1990

Table-Top Physics


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The work of UW physicist Norval Fortson and colleagues is proving that it's possible to use small-scale, table-top experiments to probe physical phenomena that otherwise can be studied only by the highest-energy, most massive accelerators.

Fortson's successes come at a time when, as one author in a recent Science article claims, "much of particle physics is in the doldrums," footnote 1 due in part to the fact that the Superconducting Supercollider in Texas was axed, and a new facility in Switzerland won't be fully rolling until well into the next century.

Some physicists' jobs, like Fortson's, boil down to putting new bounds on what "zero" really means. Fortson's lab has made the most sensitive test ever for an electric dipole inside the atom. A dipole is a tiny separation between centers of positive and negative charge.

Fortson's work is the latest in a long history of testing symmetries in atomic and subatomic physics at the UW. In the late 1950s, Boris Jacobsohn and Ernest Henley suggested how to test the symmetry of time reversal, that is, how to tell if you are watching an event as it naturally unfolds in time, or the reverse process (for example, how to tell whether you are watching a movie running forwards or backwards). As a result, experiments were undertaken by David Bodansky, George Farwell, and students at the Nuclear Physics Laboratory with an unprecedented precision for that time--about 1 part in 1,000. Ever since then, other tests of symmetries, such as differences between right-and left-handedness, have continued to be carried out by UW researchers, including Fortson and Eric Adelberger. In the case of left-right asymmetry, effects are actually detected and measured to high accuracy, and compared with predictions of elementary particle theory.

In the 1960s, experiments in high energy physics found a very tiny asymmetry in the time reversal property of one system. Now, more than 30 years later, this asymmetry has yet to be found in any other system, despite many attempts to detect it world-wide. Among the best limits are those established by Fortson and others at the UW, including Blayne Heckel and Steve Lamoreaux.

The search for an electric dipole moment is a sensitive test of this symmetry. Fortson and colleagues line up the magnetic spin axes of mercury atoms which are placed in a magnetic field. These atomic "tops" precess about the magnetic field lines much like toy tops do under the effect of the gravitational field of the earth. That is, the the spin takes on a sort of circular wobble, as the top's axis migrates slowly at an angle about the magnetic field lines. If the speed of that precession changes when an electric field is applied, it's a tell-tale sign the mercury atoms have an electric dipole. Furthermore, when the electric field is reversed, the opposite effect on precession speed should be observed if the mercury atoms have a dipole.

If Fortson were to detect a change in the speed of precession of the mercury atoms, it would revolutionize the field of physics, calling for the development of new theories, possibly supporting the theory of supersymmetry.

But so far, the mercury atoms appear dipole-less, down to about 9 x 10minusexponent 2exponent 8 centimeters. That is, if a tiny charge separation exists, it is smaller than 0.0000000000000000000000000009 centimeters. Another way of putting it is that if the mercury atom were blown up to equal the size of the earth, the dipole (charge separation), if it exists, would have to be smaller than the size of the original mercury atom.

Meanwhile, Fortson and colleagues are also pursuing another kind of precision measurement that would test an aspect of particle physics called the weak force.footnote 2 It's not the Force to which Darth Vader owes his allegiance in the popular movie Star Wars. It is, in fact, the force that is responsible for processes inside the nucleus of an atom such as radioactive decay.

According to standard theories, the weak force acts on particles in a way that distinguishes between right and left. The effect is called parity violation. For instance, a cobalt-60 nucleus decays to create a nickel-60 nucleus, an electron, and an antineutrino. In that process, the electron is ejected preferentially in one direction with respect to the cobalt-60 spin axis. The existence of a favored direction in such interactions that involve the weak force gives the universe a "handedness," like people have.

Fortson and colleagues looked for parity violation in the behavior of the outermost electron in a thallium atom. They found good agreement with the standard model. For now, at least, a revolution will have to wait.


  1. "Edging Toward Supersymmetry," Science, 268, 641 (1995).
  2. "Probing high-energy physics inside an atom," Science News, vol. 147, p. 278.

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