Practically Perfect

A mathematical theorem is either perfectly true or it is false; a circle, by definition, is perfectly round. Physical theories, in contrast, are never exactly true. One finds new physics by probing the limits of presently known physical laws.

These ideas thread through an area of atomic and optical physics that is loosely described as fundamental tests and high-precision measurements. Its origins can be traced to the 19th century, when Michelson developed an interferometer to detect a predicted spatial anisotropy of the speed of light. His failure to find any effect was actually a success: It is often taken as the starting point for explaining Einstein’s Special Theory of Relativity.

The tradition of fundamental tests leading to new science and new technologies is alive today. For example, experiments on the spectroscopy of hydrogen have long been a testing ground for basic theory and a driving force for new experimental techniques. The development of the needed laser technology has made it possible to measure the frequency of an absorption line of atomic hydrogen (even though light waves oscillate with a corresponding period of about a femtosecond, a billionth of a millionth of a second) to better than 2 parts in 1014 (1 followed by 14 zeros)— about the same as one drop of ink in 10,000 swimming pools! This new capability has potentially revolutionary implications for the science of measurement and atomic clocks, as well as in the communication and navigation applications discussed in the chapter “AMO Science Impacting the Economy.”

Another example of fundamental tests is related to theoretical predictions about the nature of the force between the nucleus of an atom and its orbiting electrons. In addition to the dominant electrostatic force, there is the small contribution of a short-range force called the weak interaction. According to the Standard Model of high-energy physics, this interaction has handedness—a property that distinguishes your hand from its mirror image.

In addition to its color and direction of propagation, light is characterized by its polarization, which can be conveniently thought of as an arrow rotating either clockwise or counter clockwise. A consequence of the handedness of the weak interaction is that atoms absorb light that rotates in one direction more easily than they absorb light rotating in the other direction. This effect is minuscule, no more than a part in 1012 (one drop of ink in 100 pools).Yet it not only has been detected but also has been measured accurately using laser spectroscopic techniques. Thus, a tabletop atomic physics experiment has provided a test of a fundamental theory; such tests normally require gigantic particle accelerators.

Another puzzle that physicists are interested in is the observation that the basic laws of physics do not seem to depend on the direction of time. To test this observation, AMO scientists are looking at the shape of the neutron. If physical laws do depend on the direction of time, the neutron would behave as if it were not perfectly spherical. The test of this physical law, performed with AMO techniques, is among the most sensitive ever carried out in physics. It shows that if a neutron could be enlarged to the size of Earth, it would be perfectly round, to within the thickness of a human hair. As a sphere, the neutron seems to be practically perfect. When any imperfection is found, physics will take another step forward, and our understanding of the physical world will deepen.

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