Hans Bethe lecture, My Relation to the Early Quantum Mechanics, November 21, 1977

about 11 months ago i decided that it would be a nice idea to have some of the distinguished scientists around and i should say in some sense still around who were actual uh participants and uh creators during the early years the formative years of quantum mechanics during the 20s and 30s during the last iap professor weiskopf from here gave uh two delightful talks uh really as he called it on my life as a physicist but his life as a physicist and comfort encompassed such a large range of the formative years and creator creators of quantum mechanics that it really was a personal reminiscence history and at that time i also had asked professor beta if he could come down and give such a talk and he was uh otherwise occupied last year but he wrote and asked after 11 months if he uh since he was going to be at mit at this time if he could now give such a talk and it's my pleasure to uh to say that it's been arranged uh this is kind of a second order introduction because i'm just going to let professor vicekov do the proper introduction of professor beta so professor vicekov if you will [Applause] well first of all i would like to express my own and i'm sure your appreciation to ted doc dukes he single-handedly organizes these events these happenings i should say and i think it's a wonderful thing for mit for physics and for our human relations that there is a man that took us who thinks of this and i hope he will do it many times more of course you might run out of people but until but until you run out of people i hope you do as well as you did and thank you all now why should i introduce hans peter everybody knows who aunt who hans peter is so what should i say but i'm supposed to say something so maybe i should tell the story which some of you know because i think i even published it somewhere namely maybe my first or almost first encounter with beta i was at that time an assistant of paulie and we did some nice calculations about the quantum field theory of new of scalar particles which at that time were not yet known to exist and and i was supposed to calculate the peer creation of those particles and in a peer creation by gamma rays and i everybody knew of course at that time also that beta has beta and hydro indeed have calculated the these things for the real particles for electron and positrons at that time real particles they're still real pastors we've got others too now and uh and so i went to him and said tell me how how do you do this what techniques did you use and hans in his usual clarity and simplicity explained to me how to do it and it was at that time a pretty uh difficult calculation so i asked him how long will it take for me to do this well then he said well i would take about three days you will take three weeks now let me right away say that i did take it did take me three weeks indeed and i did it wrong at the end well that was my first encounter uh at least my first serious encounter with hans i saw him at meetings but and then however very soon afterwards uh both of us came to this country and he as you know came to cornell and i came to rochester two places that are only 90 miles apart from each other and then started a wonderful personal collaboration and personal relations which i value infinitely much and which certainly coined my own life as a physicist and these were the days when nuclear structure physics began and then later on the days when the renormalization theory of field electromagnetic field theory was born lamp shift etc and the days uh these days and weeks and discussions of collaboration uh were just quite essential for me and i hope a little of a pleasure for hans also and so i would thank him for all this and for so much more because then afterwards our friendship grew and became deeper and deeper and it was not only how to calculate an integral but how to face as a physicist or let me see as a human being the world which we are have to face in which he helped us and me so very much and this was one of the most important things that happened in these post-war days where hans was our leader and hans was the man whom we always looked for the reasonable serious sometimes humorous advice you know that mixture is necessary uh niels bohr once said that there are things that are so serious that you can only joke about them and therefore sometimes one has to use humor and hans has that humor now let me before i let hans talk i know i you are all waiting for him but i would like to say one word very near to my heart there's one danger about the series that ted lucas is organizing and the danger is that you think that physics was so wonderful at that time and is so dreary and boring today this is not true there's nothing of this kind of course it is true that the years of 23 24 25 26 were extremely exciting and a new idea namely quantum mechanics came up but quantum mechanics is a great idea and it didn't come up well it was invented at that time of course but we didn't really understand it i'm not even sure whether we understand it today and that's what i i mean this goes on and on and i think every one of you had had and should have that pleasure of insight what i like to call the joy of inside at the moment when he understood it i'm not saying when he's when he has been taught because that's not the same thing when he understood it when he got the inside of how wonderful he how deeper into the mechanism of nature we are now we were looking through quantum mechanics this you have today and ever in addition to this of course all the development of quantum mechanics i'm sure we'll hear from hans uh based on and a lot of dreary work in fact more dreary than some of the work now which happened before or during the time of quantum mechanics i remind you of spectroscopy of the another analysis of every single atom and the and that one understood it only half on the quarter and only vague ideas etc and in so physics is just as interesting in fact in some respects more interesting today because today we live perhaps not in an explanatory age like at that time but in an exploratory age and what comes up today in almost all fields of physics laser physics low temperature physics plasma physics astrophysics uh special nuclear new nuclear physics and of course particle physics all these new phenomena new worlds that are opened today due to the development of technology since then are so fantastically exciting just because we don't understand them yet that anybody who gets the impression and i hope you won't get the impression that physics was wonderful today and really wonderful at that time and yuri today is simply wrong it was wonderful and dreary at that time and now and now one more remark i always you must have heard me many times say we shouldn't be specialists in the definite in a small field of physics we should be generalists and i try all my life to be generalist it means i try you know to understand nothing about everything instead of everything about nothing and now let me say one thing i have learned in my many many decades with hans peter he is really but truly a man who understands everything about everything hence me [Applause] dr dukas vicky and friends of mit thank you very much for this wonderful introduction i am not quite sure whether it's me you were talking about and one of the things i was very fortunate about i must say is that after this rather inauspicious first meeting with the three days and the three weeks we became such good friends i must make a small disclaimer i was not present at the creation [Applause] uh i was not present at the birth of quantum mechanics it was created in 1925-26 and i really became aware of it only in 26 when it was a very lusty infant but completely born i my story will be mainly a personal story namely my relation to the early quantum mechanics i graduated from high school in 1924 and went to the university of frankfurt which was not a very good university at the time in particular the uh nobody there really understood quantum theory as it then existed it was being talked about in a mysterious way the professor of theoretical physics maddening who was a good theorist obviously did know some quantum theory but the main impression that his students got was that this was something which nobody could possibly understand the old quantum theory which started with bohr's theory of the hydrogen atom in 1913 had a lot of successes early in its life it explained not only the hydrogen atom but also its fine structure then the x-ray spectra the great success of the theory bands factor of molecules and the periodic system there was a little bit of swindling in this in the theory of the periodic system but we let that go but in the years the two first years of my studies 1924 and 25 the it really was a very confused system nothing would work there were contradictions everywhere and some people lost hope at my university of frankfurt i got the main stimulation not from the professor of theoretical physics but from the associate professor of physics there was only one professor and one associate professor in physics the associate professor was first gala of the stan gala experiment and later meisner a very good spectroscopist who is not very well known no that was a different meisner who was in munich meisner was particularly kind to me he said well frankfurt is not a place for you you should go to a place where you can really learn theoretical physics and the best place was munich in munich was sommerfeld who had been the teacher of the best of the young theoretical physicists especially paulie and heisenberg who had previously been the teacher of also quite excellent theorists like evil and especially dubai and who had a school visited by most of of the young theorists to to be sommerfeld had written a book called atomba and which summarized all the successes of the old quantum theory and which i read only while i was studying there i should have read it before i got there but that's all right anyway he accepted me in his seminar which was somewhat unusual i had two years of study behind me but you should remember that in germany at that time and i think even today in most european countries high school graduation meant far more than it does here for instance i had a working knowledge of calculus when i left high school so and the first years of college are concentrated on subjects of your specialty so that after two years you might be considered ready for graduate study some of that seminar was most fortunate in the year 1926 because schroedinger's papers on wave mechanics were just coming out and somerfeld being a big shot got all the galley proofs of joining us papers as soon as they came from the press and so the gallery proofs were available for the members of a seminar every member had to take one part of the five papers by schroedinger not one whole paper but a fraction of it and had to report on it it took i think all semester to get through it and this was a very good thing because everything was done most carefully some of it would not stand any sloppiness if there was anything sloppy he would ask some very stupid questions and everybody was aware that this was an extremely stupid question but it brought down brought out that the student or other person really hadn't understood the paper and now had to explain it much better it was a further advantage for me that i did not know the oil quantum theory because if i had known that then i would constantly have searched for connections between the old quantum theory and wave mechanics and as it was this was a new subject and a new theory which explained everything people wanted to know some of it also was very helpful because he told us the polynomial method to solve differential equations to solve for instance the wave equation for the radial wave equation for the hydrogen atom which is a far easier method than most which were in the literature and made it clear that the solution was not difficult differential equations were sommerfeld's great love he was at home with them and had been for about 30 years of his working life he had written books about them and in particular he gave a one semester course on differential equation which was the capstone of his six semester series of lectures on theoretical physics we had a much harder time learning theoretical physics because it took us six semesters to get through classical physics and by the end of that we didn't hadn't heard a thing about quantum theory schrodinger's papers as you know solved the hydrogen atom he then proceeded to apply perturbation theory to get the stock effect it was my task to present his theor his perturbation theory to the seminar which sounded perfectly straightforward and i have used perturbation theory ever since whether it was applicable or not there was also an american postdoc whose name i have forgotten who asked my help in understanding the german of trading as paper but it turned out that he want needed much more help in understanding the physics than in in understanding the german and so i gave really two talks in that seminar [Applause] some of its place was very useful also in other ways i met many good graduate students some of whom became very well known like piles and tele or unsult who became one of the important astrophysicists in germany there were always large numbers of american postdoc visitors and they were even more interesting there was for instance lobby and condon robertson van vleck and later a man by the name of lloyd smith whom you probably have not heard about but who was very important for me later on because he got me later on my job at cornell university in 1935 when i was a refugee from germany rabi and continent in particular were often very much perturbed because piles and i were constantly talking to each other and giggling and they thought that we were laughing at them which we were not from some of it i learned to solve problems exactly i learned mathematical physics i learned differential equations which suited me very well much later from fermi i learned how to look at a problem in a simple way and see the solution in a few minutes and then sit down and actually solve it schrodinger theory was not very well founded and seemingly had no connection with the early theory either quantum or classical mechanics but it was eminently successful on the other hand it had been preceded by some paper papers by heisenberg some in collaboration with bonn and jordan heisenberg's theory was based on the old quantum theory and made it viable i might say quantum theory in the years from 22 on was trying to develop a method to calculate the intensity of spectral lines and they decided one thing to do was to take the coordinate and expand it in fourier series uh in terms of the fundamental frequencies of the system to be considered q sub n times e to the minus i n omega t and then the problem was to determine q sub n the old quantum theory was hopelessly unsuccessful about this so heisenberg in the summer of 1925 said one really shouldn't talk about the harmonics of a frequency omega but one should talk of the frequencies which actually exist in an atom and those as we all know are differences of the energy levels of the atom so that he replaced this by the sum over a double sum over energy levels k and n e to the minus i times the frequency in going from level k to level n times t and of course omega k n is e k minus e n over h and he said where one should make a similar expansion for the momentum and he would know what the coefficient should be for the momentum namely minus i omega q k n and we'll call that p k n with the same exponential and m belongs here to the mass so he said we should only talk about experimentally observable quantities both the frequencies and these amplitudes but should forget about the motion of electrons in orbits then he discussed just how these quantities q and pkn should occur should occur in the theory how would you form kinetic and potential energy from them and he found that these quantities obeyed very strange multiplication rules which he had never seen before but he wrote down these multiplication rules p and q just did not behave like all the algebraic quantities when he came home to getting in where the senior theorist was born max bond told him but heisenberg what you have there is simply the multiplication of matrices now you are learn about matrices probably in your freshman year or at most in your sophomore year but heisenberg had never heard about matrices so then born and jordan made his theory respectable from the mathematical point of view however this matrix algebra was very complicated and you could solve only the simplest problems like the oscillator and angular momentum this these two problems can still be better solved with matrix mechanics than with trading and wave mechanics but of course with some effect we did it all with waves now so heisenberg had a very clear connection with classical mechanics and the old quantum theory his theory seemed well founded it seemed to be correct but for instance to get even the hydrogen atom took extreme mathematical skill it was solved by paulie in a paper which appeared at the same time as the first paper of schrodinger but nobody of less skill than paulie could have used it at that time so as ammaf said uh we believe in heisenberg but we calculate with schrodinger and you will see in a few minutes that heisenberg himself adopted this maxim very soon schroedinger may very well have been motivated partly by the heisenberg matrix theory there is a legend i'm not sure whether it is correct that uh the buy who was at the same university as scholdinger at the time named it zurich told schroedinger now look here there is the matrix mechanics of heisenberg and i know that matrices are somehow related to differential equations why don't you find the differential equation which corresponds to heisenberg's theory another source of schrodinger's theory was the body the bodies waves everybody made fun of the boys waves which were published in 24. heisenberg got annoyed by that i mean schweidinger got annoyed by that and i understand and this on good authority that one winter by skiing i suppose it was the winter 24 to 25 schweitzinger said well i'll show you i will make the boys wave waves respectable and so he did the uh well schrodinger wanted his waves to be interpreted literally he wanted the absolute square of the wave function to be the density of electrons he was thinking of an electron smeared out let's take the hydrogen atom smeared out over space in the way the wave function indicated but his own equation for the many body atom let's say for the lead atom showed that this was impossible because the lead atom was 82 electrons would have a schrodinger equation in three times 82 coordinates in configuration space and clearly you can't speak very well as an actual density of electrons in 246 dimensional space it makes no sense not enough with that the final decisive point was scattering the schrodinger equation must describe not only stationary states but also scattering this problem was solved by max bond in 1927 and max borne pointed out together with the theory that [Applause] in a scattering experiment you can detect single particles electrons protons what have you so it could not possibly be true that psi absolute square determines the density of the electron because when you put in a detector you would also always detect a single electron which goes either into this counter or into that counter or into that counter but never part of it in into this and part of it into that so therefore born proposed in his important paper in 1927 the statistical interpretation of quantum of the schrodinger theory and this statistical interpretation is what we now accept and use and teach in our courses the bonds paper was a great paper it's the basis for all collision theory which followed and practically everything we do nowadays in nuclear physics high energy physics you name it is a scattering problem the problem which he solved was a problem which had been insoluble in the old quantum theory nobody knew how to deal with the with a periodic phenomena with anything but stationary states you could treat collisions classically but there was no way of putting the old quantum theory into it the further point in interpretation was equally important this was heisenberg's uncertainty principle the uncertainty principle of course is familiar to you and is usually written that the product of the uncertainty of momentum and of position is always greater than or equal to h bar over two this was as most of you will know essential for the reconciliation of the particle picture and the wave picture a particle has an orbit where the position is given as a function of time and in the old quantum theory people assumed that there was an orbit for an electron in the atom this was untenable incompatible with the quantum condition it was the uncertainty principle which smeared out these orbits sufficiently so that you could tolerate the quantum condition at the same time as you had the motion of particles the complete uh well of course the wave picture as i said before should not be taken too seriously either a heisenberg uncertainty principle is based on representing the particle by a wave but the wave picture should also not be taken too seriously there we have forms statistical interpretation after all we do measure single particles full particles and not fractions the complete reconciliation of the wave and particle picture was accomplished finally by the second quantization the first example of which was given by the iraq i think also in 1927 when he quantized the electromagnetic field and solved the eternal previous puzzle of the duality between light waves and light quanta before that in the years of the oil quantum theory eddington's statement applied namely that on monday wednesday and friday we believe in the quantum picture and on tuesday thursday and saturday in the wave picture of the electromagnetic field and on sunday we pray that there be some solution to this puzzle [Applause] well where was i in all this development i did not contribute to either of these to any of these fundamental interpretations of quantum mechanics i was involved with the theory of an experiment by davidson and gurman which you may have may know about in which they established i think in 1926 that the body waves were real for the electron that indeed electrons which interacted with some crystal were reflected as if they were waves as if they were x-rays there were maxima diffraction spots like the lowest spots from x-ray diffraction and you could deduce from the spots the wavelength of the electrons and indeed the wavelength came out to be h over mv as predicted however it didn't quite fit the wavelength with the the spots were not at exactly the right point they occurred at somewhat the wrong energies and some of that asked me for our thesis to look at this and i found out fairly quickly that you could explain the davidson grammar experiments by assuming that in a meta the electrons were subjected to a potential energy of about minus 20 electron volts if you assume that this attraction into the meta you could explain the observed electron diffraction experiments now the question then arose how what was this connected with how does this relate to the observed work function of metals which is generally of the order of five electron volts and that was very soon cleared up namely electrons in metals are essentially free i come back to that in a few minutes and obey family statistics so you have a certain fermi energy if you calculate the fermi energy from the electron density it's about 15 ev and so that explains the difference then having done this i was to develop a complete theory of the scattering of electrons by crystals which i did as a thesis unfortunately i tried to do too much the interaction of electrons uh with metal ions is very strong and i had no no method to treat it except perturbation theory as i told you earlier and perturbation theory is not applicable in this case so i got a first approximation giving the scattering telling also about interference between two scattered waves and all that all of which i am modeled after a theory of x-ray diffraction dynamic theory of x-ray diffraction which was due to eval who afterwards became my father-in-law [Music] so this part was all right and another part was still all right namely i found out that the form factor for electron scattering is the charge of the nucleus minus the form factor of the atom of the electrons in the atom which depends on the momentum change in during the scattering this is a useful result but i didn't emphasize it at all so paulie was quite right when he first saw me which was a few months after i took my phd first thing he told me was mr baker i was very disappointed in your thesis [Music] i uh a little later in 1930 two years later i continued the scattering theory which had been started by born i saw that certain things could be simplified in actually using that theory in that theory in the first born approximation you encounter an integral which goes as follows if number one is the particle which is being scattered then you have the incident wave function of the scattered particle multiplied by the complex conjugate of the of the final wave function and let me call that r1 integrated over the volume of particle 1 times the interaction 1 over r12 times the final and the initial wave function of particle number 2 psi initial of r1 this is inelastic scattering going from an initial to a final state for that particle and having a change of momentum of the scattered of the scattered particle this of course refers to the electron in the atom now i made the mathematical discovery that this integral can be transformed and i'm sure mathematicians had known that for at least a century uh in the following way by first integrating over r1 and then over r2 4 pi over 2 square q is this difference the momentum change times the integral psi final star psi initial times e to the i2 dot r2 integrated only over the atomic electron where this also of course is now presented in every course but it was very useful because of course this is eminently simpler you have to do only one integral instead of having to deal with two variables at first and integrating twice and so everything became very simple and it was possible to calculate electrons scattering elastic as well as inelastic from the hydrogen atom and from more complicated atoms it became further possible to sum this absolute square of that giving the cross section to get the total cross section summed over our final states of the atom to get and especially to get the expected energy loss of a particle which is the difference in energy that is the energy transferred to the atom multiplied by the cos differential cross section which is the absolute square of this and that could be nicely summed by means of a sum rule and so i could calculate the energy lost by a charged particle when going through mata well that seemed to be nice but i didn't realize that it was important until i came to cambridge england when professor blackett later lord blackett told me well that's very important for all of nuclear physics please calculate for us the relation between energy and the range of a particle well i proceeded to do so and to my great surprise it agreed with experiment the uh it has been used it was used first for those old experiments nowadays it is mostly used in particle experiments where you determine the momentum of a particle from the curvature in the magnetic field and then the velocity from the energy loss that is from the number of ions that the particle forms per centimeter path so this was very satisfactory as a matter of fact i consider this my best paper the world does not in the meantime atomic physics was marching on clearly uh once you had done the hydrogen atom you wanted to do the next atom namely the helium atom many physicists including bohr himself and heisenberg had tried to solve the helium atom on the basis of the old quantum theory they failed completely they could get any number of different results for the binding energy of the helium atom none of which agreed with experiments heisenberg who had an investment in the helium atom proceeded to solve it in wave mechanics and did what zomerfeld had recommended we believe in heisenberg but we calculate with schrodinger so he's he succeeded and what did he accomplish this was also i believe still in 1927. first he got close to the binding energy this was later improved by using the joining and variational method by hillary's and later by many other people the best calculations have been done by an israeli petrus and he gets the non-relativistic energy of the helium atom to an accuracy of one part and ten to the twelfth of course that's relative to the binding energy of the hydrogen atom but it also agrees with experiment by the way heisenberg secondly explained the long standing puzzle that helium seemed to have two spectra ortho helium and power helium and the only spectral lines you observe correspond to transitions within ortho helium or within parahelium in the old days it people thought that maybe there were two really distinct gases heisenberg showed and you all know that like these two spectra come about because the wave function can be either symmetric or anti-symmetric in the spatial coordinates of the two electrons of the helium atom third he got the connection with spin he postulated that the total wave function must be anti-symmetric the iraq had found that independently and that was the rocks basis of the fermi statistics if the total wave function is antisymmetric then the anti-symmetric spatial wave function goes with a spin of one parallel spin off the two electrons and gives you triplet states and the other one gives you singlet states with total spin zero this was not so easy to say in those days because spectroscopy was incapable in those days to resolve the triplet uh states they could only show the spectroscopist could only show a doublet and it was later on found that indeed two of the states are very close together and nowadays you can resolve them by modern methods modern spectroscopic methods and you can do a theory during the calculating the splitting in detail heisenberg's calculation of helium opened the way in two directions namely to treat molecules and to treat complex atoms the hydrogen molecule the simplest of all molecules was solved by hydra in london and they showed that it is important to have two the two spins of the two electrons in opposite direction just as they are in the ground state of the helium atom so you have a saturation of spins this theory uh gave uh a theoretical foundation to some ideas which lewis a great chemist had had previously about the chemical bond this was the theory of the homopolar chemical bond and it in the hands of pauling and many other chemists it was developed into a real understanding of chemistry it is the foundation of perhaps the most important part of theoretical chemistry this understanding of chemistry was further extended to the solid state especially to metals whose binding is again different from both the ionic bond and the homopolar bond in in chemistry the first step as so often was taken by paulie who said that the electrons in a matter might be free but then would obey family statistics i mentioned that before in connection with the work function sommerfeld took this up and developed a complete theory of three electrons and showed that three electrons obeying fermi statistics would solve all the puzzles of the old electron theory of metals which had been invented 20 years earlier by drude and never gave the right answers he could calculate conductivity the ratio of thermal to electric conductivity and all sorts of thermoelectric and magnetoelectric effects which nobody had understood before but he did not explain why the electrons could move freely in the metal after all there were ions in the metal which exert very strong forces of the elect on the electron how could the electrons go through that lattice without paying any attention to it this was solved in 1929 by felix bloch in his thesis he showed that the periodic structure of the method made it possible for electrons to move freely and to move without any resistance in fact i'm told that bloch got that result very quickly in his thesis and went to heisenberg and said this is an elementary answer i got that can't be very interesting and heisenberg said but you have solved the main problem and then the rest was more catalytic [Applause] this doesn't explain why liquid metals also are good conductors but vicky weiskopf's theory of liquids makes us understand that liquids at least near at least at relatively low temperature do behave quite a lot like solids and therefore this becomes explainable the quantitative theory of electrons in metals was started by wigner and sites a couple of years later and it felt to me to write an article in the hanbok de physique on the theory of matters it was written together with sommerfeld sommerfeld was the man who was asked by the editors to write this sommerfeld said well i write it if beta does 90 of the work and takes 90 percent of the honorarium that was signed with me especially the latter which became it was quite a good honorarium and so when i had to emigrate from germany i had at least a little bit of money to fall back on the other theoretical area opened by heisenberg's paper was that of complex atoms how do you explain let's say the spectrum of the silicon atom there are multiplets which the spectroscopists had found much before but already calculating the lithium atom which heisenberg still did you got extremely complicated wave functions the solution to this was found by wigner and von neumann and the solution was in terms of group theory which at least enabled you to understand the multiplets and to calculate which multiplet would lie lowest in energy it confirmed hund's rule namely that the highest multiplicity always lies lowest and but it was an extremely complicated theory i wanted to understand other the complex atoms the quantitative side always interested me so i learned group theory and then there had to be something to apply group theory to so i did a paper on the group theory of atoms in the somewhat unsymmetric fields which an atom is exposed to in a crystal and i found that the energy levels would be split into several levels and since then people have found that this is useful and that indeed it does apply to the behavior of atoms in in a crystal the group theory was not needed for the free atom slater whom many of you remember as a professor of theoretical physics at this institution did it the correct way in 1930 he showed that all you need to know is that the total wave function is antisymmetric and so he wrote down what is known as a slater determinant putting all the wave functions in a row and all the electrons in the column and everybody now uses the slater method to treat the complex atoms the quantitative calculation of atoms required still a different approach and this was suggested by hartree and fox hartley did many of his numerical calculations on the bush analyzer which also had its home here at mit to solve the hearty fog equation for an atom is extremely difficult and half these methods were quite insufficient in fact even modern computers are generally insufficient as was found by the atomic theory group at los alamos scientific laboratory the calculation is unstable and therefore it is necessary to first use an approximate method that again was invented by slater and then improved by corn and st and sham and once you have the approximate solution then you can do the accurate calculation the hearthy fog method as you will know treats all electrons in the field of the general field of the atom it doesn't treat the interaction of the electrons except in an overall way this was solved only in the 1960s especially by nespet at the ibm research laboratory in san jose california and he can calculate the ionization potential of an atom to an accuracy of about 0.03 electron volts out of some 10 or 20 electron volts which is an exceedingly good result of the theory it is so good that you can now predict the energy levels of atoms you have never seen and especially of ions you have never seen which is useful for guiding spectroscopies an important part of this of course was the spin this was discovered in 1926 by goldsmith and julian beck there was a celebration in the american physical society in 76 it was foreshadowed by the pauli principle which was published in 24 in which paulie found out that it was necessary to assume that always two electrons could occupy one orbit this was putting some theory behind bohr's previous explanation of the periodic system you had to explain why two electrons could sit into in one orbit of course paulie couldn't but goldsmith and urine back could even though paulie had discovered this two electron business he very much discouraged koenig who had previously had the idea of the spin and paulie looked at koenig's idea and said that is nonsense and koenig never published his paper probably however a tone for this by inventing the quantum mechanical equation which in includes spin in which he invented the spin matrices which you now all use and which of course are useful for many other purposes also such as isotopic spin well i think i have given you enough of a picture so you can understand that we were convinced back in those late 20s that every problem in physics can be solved at least every problem which involves non-relativistic quantum mechanics and this is the feeling which vicky weiskopf talked about in his introduction you shouldn't expect you shouldn't think that theoretical physics is now uninteresting it is as interesting as ever one of the differences be in the physics then and the physics now is that at that time we had a tremendous backlog of experimental information from spectroscopy from scattering experiments which nobody had been able to explain and so theory was exceedingly successful in the space of some five years to explain essentially all the accumulated information we were also very lucky that we had an equation which really worked the schrodinger equation with the pauli spin edition it was a very exhilarating time and it made all of us of our generation great optimists about physics namely that problems can be solved and this part of our conviction i think i want to impart to you and i think you shall you should believe it today also in the much more difficult problems of particle physics and similar problems unfortunately most of the simple problems we solved in those years and i apologize taking them away from you one problem we didn't solve in those days and that was superconductivity everybody was convinced that it was a problem of non-relativistic quantum mechanics but it was solved only in the 1950s by bardeen cooper and schrieffer it was shown that indeed it was a problem of non-relativistic quantum mechanics so it was soluble but it took a long time and so also in your work you should not be too disappointed if it takes a long time finally in conclusion relativity theory of cause had to be combined with quantum theory this also was still done in the twenties by the iraq in the the arc equation 1928 which gave the spin automatically and in the meantime of course we know that even the schrodinger klein gordon equation on which bt weiskov worked a little later also has its place in physics because it explains the behavior of particles of zero spin of which we have a fair number the dirac equation explains the hydrogen atom later the helium fine structure gave the spin automatically gave the g factor of the spin and therefore the anomalous zeman effect it predicted the positon but the iraq did not have the courage to make this prediction he thought he had an equation for the proton it also predicted thereby that particles can be created and annihilated which is as the bread and butter of today's physics and has remained so well i hope to have given you some picture of what we were interested in in the late 1920s what a wonderful time it was how quantum mechanics not so much was born but developed in a very short way to explain the known facts of physics in fact one might say that it explained most of the physics that you see in everyday life and most of and of course all the chemistry and we expect the biochemistry and so on but much but physics remains always new there are always new problems and so i wish you the best of luck in solving those thank you hello [Applause] don't think they consented in having a little fresh period but i would like for before that to tell you that if you think that this is all han stated did you are wrong he didn't talk about his tremendous and fundamental part in nuclear structure in explaining why the sun is hot when it shines he didn't tell his tremendous contribution to field theory and quantum electrodynamics and so many other things so this is only a small small part about enhanced painting gameplay was wondering uh in the period of 1924 to 1929 what were the experimentalists doing nuclear physics you mean 34 to 39 or 20 24 to 29 24 to 29 well there were quite a number of experiments in spectroscopy [Music] a large number on for instance in the place where i worked in munich the great thing was canal raise that is to get protons or other i mean yes or other atoms ions separated into a tube and then do experiments on them to find out for instance the lifetimes of of atomic states and people did a lot of experiments on the diffraction of particles of electrons most of them write one set of them quite wrong they did the hyperfine structure measurements that is people began in to be interested in in the behavior of the nucleus where there was lots to do and radioactivity in indeed but the set of experiments you're talking about was done before these were done before yeah you mentioned before about fermi and how fairly when you interact with fermi detection and salvage very quickly and then go back and work through the solution i wonder if we had a story or two about fermi that illustrate that um well uh one of one one feature of my cooperation with fermi was when we were both interested in quantum electrodynamics there had been three separate formulations of the interaction of electrons between two electrons one was left by heisenberg and paulie which was made more palatable by fermi himself in his article in the reviews of modern physics which people can read quite easily the uh which in which the field was quantized and everything was done beautifully and in in detail the second was a proposal by muller of denmark in which he said well let's represent an electron by an initial and final state both having wave functions so let's take the transition density psi initial times psi final star and consider this a source of the electromagnetic field you then calculate the scalar potential and the vector potential which you can do in a perfectly relativistic way and this was a different and then you can let that act on the second electron so period was the theory by bright of the interaction of two electrons in the approximation v square over c squared so not fully relativistic fairy and i sat together for a day or two now in order to find out how these are related and he saw immediately how to do this and then on the third day he said well now let's write the paper [Applause] so he went to the typewriter there were no secretaries at the institute in rome he went to the typewriter and said what he was going to write and occasionally i would criticize that and whenever we wanted to write an equation i would write this equation by longhand and in on the third day in fact fraction of the third day the paper was written and then published in psycho physique so this was one of the examples of our collaboration the other was that when anybody had any problem in the lab at home they would come to fermi and particularly when two of the younger people disagreed they would refer their disagreement to the pope the army was called the pope and the pope would then sit down and say well now let's look at it this way and in something like 20 minutes he would have all of it cleared up and made the the fundamental physics clear and then then people could go ahead and do the explicit work i have no idea i never met max planck i knew he was a great man i don't even know how he reacted to the to quantum mechanics either highs and backs or schrodingers do you vicky yeah you know i think he was sympathetically inclined there is let me tell you with anecdotes here if i may maybe [Applause] uh the anecdote is this that uh i was in berlin at that time it must have been 19 in the late 20s and there was a seminar by hans kopfermann who is an experimental physicist by the way and who at that time did spectroscopic studies of age you know the different ways of determining planck's constant by the photo effect by spectroscopic and so on he had a whole series of maybe eight or ten different ways and always got the same value and in this in the first row of the seminar setting einstein planck and nanced and a lot of other famous people and i overheard announced well nance asked a few questions to coppermine i don't understand how could you measure this so accurately and kopferman explained it and then nernst said to plank well her colleague i said in german and then in english then you have still a few chances of being so i know i think planck was very sympathetic to modern quantum mechanics he did not work in it he was after all i don't know how old he was at that time but uh in the 70s and and i know then you can't work anymore and but in contrast to nast and to other people of his age he he was very sympathetic to quantum mechanics another question whether you've worked for the ancients i have never worked with einstein except politically i met him only in this country in princeton and i never discussed physics with him he worked at that time on the unified field theory he was a great opponent of of bohr's interpretation of quantum mechanics as you may know and this i think was one of the few occasions in which einstein was quite wrong hitler came to power in [Music] 1933 i was half jewish i still am and that meant that i could not get any position at any university in germany and since i wanted to do pure physics work i couldn't stay in germany so it was as simple as that also i didn't like him very much uh there some of you may have seen yesterday's new york times it has in the book review section a review of some books which try to rehabilitate hitler and say that he is was not such a bad man the reviewer agreed with me he was as bad a man as you ever would want to [Applause] at some see we realized that all the phenomena in one's everyday experience could be explained by this non-humanistic quantum mechanic and i i sort of envisioned some small group of physicists i guess you must have been an elite actually at the time who understood this and sort of could see all the immediate experience becoming a parent did you feel lonely i mean did you sort of want to tell the world um your your your friends who your very best friends who just happen to not be physicists well of course we wanted to tell and i guess it was told to a quite considerable extent the feeling of of understanding certainly was transmitted to the chemist and very well transmitted to them they made use of quantum mechanics as much as the physicists did the uh of course explaining the world is uh to be understood the same way as laplace understood it back 150 years earlier namely if you know if you can solve all the mathematics then you might explain any phenomenon that you choose to but even today's computers are by far insufficient to solve other mathematics i mentioned that even the healthy focus equation for an atom is quite quite difficult to solve i don't believe that i at least and most of the physicists that i knew had any uh intention to tell it to the world at large one of the results namely the heisenberg uncertainty principle became very famous and was taken up by philosophers and and i think is fairly generally known if not understood but certainly we never thought we we could solve the human problems that people have and and i still um believe firmly that the human problems are on a different plane and cannot be solved by solving the schrodinger equation [Applause] i think we should now really thank hans peter for this unusual unique experience of listening to him thank you you

Channel: AIP History

Views: 43,132

Keywords: Physics, history, history of science, AIP, American Institute of Physics

Id: E61UR4Lbifo

Length: 87min 58sec (5278 seconds)

Published: Mon Apr 04 2022