Eleven years after first introducing the world to relativity, Albert Einstein (1879 – 1955) finally published his Theory of General Relativity in 1916, owing to its mathematical and conceptual complexity. It expanded on his Theory of Special Relativity to include a new theory of gravity. However, others in the scientific community were understandably skeptical at the time of its publication. It was one thing to find a mathematical model that worked, but another to actually put it to the test.
There were three experimental tests performed that led to the widespread acceptance of general relativity: the precession of Mercury’s orbit, the bending of starlight by massive objects, and gravitational redshift. The most well known of these involved British astronomer Sir Arthur Eddington (1882 – 1944) who, inspired by the theory, remarkably observed the Sun’s effect on background starlight (as is discussed on “The Sun” page).
General relativity led to a revolution in our understanding of gravity, and the theory has stood the test of time for a century. It has interesting (and very non-intuitive) implications. Gravity is not to be treated as a force, but rather a system by which objects simply follow the shortest distance between two points in space (a curved four-dimensional space, that is).
To illustrate with an example of simpler dimensions: envision a tourist flying across the Atlantic Ocean between two airports on Earth at the same latitude, starting in New York City and landing in Istanbul. Sure, the tourist could just travel eastward on the line of latitude to get to the second airport, but is that actually the shortest path? No, we must remember that the surface of Earth is curved, so the optimal path is instead an arc with the ends curved downward (more precisely, this arc would be part of a great circle). This is why most transatlantic flights will typically take a traveler northward toward the Arctic (not to mention the added benefit of being near land in the event of an emergency).
Just like airline pilots try to minimize the distance of their flight paths, objects naturally follow this warped fabric of spacetime. And what causes this curvature? Mass, and more of it means more curvature. And curvature tells matter how to move. It’s with this relationship that we can relate mass and energy with the geometry of spacetime.