Clocks test Einstein vs. Mach.

Clocks test Einstein vs. March

It is almost 70 years since Albert Einstein published his general theory of relativity. The theory has become physicists' standard theory of gravity and the basis of modern cosmology. Yet that has not prevented a steady stream of physicists, discontented with some of its provisions or some of its principles, from trying to amend, rearrange or replace it. So far Einstein's theory has survived experimental tests. The latest test, done recently at the National Bureau of Standards (NBS) in Boulder, Colo., also corroborates Einstein's theory. The experiment concerns one of the foundation principles of the theory, Einstein's idea of the masses of material objects.

Mass is a basic property of material bodies and a number that goes into any calculation of their motions, but the working definition of mass that physicists use has never really satisfied philosophers of science. During the years in which Einstein was working up his theories of relativity, Ernst March, who was both a theoretical physicist and a philosopher of science, worked out what became known as Mach's Principle. As Mach was a philosopher, different commentators differ as to what it was he actually said, but in the most general sense Mach's Principle proposes that the mass of any body is related to the masses of all the other bodies in the universe. This means that if the geometrical relationship between a given body and the rest of the universe changes, the mass of the given body will change.

Some commentators say that Einstein disregarded Mach's Principle; some say that he interpreted it in a very special way that caused it to cancel itself out. In either case Einstein's theory denies any relationship between the mass of one body and those of others. Experimenters continue to test for such a relationship, however, one reason being that Mach based his principle on a philosophical question that still rankles: Suppose there is only one object in the whole universe; how do you measure its motion? You need other bodies against which to measure its motion, and from this necessity Mach deduced that the mass of one object has meaning only in relation to the other masses in the universe.

The NBS experiment, done by John D. Prestage, John J. Bollinger, Wayne M. Itano and David J. Wineland, tested a property of Einstein's theory known as Local Lorentz Invariance. According to Wineland, one possible way of applying Mach's Principle is to say that if there is a relationship between the mass of a body and its velocity with respect to a frame of reference fixed on other bodies (for instance, the fixed stars of our galaxy), then that mass ought to change as the orientation of its velocity changes with respect to the fixed stars. Local Lorentz Invariance denies that this will happen.

The experiment used two atomic clocks, one based on hydrogen and one on beryllium. The two clocks were located in the same place (expressing the "local' in Local Lorentz Invariance). As the earth turns, the two clocks have a velocity with respect to the fixed stars, and the orientation of velocity is constantly changing. If Einstein was wrong, the difference between the mass of the hydrogen nucleus and that of the beryllium nucleus should change also, as the two masses do not necessarily change by the same amount. The timing rates of the clocks depend in part on the nuclear masses, and so such a difference should change their rates of timekeeping with respect to one another. The experiment found no such change.

The experimenters say this test was 300 times as precise as the last test of the same principle, done in 1961. Wineland says they can thus say that Einstein's theory is good to a certain level. If it breaks down below that level, a more precise experiment must be mounted to find out.