General relativity is just plain weird.
The basic idea of gravity we are taught in school comes from Isaac Newton’s “Principia” in 1687. Gravity is a force exerted by objects with mass. The greater the mass, the greater the gravitational force. The larger the distance between objects, the lesser the force (it decreases with the square of the distance). The gravity of the Sun pulls on Earth and holds it, along with the other planets, asteroids, comets, etc., in orbit.
Not so, according to Albert Einstein in 1915. He came up with a completely new, and quite radical, alternative explanation.
Einstein’s crazy idea is that the presence of mass warps the fabric of space around it. Then, that warped space controls the motion of other masses nearby. Newton’s idea of a gravitational force is thus replaced with four-dimensional space-time geometry. Planets orbiting around stars, and stars traveling through galaxies — these are space-time distortions moving within other space-time distortions. As one famous description puts it: mass tells space how to warp, while warped space tells mass how to move. Yeah, weird.
On the face of it, Isaac and Albert are just describing the same phenomenon from two different points of view: the former sees a force, while the latter sees geometric distortions. And, since the algebraic equations of the gravitational force are so, so, so, so, so very much simpler than the tensor calculus of general relativity, why go to all the relativistic trouble?
The answer is that there are certain situations, generally involving very large masses, where Newton’s gravity is demonstrably wrong. The most famous of these is the precession of the perihelion of Mercury.
The orbit of Mercury is not fixed in space. Each time Mercury orbits the Sun, its orbit rotates by a minuscule amount. The position when Mercury is closest to the Sun, called perihelion, is used to measure this orbit rotation, called precession. While Newton’s gravity predicts a precession of the perihelion of Mercury, the measured value is significantly higher. This mismatch between prediction and observation is resolved by Einstein’s general relativity in that the warping of space at such a close distance to the Sun produces a slightly stronger precession than gravitational force.
The other famous demonstration of general relativity is the bending of light as it passes a massive object. Light rays also have their paths changed by passing through warped space. A total solar eclipse on May 29, 1919, served to test this effect. During the eclipse, astronomers could see stars whose light had passed close to the Sun. Their apparent position on the sky would be shifted from their normal position due to passage through the warped space around the Sun. By observing the precise positions of such stars both before and during the eclipse, astronomers measured the effects of general relativity. (See the image accompanying this post.)
Those 1919 observations did much to confirm that this crazy idea of general relativity reflected the reality of the universe. We now have many tests of general relativity. Most are subtle and require significant explanation. However, there is one that is visually striking, and which is critical to the scientific underpinnings of some important (and very cool) Hubble observations. I’ll address that in my next blog post.