FEATURES Did the great experimentalist doctor his own findings to suit his beliefs? Perhaps not, as aclose look at his beliefs reveals. Millikan's struggle with theory Gerald Holton n 1916 Millikan published his experi­ by any other experimenter. rectly at the ordinates, he decided"simply I mental determination of the value of Several factors were responsible for the to cut the feet off"where those curves were Planck's constant, with remarkable preci­ success of the experiment, performed at trailing on too far. sion. Itwas later cited as part ofhis Nobel the Ryerson Laboratory ofthe University In short, a triumphant work by a su­ Prize award. But as a study of that paper ofChicago. First ofall, Millikan's use ofan perbly confident scientist, ofhighest im­ together with Millikan's subsequent com­ ingenious device he termed "a machine portance in its day, and richly deserving to ments over the years reveal, he initiallyre­ shop in vacuo;' assembled thanks to "the be cited as part of his Nobel Prize award fused to accept the interpretation of the skill and experience of the mechanician, in 1923, "for his work on the elementary theoretical meaning ofhis work, as seen Mr. Julius Pearson" .Essentially, the device charge ofelectricityandthe photoelectric from the perspective of a present-day allowed a rotating sharp knife, controlled effect". If historically minded scholars physicist.Yet, over time, Millikan adjusted from outside the evacuated glass contain­ look through that 1916 volume of the his view retroactively to accept the mean­ er by electromagnetic means, to clean off Physical Review inwhich Millikan's paper ing ofwhat he had done - namely to pro­ the surface of the metal used (sodium, appeared, they will notice that physics in vide crucial support for Einstein's heuris­ potassium, lithium) before exposing it to America at that time was still a mixed bag. tic point of view on the quantum theory the beam of the fairly monochromatic One finds in the volume a number ofcon­ oflight (1905). This case also serves as a light coming from a quartz-mercurylamp. tributions by scientists such as A.H. test whether scientists, as is now often al­ Second, the frequencies of light Millikan Compton, P.W. Bridgman, Irving Lang­ leged, as a rule arrange to obtain their re­ used covered a much larger span,between muir, E.C. Kemble, among others (with sults in the light oftheir own beliefs. 2399 A, and 5461 A, than had been A.A. Michelson listed on the Board ofEd­ achieved previously, permitting the rela­ itors). Their papers show thatthe mainat­ Robert A. Millikan's historic paper of1916 tion of the kinetic energy of the electron tention was the experimental part of sci­ in Physical Review on the measurement of and the frequency of light to be deter­ ence, in which Americans were regarded Planck's constant (A Direct Photoelectric mined with a "maximum uncertainty" of as mostinterested and competent. But the Determination ofPlanck's h; Physical Re­ 0.5%. In addition, the kinetic energies of volume as a whole, published in monthly view [1]) lends itself to two different yet the photoelectrons were found by mea­ parts ofsome 200 pages each on average, complementary readings-first through suring the potential energyVe ofthe elec­ indicates that a good deal ofthe work go­ the eyes of a physicist oftoday, second by tric field needed to stop the electrons­ ing on in physics in the US in the early an historian ofscience, reconstructing the andthere Millikan was inthe excellent po­ years of this century was still narrow and dramatic meaning the paper had in its sition ofbeing able to confidently use the unambitious, even tending to descend to own time. value for the charge e of the electron he lengthy descriptions of improvements in To a physicist Millikan's paper is a pa­ had published in 1913 with unprecedent­ power supplies, pumps, galvanometers, tient and eloquent essay on how he came ed accuracy, obtained with his oil drop ex­ and the like. to the conclusion that has been in our periment. Next, on looking at the early pages in physics textbooks ever since: While it had Last but notleast, shining through it all that same volume, and the segment pub­ been known for a long time that light were Millikan's typical characteristics as lishedtwo months earlier mJanuary 1916, falling on metal surfaces may eject elec­ experimenter and person: His penchant we come upon another article byMillikan, trons from them (the photoelectric effect), for experimenting in anareainvolving the onthe same subject,inwhich the veryfirst Millikan was the first to determine with hottest question of the day - in this case sentence announces Millikan's opinion great accuracy that the maximum kinetic the value ofa fundamental constant ofna­ that "Einstein's photoelectric equation... energy of the ejected electrons obeys an ture (as he had done in his 1913 paper) cannotin myjudgmentbelooked upon at equation equivalent to the one Einstein and, as we would now put it, the quantum present as resting upon any sort ofa satis­ had proposed in 1905, namely theory oflight; his energetic persistence ­ factory theoretical foundation;' even 1/2mv2= hv - P, where h is Planck's con­ this paperwas the culmination ofwork he though "it actually represents very accu­ stant,v the frequency ofthe incidentlight, hadbegun in 1905; his passion for obtain­ rately the behavior" ofphotoelectricity (p. and P "the work necessary to get the elec­ ing results "with verygreat precision" (Fig. 18). tron out ofthe metal" in Millikan's words. 1) - even atthe cost ofsome practices that Indeed, knowing this, and rereading Moreover, Millikan determined h to have are perhaps questionable from our current Millikan's subsequent (March 1916) paper the value 6.57 x 10-27 erg-sec, "with a pre­ perspective, for instance, his frank re­ on Planck's constant, we see more clearly cision of about 0.5 per cent"-avalue far mark that to get the curves derived from that he is emphatically distancing himself better than had been obtained previously his experimental results to intersect cor- throughout from Einstein's attempt in 12 Article available at http://www.europhysicsnews.org or http://dx.doi.org/10.1051/epn:2000303europhysics news MAY/JUNE 2000 FEATURES 1905 ofwhat Millikan calls a "coupling of he so characterized any ofhis work, using forms of quantum theory then being dis­ photo effects ... with any form ofquantum the phrase because his was still only a cussed, siding with the "least concentrat­ theory" (p. 355). What we now refer to as "heuristic" proposal, devoid of sufficient ed:' namely that ofPlanck, as against"the the photonwas, in Millikan's view, a"bold, theoretical grounding. most concentrated;'thatofEinstein,which not to say reckless, hypothesis" - reckless As it happened,Millikan was in Europe he called "radical" because it could not be both because it was contrary to such clas­ in 1912 for six months, primarily to meet reconciled with "the facts of diffraction sical concepts as light being a wave propa­ the foremost physicists, and particularly and interference, so completely in harmo­ gation phenomenon, and because of the to attend in Berlin Planck's lectures on his ny in every particular with the old theory "facts ofinterference" (p. 355). Inthe back­ theory ofheat radiation. He also took the ofetherwaves:' Indeed,he confessed a cor­ ground we glimpse the presence of opportunityin June 1912 to lecture on the puscular theoryoflightwas for him"quite Michelson, the (~tist of Light;' who was results of his oil drop experiment before unthinkable". In short, Millikan's classic Millikan's admired patron and colleague the Deutsche Physikalische Gesellschaft. 1916 paper was purely intended to be the at the Ryerson Laboratory, the 1907 No­ Planck was delighted with this relative verification of Einstein's equation for the belist, famous for his interferometers, the newcomer who was bringing the best photoelectric effect andthe determination work carried out with their aid - and for available value for the charge of the elec­ ofh, without accepting anyofthe"radical" his adherence to ether physics until his tron at the time, which fitted well with implications which to us nowseem so nat­ death in 1931. Planck's own deduction ofe from his for­ ura!. So Millikan's paper is not at all, as we mula for radiation. Two of the most im­ Recently opened records of the Nobel might now naturally consider it to be, an portant universal constants, hand e, were Prize Foundation showthat from 1916 on, experimental proofofthe quantum theo­ finding common ground through this per­ Millikan was nominatedconstantly.When ry oflight. Although Millikan admits that sonal encounter. the award at last came to be in 1923, his the facts seem "to demand some modifi­ On returning to the United States, Mil­ Nobel address ofMay 23,1924, contained cation of classical theory:' (p. 355) that is likan gave a lecture in December 1912 at passages that showed his continuing notwhat he is willing to attempt here. And the Cleveland joint meeting ofthe Ameri­ struggle with the meaning of his own intruth,Einstein himself, from the begin­ can Association for the Advancement of achievement: "After ten years of testing ning, was not comfortable with the quan­ Science and the American Physical Soci­ and changing andlearning and sometimes tum theory oflight, calling it, in a letter of ety, inwhich he clearly regarded himselfas blundering ... this workresulted, contrary May 1905 to his friend Conrad Habicht, the proper presenter ofPlanck's theory of to my own expectation, in the first direct "very revolutionary:' Itwas the only time radiation. Millikan compared the various experimental proof...ofthe exact validity, °':F~';": ;};~~"~'···~·::::~:~;::~':::I~,~i;:"'" ~i X 13 , ij:;; =¥. Vo 43.9 10 .