Daniel Kleppner

Cavity Quantum Electrodynamics

As Casimir pointed out many years ago, conducting surfaces and cavities alter the structure of the vacuum states, and these alterations can have physical effects. This is the underlying principle of the Casimir force and also the point of departure for a series of recent studies on atom-vacuum interactions in the microwave and millimeter wave regimes. The natural scale for such effects is set by the spontaneous emission rate. At microwave wavelengths this rate is normally too small for spontaneous emission to be observable, In Rydberg atoms, however, the rate is enhanced by a factor n4 (n is the principal quantum number, typically 20-40). Advances in experimental techniques for Rydberg atoms have opened the way to the study of the atom-vacuum interaction at long wavelengths. The result has been a renewed interest in physical effects due to the vacuum, essentially a new area of macroscopic quantum phenomena. Recent experiments with Rydberg atoms and also with free electrons are described. I will attempt to set the stage by reexamining some of the physical effects of the vacuum and illustrating the ideas with an experiment in which spontaneous emission was effectively turned off by tuning below cutoff a waveguidelike structure that surrounded the atom.1 (Invited paper, 25 min)

Time too good to be true

W wishing to cause unnecessary distress, I would like to call attention to a couple of issues concerning time. The first is merely calendraic but the second concerns the future of time itself. The first issue is that we may have to say farewell to leap seconds. Leap seconds, as you might recall, are the occasional one-second adjustments of our clocks that are made to maintain harmony between the astronomical and atomic time scales. Personally, I would be sorry to see leap seconds go because that would cost me the pleasure of mulling over the best way to spend my next one. Although a mere second might seem to be too short to cause jubilation, I believe any gift of time deserves to be treasured. Also, one second is not really that short. It is long enough to record a few million high-energy scattering events, and in femtosecond physics, one second is virtually an eternity. Also, one second is sufficient for a word or quick kiss that might change your life. The argument about whether to retain leap seconds is reminiscent of the argument about standard time versus daylight savings time: What is convenient for one community can be inconvenient for another. City dwellers generally favor daylight savings time and farmers generally oppose it. Astronomers favor leap seconds because they keep clocks in synchrony with the orientation of the Earth. Synchronization is helpful in deciding where to point telescopes and in interpreting the data in astronomical records. Celestial navigators—that vanishing breed—also like leap seconds. The Global Positioning System, however, cannot tolerate time jumps and employs a time scale that avoids leap seconds. Moreover, all large-scale systems that require precise synchronization are likely to have trouble with leap seconds. For instance, any attempt to introduce a one-second hiccup in the phasing of North American power grids would likely cause a hemispheric blackout.

Cold collision frequency shift of the 1S-2S transition in hydrogen

We have observed the cold collision frequency shift of the 1S-2S transition in trapped spin-polarized atomic hydrogen. We find Delta v(1s-2s) = -3.8 +/- 0.8 x 10(-10)n Hz cm(3), where n is the sample density. From this we derive the 1S-2S s-wave triplet scattering length, a(1s-2s) = -1.4 +/- 0.3 nm, which is in fair agreement with a recent calculation. The shift provides a valuable probe of the distribution of densities in a trapped sample. [S0031-9007(98)07613-3].

A Beginner's Guide to the Atom Laser

An atom laser is explained using the similarities and differences with optical lasers. (AIP) {copyright} {ital 1997 American Institute of Physics.}

Research in Small Groups

In Munichs Deutsches Museum one can come upon an Italian Renaissance study, meticulously recreated and handsomely furnished save for one intrusion: A plank, one end propped on a trestle, the other resting on the floor. With such a crude apparatus and using his pulse to clock how long a brass ball takes to roll various distances, Galileo first explored the nature of uniformly accelerated motion. With the aid of these measurements he discovered the basics of dynamics; in so doing he also established a tradition for experimental research that has animated physics ever since.

Rereading Einstein on Radiation

The concepts of spontaneous and stimulated emission are well known from Einstein’s 1917 paper on radiation, but his theory of radiation comprises many other concepts—the paper is a treasure trove of physics.

Trapped Atomic Hydrogen

The rapid development of techniques for cooling and trapping atoms using laser light has created a new subfield of atomic physics. Research opportunities include the study of matter at ultra low temperature, ultra precise atomic spectroscopy and the study of light-matter interaction in a new quantum regime.

Boost-Phase Defense Against Intercontinental Ballistic Missiles

An American Physical Society study concludes that disabling ICBMs before they release their munitions would be very difficult at best and, in some cases, impractical.

Roundtable: Physics in Transition

The question before us can be stated simply: Where do we go from here? The “we” is the physics community, though its our hope that the discussion will range more widely into science and technology, so well be able to comment on the changes likely to take place in research universities, national laboratories and private industry. Our subject is physics in transition to the 21st century. Some say physics has been in a state of transition for well over a century—certainly since James Clerk Maxwell in the 19th century. In every decade of the 20th century, physics has experienced momentous turning points. In this last decade of the century we seem to have reached another turning point with the end of nuclear weapons rivalry that was given the name of cold war and the increase of global industrial competitiveness. In the past year, pressures by the Federal government and by commercial companies have increased to make physics and the rest of science more relevant to business and to society. We are already witnessing the reduction of physics research at some major corporate laboratories and sensing a shakeup for physics facilities at some national labs. So this discussion of the future of physics is timely.The question before us can be stated simply: Where do we go from here? The “we” is the physics community, though its our hope that the discussion will range more widely into science and technology, so well be able to comment on the changes likely to take place in research universities, national laboratories and private industry. Our subject is physics in transition to the 21st century. Some say physics has been in a state of transition for well over a century—certainly since James Clerk Maxwell in the 19th century. In every decade of the 20th century, physics has experienced momentous turning points. In this last decade of the century we seem to have reached another turning point with the end of nuclear weapons rivalry that was given the name of cold war and the increase of global industrial competitiveness. In the past year, pressures by the Federal government and by commercial companies have increased to make physics and the rest of science more relevant to business and to society. We are already witn...

Their Most Productive Years: Young Physics Faculty in 1990

We live in a society shaped by science and technology. As our society evolves it will draw more and more upon science and technology to generate economic growth, to improve health and to enhance the quality of life. In order to provide adequate scientific and engineering personnel for our national needs and to sustain the knowledge base from which growth derives, careers in science and engineering must be attractive to our youth.