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.

Scientists now find themselves in a situation quite similar to the one right before Einstein came along, when Newton's equations, while still wonderfully powerful tools to understand the universe, were starting to show a little wear. The same is now true with Einstein. His description of gravity still works incredibly well, good enough for anything we encounter on Earth or the near universe. But there seem to be some problems with Einstein's equations when conditions get really extreme, when gravity gets out of control.

One example that springs to mind is a black hole, a collapsed star that has formed a "bottomless pit" of gravity. Einstein's equations pretty much give up and go home when you try to calculate what conditions might be like near the center of a black hole. Einstein also doesn't work quite right when you're dealing with the world of the very small, scales the size of an atom or smaller. In those realms, scientists use the laws of quantum mechanics to guide their experiments, which don't always agree with Einstein's theories.

For decades now, physicists have been trying to come up with a new theory of gravity that works better on extremely small scales or at extremely intense levels of gravity. Some of the work has been promising, but nothing has really broken through as a new comprehensive description of gravity.

That may change in the near future, as scientists are set to conduct a rigorous examination of Einstein's theories (and any discrepancies therewith) up in the International Space Station. The idea goes back to Einstein's prediction that time is stretched and warped by the presence of a gravitational field. Scientists are now looking for tiny errors in Einstein's predictions as to exactly how fast or slow clocks will tick up in space, as opposed to down on the ground.

The problem in the past has mainly been with our technology; we've never been able to build an accurate enough clock to measure the tiny shifts in time. Now, several teams have designed mind-bogglingly precise timepieces that make use of the weightless conditions aboard the Space Station.

In one design, the clock will keep time by watching the vibrations of Cesium atoms kept at about a millionth of a degree above absolute zero (-273 degrees Celsius). The low temperatures will keep the atoms very still and well-behaved, so no collisions or interactions mess with their natural vibration rates. Another clock will use a maser (just like a laser but with microwaves instead of visible light) to give off highly uniform light, the wavelength of which can be used for the "ticks" of the clock. Yet another will use super-cooled Rubidium atoms.

But all the clocks have the same goal: prove Einstein wrong. If a tiny, tiny shift in time is measured that can't be explained by Einstein's equations, we'll be hot on the trail of the next big discovery in modern physics.

So was Einstein wrong? To me, that's a useless question. I doubt that any physicist alive today thinks that Albert Einstein was the totally correct, last word in physics forever. I seriously doubt that Einstein himself would be surprised that we're now finding some exceptions to his rules. If we'd figured everything out perfectly at the beginning of the 20th century, I'd consider the universe to be a disappointingly simple place to live.

Einstein gave us an incredibly powerful model of how the universe works, but it was just that: a model. As time progresses, we'll find better and better models that take us closer to the actual truth of the universe. Whatever our next new great theory turns out to be, you better believe there will come along an even better one that describes the universe just that tiny bit more accurately. And I doubt we'll ever reach the end of that cycle. I'm counting on the universe to always have one more mystery up its sleeve.