Lamb and Retherford Discover the Lamb Shift

Willis Eugene Lamb, Jr., and Robert C. Retherford discovered a disparity between the energy levels of electrons in two different states previously believed to have exactly the same energy levels. The discovery of this disparity, which came to be called the Lamb shift, paved the way for the theory of quantum electrodynamics.

Summary of Event

By the end of the nineteenth century, classical physics sought to explain the physical world as the interaction between discrete particles of matter or by wave activity. The success of classical physics prompted many scientists to predict the end of physics as an investigative displine, because all the significant problems of physics seemed on the verge of solution. Problems began to occur within this system, however. These problems led Max Planck eventually to discover bursts of energy that looked like particles, in contexts in which a continuous flow of energy was expected. This discovery marked the beginning of quantum mechanics Quantum physics , which states that electromagnetic waves come in discrete units of energy rather than as one continuous flow of wave action. In terms of classical physics, quantum theory violates one of the fundamental assumptions of physics: that light, or radiation, spreads continuously and is evenly distributed through space. [kw]Lamb and Retherford Discover the Lamb Shift (1947)
[kw]Retherford Discover the Lamb Shift, Lamb and (1947)
[kw]Shift, Lamb and Retherford Discover the Lamb (1947)
Lamb shift
Particle physics
Lamb shift
Particle physics
[g]North America;1947: Lamb and Retherford Discover the Lamb Shift[01960]
[g]United States;1947: Lamb and Retherford Discover the Lamb Shift[01960]
[c]Physics;1947: Lamb and Retherford Discover the Lamb Shift[01960]
[c]Science and technology;1947: Lamb and Retherford Discover the Lamb Shift[01960]
Lamb, Willis Eugene, Jr.
Retherford, Robert C.
Dirac, Paul Adrien Maurice
Feynman, Richard P.
Tomonaga, Shin’ichirō
Schwinger, Julian Seymour
Planck, Max
Einstein, Albert
Broglie, Louis de

By 1905, Albert Einstein had worked out the details of the theory of special relativity and had begun to study the photoelectric effect, wherein light waves falling on certain metals would release electrons from the metal. His studies showed that the release of electrons from metal depended solely on the wavelength of light. Regardless of the brightness of light, shorter wavelengths of light released more electrons, while longer wavelengths of light lacked the energy to release any electrons. For Einstein, the photoelectric effect meant that specific wavelengths of light possessed “quanta” of energy.

In 1913, Niels Bohr Bohr, Niels introduced a new quantum view of physics. Bohr retained the classical model of the atom, where the electrons spun in an orbit around an atomic nucleus. As a result of this spin, the electron generated electromagnetic waves according to the laws of classical physics. Bohr used Planck’s theory of quanta and constructed a model of an atom in which electrons emitted radiation only when they changed orbits. To jump to a higher orbit, an electron needed to absorb a quantum of energy. In returning from that higher-energy state, the electron would release a quantum of energy. The orbits were set at specific distances from the nucleus, and it required a quantum of energy to make the jump.

The differences between the Bohr model and that of classical physics lay in the fact that the Bohr atom did not radiate energy when it was in a stable orbit, whereas in classical physics, a spinning electron was required to radiate electromagnetic waves continuously, with the result that the electron lost energy until it fell into the nucleus. All the components for a new view of the physical world were now available, and in 1924, Louis de Broglie proposed the wave-particle duality: Not only could waves act like particles but also particles could act like waves. Erwin Schrödinger used this idea of the dual nature of matter to develop a theory of wave mechanics.

The wave-particle duality Wave-particle duality[Wave particle duality]
Particle-wave duality[Particle wave duality] became one of the foundations of modern physics, and Paul Adrien Maurice Dirac provided the theoretical structure for this physics in what is often called the “Copenhagen interpretation.” As the leading advocate for this point of view in the 1930’s, Dirac claimed that neither the particle nor the wave aspect of matter was subordinate to the other. In 1928, Dirac published an equation Dirac equation that described all the properties of the electron and satisfied the requirements of both quantum mechanics and relativity. By 1930, Dirac had formulated a mathematical transformation theory that became the basis of future research programs in quantum mechanics and quantum electrodynamics (QED).

The development of QED, describing electromagnetic radiation and properties of the electron, was based on the work of Dirac in conjunction with the contributions of Werner Heisenberg and Wolfgang Pauli. Although the Dirac equation was a critical discovery in physics, the mathematical description produced barriers to further development. During World War II, the experimental use of microwave techniques prompted new explorations in QED. It was at this juncture that Willis Eugene Lamb, Jr., and his longtime collaborator Robert C. Retherford entered the picture.

Lamb had completed his doctoral dissertation at the University of California at Berkeley in 1938 and accepted an appointment to teach physics at Columbia University in New York City. His research work centered on the metastable states Metastable states, atomic of atoms. A metastable state is a prolonged version of what is normally an ephemeral state. For example, as a hydrogen atom absorbs energy, entering what is called the excited state, its electron jumps temporarily to a higher orbit then quickly decays to its original state, emitting a single photon in the process. The electron is in its outer orbit for an infinitesimal fraction of a second. Metastable conditions last several million times longer than that because of a property called parity, which requires the electron to emit two photons.

Lamb worked on microwave absorption and emission of atoms in order to determine the “fine-structure” of the atom. He was able to excite the electrons of hydrogen atoms by bombarding them with microwaves that were equal to the energy difference between their two orbital levels. When each electron returned to its original orbit, it emitted a photon that provided the characteristic spectrum of hydrogen. The Dirac equation predicted that in the hydrogen atom, the two states of heightened energy—the momentary excited state and the metastable state—would have precisely the same energy levels. In 1947, working with Retherford, Lamb demonstrated that these two energy states were not exactly equal after all. The tiny gap in energy level between excited and metastable state was a significant complication in the developing models of quantum physics. It came to be known as the Lamb shift.

The Lamb shift began a revision of Dirac’s equation that in turn led to the development of a theory of quantum electrodynamics. Lamb followed up his experimental discovery with contributions to the new theory. In 1955, he shared the Nobel Prize in Physics with Polykarp Kusch for their independent work on the interactions of electrons and electromagnetic radiation.


The experimental discovery of the Lamb shift identified a problematic area in the Dirac equation and produced theoretical reevaluations of the quantum effects of the electron that led to the growth of quantum electrodynamics. The most imaginative theoretical approach belonged to Richard P. Feynman. While sitting in a cafeteria at Cornell University, Feynman watched someone tossing a plate in the air. He decided to formulate a mathematical description of the spin and wobble of the plate and from this serendipitous event produced a new view of electron dynamics. Feynman’s resulting diagrams predicted the Lamb shift with great precision; they became a powerful tool in many areas of physics.

Working independently from each other, Julian Seymour Schwinger and Shin’ichirō Tomonaga chose to work out the consequences of the Lamb shift from another point of view. Earlier physicists had ignored the charge and mass of the electron. Schwinger and Tomonaga decided to measure both these quantities. The mathematical technique they developed to measure both the mass and charge of the electron was called renormalization. They argued that the infinite “bare” mass of the electron is canceled out by the mass of the photon and the particle cloud that surrounds the electron, except for a small residual mass. This residual mass and similar residual charge are the only finite quantities that can be measured. Later experimental results agreed with the predictions of the renormalization process.

In 1965, Feynman, Schwinger, and Tomonaga shared the Nobel Prize in Physics. Their work opened the door for future studies in quantum electrodynamics, such as better understanding of the fine-structure of the atom, the nature of the electromagnetic field, and the interaction between radiation and electrons. Indeed, in a brief period of time, discoveries in physics reshaped the understanding of the nature of matter, enabling scientists to produce a new vision of the fundamental organization of the universe. Lamb shift
Particle physics

Further Reading

  • De Broglie, Louis. The Revolution in Physics: A Non-Mathematical Survey of Quanta. Translated by Ralph W. Niemeyer. New York: Noonday Press, 1955. A highly recommended text for those seeking nontechnical information on quantum mechanics. De Broglie is one of the founders of modern physics and shares these discoveries with his reader.
  • Einstein, Albert, and Leopold Infeld. The Evolution of Physics: The Growth of Ideas from Early Concepts to Relativity and Quanta. New York: Simon & Schuster, 1938. One of the most accessible single-volume histories on the development of modern physics available to the general reader. There are virtually no technical terms and no mathematics are required. The final section of this book is on quanta.
  • Feynman, Richard. QED: The Strange Theory of Light and Matter. Exp. ed. Princeton, N.J.: Princeton University Press, 2006. The definitive work on quantum electrodynamics for audiences of any level, by one of the foremost physicists and writers in the field. Index.
  • Fritzsch, Harald. Elementary Particles: Building Blocks of Matter. Translated by Karin Heusch. Hackensack, N.J.: World Scientific, 2005. Brief explanation of quantum electrodynamics for a popular audience. Index.
  • Schweber, Silvan S. QED and the Men Who Made It: Dyson, Feynman, Schwinger, and Tomonaga. Princeton, N.J.: Princeton University Press, 1994. This massive, comprehensive history of the development of quantum electrodynamics includes several chapters on Lamb, the Lamb shift, and their importance to QED. Bibliographic references and index.
  • Segrè, Emilio. From X-Rays to Quarks: Modern Physicists and Their Discoveries. San Francisco: W. H. Freeman, 1980. Segrè was one of a few physicists who both participated directly in nuclear physics (and received a Nobel Prize for his work) and wrote a number of popular accounts on the history of physics. The earlier sections of this volume cover the discoveries and theories of those who produced a coherent picture of the atom.

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