Presentation Speech by Professor Ingvar Lindgren of the Royal Swedish Academy of Sciences
Translation from the Swedish text
Your Majesties, Your Royal Highnesses, Ladies and Gentlemen.
This year’s Nobel Prize in Physics is shared between three scientists, Professor Norman Ramsey, Harvard University, Professor Hans Dehmelt, University of Washington, Seattle, and Professor Wolfgang Paul, University of Bonn, for “contributions of importance for the development of atomic precision spectroscopy. “
The works of the laureates have led to dramatic advances in the field of precision spectroscopy in recent years. Methods have been developed that form the basis for our present definition of time, and these techniques are applied for such disparate purposes as testing Einstein’s general theory of relativity and measuring continental drift.
An atom has certain fixed energy levels, and transition between these levels can take place by means of emission or absorption of electromagnetic radiation, such as light. Transition between closely spaced levels can be induced by means of radio-frequency radiation, and this forms the basis for so-called resonance methods. The first method of this kind was introduced by Professor I. Rabi in 1937, and the same basic idea underlies the resonance methods developed later, such as nuclear magnetic resonance (NMR), electron-spin resonance (ESR) and optical pumping.
In Rabi’s method a beam of atoms passes through an oscillating field, and if the frequency of that field is right, transition between atomic levels can take place. In 1949 one of this year’s laureates, Norman Ramsey, modified this method by introducing two separate oscillatory fields. Due to the interaction between these fields, a very sharp interference pattern appears. This discovery made it possible to improve precision by several orders of magnitude, and this started the development towards high-precision spectroscopy.
One important application of Ramsey’s method is the cesium clock, an atomic clock on which our definition of time has been based since 1967. One second is no longer based on the rotation of the earth or its movement around the sun, but is instead defined as the time interval during which the cesium atom makes a certain number of oscillations. The cesium clock has a margin of error equivalent to one thousandth of a second in three hundred years. Compared with this clock, the earth behaves like a bobbing duck.
The dream of the spectroscopist is to be able to study a single atom or ion under constant conditions for a long period of time. In recent years, this dream has to a large extent been realized. The basic tool is here the ion trap, which was introduced in the 1950s by another of this year’s laureates, Wolfgang Paul in Bonn. His technique was further refined by the third laureate, Hans Dehmelt, and his co-workers in Seattle into what is now known as ion-trap spectroscopy.
Dehmelt and his associates used this spectroscopy primarily for studying electrons, and in 1973 they succeeded for the first time in observing a single electron in an ion trap, and in confining it there for weeks and months. One property of the electron, its magnetic moment, was measured to 12 digits, 11 of which have later been verified theoretically. This represents a most stringent test of the atomic theory known as quantum electrodynamics (QED).
In a similar way, Dehmelt and others were later able to trap and study a single ion, which represents a true landmark in the history of spectroscopy. The technique is now being used in development of improved atomic clocks, in particular at the National Institute for Standards and Technology (formerly the National Bureau of Standards) in Boulder, Colorado.
Another technique for storing atoms and observing them for a long period of time has been developed by Ramsey and his co-workers at Harvard University, the hydrogen maser. This instrument is mainly used as a secondary standard for time and frequency with a higher stability for intermediate times than the cesium clock. It is used, for instance, for the determination of continental drift, using VLBI (Very Long Base Line Interferometry). Here, signals from a radio star are received with radio telescopes on two continents and compared by means of very accurate time settings from two hydrogen masers. Another application is the test of Einstein’s general theory of relativity. According to this theory, time elapses faster on the top of a mountain than down in the valley. In order to test this prediction, a hydrogen maser was sent up in a rocket to a height of 10,000 km and its frequency compared with that of another hydrogen maser on the ground. The predicted shift has been verified to one part in ten thousand.
The continued rapid development of the atomic clock can be foreseen in the near future. An accuracy of one part in one billion billions is considered realistic. This corresponds to an uncertainty of less than one second since the creation of the universe fifteen billion years ago.
Do we need such accuracy? It is clear that navigation and communication in space require a growing degree of exactness, and existing atomic clocks are already being utilized in these fields to the limit of their capacity. The new technique may be even more important for testing very fundamental principles of physics. Further tests of quantum physics and relativity theory may force us to revise our assumptions about time and space or about the smallest building blocks of matter.