I\TRODI:(TIO\ TO FEATURE OH THE MECHANICAL EFFECTS OF LIGHT The idea that light carries momentum and therefore can exert a pressure was proposed by Kepler and later elaborat- ed on by Newton. But it is James Clerk Maxwell's 1873 theory of light that consistently implied the existence of radiation pressure. Indeed, Maxwell's theory of electro- magnetism provided the first classical expression for the momentum carried by a radiation field. Its initial experi- mental verification was achieved in 1901 by the Russian

in the potential well provided by the combined effects ofthe light and gravitational forces.

In parallel with this 'konventional" work on macroscopic objects, a recent development of considerable importance has been the use of strong resonant enhancement. This provided by light scattered resonantly from the quantum levels of atomic particles. Here, the momentum carried b3. light is used to cool and possibly trap free particles, with

0 .3

f

their subsequent manipulation and further cooling. The ~ 1 . timate goal of this area of research is that of achieving the spectroscopic ideal of a single particle a t rest in an empty universe, a theorist's dream. This may eventually lead to hitherto unknown spectroscopic accuracy in the determina. tion of fundamental constants and a concomitant improve. ment in time standards.

An interesting new phenomenon is the resonant Kapitza- Dirac effect, that is, the scattering of atoms from the period. ic structure provided by a standing laser wave. The reso- nant nature of the interaction makes diffraction effects ob. servable for particles as heavy as ordinary atoms. This process can be regarded either as Bragg scattering from the standing-wave potential or alternatively as an emission-ah- sorption sequence of photons alternatively from the two traveling-wave components of the standing wave. Thus this phenomenon properly belongs to the present feature.

Beside the interest in light pressure owing to its potential applications, this topic provides a fascinating area of funda- mental research. I t combines the internal quantum struc- ture of the atomic particles with their translational degrees of freedom in an essential way. Hence the investigation of the combined dynamics of the internal levels and the motion in real space is a challenging problem in quantum mechan- ics. I t also provides interesting insights into both how quan- tum mechanics approaches classical mechanics and the emergence of statistical mechanics. Some of the papers address these fundamental questions.

The collection of contributions assembled in this special issue mirrors the range of interests in both Eastern and Western research groups. We have organized them in four categories: Dynamics of Light-Pressure Forces, Cool- ing of Atomic Beams, Deflection by Photon Scattering, and Forces on Macroscopic Bodies, thus following a windy path from the microscopic to the macroscopic effects. In all there are 16 papers, but as the reader will notice most

of them are purely theoretical or provide the analysis of preliminary experiments. We do not doubt, however, that this situation will be changing fast, and a large number of experimental investigations will follow the present vivid in- terest in this field. This, in fact, is one of the major motiva- tions for preparing this special issue a t this particular time: It is useful to summarize the present state of knowledge just

as the interest is soaring. We hope that the papers included in this collection provide a modest but important step to- ward this goal.

P. Meystre Stig Stenholm

physicist P. N. Lebedev, followed soon after by the Ameri- cans E. F. Nichols and 6. F. Hull. The early experiments were carried out using incandescent light sources and nonre- sonant light.

Over the years, the concept of radiation pressure was used successfully in connection with physical problems as diverse as the internal stability of massive stars, the tail of comets, the orbital motion of artificial satellites, and the early uni- verse. More recently, inertial confinement by laser light and the invention of the free-electron laser as well as the novel concept of laser accelerators (inverse free-electron la- ser) have made considerable use of the mechanical effects of light.

In quantum theory the momentum carried by the photon became one of the central issues. The detailed conservation of energy and momentum in the individual quantum event was investigated in Compton scattering. For x rays and gamma rays the recoil momentum is too large to be ignored, and indeed the Mossbauer effect was conceived precisely to circumvent the limitations imposed by photon recoil. For visible light, however, the effect is so small that it has usually been ignored. This situation changed with the introduction of modern laser sources of high brightness.

This special issue tries to give a balanced overview of recent work on the mechanical manifestations of the mo- mentum carried by light with a main emphasis on those applications of relevance to laser spectroscopy and atomic physics. This implies that a vast amount of work in plasma physics, astrophysics, and the free-electron lasers is omitted.

This restriction is certainly unfortunate, but it was necessary in order to keep the volume to a manageable size. The systems considered include both light-weight macroscopic bodies, where nonresonant scattering is utilized, and microscopic systems of atoms or ions, where explicit use is made of resonant enhance- ment.

The use of laser sources leads to such a strong light pres- sure on macroscopic bodies that they can be utilized for actual manipulation of the particles. Not only the linear forces but also the torques provided by circularly polarized light can be observed. I t was Ashkin who first suggested and demonstrated that small neutral particles, dielectric balls, can be accelerated and trapped using the radiation-pressure forces associated with focused laser beams. In later experi- ments, these particles, weighting on the order of a micro- gram, were levitated against the gravitational field. More- recent work showed that the resonant enhancement of the radiation-pressure force provided by an optical resonator permits us to trap systems of considerably larger mass with-