R EFERENCE F RAME

PhysicsI in 50 Years Daniel Kleppner

HYSICS TODAY invited me to talk traviolet catastrophe struck-and I?about the future of physics at its A. A. Michelson claimed that the “ma- fiftieth-anniversary celebration. Over- jor laws of physics are pretty well come by the desire to see old friends known.” That was in 1894 at the Uni- and the promise ofgood food and drink, versity of Chicago, at the dedication of I agreed. the Ryerson Laboratory. Exactly one Let me start by expressing my decade later, Einstein published his pleasure in participating in PHYSICS papers on the electrodynamics of mov- TODAY’S fiftieth-anniversary celebra- ing bodies, the photoelectric effect, tion, and expressing my deep appre- Brownian motion and the quantum ciation to Charles Harris, Steve Benka theory of solids. and Gloria Lubkin for providing me However, to be candid, I should with this evidently irresistible opportu- point out that if you are really smart, nity to humiliate myself publicly by at- inkling that the most exciting advance you may be able to say something tempting to say something intelligent in atomic physics for decades was intelligent. Lord Kelvin, for instance, about physics in the next fifty years. about to take place-Bose-Einstein was really smart. In 1900, he pre- As everyone knows, long-term pre- condensation in a gas. Another missed sented a lecture at the Royal Institu- dictions in science are hopeless and topic was quantum computation, which tion entitled “Nineteenth Century even short-term predictions are usu- was a hot topic within a couple of years. Clouds over the Dynamical Theory of ally wrong. Fortunately, they are usu- Such omissions are not due to lack of Heat and Light.” He spotted two ally wrong the right way, for in phys- imagination or shortsightedness. If any clouds. Cloud one was the problem of ics-unlike the common situation in blame is to be assigned, it must be as- the ether-there seemed to be no way human affairs-reality frequently ex- signed to Nature for being too generous. to account for the effects of motion ceeds expectations. I documented this Progress in technology ought to be through it. Cloud two was the problem phenomenon some years ago in a PHYS- easier to forecast than discoveries in of specific heats: He emphasized that ICS TODAY “Reference Frame” column basic science, but even here the pre- the equipartition theorem gave incor- (“A Lesson in Humility,” December dictions are likely to be askew. One of rect values for specific heats of mole- 1991, page 9), showing that if one my favorite childhood books was a 1912 cules unless one arbitrarily excluded compares the best forecasts made by edition of the Book of Knowledge. certain motions. He characterized both a group of responsible scientists with There was a splendid article on the of these clouds as being pretty dark, and what actually happens, the forecasts latest technical wonder, the airplane, of course he was right. Nevertheless, are pale compared to the reality. The with a full page devoted to illustrations even Kelvin had no way to foretell the particular group of responsible scien- of the airplanes of the future. They revolution about to take place. tists was the Physics Survey Commit- were not mere biplanes. They were Since it is essentially impossible to tee, headed by William Brinkman, triplanes, quadraplanes, and airplanes predict scientific discoveries, it is which prepared a report in 1986. with up to a dozen wings. And at the tempting to go in the opposite direction Looking back on our omissions five 1939 World’s Fair, the General Motors and predict things that will not hap- years later, I found the following un- Futurama displayed a gorgeous model pen. However, this is also most un- predictable discoveries and overlooked of teardrop-shaped cars whizzing wise, since it practically guarantees advances: Supernova 1987A, high- through pristine cities on highways that they will happen. Perhaps you temperature superconductivity, atom with fantastically complex intersec- have your own pet list of failed predic- cooling and laser manipulation, bucky- tions and overpasses. The Futurama tions. On my list are Rutherford’s balls, complexity, chaos and non- actually provided a pretty good picture claim that anyone who thought nuclear linear dynamics, superdeformed nu- of today’s highways, but thanks in energy would be useful was talking clei, large-scale structure of the uni- large part to automobile emission, the moonshine, and the prediction made verse and mesoscopic physics. cities are hardly pristine, and the cars, to Charles Townes that the maser In my own field of atomic, molecular of course, are not whizzing at high would never work-this by some re- and optical physics, there was such speed-much of the time they are spected physicists at Columbia. I re- rapid progress after the Brinkman re- crawling bumper to bumper. call a talk at an American Physical port that a new committee set out to In spite of the obvious pitfalls of Society meeting by President Reagan’s prepare an up-to-date survey. The re- prediction, there is a long and honor- science adviser, George Keyworth, sult, Atomic, Molecular and Optical able tradition of physicists mis-fore- shortly after the President announced Science (National Academy Press, casting scientific progress. Toward the the Strategic Defense Initiative whose 1994), was about as up to date as end of the 19th century, physics was technical goal was an impenetrable possible. Nevertheless, it gave little so impressive that some respected missile defense. There had been much physicists thought the job was pretty opposition from the scientific commu- DANIEL KLEPPNER is the Lester Wolf well finished. Oliver Lodge stated that nity, and to counter that Keyworth Professor of Physics and associate director of “The whole subject of electrical radia- produced a long list of things that the Research Laboratory ofElectronics at tion seems working itself out splen- experts said could not work but even- the Massachusetts Institute of Technology. didly”-just a few years before the ul- tually did work, from airplanes to tele-

0 1998 hmerican Insmute of Phyncs, S-0031-9228-9811-210-3 N OVEMBER 1998 P HYSICS T ODAY 11 vision. The argument appeared to be covered, and fluctuations that illumi- description as being quantitative. that because so many experts said SD1 nate the earliest stages of the universe Second, I predict that new experi- was not technically possible, we should were detected. Optical telescopes, en- mental techniques will continue to be assured that it was technically pos- hanced a hundredfold by the invention flower, and that whenever we acquire sible. Unfortunately, we can’t be abso- of the CCD camera, revealed large- some new tool for looking at the natural lutely sure that universal disapproval scale structures in the universe and, world, we will see marvelous things. by experts guarantees success. more recently, evidence for the accel- That thought is expressed most beau- So, I will refrain from predicting eration in the expansion of the uni- tifully by a Jesuit paleontologist and what will not occur in the future. Fur- verse. And with the creation of x-ray philosopher, Pierre Teilhard de ther, I won’t even hazard a guess about telescopes, the mysterious x-ray bur- Chardin, in his book The Phenomenon which fields will decline, for my own sters were discovered. The list is too ofMan (1955). Teilhard was concerned field, atomic physics, appears to have large to summarize, and it is growing with the development of living forms, actually died several times in this cen- too rapidly. Thanks to these new tools the evolution of systems of higher con- tury. Fermi instantly abandoned it for seeing the cosmos, we now live in sciousness and ultimately human de- when he learned about the neutron. a golden age of astrophysics. velopment. He was certainly not con- Norman Ramsey told me that when he This golden age is not merely for cerned with progress in physics. Nev- approached I. I. Rabi in 1937 about astrophysics, for all of science is linked. ertheless, what he wrote fits physics doing graduate work, Rabi declared Rydberg atoms, for example, were first exactly: “The history of the living that the field of molecular beams was observed by radio astronomers. My world can be summarized as an elabo- pretty well washed up. That was a interest in them grew directly out of ration of ever more perfect eyes within few months before Rabi invented mag- that discovery, and they have been my a cosmos in which there is always netic resonance. In the 196Os, when bread and butter ever since. something more to be seen.” In phys- it seemed that spectroscopy was get- So there are a few examples from ics, the creation of new and more pow- ting pretty routine, laser spectroscopy astrophysics of what can happen when erful experimental methods is indeed transformed atomic physics. And just you have new techniques for seeing an elaboration of ever more perfect a few years ago, there was no inkling things. There is no reason to believe eyes. I predict that we will continue of the tremendous excitement that la- that experimental advances in astron- to create new eyes, and that whenever ser cooling and trapping were about to omy are about to cease. Gravitational we do, we will not merely see more generate, though last year’s Nobel wave astronomy, for instance, seems a things, we will make breathtaking dis- Prize in Physics leaves no doubt as to good bet to be realized long before coveries. the scientific interest. PHYSICS TODAY'S centenary. And my final prediction is this: If For these reasons, I will not predict Because physics is driven by experi- fifty years from now you should happen that high-energy experimental physics ment, and new experimental tech- to look at the centenary issue of PHYS- will dwindle because of the time scale niques are continually created, there ICS TODAY, you will be amazed! and cost of large machines, and the is every reason to be optimistic about I thank Susan N. Coppersmith and An- need to work in corporate-size research our scientific future. This simple thony P French for historical references. W groups. It would not surprise me if in thought gives me the courage to make hfty years there are working groups a few predictions. devoted to the VNL-NLC, the ‘Cvery Next First, I predict that in fifty years Thinking About the Future to Last Next Linear Collider.” More there will be physics. By physics, I otwithstanding the perils and pit- likely, particle research will be done with mean the science of understanding the Nfalls of predicting the future of some totally different approach. In any physical world in quantitative detail. physics, one cannot intelligently plan case, from having read graduate appli- However, in some areas what is meant for the future by merely extrapolating cations at MIT for a few years, I know by “quantitative detail” today may not from the past. A celebratory parry is that there are bright students interested be the same fifty years from now. Prior not the forum for addressing this issue in experimental high-energy physics. As to the creation of quantum mechanics, seriously, but one might try bringing long as bright students are attracted to no thoughtful physicist would accept together a small group of leading physi- a field, it will do okay that a language of mere probabilities cists with diverse interests to share their Having argued that one cannot pre- could provide a quantitative descrip- views. Such an exercise was carried out dict what will happen and what will tion. The very first intimation of prob- by the Princeton University physics de not happen in physics, I appear to have abilistic behavior-radioactive decay- partment in November 1996, in its cele- totally welshed on my assigned subject. was deeply troubling. Today, most bration of Princeton’s 250th anniversary. However, I believe that there is actu- physicists regard quantum descrip- There were great talks on nonequilibrium ally something to say. It follows from tions as natural and intuitive, and they physics, computation and neurobiology, the simple observation that physics is are not at all troubled by the language complexity, high-temperature supercon- built upon experiment. Whenever of probability. ductivity, medical imaging, cosmology, ex- there is a major advance in experimen- Lots of people, myself included, ex- perimental and theoretical gravitation, tal technique, new physics comes tum- pect that physics will have increasingly neutrino oscillations, high-energy collid- bling out. The recent history of astron- important things to say about biologi- ers, string theory and particle theory. In omy and astrophysics demonstrates cal processes, but the language of that an introductory talk, Sam Treiman de this pretty convincingly. physics may turn out to be different scribed a similar conference held fifty years Within the last fifty years, cosmol- from the language we know. And if earlier. He too stressed that the fruits of ogy has been transformed from meta- physics comes to deal with large com- science are more abundant than anyone physics to hard science, and our vision plex systems (by which I mean some- can predict. of astronomical and astrophysical proc- thing neurological, possibly the brain The proceedings of the 1996 Prince- esses has been incredibly expanded. of a fruit fly or something like that), it ton conference make excellent reading: The creation of radio telescopes led to may well use a language that we could Critical Problems in Physics, V. L. Fitch, the discovery of quasars, pulsars, not accept today as being quantitative, D. R. Marlow, M. A. E. Dementi, eds., gravitational lenses and black holes. just as a turn-of-the-century physicist Princeton l-J. I’., Princeton, N.J. (1997). The 2.7 K cosmic background was dis- could not honestly accept a quantum

NOVEMBER~~~~ PHYSICSTODAY 13