100 years ago, the way we viewed our Universe was vastly different than the way we view it now. The night sky, with stars, planets, comets, asteroids, nebulae, and the Milky Way, was viewed to make up the entire contents of the Universe.
The Universe was static, governed by two laws only: Newton’s Gravity and Maxwell’s Electromagnetism. There were the first hints that the Universe was made up of quantum particles, such as the photoelectric effect, Rutherford’s first hints at the existence of the nucleus, and Planck’s view that energy was quantized. But other than that — and Einstein’s new Theory of Special Relativity, there were very few mysteries about the Universe in 1909. But one of them would change our view of the Universe forever.
You see, there was a tiny, tiny problem with the planet Mercury. Its orbit just wasn’t quite right. Kepler’s Laws (which can be derived from Newton’s Gravity) said that all the planets should move in ellipses around the Sun. But Mercury (above) doesn’t quite do that. Mercury makes an ellipse that precesses — or rotates — ever so slightly. Specifically, it precessed at a rate of 1.555 degrees per century. A greatly exaggerated example of precession is shown below:
Now, physicists and astronomers have always been very detail-oriented people. So they calculated what the effects of the Earth’s equinoxes precessing were, and were able to account for 1.396 of those degrees. They realized that there were seven other major planets (and the asteroids) acting on Mercury, and that was able to account for another 0.148 degrees. That left them with only 0.011 degrees per century that was different between their theoretical predictions and their observations. But this minuscule difference was significant enough that it led some to consider that Newton’s Law of Universal Gravitation might be wrong.
The same guy who discovered the photoelectric effect, special relativity, and E=mc^2 came up with a new theory of gravity. Instead of an “action at a distance” due to mass, this new theory said that space gets bent by energy, and causes everything — even massless things — to bend beneath what we see as gravity.
Now this new theory was very interesting for a few reasons. First off, it accounted for those 0.011 degrees that Newton’s Gravity did not. Second, it predicted — as a simple solution — the existence of black holes. And third, it predicted that something very exciting and testable would happen: that light would be bent by gravity.
Big deal, said Newton’s advocates. If I take E=mc^2, and I know that light has energy, I can just substitute E/c^2 for mass in Newton’s equations, and get a prediction that Newton’s gravity would bend light, too. It just so happened that Einstein’s bending was predicted to be twice as much as Newton’s bending, and that there was a total Solar Eclipse coming up in 1919. The stage was set for the most dramatic test of gravity ever.
The director of Cambridge Observatory, Sir Arthur Eddington, led an expedition to observe the total solar eclipse of May 29, 1919. During an eclipse, the sky gets dark enough that you can see stars, even close to the Sun. So Eddington set out to map the position of the stars when they were close to the Sun, and see how the Sun bent the light. Would it match up with Einstein’s prediction, Newton’s prediction, or would it not bend at all?
Lo and behold, Einstein’s prediction was spot on. Just like that, Newton’s theory of Universal Gravitation, the most solid foundation in all of physics — unchallenged for over 200 years — was obsolete. All of this was done in the years 1909-1919, and it was just the start of changing how we view the Universe.
And (FYI) so far, in the 90 years since, every single prediction of Einstein’s gravity that’s ever been tested — from gravitational lensing to binary pulsar decay to time dilation in a gravitational field — have confirmed General Relativity as the most successful physical theory of all-time.