Richard Feynman on Quantum Mechanics Part 1 - Photons Corpuscles of Light

e we have a a theory which is called Quantum electrodynamics which is our pride and joy it's so successful and it's such so wide in its application and what I would like to tell you about is that theory how it works or how it looks like it works what the world looks like from that point of view the uh physics has got a history at least it's theoretical history of uh synthesis perpetually of course the experiment is always finding new phenomena problem is to work them together and sometimes we see that they're different aspects of the same phenomenon an example is of course simplest and earliest one is that the laws of motion became in the theory explained the properties of heat because heat was motion and if you knew how motion worked you could understand the thermal thermal properties it also explained the properties of sence s which is otherwise a mystery as the motion of the atoms and waves in the gas onside from that knowing the laws of motion you have to know Newton who gave us the laws of worked out the laws of motion also gave us another theory about the forces between large masses and distances from one another called the theory of gravity well that's just a that thing the theory of gravity is not as well known and understood pretty well but is not what I'm going to talk about as the time went on phenomena associated with electricity you know rubbing combs in your hair and things like that and magnetism became uh interesting to the experimental physicists and they discovered relations between them experimentally until they saw ultimately there were not two different phenomena but different aspects of the of the same thing another phenomenon that Newton had studied was light so in that time it looked at first like there were many things motion and gravity electricity and mag magnetism later and light but when Maxwell put together the laws of electricity and magnetism he found out that the behavior the equations that he had produced expectation that it would be behavior of waves that would propagate at a speed which was figured out from electrical measurements but came out the same as a speed that light actually propagated and so there was a new theory of Light which is that it was an electromagnetic wave and Maxwell's great synthesis in 1873 was to connect electricity magnetism and light light is just one aspect of the electromagnetic wave which can have different kinds of wavelengths from that point of view and if you have different wavelengths if the wavelength is very short between about four 100 millions of a meter no of a centimeter 400 millions of a centimeter and 700 millions of a centimeter then you see it directly with the eye but if it gets longer the wave is well it's a long end it's red and then the other end it's blue and if it gets more longer than the red we call it infrared the rays are there but the eye doesn't seem the pit viper has a eye that sees the infrared and if you go in the other direction into the Beyond The Violet then we can't see it again but the be has an eye that sees the ultraviolet and uh if we go still further to the far ultraviolet no animal has it that can see it but we can make instruments that detected or photographic plates and so on up into X-rays and so forth and down in the other direction far infrared uh get into radio waves and we can build instruments that detect them and we can use them to advertise soap is it in addition so that there is an enormous range of one property the wavelength a range of phenomena that's a complete enormous Spectrum the Spectrum we see with the eye is very narrow range and it's the entire Spectrum it's all put together with the one theory of electromagnetic waves I'm going to talk about that part of it like I'm going to call it light instead of saying electromagnetic radiation light is what we see is only one little part but from the physicist point of view the accident that the human eye happens to be sensitive to ways from here to here is not essential a phenomena are the same over the whole range and that we call I'm going to call them all light but it could be rways or x-rays or what have you next thing that was discovered was the structure of the atoms and that I'd like to remind you that you have I believe a Nobel Prize winner from New Zealand before Mr Rutherford who was I believe a New Zealander who worked out that the atoms had nuclear you know seem always here I've only been here a few days everybody's talking themselves down I thought this would be a happy country but something's happened to you they got plenty of room and not too many people and it looks like it ought to be good anyway you do don't forget you had Rutherford so it's okay anyhow he had a theory of the he developed the our understanding of the atoms that's having a tiny core it's very heavy with the particles going around it electrons now supposing that the electrons went around according to the laws of motion of Newton some properties of matter could be understood exposing the atoms were made that way but most of the time it failed and it became more and more of a crisis in physics to understand what matter was light uh because it looked so obviously right that it had to be electrons going around nuclei and yet nothing worked when you worked it out and the discovery was made then in the discovery of quantum mechanics first in the behavior of light and then in the behavior of matter and finally culminating in 1926 with the full equation of quantum mechanics which has told us that the laws of motion of Newton were not right and had to be modified through other laws which are quantum laws of motion and when this Quantum Laws of Motion were applied to electrons to explain the properties of matter it was a fantastic success the properties of the atoms can be all worked out mathematically in principle at least in the simple atoms in detail and therefore the theory behind chemistry which atoms combine with which at what rate and so forth is in principle theoretical chemistry deeply is physics it's not a joke it's a direct the chemist will admit that's exactly his point of view that the understanding of the atoms in the deepest level is physical physics except that the atoms have so many particles in them it's very hard to calculate what's going to happen so he has to use a lot of empirical rules to help him but in as far as we can tell there's nothing about chemistry that's not understood ultimately as the behavior of electrons following the laws of motion of quantum mechanics this defines the properties of all substances also and so that the whole theory of the properties of ordinary substances and the chemical properties and so forth have all been reduced to the motion of electrons in the meantime the theory of light and its interaction with matter which was Maxwell's equations had to be modified to become a quantum theory also oh I forgot to mention that during this time one to somewhat to one side of the way I want to go in these lectures I would not going to discuss much about relativity but the theory of relativity was developed and that just makes it easier for us to guess laws it tells us if we know how something varies in space then we know how it varies in time or vice versa that there's a nice relationship between a behavior in space and behavior in time and that it's all sort of different aspects of the same geometrical thing called SpaceTime at any rate D using the principles of Relativity and new Quantum and the new quantum mechanics found a wonderful Theory as simple as possible thing you could write down for the motion of electrons D theory of electrons and that was the situation about 1927 or 8 but uh the problem of the interaction of electrons with light which was a complete quantum theory of electrodynamics other words make Maxwell's equations of light of electricity and mag and the theory of motion of electrons all into one grand theory was accomplished in 1929 in the theory was that was called Quantum electrodynamic trouble with it was nobody could figure anything out or better when they did figure it out they got nutty answers if you didn't do it too carefully you got a reasonably good answer for a problem but if they carried it out very carefully you would get some silly answer like zero or infinity or absolutely absurd results this strikingly lasted for 20 years while people tried to figure out what the correct theory was during this time experimenters were measuring things more and more accurately the theory of uh one of they measured a they found a few things with very subtle effects that the theory of interaction of light with electricity should explain and then they measured them and they found these effects but they couldn't explain that is the theory didn't explain it quantitatively because when the people made the calculation they got Infinity instead of the right result as an example an electron in a magnetic field recesses at a certain rate and the rate according to D was a certain amount but when you he didn't take into account the interaction of electricity and light and when he took it into account we knew that the answer should be wrong let's say that the rx's answer was right but when experiment was made it came out not to be one but to be a little bit more uh this number here the experiment is weren't good enough to tell us exactly it's somewhere between 5 and 21 here we write it at eight plus or minus three is the next digit that means this is not measured accurately so in 19 uh I'm Sorry by 1948 20 years we had at last measured something which showed that the original theory was without interaction is incomplete this is supposed to be the result of interaction when you went to calculate it you got Infinity so there was a very strong effort made then in 1948 because of the fact that experiments were showing such accur to try finally to get that theory straightened out and it turned out surprisingly it was worked out more or less independently by three guys who got Nobel prizes if one of which you see here and uh uh Professor swinger that's not me with the other one of the other ones first worked out a correct way we we found out that the original theory that was written in 1929 by Heisenberg and dur and poy was very nearly correct and the problem was that there was something wrong with the way they handled doing the calculations and we straightened it out and then we could do calculations and schwinger for example calculated this and found out that it was something like this theoretically and that was a tremendous achievement it's a great excitement because that meant we really understood more more subtle details and that the original theory of Heisenberg and d and so on were fundamentally was fundamentally right just a little switch on how you calculated things now this is the theory then that we're going to talk about this theory has lasted now for 50 years uh 50 years 30 years I can't add from 1979 1949 oh it's 50 years yes from the time that direct and and Heisenberg wrote it and it took 20 years to figure out how to calculate with it then the remaining 30 years the methods of calculations improved they calculated things much more accurately in the experimenters became more and more Adept at measuring things and this particular rate that they uh measured in 1948 to here now in 1979 has been measured to be in fact 10159 65 24 and the four may not before four it could be somewhere between two and six after all the more write the some way you have to stop right and this I write to show you the tremendous achievement of experimenters during the last 30 years in order to test with the Precision the correctness of the theory in the meantime poor guys using calculators and sweating and writing marks on pieces of paper so I'm calculating the results of the theory under the same circumstances for the same phenomena produce the predicted value that it should be 6523 within min plus or minus 3 why should the theories have a plus or Min they get exhausted in Computing the number of decimal places that they eat to to keep up with experiment there are this is not atypical there are two or three or four perhaps different places where it's been measured and checked to that degree of accuracy this degree of accuracy that number of decimal places corresponds to a Precision something like this if you were measuring the distance of Me to the Moon the question would come up do you mean from my chin or from the top of my head the difference between whether it's from the chin or the top of my head to the Moon is the plus and minus pool on the end of that number in proportion all right that is a to intimidate you that the theory is correct that in high accuracy I have don't need to produce a large number of other experimental results they all have this feature it is remarkable that at this time it is possible to say that there's no experimental discrepancy known between the predictions of the theory anywhere and the results of experiment that doesn't mean we can compute everything the rules of the game by which we make the computation the laws underneath everything that makes nature work are simple it doesn't mean we can really figure everything out to give an example if you play the game of checkers I think you call it Checkers here maybe draws something the rules of the game are very simple the way the pieces move is simple and if you want to make it even simple make the rule no Kings but when you come to the end of the board you start at the beginning doesn't make any difference the rules are simple but imagine a checker board with 100 million piece squares on each side hundreds of millions of these Checkers in different positions moving through the board taking pieces and being taken from the other which way are they going to swir which way is the game want to go a lot of think difficulty of the rules that's involved but the multiplicity of its action and interconnections matter as you all know be May is May atoms and all this stuff is such a multitude of little particles that in ordinary circumstances so much is happening that in spite of the fact that the rules are relatively simple not quite as easy as the Checker rules but pretty easy it still is very difficult in almost all circumstances to figure out what could happen exactly when we can't figure it out exactly but can do a pretty good job of approximating the phenomenon is in the range that we expect when we have a situation that's sufficiently simple a corner of the board where there's only a few pieces then we can compute exactly what ought to happen and when we do experiment in those circumstan it fits exactly and that's all we can say at the moment about this Theory this uh theory has been design was originally designed in ideas of space and time and a geometrical framework the question is how small a scale can we go down to and during the time of this this period of time we not only meas tried to measure accuracy but also tried to see how small a distance the theory would be correct at and I can only tell you the distance is 10 to the minus 15 cm that means Point Z 15 zeros before you get to 14 zeros before you get to a first digit of distance in centimeters we that thing is that ACC we know that the laws are that accurate to put it in another way it used to be thought that atoms were small that was a limit of measurement but uh at the present time with the new instruments and divine during all this time we've been able to make instruments that can test this Theory down to distances that could be described this way if the atom is made 100 kilom on a side then we're measuring with 1 cm accuracy inside so the theory is right the distances corresponding to ctim when an atom is 100 kilm so altogether I can only emphasize with delight and excitement the fact that so much of nature is so accurately describable by one Theory it's enormous range of phenomena all the things you ordinarily see the best way to describe what the phenomena are colors of things this sness of materials the weight of things the way they temperature when you change the temperature how much heat it takes sounds in a whole these phenomena yeah only best way to describe is to describe the phenomena that are not included in this Theory and one of them is the accelerations produced by gravity the force of gravity is in another theory gravitational Theory or general relativity another range of phenomena have to do with exciting the interior of nuclei a nuclear physics protons and neutrons radioactivity and nuclear phenomena that excluded all the rest of the phenomena of nature are contained in this one Theory now you can see why it is that I feel a bit uncomfortable when come someone ask to give a talk please tell us the latest things because then I'm talking about our problems that we have in trying to understand the insides of a proton for example for a proton to tell you a contrast our understanding of the outside of the atoms the electrons and light that I'm going to talk about the theory it's our Jewel and great achievement to the things that people ordinarily have to talk about at these lectures it's as follow you take the corresponding number for a proton I don't remember it it starts at 273 I think 1 n I'm not sure or is it 2719 I don't remember and it goes on for a number of decimal places because the experiment that can measure it so well now we have a theory of the protons recently developed involving quarks and so on we can't make any calculations yet we haven't developed the technique good enough so the best I could say really and probably exaggerating is that the theory does say that this number should be around three and the error and that's what I think I must be bigger I can't prove that it's as small as it might be three uh that gives an idea of how sloppy our understanding of protons is compared to that due Precision that we understand electrons all right so that gives us some idea of why it is I feel so uncomfortable talking all the time about what we don't know much about and nobody asked me about the stuff we do know everything about so therefore I'm forcing upon you a lecture on the things that we think we know something about okay so that's our job now the question is what am I going to do I'm going to tell you what the theory is I'm going to tell you what it looks like what we do to make the calculation just what the thing is because otherwise is how you're going to understand what world picture in other words this thing is and it is a world picture because it describes all the phenomena except for radioactivity and gravity in the world that's a lot of phenomena it's possible even it might explain and should explain if everything is Thoroughly understood the laughter of the audience when you make a dumb remark now if I'm couldn't explain this Theory the question is are you going to understand it will you understand understand uh the theory when I tell you first that the first time we really slowly explain it to our own physics students is when they're in the third year graduate graduate physics then you think the answer is going to be no and that's correct you will not understand but this business about not understanding is a very serious one that we have between the scientists and aians and I want to be AB work with you because I want going to tell you something the students do not understand that either and that's because the professor doesn't understand which is not a joke but a very interesting I think and I would like to explain it my task really as a to explain all this is to convince you not to turn away because it appears incomprehensible that's what it takes four years of us to do to the student is to get him so he doesn't run away because it looks crazy the thing that's exciting about this is that nature is strange as it can be in this sense that the rules that are going to be obeyed that I'm going to tell you about by which this stuff is analyzed by which we understand nature the rules of the checkers yeah are so screwy you can't believe them nevertheless if you follow out the consequences and see what they do sure enough all the ordinary phenomena that happen you can understand that's hard to do because you have to know how to count big numbers and do lots of arithmetic and so forth to see how it is that these rules really explain common experience that will be more difficult for you to understand what is not difficult is this well that's difficult enough yes because it's so strange but it's no more difficult for you than for the students and no less difficult I know sometimes I hear people coming to my lectur to say oh I'm going to come to the lecture although I know I'm not going to understand anything it makes me feel bad or when they come up afterwards they say oh I enjoyed your lecture it was lots of fun but I didn't understand anything you're saying I really am trying to make myself clear so I would like to discuss this with you will you please keep coming in spite of the fact that you don't understand it because I don't understand it either and the fun of it is that we it's so mysterious okay that's the fun of it so this's business about understanding requires just a few words and so I'm going to say something about the relationship and I would hope we get some of your cooperation sometimes you don't understand because say the language is the fell com from America and he talks too fast that's my fault and I apologize I hope it's all right that's a kind of trivial difficulty relatively next kind of not understanding is because you perhaps use new words that's an accident that comes because I'm working technically and I use the words all every day and I forget that everybody doesn't know what they mean and I have to be very careful again my job then there's a kind of saying that you don't understand it meaning I don't believe it it's too crazy it's the kind of thing I just I'm not going to accept it uh well the other POS well this kind I hope you'll come along with me you'll have to accept it because it's the way nature works if you want to know the way nature works we looked at it carefully look at that's the way it looks you don't like it go somewhere else to another Universe where the rules are simpler philosophically more pleasing more psychologically easy I can't help it okay if I'm going to tell you honestly what the world looks like to to the to human beings who have struggled as hard as they can to understand it I can only tell you what it looks like and I cannot make it any simp I'm not going to do this I'm not going to simplify it I'm not going to fake it I'm not going to make tell you it's something like a ball bearing on a spring It Isn't So I'm going to tell you what it really is like and if you don't like it that's too bad okay there's also the possibility that you don't understand because you're con you get a bit confused and you're sure that you must have misinterpreted what I said or uh something like that and you get turned off and that's of course a difficulty let me assure you that most of the time you did interpret correctly what I said because if it does I'm going to it's going to be so shocking the way nature actually works that you're not going to believe that either I faked it I'm not telling you the full story that's for the students I have another way of explaining it or something like that it is not true I'm going to be honest okay so I'm going to ask you to try not to get turned off and not to be afraid relax and enjoy it realize that nobody understands it what the hell are my students learning for four years if nobody I'm going to explain and I'm going to explain by a kind of an example uh I like to take the Mayan Indians they had a writing system and we know some of the things they wrote were astronomical things and they had a scheme for predicting many things in the sky eclipses and so on let's take the example of when Venus which was important to them because it represented evil of some sort was a Morning Star and when it was an evening star so they could predict ahead of time whether this bad influence was going to be in the morning or in the evening and so they discovered that if they waited that this cycle of morning evening morning evening morning evening five of those occupied just exactly the same time as eight times a certain period that was important to them 365 days it's not exactly a year and they knew the difference but they still counted in 365 day intervals which they called the tomb so they said that five of these Cycles is eight tuned then they uh discovered of course Very quickly that if they did this five cycle bit for eight tunes 10 times they were off by about 6 days and so they had a rule for Shifting the making corrections as they went along and thus had a very good way to predict when Venus was coming off okay now let's look at this thing from a point of view suppose that the professors the priests in those days who wrote this stuff and taught their students these rules were're giving a lecture to try to explain what they did in order to make these wonderful predictions about Venus then if the fellow was any good at Exposition and really knew what he was doing he would say what we're doing is we're counting the day just like you're putting nuts in a pot and we keep on Counting 58 uh 365 nuts and then another 365 and another 365 and another 36 guys what a lot of work and when we get all finished we said that's five of these periods now they understood what he said that's easy they did not know a quick and tricky way to add 365 time 8 I'm sorry I said five times I meant eight times uh the students were learning in the meantime the laws of arith something which is to us now because we have public preed general education almost everybody has to struggle through and learn how to add numbers by a tricky scheme of writing him in place system and making carryings and so on so that if you buy wine for $415 and your meal is 287 or vice versa it cost 702 and the girl who does this the waiters just ordinary person in two minutes does that how did she do it what is she doing when she's adding 415 to 287 she's doing this counting out 415 pennies then counting out 287 more pennies and telling you how many pennies you would have got if you counted them all from the beginning to the end but it's a highly educated and very trained to be able to do that with those large numbers quickly this training is is something in spite of the fact that everybody's G it it's something pretty good because in the 14th century mathematicians were they were called who could do that almost everybody in our civilization can do that but I would like took this example you can understand what's involved what the students are taught you see in our particular problems now about physics there are many bigger numbers the numbers are much bigger it's hard because numbers are so enormous you can't count them directly and so we've invented a fantastic array of tricks and gimmicks for putting together the numbers adding counting checking and so forth without actually doing it the way I could describe what we're trying to do if I say I draw this and I draw that and I draw this and I draw that and I see where the end point is we don't actually sit down and draw 7,000 arrows and find out where the end point we have a way of figuring out where it come just like we don't actually count 415 pennies and 287 pennies to find out that you owe me 702 pennies we do it by another trick this is the tricks of mathematics and that's all so that's the part I'm not going to worry about we're not going to worry about that so they relax you don't have to know mathematic all you have to know is what it is all it is is tricky ways of doing something which would be laborious otherwise so what that it's true that in the years we've developed enormous abilities in mathematics and it takes a long time to train the students and so therefore they're very highly educated in that but if you ask him why now we go back to the mays we ask them why the rule why when you wait fill up a tub eight times with 365 day markers it comes out that the Venus is up five times they don't know they don't understand at all the more accurately they can do it the fact that they know that they have to change it by six days and so for adds nothing to their understanding of it the student who has learned all this mathematics and is able to make these calculations not only of Venus of the Mars the Sun the the eclipses and everything else is a super priest doesn't know why any better and he would explain if nothing but counting days he would be reduced to the truth on the one hand and to an honest statement that he doesn't understand it the other hand and could tell somebody all about it who doesn't know how to count all these numbers so trickly and so cleverly as the priest students knew okay now probably I don't know about philosophy of Mayers we have very little information due to the efficiency of the Spanish conquistadores and uh well mostly their priests who burned all the books they had hundreds of thousands of books and there's three left and one of them has this penus calculations in it so that's how we know about that and uh just imagine our civilization reduced the three books the particular ones left by accident which one see so uh anyway I get off the subject I make this up now that what I'm saying now is just a story suppose now that the students would discuss or people would discuss the possible meanings of this why then they would begin to think about well 8 * 365 is 2920 that's got two twos in it now two is a lucky number and it has two twos in it and then the nine represents the god of so and so which is related to Venus and so for and that would be a good argument then but in another city some other guys getting together have a different kind of an argument about it they'd say look now the fact that there's a 20 at the end if I subtracted that away first I get 2900 which is especially good number from blah blah blah and so on and they would have different theories and then someone would come along and say you know it doesn't make any difference which one of these theories is right we still have this fact to go along with and that is our modern scientific point of view in the earliest days of science we got confused arguing philosophically what was a reasonable reason for nature of ho of vacuum or it seemed to be nice to gods were doing it different kinds of psychological reasons for thinking it probably is all right after you discovered what it was these things were never useful for predicting what should happen next and we soon learned not to make these arguments it's useless it doesn't add anything and so we're not going to make my imaginary Mayan uh argument about the various gods that make the numbers and so I'm left if I'm a modern scientist with a description of the situation all right now I prepared the audience used up all my time to prepare the audience doesn't make any difference I will continue anyway in spite of the fact I use up all the time to uh describe the theory and in describing it I will first describe some part of it the theory is the properties of light electrons and the interaction of light and electrons it's all one Theory I cut it into three parts that way and the first thing I'm going to start with is the uh properties of light okay I'm going tell you some of the properties of light and I hope if I can do it to get to the key point and then uh we'll continue in the following lectures to elaborate on it first uh I don't go through the history of the theory of light it had various things the first and most important thing I would say is that Newton found out that what we see is white light is a mixture of otherwise Pur stuff that's easy to understand and if you understand how each of the parts works you just put the mixture back together again and they separate the light in prism which is done automatically in ockland very often le as far as I can tell in rainbows and uh the various colored if you would separate light in the prism and take out the part that looked say yellow then you couldn't separate that any further in another prison it just stayed yellow and that's called monochromatic light light of one color so I'm going to discuss all my phenomena for a while with light of one color because it's simpler the first thing Newton believed that light was a corpuscular thing and had very turned out to very strange properties from that point of view and it was then explained that many of these strange properties was because in fact it was the wave which was wrong it turned out he was right it was a particle it is corpuscular uh the reason that he said it was corpuscular was based on an incorrect guess as to the behavior of waves and this argument was wrong logically but it turned out in the end that it was particles now how I talk about how I know it's particles is this if we make an instrument to detect light that's as sensitive as it can possibly be made we make it more more sensitive in fact this thing is called a photo multiplier and that's not the only instrument I just take one for an example and doesn't make any difference how we do it when we get to light that's sufficiently weak an instrument to detect it here is clicks pulses uh as if it was rain falling on something you get bang bang bang bang when a light is bright the rain goang lot of them when the light is very dim boom boom boom boom boom small the particular boom booms and the bang bang bangs and so forth are completely out of proportion the actual rate is enormous okay and a little bit less when there less light it's very difficult to get it to a boon Bo Boon it could so dark in here you wouldn't know what but uh this device to show you an example of how it works just so you understand what happens when we detect the weakest possible light is it works like this is a metal plate here made cesium or something when light shines on it it knocks an electron out then you have another plate here with a voltage that attracts the electron so the electron so to speak Falls and subtracted and speeds up and hits this plate when it hits this plate got about 100 volts of energy it splatters other electrons get knocked out at two or three five perhaps on the average now those are attracted to another plate and they all go sailing down here with another 100 volts five of them this time and each one of those knocks out on the average five or six other electrons now I got 25 yeah and that subtracted to another plate and that hits those and so on and you have a maybe 10 or a dozen plates by the time you get out the other end you got such a chunk of charge in electrons so many of them you multiply five Time 5 Time 5times 5 You' be a long time counting pins to discover how much that is you get such a tremendous pulse that you can number of electrons high enough it can go directly through C circuits and so forth and turn on and off switches and do all kind of things make voltages to pin make noises do everything that's Amplified now what happens when we have a device like this and we put it in the dark is it goes click click click click click click every once in a while a light particle comes in a photon this is a particle in every sense the experiment of all the right properties is follows and if you have a very weak light and you have one or two just one of them every once in a while if you put two cells out and there's just a few of them coming then it goes on one or the other they don't go off together they go off together you got too many coming and you can't resolve it but if it's very weak the particle is either here or there and it comes in particle I don't know how I can much I can emphasize this especially to Young students who have learned its waves it is particles in every way whenever you can detect it it's unfortunate for us that we can see the light I mean it's unfortunate for us no not quite not quite that we if we were 10 times more sensitive to light than in the dark we would see that what we're seeing is little flashes little tiny dips dots of light the nerve would go off just like this photo multiplier in spots but the human eye is not quite that sensitive and takes five or six of these particles photons five or six photons to make one nerve fiber go off so it isn't so we cannot detect with the eye light quite low enough to notice the fact that that it comes in the form of raindrops all right got that they're particles right you can detect them with an instrument you can count them so and so many per second bright light more per second dim light less per second okay now we start to describe the properties of light a little further next property I want to talk about is Reflection from a glass surface or a water surface I believe everybody knows that you can see the Sun or the moon let's say in the sea as it settles Reflections are a happy thing in art pictures the moon light reflected from the water you have a a window uh when you look through a window you there must be millions of examples right back there there's one you look at a window you can see through it but also some reflection now already there's a problem because the light that's reflected is not as intense as the light that's shining some of the light goes through the window say well through the water down in only some of the light comes back if the light is headed for water for example straight down only about 2% reflects what does that mean only 2% reflects that means that if we had a phon counter here let's say uh draw the experiment so you know what I'm talking about it's hard to make a water surface that's vertical yes so we'll make the water surface horizontal and uh lights coming down and some of it's reflected and we put a counter here to the one of those photo multiplier things and we count the count and we know how much we should have got and we find one how can it be partly reflected when I forgot to say when I was talking about the prod when you have light of a definite color the energy that the first one knocks out is always the same each particle is the same strength there's not half particle it's a full photon what you get is full Photon which you only if the light is dimmer what does that mean there are fewer of them right simple so of a 100 that come down here perhaps uh four about this way and 96 go through what determines which four how does it which one that's coming down of the 100 knows with they come back up right so the situation is that the phenomena is probabilistic it takes odds it comes down here and has a 4% chance one out of 25 trials you I think you know what odds are if you have for instance a d and you want to get a a one and you roll it well it doesn't come out so often but it comes out sometimes one out of six and uh what that means if you roll die a 100 times let's make it 600 times little easier you roll a D 600 times you might get 1051 or maybe 90 two ones or something like that right you get about 100 and in the if the numbers were bigger the accuracy and percentage is bigger not accurate so if you have billion six billion no not six cuz I got one 125 25 billion of these coming down about a billion will come off okay now let's see if the next feature is how can it be probabilistic suppose that I had a light so weak that I had only one Photon coming every few minutes will this counter go off or will the one down here go off one out of 25 times this one goes off which time what determines that possible theories nothing pure chance the world is made of chance that would mean that the physicist can't predict the future it would mean that if you set up an experiment with exact conditions you cannot predict what happens in the future because you can't predict whether it's going to go up or going to go down you just got 4% odds your whole beautiful structure of science is reduced to Computing odds nature instead of being definite does everything by chance not so good other possibility there little spots on here and all has to Photon has to hit a spot on the surface and Newton had several things you'll find out later when I give you more phenomena how these various explanations which way it's going to go and why the spot one doesn't work but I'll give an argument that Newton made about that he said it can't be that because he said you can polish play it's wonderful I love to read these old guys you know they they knew you could polish glass but they had the intelligence to deduce from the fact that you can polish glass that there's no spots on it like why what's polishing see he polished he called his own lenses he ground his own lenses so he knew what it was doing he takes a coarse grain sand like stuff for what do they call it the the the polishing powder I mean the grinding powder and that shapes it it cuts but it cuts fairly obvious grooves then you take a finer one with finer particles and you cut the grooves are finer and you make the grooves finer and finer and finer and after they're fine enough suddenly it's smooth and the light comes right through when it's co it's bounced around and so he concluded that light cannot see the gooes that when I polish it it's not that it's smooth it's it's still bumpy but on a small scale whereas when I the big grains it's on a big scale it can't be any different when I polish with the small grains it's just a small scale regularity but somehow light doesn't see anything on a small scale correct all experiments have shown that this is absolutely not the right explanation if it was there'd be all kinds of ways of testing that I told you we can measure down at 10 Theus 15 cm and so forth and that what would happen would be you'd be able to find some area to focus very carefully the light so that the two that the reflection coefficient would be higher than 1 and 12 25 because you happen to be near a place where there was a spot can't do it no matter what you do it's one and 25 other possibility the light that's coming down here the photons are doing are well uh like footballs and they spin and depending upon whether they hit with the nose point or with a flat Point depends on whether they bounce back in other words something inside the photon is determining which way it goes again no because you can if that were the case the light that went through would be all of a certain kind of football and if you try the reflection again you'd expect a different number than 4% because of the fact that they're all lined up and you can't line light up you can't fiddle around up here by any kind of filtering that'll change that percentage all the lights photons are identical if they're the same color in one color they're all identical and they behave with 4% are we therefore reduced to this horror that physics has got reduced not to these wonderful predictions but to probabilities yes we have that's the situation today in spite of the fact that philosophers have said it is a necessary requirement for science that uh setting up an experiment exactly similar to will produce results exactly the same the second time not at all one out of 25 it goes up and sometimes it goes down unpredictable completely by chance all right I already see you turning off I can see you say you don't understand understand me you can't understand that it could be chance I don't like it tough I don't like it either but that's the way it is okay I don't understand it either I don't understand it it must be that nature knows whether it's going to go up or down no it do not be that nature know we are not to tell nature what she's going to be that's what we found out every time we take a guess as how she's got to be and go and measure she's clever she's always got better imagination than we have and she finds a clever way to do it that we have thought of and in this particular case the clever way to do it is by probability by odds and so the first aspect I have to tell you about then is that light works by probability all right by the way just incidentally the wave theory had no difficulty with this at all because what happened would be like the waves of something are coming down here and they shake and they just keep going but some of them bounce back something like a sand bar sometimes partially reflects the way waves in the seene so some of the energy went back but waves don't operate this thing by the way when you turn the light on this might go off at any moment you don't have to wait for a certain length of time of course there's odd that it goes off at any instant but it could accidentally go up the moment you open the slit so it's not a question of waiting for the energy of the wave to pile up or anything it's hopeless I have to start with light as particles waves explain many things but not right it's particle okay okay it's photon the problem is they have reflected with probability now the next feature having to do with reflection again that I would like to describe is something that you're all so familiar with perhaps not so directly but perhaps you've seen the colors in soap bubbles it's made out of soap water which has no color no color I've seen the color mix it together get a lot of soap water together and look at it no color you take oil which is a sort of a yellow fluid and you it drip with blackish yellow junk it drips out of an automobile on a rainy day you must have experience here with that and you look at the puddle to your Delight there are colors colors these colors are produced by Reflections from two surfaces near each other very close together if the surfaces are far apart we'll discuss it later the same phenomenon really occurs but it's much harder to see under normal circumstances what happens there is that in a soap bubble for example there's a layer of water and so we have two surfaces at some distance from one another so and then the photon can either come down here and reflect right away or it can come down this way and reflect from back there okay now as it turns out that's about the same percent if it's just water between air and air it's about the same one and 25 both time what's the colors if I'm going to do it with monochromatic light that light of one color what can you see in colors what you see on an oil film on a mud puddle or in a soap bubble when you use light of exactly one color is not colors but bands bright and dark bands places where the light is reflected very well and places where it's not reflected at all across the bubble now the different the position of those bands if you first looked at a bubble with red light you see bands black and black and red black and red band all over you look at it with blue light you also see blue and white blue and black blue and black bands but the pattern is not the same they're displaced from the where the red ones were and so when you have both red and blue you get purple and red and black and purple and so on now if you add to that yellow with its pattern and so forth you get all these colors mixing okay so we're going to simplify experimentally as as Newton did and look at these uh this reflection what Newton uh most of his experiments he did this clever way he had a beautiful curved piece of glass which was a lens and a flat piece and put them next to each other and then this was only a very slight curvature and so he had this gap between the two and when he shine shown light this way and looked at reflected light which depending on where you looked see this is a cross-section of the lens which is more completely like that there is some Reflection from here which is irrelevant at the moment there another glass plate down here when he looked in different places the light came down all over when he looked in different places he was looking at cases that corresponded to reflection with one Gap or another gap between the sides and when he looked at this with monochromatic light look down on it since it was a circular l a spherical lens he saw in what I called bands in the soap bubble but more organized he saw Rings called Newton's rings with a black area starting and then red if it was red light he was using and black red and so forth these looked like they were coming closer and closer together but he was a clever man and understood immediately what it meant if he measured the distance from here to here and plotted the answer for whether it's red or dark against this distance not against this instead of measuring away from the center he measured how far apart these plates were then he found like our friends the Mayans every time you had 365 cons went black in other words if this was black and this was red then double that distance was black again and yes and triple the distance was red and four times the distance was black and five times the distance was red and so on nice and even in other words instead of measuring by this Distance by measuring the thickness you find the following experimental result reflection coefficient against thickness between the two layers okay the spacing between in this case if it's an oil film bubble with that that's the distance in this case it's the spacing between the lens and the plate and if you could change it it's hard it to change in a soap bubble under control but it changes automatically as it dries out you get the following you get no reflection if the thickness is zero then you get a strong reflection for second thickness and then if you make it thick you get no reflection reflection then you no reflection you no reflection and so so on and now it's getting thicker and thicker does this go on forever yep if you got good enough monochromatic light and get the experiment under control you can make it go almost indefinitely you can make this work for distances of a meter or more still catching keeping track whether it's even odd even odd bump bump bump okay there's nothing that's interesting very interesting but you know that's daming why because what theory were you're figuring for the Reflection from that that made the reflection be 4% the odds 4% whatever you put another layer down here at the right thickness expecting to get 8% and you get nothing how does the layer down here turn off the Reflection from the layer up here or if you figure that out and you make the layer just a little thicker it doesn't turn it off but in fact the reflection is more than twice on this scale here this line would have been what you would have expected if you expected it always to be 4% plus 4% or 8% from the two surfaces this is what you would have got if you disregarded all this nonsense that actually happened but common sense it reflects with a certain odds and it reflects from the other surface with a certain odds and so altogether you got twice as many coming back this is the twice as men but for some if the thickness is zero you don't get anything if the thickness hey that's not such a bad idea is it if you don't have any water there at all you wouldn't get any reflection starts out right anyway that theory was kind of dumb now that I come to think of it and that helps to explain Newton's OB when the distance is too small the light doesn't know anything about it if the thickness is sufficiently small it doesn't reflect it does exactly the same as it there's nothing there but the horror of it is it's all right that it increases as the thickness goes up but it overshoots you see and then it comes back to zero again when the thickness is just right it's very difficult to invent a probability thing if I had these spots on this surface it's very hard to see how you're going to have spots on this surface which turns the spots on that surface off when they right right distance away and so Newton went a little bit nutty and he talked about fits of reflection and transmission and so forth and I would like to now just finish this by telling you what is the answer now here's the answer this is the way we figure this out it goes as far what we're going to calculate is a probability the probability if a particular question and probability that this counter goes off the probability that if I had one Photon coming down it'll come back to this counter the probability which is measured by this curve and here's the way the rule is for finding the probability now listen hold your seats now hold on don't be afraid just just go along all right never mind it don't like it huh just hold on it works like this what you do is you take a piece of paper a piece of paper has nothing to do with the original thing the piece of paper is only make marks on it's got nothing to do with the light right how you following rules that you make an arrow to represent well for each reflection you make an arrow this arrow for example is for the Reflection from the front surface and this Arrow might be the Reflection from the back surface and I'll tell you in a minute how to make those arrows and then what you do is you tie the arrows together this way you make one and then the other all right that means you make this arrow and then you put the tail of the other one on the head of that one there's the first arrow is the reflecting Arrow from the first surface and this is the second arrow is the reflecting arrow for the second surface and you put these two arrows together by this Rule and then look at where how far off you've come to the end yes to count the pins in the Hool to count the number of beans you put in the barrel I mean you make these pictures and then you ask how big is the circle an area that area represents the probability that you get the thing back if the circle area is Big then you get a high probability if the circle area is small you get a small probability and I just have to say one more thing of how to make the arrows and well the size of the arrow depends upon the particular materials I won't come into that right now the absolute size 4% is another matter what you do is you make an arrow and depending upon the time it takes for the light to get from The Source where it started to the place where you want to count it you turn that out like a clock depending on how much time so if it takes a long time you see you start out at the source there's the arrow for the source but that's not where you that's not the arrow you're going to draw this is just a thinking now and then you say turn turn over D round round round round round round depending on how much time it takes at a certain rate every second that goes around 75,000 goes around a hell of a lot of times it goes around one followed by 15 zeros time in one second but it doesn't take light very long to get to the from The Source turn but still turns around a lot of time you're turnning around okay it's like the wheel and just at the moment it hits the counter it happens to be sitting at some angle all right that's for the reflecting from the first surface now what about that was the That was supposed to be this one I I've gotten too many arrows on here what happened is you take the first surface reflection turn it through it this angle depending upon the time and it ends up here say that's not very big angle you said it was a big angle it is a big angle you know if you keep on turning it look can look like a small angle when you're done but you had to turn around and around and around and around around right you know what I mean it goes around like a clock hand after 25 years it's still saying two can start at two and end up at 215 that's 25 years it went around so here it is having been turned around a lot of times okay this is the from the first surface I should say that's the one the arrow for the first surface now the arrow for the second surface rule same as the arrow for the first surface but in the other direction it's just an accident it happens to be the opposite direction it starts because well but because the rule turn out later when you go from air to Glass it's one way from going glass to air you're change it around anyway you start this way for the second surface and you turn this one no which way did I turn yes for the time and when you get finished with this roulette wheel and the second one it comes out so and then you add them together the way I said you tie one on the other end and make this C and that's the laws of elect light and that'll tell you whether it reflects or it doesn't reflect and what difference does it make why do I get these ups and downs well sure if you move this kept that the same and move that what is that going to do to my exercise over here answer the first hour the time doesn't depend upon where the other surfaces is so it does exactly as I did before I'm doing the same experiment over with the bottom surface a little further along okay top the same place now the second one however when I went to turn it it's a little bit further it takes a little longer to get there and therefore it's turned to a new position so this second time the picture would look if the thing was thicker the picture might look like this instead of being here since it had a turn further perhaps it's turned to here the second time and then when I add these two things together and Tie One On on the end of the other and I'll just do that again here you see that this line is now a long line whereas before it was a relatively Short Line This remember was the answer line the answer line is a longer line and the area here is much bigger of this circle and the probability is larger and this upping and DOW Jing Jing and the answer is comes out exactly predictable just by this little game all right that's a shot huh that's you say yeah he's going to explain why it's like that that's exactly what I'm not going to explain I don't understand it that's the way it works the what I'm going to do in the next lectures is to tell you sort of the generalization of this is this a special example for reflection I'm going to tell you how the rules go about turning arrows is in fact somewhat simpler than this example it's not bad at all it's easy it's not hard but much more generally and I'm going to what I've done is this is a prototype of the general result there is no secret behind it that we can do any better than give you that result all I'm going to do is generalize it it's going to sound something like this this part you need to understand right away it's what I'm going to elucidate in the next lecture the idea goes as follows in general but just a statement then I'll come back and do it again so don't worry to calculate the probability of an event which can happen in a number of different ways the probability of an event is always what we call the square of an amplitude more in this model it means the size of a circle corresponding to an arrow an arrow is called an amplitude for every event you calculate an amplitude which is an arrow on a plane the probability is the area corresponding to that Arrow right that's the first thing second how do we calculate the amplitude for an event if the event is simply a particle going a phon going from point point to another of a definite color then it's simply an arrow which turns depending on the time that's the amplitude notice by the way there's no Reflections and no trouble it just turns the area stays the same the probability of finding a photon is not altered even though the arrow W is its areas it's when we get in trouble when we have more than one way to occur then the rule goes as follows if the thing can happen in more than one way then you find the amplitude what that was the arrow for each way see if it could happen in two ways you got the arrow one way and the arrow the other way and if it can happen three ways if it was a double layer stuff and so on then you put another arrow for the Third Way maybe that one's a short one and if there's a fourth way I put another arrow and when you all finished you put them on tail to tail all the possibilities and find the total what we call the total or the net result of making all these little arrow steps and that final arrow is a total amplitude we say for the event to occur the probability is as always the area corresponding to that Arrow that's kind of stinky right well it's fun and that's strange that nature is like that and I hope you come back to hear how it works in general a little bit better statement of this to review it and one other thing I am going to try to explain to you how this rule explains several of the ordinary phenomena that you used to in light such as angle of incidence equals angle of reflection on reflection that light bends as it goes from air to water and it travels in straight lines from point to point and so on it's all hidden in that one rule and how that one rule carries all this information is part of the next lecture as well as uh I would tell you right away one more thing that this rule that to figure out what happens in nature you have to calculate an amplitude is not just for light it doesn't make any difference what happen an electron does something a nucleus explode doesn't make any difference how do you find it you can only calculate the probability it's going to happen and how do you do that you can guess you calculate an amplitude that's a lousy little arrow and the probability is the square of the amplitude we call it that's really the you should call it in our case the circle of the amplitude the circle that represents the amplitude measures the probability and that's true not just of light but of the whole structure of nature as far as we can tell and although Quantum electrodynamics is only about electrons in light we discovered that part of the rule at least is valid also for nuclei nuclear particles quarks and everything else the thing that is special about electrodynamics is our complete knowledge of exactly what the rules are for drawing the arrows but the fact that you have to draw arrows and end up calculating probabilities like that which is such a shocking and horrible form for nature is something that I will talk about next time in further detail thank you very much these rotating arrows are all very well but is there another model the question is is there another model naturally we struggle to find it I tell you the answer what's going to be in the fourth lecture nobody can find one it's worse there's we're not so dumb as you know we're pretty Advanced compared to the May we've analyze it very T you almost prove it's impossible to find one over a wide class of ordinary possibilities if it's going to be any kind of a model it's going to be at least as weird as this thing the reason is that the answers from this are so simple that is looks a little complicated you're not used to it but mathematically those forms in those curves are mathematically so simple and the rules are so simple it's hard to make any mechanism at all that can reproduce such Simplicity the Mayan thing was really fairly complicated the numbers were peculiar there's no explanation of them this analogy our numbers are not peculiar those they have an explanation for it's a different situation one more thing about gravity I would just remind you we don't really have a good bottle because what comes to why is it that there's a force invers is the square the and what do you mean invers is the square of the distance that's mathematical and Newton was the one who taught us that we can make progress if you stop arguing about that he said I make no hypothesis I don't explain the gravity law I tell you what the law is that tells you how the things look and you can predict where the stars are going to be and that's the pattern but I don't at the moment know but he left open the question just like you asked maybe tomorrow somebody will figure it out on your particular question it's all always possible if tomorrow somebody will figure it out but it's going to be very difficult and very strange do you like the idea that our picture of the world has to be based on a calculation which involves probability not really if I get right down to it I don't say I like it I don't say I don't like it I have very highly trained over the years to be a scientist and it's a certain way I have to look at things when I give a talk I simplify a little bit I cheat a little bit to make it sound like I don't like it what I mean is it's peculiar but uh I never think this is what I like this what I don't like I think this is what it is and this is what it isn't okay and whether I like it or I don't like it is really irrelevant and believe it or not I have extracted it out of my mind I do not even ask myself whether I like it or I don't like it because it's complete irrelevance it's a kind of a dumb answer but it's true and when I'm lecturing I shouldn't have said I don't like it what I was trying to say is you probably don't like it there's nothing I think we can do about it it might have been something else I I don't know how else to express it it's not really personally a dislike have you left out anything in this lecture which you need to add later it it's a very difficult and I work very hard on the lecture to try to use as my examples things that I didn't have to change later the interpretation slightly you know I couldn't get an elementary enough process that I didn't have later on I have to make a little change you see we talk and we see as though it's reflected at the surface actually what's really happening and a deeper understanding which I should do later on in the lecture but I might forget is that it's reflected by cause it affects electrons in here which Reit light here or here or here or here or here or here and what really is the things that we have to add is not the the arrow from here and the arrow from here but a whole lot of little ones from all the distances from there to there but believe it or not you get the same answer okay so what I really ought to do and I would do if I were doing it correctly would be to talk about the Reflection from every interior part and adding arrows but I would rather add just two arrows the first time that an Infinity of them so I cheat didn't cheat a little bit but I got myself in a slight hole which you picked up the actual reflection is from the material the electron in the material in the case of Newton's thing it's because the electrons which you're reflecting from the glass here and here are interrupted in their pattern and when you add the arrows it comes out to be believe it or not the same as if you just take the one one from here and one from there perhaps just for those who can I don't want to make it too hard but just to give you a clue of The Marvelous way it works if you added tiny arrows to represent Reflections from everywhere instead of just talking about it reflected from here to here which is a good equivalent way but if you took a whole lot of little baby arrows each one a very very tiny angle from the other all the same length which is the reflections from all these places you see you generate a circle and what I was using instead of the circle were the two arrows which were the beginning arrow and the end Arrow of the circle and what you told me is I shouldn't talk about the reflection in the front surface and the reflection in the back surface I should talk about the reflection of all the stuff in the between and it's but it's equivalent the net result of going around here is the same as going up here and back on that one it even accounts for the minus side that is the distance from here to here is what you get by putting onto this Arrow This One backwards well I got to the distance is the same see I get I got to upside down in my but uh this line is the same line as you would get by going in a circle so it's really reflected from the interior but in the first lecture I thought I'd just get two arrows instead of an infinite number does your picture apply to anything besides electrons and light this aspect is universal over the whole of the world's phenomena as far as we can tell not just light and electricity this phenomenon of drawing arrows and making areas for probabilities probability amplitudes we call them the amplitude and the probabil are the squares of these amplitudes or the circles of the that's Universal the next problem is a rule for how to draw arrows under different circumstances what kind of arrows do you draw under different circum what are the rules one of the rules I told you is it turns at a certain rate for light and so on those are the rules that we do not know well in the case of the nuclear phenomena but we know ex virtually exactly or at least as far as we can tell experimentally I lost my numbers uh for electrons and light the rule for how to make the arrows is completely apparently completely not nothing is ever complete but within the accuracy and so on it's known what's not known is how rules for Mega the arrows when it's protons that are moving around and so on okay when you are looking at something do you see only light or do you see the object the the question of whether or not when you see something you see only the light or you see the thing you're looking at is one of those Dopey philosophical things that an ordinary person has no difficulty with even the most profound philosopher in sitting eating his dinner has many difficulty making out that what he looks at perhaps might be only the light from the steak but it still implies the existence of the steak which he's able to lift by the fork to his mouth the philosophers that were unable to make that analysis and that idea have fallen by the wayside through hunger can you tell us whether in the future your theory will be found to be wrong or is it complete no of course not how can we know what the final thing is I tell you only what we know today can I tell you more do you want me to tell you more would you like me to tell you what we know tomorrow I'm sorry I have the Nobel Prize from the past not from the future I do not know the future and I answer in the similar way to your likes and dislikes if you ask me what I think will happen in the future I'll tell you I do not think I do my best to understand what I supposed to understand what we know so far I do not know what you're going to discover next okay you I can't I know only is you're talking about the edge of the discovery business and this it's impossible to say what Beyond The Ed so I can't answer you all right except to say that the history of physics has been that the things that looked like they were nicely set aside were and it turned out to be erroneous upon further Discovery and since Society is continues to be vigorous enough in its Endeavors to investigate nature it's almost sure from a social point of view not from a theal physics that new things will be found out which will not fit in and we have no way to tell whether some young man perhaps from New Zealand or somewhere we'll find another way or more about this stuff so it'll have a different picture in the future obviously I can't say I tell you what it looks like today you call it my theory it's not my theory it's Theory everybody uses

Channel: Narayan Behera

Views: 1,723,271

Keywords: Feynman

Id: xdZMXWmlp9g

Length: 77min 57sec (4677 seconds)

Published: Sun Oct 16 2011