General Relativity III

When Albert Einstein presented his theory of gravity 100 years ago, he included the solution to a problem that had vexed astronomers for decades.
As the planet Mercury orbits the Sun, its orbital path migrates as well, so Mercury’s closest point to the Sun inches forward a tiny bit with each orbit. Astronomers tested several possible solutions to the problem, including searching for an unseen planet inside Mercury’s orbit, but nothing worked.
But Einstein’s general theory of relativity did. The theory said that massive objects like the Sun “warp” the space-time around them — and by an amount that matches the change in Mercury’s orbital path.
It was the first confirmation that general relativity was right — but not the last. In fact, it’s withstood every test that scientists have thrown its way.
Relativity predicts, for example, that a massive object deflects the path of any light that passes close to it. Astronomers tested that idea during a solar eclipse in 1919. They measured th