Fine-structure constant
The fine-structure constant, often denoted by the Greek letter , is a dimensionless quantity frequently encountered in atomic physics; it is generally considered to be a fundamental physical constant. It was introduced into physics by A. Sommerfeld in 1916, and is sometimes called the "Sommerfeld fine-structure constant". In the theory of quantum electrodynamics, it represents the strength of the interaction between electrons and photons.The fine-structure constant is defined as
In cgs units, electrical charges are measured in a way which results in the factor 4πε0 becoming equal to one:
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The small value of the fine-structure constant is important in allowing calculations using quantum electrodynamics. Quantum electrodynamics allows one to break up a quantum mechanical problem into a power series of α and the small value of α creates a situation in which the terms corresponding to higher orders of α become unimportant. By contrast, the large value of the corresponding factors in quantum chromodynamics make calculations involving the strong force extremely difficult.
One controversial explanation of the value of the fine-structure constant invokes the anthropic principle and argues that the value of the fine-structure is what it is because stable matter and therefore life and intelligent beings could not exist if the value were something else.
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Physicists have been wondering whether the fine structure constant is really a constant, i.e. whether it always had the same value over the history of the universe, as some theories had been suggested which implied this not to be the case. First experimental tests of this question, most notably examination of spectral lines of distant astronomical objects and of the Oklo natural fission reactor, have not hinted any changes.
Recent improvements in astronomical techniques brought first hints in 2001 that α in fact might change its value over time. (For a brief article see (1) ). However in April 2004, new and more-detailed observations on quasars made using the UVES spectrograph on Kueyen, one of the 8.2-m telescopes of ESO's Very Large Telescope array at Paranal (Chile), puts limits to any change in α at 0.6 parts per million over the past ten thousand million years. (See
ESO press release or (2) ).
As this limit contradicts the result of the 2001 results the question on whether α is constant or not is open again and the correctness of the contradicting experiments is currently (as of 2004) hotly debated by the scientists involved.
The physicist Arthur Eddington at one time thought that α, which had been measured at approximately 1/136, should be exactly 1/136, based on aesthetic and numerological arguments. Measurements have currently shown this not to be the case. Around 1938, when another measurement showed α to have a value nearer 1/137, Eddington constructed an argument relating the number 136+1 to the Eddington number. This was his estimate of the number of electrons in the Universe.Is α really constant?
Arthur Eddington and the fine structure constant