No one changed the fundamental landscape of physics more than Albert Einstein. Before Einstein, space and time were fixed, and separate. Einstein showed they were elastic, and intertwined. Before Einstein, solid matter was different from insubstantial energy. But Einstein showed they were the same, and matter could melt into energy, according to E=mc2. Before Einstein, gravity was considered a force, like electricity. He showed it was the warping of space-time into an unseen fourth dimension.
Here are four key theories he developed:
* 1905: Photoelectric Effect. Einstein showed that light has concrete, particle-like properties in addition to its well-known wavelike nature. Among other things, this discovery led to the invention of nifty devices such as photoelectric detectors that keep the closing elevator door from squashing you into spaghetti. It was for his work on the photoelectric effect that Einstein received the Nobel Prize in 1922.
* 1905: Brownian Motion. Before Einstein, the idea that matter was made of molecules was generally accepted, but far from proven. Molecules themselves weren't visible. But Einstein calculated that molecules moving around in a liquid would cause little grains sitting on the surface to dance around in a jittery way. His calculations accurately described the erratic behavior of pollen grains on water previously noticed by botanist Robert Brown. Hence, Einstein offered the first convincing proof that molecules were real.
* 1905: Special Theory of Relativity. Einstein came to the startling conclusion that the speed of light is always constant, no matter whether you're moving toward the light, or running away from it. That meant that space and time were elastic; that is, the space and time you perceive depend on your point of view. If you're moving almost as fast as light (imagine running alongside a light beam, as Einstein did), then the light won't appear to go any slower. However, your space will contract, and time will slow down. What's more, he showed that traveling fast also increases your mass. As you travel close to light speed, you get infinitely massive. Since nothing can be infinitely massive, nothing can travel as fast as light. One consequence is that energy can turn into mass, and vice versa. Among other things, this theory explained how the sun shines (by converting mass into energy, through nuclear fusion). Alas, it also led to the development of the atom bomb.
* 1916: General Theory of Relativity. This was an extension of the "special" theory because it brought gravity into the fold. That is, it showed "gravity" is also relative. If you are falling off the side of a building, you don't feel any gravity, because you are moving along with the gravitational field. (That's why astronauts feel weightless; they are still affected by gravity, but they are moving along with it, so it seems to disappear.) Einstein called this insight the "happiest moment of his life." It led him to the realization that gravity is not actually a "force," but rather, the effect of the curvature of space-time caused by massive objects. Imagine the sun sitting in space like a bowling ball sitting on a water bed. The sun warps space-time the same way the bowling ball warps the mattress. If you happen to be a marble rolling around near the bowling ball, you'll fall into the "gravity well" created by the bowling ball. The Earth "falls" toward the sun for the same reason.
K.C. Cole's column, Mind Over Matter, appears on The Times' Science page.