“No matter how long you stay in physics, there is a lot to learn”, Arthur Schawlow opens this lecture, his third one in Lindau. With hindsight, it is amazing to realize how much he met this claim: His state-of-the-art-talk refers to no less than three future Nobel Prizes (1997, 1999 and 2005) that draw upon his own pioneering research. Together with his brother-in-law Charles Townes he had been a co-inventor of the Laser in the late 1950s . While Townes received a Nobel Prize already in 1964, Schawlow had to wait until 1981 before the Royal Swedish Academy of Sciences (RSAS) awarded him one of the Nobel Prizes in Physics together with Nicolaas Bloembergen “for their contribution to the development of laser spectroscopy”.

“We can learn about light only through its interaction with matter”, says Schawlow and emphasizes that not only matter but also light exists in many forms. ”Even for a given wavelength the interaction depends on the intensity of light and its duration.” He illustrates what it means, for example, that lasers with a pulse duration of lower than ten femtoseconds can be made: “In ten femtoseconds light travels a distance of only three micrometers, which is less than the twentieth of the width of a human hair.” Such ultrashort pulses have made it possible to observe what happens during a chemical reaction. For this purpose, a molecular motion is initiated by a light pulse and probed by another one just femtoseconds later. “This has been quite extensively studied particularly by Professor Zewail and his group”. Eight years later, in 1999, Ahmed Zewail received the Nobel in Chemistry. The RSAS praised the technique he had invented as the „world’s fastest camera“ to obtain „slow motion“ pictures of molecular interactions.

On the opposite side of the time scale, long measurements are necessary to obtain precise absorption spectra. High-resolution spectroscopy had long been hampered by the Doppler effect, which broadens certain spectral lines and hides finer ones. In the 1970s, Schawlow together with his post-doc Theodor Hänsch found a way to overcome this obstacle, namely saturation spectroscopy, whose principle he describes in this lecture. In this context, he introduces the Balmer spectrum of hydrogen as “the Rosetta stone of modern physics, from which many basic discoveries have come from”.

“It is possible to use lasers to slow down atoms to very low velocities”, Schawlow explains with reference to suggestions from him and Hänsch in 1975, and subsequently discusses possible cooling and trapping mechanisms, thereby explicitly referring to the creation of “optical molasses” by Steven Chu who six years later would share the Nobel Prize in Physics with Claude Cohen-Tannoudji and William Phillips “for development of methods to cool and trap atoms with laser light”.

Last but not least, Schawlow foresees one of the Nobel Prizes in Physics 2005, which Theodor Hänsch and John Hall shared “for their contributions to the development of laser-based precision spectroscopy, including the optical frequency comb technique”, although the breakthrough in this technique did not occur before the end of the 1990s. “Progress has been made towards improving the accuracy of measurement of spectral line positions, directly in terms of the oscillation of the frequency of light rather than its wavelength”, Schawlow says and introduces the principle of Hänsch’s “new method that provides increasingly smaller frequency differences”. Both for theorists and for the theory of quantum mechanics “new challenges may come from these increasingly precise measurements”, Schawlow predicts and concludes his lecture with the remark that “the use of laser to study interactions of matter and light is an extremely active frontier of modern physics”.

Joachim Pietzsch