Was Einstein wrong?

I spend a good deal of time each week answering email from people who have questions about NASA, astronomy, or just general questions about science. I get the feeling that a lot of the questions come from students wanting to have their homework done for them ("Could you please define what a globular cluster is and give three examples?"), or people who feel a little neglected in life ("Einstein was wrong and I have discovered a better theory of gravity! Please alert the scientific community and give me the Nobel Prize.")

I'm bemused by the "Einstein was wrong" letters, as they almost always come from people who have little or no formal training in physics. The plain truth is, yes, we're pretty sure that Einstein was wrong, at least in some very special circumstances. But he was also very, very, close to being right, and probably always will be. And scientists are now on the cusp of making the first measurements that will tell us how close to being right Einstein was, and just how wrong he was, too.

What we're talking about here is gravity. There's always been something special about that particular natural force. We still don't really understand how it works, and that's been a problem for some time. Isaac Newton was the first person to come up with a good description of gravity, but even he didn't try to come with a reason why gravity works the way it does. What Newton did do was come up with a series of simple, easy to use equations that described how mass attracts other mass. Want to calculate how long it will take for an apple to fall from a tree? Newton's your man. In fact, Newton's description of gravity worked so well that it held sway for over 200 years.

But toward the beginning of the 20th century, people started noticing small discrepancies between the predictions of Newton's equations and what was really going on in nature. As it turned out, Newton's laws didn't work so well when the force of gravity got really strong, stronger than we normally experience on Earth.

Take the orbit of Mercury. Newton's equations are really good at describing the orbits of the planets around the Sun, but Mercury didn't work quite right. Its orbit seemed a bit skewed, more of a spiral than a complete ellipse. For a time, no one could figure out what was going on.

Then along came Einstein with a staggering proposition. Not only could he describe the orbit of Mercury much better than Newton, but he even had a reason why. The secret, according to Einstein, was to think of gravity as a curvature in space and time. Anything with mass curves space (and yes, incredibly, time as well) to some extent. Larger masses curve space and time more dramatically, so when you observe something right up close to the Sun (like Mercury), you have to take the warped space and time into account before you make the calculations. Take Einstein's new curvature into account, and bingo, you can predict the orbit of Mercury perfectly.

The idea of space and time being curved or warped takes a little getting used to, but it is a real, experimentally verified fact of the universe. Clocks in strong gravitational fields run measurably slower than clocks we've put up in space, and rulers (or any means to measure distance) are actually longer or shorter depending on how deep into a gravitational field they are. Really.

The reason we don't notice these changes in everyday life is that they are tiny. A ruler really is a different length on top of the Empire State Building as opposed to the basement, but the difference is many times smaller than a single atom. And a clock on Mount Everest really does run faster than one in Death Valley, but by much less than a millionth of a second a year.

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