Einstein's theory still beats the competition, new space data show
Albert Einstein would be pleased. The latest data from Mars support his famous relativity theory, rather than its latter-day challengers. New solar system measurements - the most precise ever made of the orbits of Earth and Mars - agree with Einstein's formulas for gravitational attraction among the planets, Ronald W. Hellings of the Jet Propulsion Laboratory (JPL) in Pasadena reported last week at a meeting of the American Physical Society. The analysis fails to show evidence of some subtle influences on planetary orbits predicted by two major alternative theories proposed in recent years.
Einstein theories, among other things, provided a refinement of classical equations describing gravitational force. They arose from his radical conception that gravity was formed as a result of the basic structure of space itself.
One theory, first advanced by the eminent British physicist P. A. M. Dirac, is that the speed of light and other apparently fundamental constants used in Einstein's calculations are not actually constant, but changing extremely slowly over billions of years. If that were true, it would require a fundamental change in Einstein's theory.
Other recent work has questioned relativity's first observational success: its prediction of the unusual orbit of the planet Mercury. Last year University of Arizona scientist Henry A. Hill suggested that Mercury's eccentric orbital path might be due to distortions in the shape of the sun rather than relativistic effects.
To check these theories, Dr. Helling analyzed radio signals transmitted from Mars by the Viking spacecraft, which the United States placed on the red planet's surface in 1976. This enabled JPL scientists to determine the distance from the Earth to Mars to within 10 meters over a six-year period. Helling says this ''unprecedented accuracy'' in solar system data ''has been responsible for a major breakthrough'' in accurately testing relativistic gravity.
Dr. Dirac proposed, among other things, that gravitational force may be slowly weakening.
Because the path of the planets is determined by their gravitational attraction to the sun and other planets, he suggested that sufficiently accurate measurements of their orbits ought to show the effect of even the minute changes. But the Earth-to-Mars distance measurements showed no such evidence, the JPL researcher reports.
Dr. Hill's challenge to Einstein's work was of a considerably different nature. It stemmed from his study of the natural vibrations at the sun's surface.
The pattern of the sun's quaking, when added to what is known about the sun's nature and structure, led him to conclude that the core of the sun must be rotating much faster than the surface.
Such rotation implies that the material in the sun would be concentrated more at the equator than at the poles. And this, in turn, would alter the sun's gravitational field enough to alter Mercury's orbit in such a way that the relativistic calculations of its path would be wrong.
But if the sun is nonspherical to the extent suggested by Hill, it should also affect the orbits of Earth and Mars to a degree that would be evident in the JPL measurements.
Helling, however, reports no such evidence in the Viking data.
This new test of Einstein's relativity calculations verifies them to figures of more than six decimal places. Thus, after 78 years, his work continues virtually unchallenged as the best scientific explanation yet conceived for the basic nature of space and gravity.