Regarding Guy Gugliotta's July 12 article, "Testing Einstein Theory in the Real Universe":

The interest in Gravity Probe B stems from a combination of its tortuous history and the fact that it tests a fundamental prediction about Einstein's Theory of General Relativity. Some of us have been following this experiment since the 1960s, when NASA began supporting development of the spacecraft and devices that would enable experiments of this sort.

Leonard Schiff and William Fairbank of Stanford University are rightly given credit for initiating the studies that led to Gravity Probe B, but the real credit probably should go to Robert Dicke of Princeton University. Dicke was a very successful physicist who in the mid-1950s switched his career from atomic physics to experiments testing general relativity. I was a graduate student at Princeton University at that time and remember quite well his seminars on the subject.

His basic argument was a simple one. When general relativity was first proposed early in the century, several successful experiments had been conducted to test the idea. New technology with vastly greater precision in the 1950s enabled scientists to repeat the experiments. A very good theoretician, Dicke was able to show that alternate forms of general relativity existed that gave slightly different predictions compared with Einstein's formulation. Several of my fellow graduate students pursuing PhDs helped Dicke perform three experiments over a decade or more. In the end, Einstein's theory held up to remarkable new levels of precision.

A number of other scientists, not just Schiff and Fairbank, took up this same idea about this time, using newly emerging technology to test fundamental aspects of physics, such as the equivalence of the electrical charges of the electron and proton -- nature's basic atomic building blocks -- or the stability of the proton.

Testing fundamental physics using advanced instrumentation is highly seductive. What could be more significant and more fruitful than testing the laws of physics with very high precision?

But after 50 years the results have been disappointing. As was Dicke's experience, others found that the fundamental laws of physics were valid to great precision.

I think I know the reason for the lack of success. Physics is very successful when there is a close connection between theory and experiment. Theory can be out ahead of experimental information, but only by a little. As the gap gets larger, the success rate of new theory gets smaller.

In the case of Gravity Probe B, the gap is huge. No one can predict with any degree of confidence how large a deviation from Einstein's theory there might be. Why then is there any expectation that Gravity Probe B will be successful in detecting one?

There have been huge advances in fundamental physics since Dicke and others set out in this direction. But they have been the result of painstaking, incremental advances made at particle accelerators or at laboratory benches, augmented by great discoveries, largely serendipitous, in astronomy.

-- Herbert Gursky

Great Falls

The writer, a physicist, directs the space science division at the Naval Research Laboratory in Washington.