else do i attract you excuse me i mean do i attract you 38:22 i was going to say excuse me do i attract you physically i didn't mean that 38:29 what i mean is it really true that two things attract each other 38:35 can we make a direct test and not just wait for the planets and look at the planet to see if they attract each other and this 38:41 experiment that the direct test was made by cavendish on equipment 38:48 which you see indicated on the next slide if i got my slides right 38:57 well i made a mistake i was talking about the the the importance of the 39:04 gravitation i was overwhelmed with my clever remark about astrologers and forgot to mention the important places where 39:09 gravitation does have some real effect in the behavior of the universe and one of the interesting ones is the 39:15 formation of new stars in this picture which is a gaseous nebula inside our own galaxy and there's 39:22 not a lot of stars but it's gas there are places where the gas has been compressed or attracted to itself 39:28 here it starts perhaps by some kind of shock waves to get collected but the reason remain there for the phenomenon 39:34 is that gravitation pulls the cloud of gas closer and closer together so big mobs of gas and dust 39:40 collect and form balls which as they fall still further the heat generated by the falling 39:48 lights them up and they become stars and we have in the next slide some evidence of the creation of new stars 39:56 it is unfortunately harder to see than i thought it was when i looked at it 40:01 before but this is not exactly the same as this this bump here is further out than here and that this 40:09 also has a new dot here there are i have found better 40:15 examples but were unable to produce a slide there is one example of a star patch a light that grew in a place in 200 and 40:22 200 days so that when this is in the same kind of condition of a gas cloud 40:28 when the gas collects too much together by gravitation stars are born and this is the beginning of new stars 40:34 so the stars belch out dirt and gases when they explode sometimes and the dirt and 40:39 gasses then collect back again and make new start sounds like perpetual motion 40:46 i now turn to the subject i meant to introduce which was the experiments on the small scale to see whether things 40:51 really attract each other and i hope now that the next slide does indicate this is a second try 40:56 yeah cavendish's experiment the idea was to hang by a very very fine quartz fiber 41:04 a rod with two balls and then put two large lead balls in the positions 41:09 indicated here next to it on the side then because of the attraction of the balls 41:15 there would be a slight twist of the fiber it had to be done so delicately because the gravitational force between 41:22 ordinary things is very very tiny indeed and there it was and it was possible then to measure the 41:28 force between these two balls cavendish called his experiment weighing the earth 41:36 we're pedantic and careful today we wouldn't let our students say that we would have to say they're measuring the 41:41 mass of the earth but the reason he say that said that as the following by a direct experiment he 41:48 was able to measure the force and the two masses and the distance and thus determine the gravitational 41:53 constant you say yes but we have the same situation on the earth we know what the pull is 41:58 and we know what the mass of the object pulled is and we know how far away we are but we don't know the either the mass of 42:04 the earth or the constant but only the combination so by measuring the constant and knowing 42:09 the facts about the pull of the earth the mass of the earth could be determined so indirectly this experiment 42:15 was the first determination of how heavy or how massive is the ball on which we stand 42:22 right it's a kind of an amazing achievement to find that out and i think that's why cavendish named his 42:27 experiment that way instead of determining the constant and the gravitational equation weighing the earth 42:36 he incidentally was weighing the sun and everything else at the same time because the pull of the sun is known in 42:42 the same manner now one other test of the law of 42:48 gravitation is very interesting and that is the question as to whether 42:55 the the pull is exactly proportional to the mass 43:00 if the pull is exactly proportional to the mass and the reaction to forces the motions induced by forces the changes in 43:06 velocity are inversely proportional to the mass that means that two objects of different mass will change their 43:13 velocity in the same manner in a gravitational field or two different things no matter what their 43:19 mass in a vacuum will fall the same way toward the earth that's galileo's old experiment from the 43:25 leaning tower i took my young son of two and a half to the leaning tower of pisa 43:30 and now he every time a guest comes he says leaning tower so anyhow it means for example that in a 43:39 satellite uh i mean a a man-made satellite an 43:44 object inside will go around the earth in the same kind of an orbit as a satellite on the outside 43:49 and thus float in the middle apparently so that this fact that the force is exactly proportional to the mass and that the 43:56 reactions are inversely proportional to mass has this very interesting consequence the question is how accurate is it and 44:04 it has been measured by an experiment by a man named ertvos 44:09 in 1909 and very much more recently and more accurately by dickie and it is 44:15 known to one part in 10 000 million the mass is exactly proportional i mean 44:22 the forces are practically proportional to the mass how it's possible to measure without 44:29 accuracy i wish i had the time to explain but i'm afraid i i cannot 44:35 it's a remarkably clever i'll give a hint how i give one hint there suppose that you wanted to measure 44:40 whether it's true for the pull of the sun you know the sun is pulling us all it pulls the earth too 44:46 but suppose you wanted to know whether you had a piece of lead here and a piece of copper here for 44:52 polyethylene and lead it was first done with sandalwood now it's done with polyethylene 45:00 whether the pull is exactly proportional to the to the inertia the earth is going around 45:05 the sun so these things are thrown out by inertia and they're thrown out to the extent that these two objects have 45:11 inertia but they're attracted to the sun to the extent that they have mass in the attraction law so if they're attracted 45:18 to the sun in a different proportion and they're thrown out by inertia one will be pulled toward the sun and the other away and so hanging on another 45:25 one of those cavendish quartz fibers the thing will twist toward the sun it doesn't twist to this accuracy so we 45:31 know that the sun's attraction for these two objects is exactly proportional to the centrifugal effect which is inertia so the force 45:38 of attraction on an object is exactly proportional to its coefficient of inertia in other words it's mass 45:46 i should say something about the relation of gravitation to other forces to other parts of nature other phenomena 45:53 in nature and i'll have more to say of a general quality later but there is one 45:58 thing that's particularly interesting that is that the inverse square law appears again it appears in the electrical laws for instance 46:05 that electricity also exerts forces inversely as a square the distance this time between charges 46:11 and one thinks perhaps inverse square the distance has some deep significance 46:17 maybe gravity and electricity are different aspects of the same thing no one has ever succeeded in making 46:22 gravity and electricity different aspects of the same thing today our theories of physics the laws of 46:27 physics are a multitude of different parts and pieces that don't fit together very well we don't understand the 46:33 one exactly in terms of the other we don't have one structure from which all is deduced we have several pieces that don't quite 46:40 fit exactly yet and that's the reason why in these lectures instead of having the ability to tell you what the law 46:46 of physics is i ask talk about the things that are common to the various laws because we don't know we don't 46:52 understand the connection between them but what's very strange is that there are certain things that are the same in both 46:58 but now let's look again at the law of electricity the law goes in versus the square of the 47:03 distance but the thing that is remarkable is the tremendous difference in the strength of the electrical and 47:09 gravitational laws people who want to make electricity and gravitation out of the same thing 47:14 will find that electricity is so much more powerful than gravity that it's hard to believe they could both have the same origin now how can i 47:21 say one thing is more powerful than another it depends upon how much charge you have and how much mass you have i'm certainly uh 47:28 well the you can't talk about how strong gravity is by saying i take a lump of such and such a size because you 47:35 chose the size if we try to get something that nature produces our own pure number that has 47:42 nothing to do with inches or years or anything to do with our own dimensions we can do it this way if we take the 47:48 fundamental particles such as an electron any different ones will give different numbers but to get an idea in them but 47:54 take electrons two electrons a fundamental particle that's an object that's not something i can't 48:00 i don't have to tell you what units i measure in it's two particles of the fundamental particles and they repel each other inversely as a square the 48:06 distance due to electricity and they attract each other inversely as a square that this is due to gravitation question what is the ratio of the 48:13 gravitational force to the electrical force and that is illustrated on the next slide 48:20 ratio of the gravitational attraction to the electrical repulsion is given by a number with 42 digits 48:27 and goes off here it's all this is written very carefully out so that's 42 digits now therein lies a very deep mystery 48:35 where could such a tremendous number come from that means if you ever had a theory from 48:41 which both of these things are to come how could they come in such disproportion from one equation has a solution which 48:47 has for one two kinds of forces and attraction and a repulsion with that fantastic ratio 48:54 people have looked for such a large ratio in other places they're looking for a large number they 49:01 hope for example that there's another large number and if you want a large number why not take the diameter of the universe 49:08 to the diameter of a proton amazingly enough it also is a number with 42 digits 49:14 and so an interesting proposal is made that this ratio depends is the same as a ratio of the 49:20 size of the universe to the diameter of a proton but the universe is expanding 49:25 with time and that would mean the gravitational constant is changing with time and although that's a possibility 49:32 there's no evidence to indicate that it's in fact true and there are several difficulties where having partial indications that it 49:38 doesn't that the gravitational constant has not changed in that way so this tremendous number remains a mystery 49:46 i must say to finish about the theory of gravitation two more things one is that einstein had to modify the laws of 49:53 gravitation in accordance to his principle with his principles of relativity the first was one of the principles was 49:59 that if x cannot occur instantaneously while einstein newton's theory said that the force was instantaneous 50:05 he has to modify newton's laws they have very small effects these modifications one of them is all 50:10 masses fall light has energy and energy is equivalent to mass so light should fall 50:16 and that should mean that light going near the sun is deflected it is and also the force of gravitation is 50:22 slightly modified in his theory so that the law has slightly changed very very slightly 50:28 and it is just the right amount to account for the slight discrepancy that was found in the movement of mercury 50:35 finally with reconnection to the laws of physics on a small scale we have found that the behavior of 50:41 matter on a small scale obeys laws so different very different than things on a large scale and so the question is well 50:47 does gravity how does gravity look on a small scale what is what is called the quantum theory of gravity 50:54 there is no quantum theory of gravity today people have not succeeded completely in making a theory which is 51:01 consistent with the uncertainty principles and the quantum mechanical principles i'll discuss these principles in another 51:08 election now finally you will say to me 51:15 yes you told us what happens but what is this gravity where does it come from and what is it 51:21 do you mean to tell me that the planet uh looks at the sun or sees how far it is takes the inverse of the square of the 51:27 distance and then decides to move in accordance with that law and movement in other words although 51:34 i've stated the mathematical law i'm giving you no clues to the mechanism 51:40 i will discuss the possibility of doing this in the next lecture which is the relation of mathematics of 51:46 physics but finally in this lecture i would like to discuss to remind just at the end 51:51 here to uh emphasize some characteristics that the gravity has in common with the 51:58 other laws that we have mentioned as we passed along the first is that it's mathematical in 52:04 its expression the others are that way too we'll discuss that 52:10 next time second it's not exact einstein how to modify it we know it 52:15 isn't quite right yet because they have to put the quantum theory in that's the same with all other laws 52:21 they're not exact there's always an edge of mystery there's always a place that we have some 52:26 fiddling around to do yet that of course is not a property probably not a property it may or may not be a property of nature 52:32 but it certainly is common with all the laws as we know him today it may be only a lack of knowledge 52:40 but the most impressive fact is that gravity is simple it is simple to state the principle 52:46 completely and have no left have not left any vagueness for anybody to change the ideas about 52:52 it's simple and therefore it's beautiful it's simple in its pattern i don't mean 52:57 it's simple in its action the motions of the various planets and the perturbations of one on another can be quite complicated to work out 53:04 or to follow how all those stars in the globular cluster move is quite beyond our ability it's 53:10 complicated in its actions but not in the basic pattern or the the system 53:15 underneath the whole thing is that's a simple thing that's common in all our laws they all 53:22 turn out to be simple things although complex in their actual actions 53:27 finally comes the universality of the gravitational law the fact that it extends over such enormous distances 53:33 that newton in his mind worrying about the solar system was able to predict what would happen in 53:39 an experiment of cavendish where cavendish is little model of the solar system the two balls attracting 53:45 has to be expanded 10 million million times to become the solar system and then 53:51 10 million million times expanded once again and we find the galaxies attracting each 53:57 other by exactly the same law nature uses only the longest threads to 54:03 weaver patterns so that each small piece of her frag of her fabric reveals the organization of the entire 54:09 tapestry thank you 54:31 [Music] 54:36 so [Music] 54:52 [Applause] [Music] 55:18 so [Music] 55:32 so 55:59 so value has changed Feynman's Lectures on Physics - The Law of Gravitation ProtonsTalk 9.21K subscribers Subscribe 2.2K Share Clip Save Download 123,377 views Premiered Oct 24, 2020 The Feynman Lectures on Physics, Volume I: https://www.feynmanlectures.caltech.e... "In this chapter, we shall discuss one of the most far-reaching generalizations of the human mind. While we are admiring the human mind, we should take some time off to stand in awe of a nature that could follow with such completeness and generality such an elegantly simple principle as the law of gravitation. What is this law of gravitation? " Transcript Follow along using the transcript. Show transcript ProtonsTalk 9.21K subscribers Videos About Facebook Twitter Instagram Linkedin Live chat replay See what others said about this video while it was live. Open panel 176 Comments rongmaw lin Add a comment... @billb207 1 month ago This lecture was presented on 9 November 1964, and was the first of six 'Messenger Lectures' given by Feynmann between 9–19 November 1964 at Cornell. The series was taped by the BBC. Feynmann was working at Caltech at the time and was yet to receive his Nobel Prize; this would be awarded the following year. 26 Reply 3 replies @dosesandmimoses 4 months ago Communication was his greatest asset.. taking incredibly difficult concepts of different theorists and comparing and contrasting with a conservation of verbiage holds the attention and is pretty awe inspiring 37 Reply 1 reply @throckmortensnivel2850 3 months ago Feynman had a curiousity. He wanted to know how everything worked. He wasn't only a theorist. He loved tinkering. And, as has been noted, he was an excellent communicator. Wish there were more of him around. 22 Reply @speckinthedark 1 year ago What a legendary person!❤ 34 Reply @oliverave1234 4 months ago This lecture demonstrates what would happen if Art Carney taught physics. 22 Reply @realmtraveller 2 months ago The students who were fortunate enough to study under this man, truly never knew how lucky they were till much later. 3 Reply 2 replies @Trebor_I 4 months ago I always love how he sounded like a NYC cab driver. 27 Reply 2 replies @richardpark3054 1 month ago (edited) What a giant! A human that makes me proud to be a human! Thank you, Dr Feynman! 1 Reply @FARDEEN.MUSTAFA 4 months ago Professor Richard Feynman's theory of Quantum Electrodynamics is really dynamic. 17 Reply 2 replies @scottgreen3807 2 years ago Once upon a time when a intellect was actually valued. 124 Reply 26 replies @rav8149 1 month ago 7.38 Higher generality over the laws themselves .. this was epic and deep in itself 2 Reply @chrisguffey7991 2 months ago Dr. Keith Honey. WVIT. Small town, no one has heard of, tiny class of three. Similar lectures. Epic. 3 Reply @gpawar 2 years ago Amazing Lecture !! 11 Reply @RaRamercy 6 months ago start at 6 :00 27 Reply 2 replies @costrio 2 months ago The "tremendous number, remains a mystery?" (with 42 digits) Wow! He just discovered the answer to "life, the universe and everything" according to the HHGTG. Feinman is "Deep Thought?" 5 Reply 1 reply @costrio 2 months ago Mr. Feynman was a fine man who drove a fine van painted with Feynman Diagrams. So cool. Are there any videos of him playing drums, I wonder. 2 Reply 1 reply @yohanannatanson4199 4 months ago Absolutely fascinating... 3 Reply @niedsonemanoel 2 years ago Amazing! 7 Reply @brunojean8007 1 month ago Has anyone asked yet where the following lectures can be found ? 3 Reply @willo7734 4 months ago (edited) If there was one scientist from history that I could meet then kick back and have a beer with it would be Mr Feynman. i wish he could come back and teach some science to the flat earth people. 10 Reply 1 reply @rogerferrao6367 8 months ago Marvelous explanation 👌 3 Reply @sieger2096 4 months ago Feynman lectures❤ 3 Reply @darz3 1 month ago @46:50 , we are still searching for the unified theory 1 Reply @pruephillip1338 1 month ago It seems we haven't moved much since Feynman's time in our understanding, have we? 1 Reply 1 reply @terrywelch-gk4xm 4 months ago Nobody in the lecture caught his joking innuendo at 23:20...crickets chirping in the silence... 9 Reply @rehmanmirza7315 2 days ago One of the most handsome physicist Reply @davidfoster5906 2 months ago The Art Carney of physics 1 Reply @guntherirlbeck8631 4 months ago We still avalaching in high speed..in the 60's it was true...nowadays Physics is in trouble. We do need new Feynmans and Einsteins... 1 Reply 2 replies @sclogse1 4 months ago If you looked at the comments and wonder about what appears to be random listing of names by different youtubers, every one of them joined youtube at the same time. 2 Reply 2 replies @ophthojooeileyecirclehisha4917 3 days ago thank you Reply @TheSimCaptain 2 months ago Amazing how science hit a dead end after scientists like Feynman. We're still trying to find a quantum theory of gravity some 70 years later. Reply @ronaldradford3971 4 months ago That was informative 1 Reply @TLH442 2 months ago Gravity, just got the idea of what it is. Surplus electrons or their absence. What is the net charge of a planet as opposed to a newton star with its degenerate electrons that help support it against collapse? Gravity is proportional to mass but something is wrong. Think of a dielectric effect that mass has on the electric permittivity of free space. Gravity is weak but it could still be a just a weakened electrostatic or dynamic force. There are examples of sub atomic particles where laws of attraction are not 1/R^2. Feynman said something and I immediately agreed and started typing this. To conclude, mass is not the cause of gravity even though its true that its a straight linear relations ship. It's the residue of electric and magnetic fields of each object causing the attraction. The spooky forces of attraction are what make physics so hard to comprehend as Dr. Feynman has noted. Observation with cause and effect are not usually truth about why it is the way it is. It's like mass is the dark area of the Mandelbrot set, an infinite set of points completely is disconnected from the other infinite domain of points in color. We have photons and kilograms. One collects spontaneously, the other is doing its best to never rest for a nanosecond at least until it get's too close to a nucleus, neutron star or black hole. Is it not rational to think that if energy wants to scatter then mass will respond by collecting. It's a balance between potential and Kinetic energy's. 2 Reply 2 replies @udhdabadhikari5997 2 years ago Mastermind ! 6 Reply @ronaldradford3971 4 months ago Art Carney comment hammered it, lol 2 Reply @Constantinesis 1 month ago I think it deserves an upscale? Reply @angelaslaney1514 4 months ago In this day and age, is it not possible to up-scale 360p to something actually watchable? Reply 1 reply @tonyduncan9852 1 month ago Wow. Sixty years ago, Just like yesterday. 😎 Reply @sonarbangla8711 1 month ago I wonder if Feynman could explain, if g=0 at the center of the earth, then why there is so much pressure there since P=mgh and not 0. Reply 4 replies @donaldwhittaker7987 4 months ago His lectures are priceless. Is it possible to render them in HD? I've seen old charlie chan films that look better. Thanks. 5 Reply 3 replies @ChandrasegaranNarasimhan 4 months ago (edited) I have been to America. They never criticize you. May be I am different. Coming back to feynman's lecture, it lacks quantitative aspects. I really do not know why. Btw: 38:31 attracted to me vs interested in me-> man's problems. Reply 1 reply @eeroiiskola5942 2 months ago They speak portuguese in Brazil. 2 Reply 2 replies @Resiprocity1 2 months ago (edited) “Gravitation” should be re-named to “Attraction” because at the end of the day the behaviors observed are akin to 2 magnets with like ends, like orientated. This would make the concept much more understandable. 1 Reply 3 replies @michaelgonzalez9058 4 months ago Can a spectogram capture a numerical sequence of the forces of a ship going to orbit -from earth too the space station 😢😢 1 Reply @grahamblack1961 2 months ago In the 60s they had Feynman. Now we have Neil deGrasse Tyson. Hmmm. 2 Reply 1 reply @dieterdreier7109 4 months ago He is the king of the Bongo,he could be the king of the Congo and all girls from Katongo... we save him.. 1 Reply @aboundlessworker.mazhar 3 months ago I am too much dopaministic😢 but i love physics😅 1 Reply @ronaldradford3971 4 months ago Ralph about to yell from the crowd, lol 1 Reply @sieger2096 4 months ago 💙 1 Reply @dsharpness 1 month ago Oh, heardsaid, electron fields never end too...😮 Reply @davidseed2939 2 months ago 6:10 start Reply @nowhereman9463 4 months ago Isn't it amazing how human beings believe we can explain the unexplainable with simple words. I prefer to just enjoy the workings of IT ALL. 1 Reply 6 replies @terrywelch-gk4xm 4 months ago (edited) Atoms will be seen to attract like atoms of the same kind and type with the force we call gravity even on intergalactic and cosmic scales. In this my theory, all hydrogen atoms everywhere, even these on earth, are aware of each other even to the farthest known limits of space, and this is what we see as entanglement. and also call "gravity". 2 Reply 6 replies @shawnheneghan4110 1 month ago (edited) Like anyone in Brazil would notice - they speak Portuguese Reply @XmanXman-jd3ts 1 year ago The first Youtuber? 4 Reply 1 reply @marksoffian5568 2 months ago I didn't know that Ed Norton knew Science Reply @dosesandmimoses 4 months ago Boom 1 Reply @clairaut 2 months ago It goes back and forth too much. It makes my head spin. Reply @AppleVsGravity 4 months ago I don't see any wokies! The good ol' days. 2 Reply 1 reply @PaulHernandez-d8i 5 months ago Williams Jennifer Rodriguez Joseph Brown Barbara Reply @barryzeeberg3672 4 months ago Wouldn't it be incredible if his wife's name happened to be "Trixie" :) 1 Reply @TGoat123 2 months ago Surely you're joking Mr Feynman. Reply @Giani369 4 months ago 😊😊😊😊😅😅 🎉 1 Reply @sabirsadiq-ys9po 5 months ago Write in google: Singularity sphere in the heart of a black hole 1 Reply @themusicofnewyork1570 8 months ago Great lecture. But I believe he did not understand the tides himself Reply 2 replies @TempleElaine-z4l 4 months ago Anderson Michelle Walker Joseph Thomas Anna Reply @leonardobrunorende5363 2 months ago (edited) Final minutes... no, professor, no... gravity is far, very far from silmpe, has a tremendous complexity compared to quantum mechanics, such that can not be unified. Reply @Hank-x5q 4 months ago I would be to embarrassed to call this lecture child's play...⚫️👽...damn! Reply @assignmentuot7963 4 months ago T Reply @saviogilbertlewisanthony1696 2 years ago Why is the planet attracted 1 Reply 4 replies @jacque4697 4 months ago Sounds Like a con artist. That’s how most of these so called scientists sound. 2 Reply 1 reply @Bultish 4 months ago 37:46 BAM 🤩😄 1 Reply @HopkinsDean-r8i 4 months ago Lopez Elizabeth Thomas Steven Jackson Sandra Reply