08 July 2009 by Marcus Chown
A SUBTLE anomaly in the orbit of the planets in our solar system could prove a controversial idea that goes beyond Einstein.
The orbit of the innermost planet, Mercury, departs from what it should be under Newton's laws. A century ago, when Einstein explained this anomaly, it confirmed his theory of gravity - the general theory of relativity.
The answer to the riddle of dark matter could be found in our own solar system (Image: NASA)
Now an Israeli physicist predicts that a similar but far more subtle anomaly in the orbits of the planets, if detected, might prove his own theory, known as modified Newtonian dynamics, or MOND. This provides an alternative theory to dark matter to explain why stars orbiting at the edge of spiral galaxies are not flung out into space. These stars are travelling at speeds too fast for conventional gravity from the mass at the heart of a spiral galaxy to hold them in their orbits, so something else must be keeping them on track.
One theory is that invisible dark matter provides that extra pull. But an alternative is MOND, devised in the early 1980s by Mordehai Milgrom, now at the Weizmann Institute of Science in Rehovot, Israel.
One of the suggestions behind MOND is that the gravity experienced by the galaxy's outer stars is somehow stronger than what would be expected under Newtonian physics. MOND has it that below a critical threshold acceleration, called a0, gravity switches from the conventional Newtonian form that weakens with the inverse-square of distance to a stronger form that declines merely with the inverse of distance.
In other words, Milgrom proposed that gravity was stronger than expected at the low accelerations experienced by the outermost orbiting stars.
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