Astronomy
Testing General Relativity Through the Precession of Mercury's Orbit
Quick fact
Before Einstein, astronomers had measured that Mercury's orbit precesses by 574 arcseconds per century, but Newtonian physics could only explain about 531 arcseconds of that. Einstein's general relativity elegantly accounted for the missing 43 arcseconds.
Why this is interesting
You've probably heard that Einstein's theory changed everything, but how do you actually test a theory of gravity? It turns out a tiny wobble in Mercury's orbit helped prove him right.
Read the full explanation
Understanding Testing General Relativity Through the Precession of Mercury's Orbit
Think of Mercury's orbit around the Sun as a racetrack that is not a perfect circle but an ellipse. The point where Mercury is closest to the Sun is called the perihelion. Over time, that point does not stay fixed in space; it gradually moves around the Sun. This is called orbital precession. Most of this precession is caused by the gravitational pull of other planets, especially Venus and Jupiter, plus a tiny effect from the Sun's oblate shape. Even after accounting for all those effects, there remained a small, unexplained shift of about 43 arcseconds per century—a tiny angle, barely visible without precise telescopes. Astronomers had noticed this anomaly in the 19th century and even speculated about a hidden planet 'Vulcan' near the Sun, but no such planet was ever found. Enter Albert Einstein. In 1915, he published his theory of general relativity, which says gravity is not just a force but a warping of spacetime itself. According to this theory, the geometry of spacetime near the Sun is slightly curved, and this curvature causes Mercury's orbit to precess by exactly the observed amount. Einstein calculated the extra precession due to general relativity and found it to be 43 arcseconds per century, perfectly matching the mystery anomaly. This was the first strong evidence that general relativity was correct, and it helped launch the theory into the scientific mainstream.
A deeper explanation
General relativity describes gravity as the curvature of spacetime. The Sun's mass curves the space around it. For a planet like Mercury, which is close to the Sun, this curvature is more pronounced than for planets farther out. In Newton's view, gravity is a force that acts at a distance, and orbits are ellipses that remain fixed. But in Einstein's view, the path of a planet is determined by the shape of spacetime, and the curvature causes the ellipse itself to rotate slowly. The effect is stronger for planets closer to the Sun, which is why Mercury shows the largest precession. Einstein's calculation showed that for a planet orbiting the Sun, the precession of the perihelion (in radians per orbit) is given by a term that depends on the gravitational constant, the Sun's mass, the speed of light, and the orbital parameters. For Mercury, this gives about 43 arcseconds per century—exactly the discrepancy that had puzzled astronomers. This agreement was a huge triumph for general relativity. It wasn't just a prediction of a new phenomenon; it precisely explained an existing, unexplained observation. This success gave physicists confidence that general relativity was a valid description of gravity, leading to further confirmations like the bending of starlight, the gravitational redshift, and more recently, gravitational waves.