"He who loves practice without theory is like the sailor who boards ship without a rudder and compass and never knows where he may cast."—Leonardo da Vinci

It's often lonely, these days, as a theorist. As soon as most people hear the word theory, in fact, they start thinking about something like this:

 (Image credit: F. Steiger.)

But if you're scientifically minded, you know just how powerful your theory is. Because your theory – if it's any good – allows you to not only explain what you've already seen, but allows you to predict something new, which you can then go look for.

By the early 1800s, there were two theories about the nature of light. One of them, going back to Newton, is that light is a ray.

(Image credit: University of Iowa.)

And, of course, it is a ray. But there was another idea – going back even farther (to Christiaan Huygens) – that light might also be a wave. And this gained a lot of support in 1799, when Thomas Young first passed light through two thin, nearby slits.

 (Image credit: Matthew Parry-Hill and Michael Davidson.)

The pattern that comes out of this – the famed double-slit pattern, below – can only be explained if light were, in fact, a wave.

 (Image credit: Benjamin Crowell.)

So this was the leading theory in the 1800s: that light is a wave. So if you're a good theorist, and you're interested in studying light, what do you do?

Well, if you're famed French mathematician and physicist Simeon Poisson, you would think of the most ridiculous configuration you could imagine in the hopes of disproving the light-is-a-wave theory. And that's exactly what he did in 1818.

(Image credit: Auburn University.)

He imagined that you took a wave source of light, and had it shine on and around a completely black, spherical obstacle, setting up a screen behind it. Obviously, he reasoned, you would see some light on the screen indicating the outside of the sphere, and darkness, or a shadow, on the inside.

But, he calculated, if the wave theory of light were correct, you would get something completely absurd!

(Image credit: Robert Vanderbei.)

Sure, you'd get light on the outside, and shadow on the inside, but what's that at the very center? Poisson predicted, using the wave theory of light, that you'd actually get a bright spot of light at the very center of this shadow! How absurd was that! And therefore, he reasoned, the wave theory of light was absolutely crazy, and had to be wrong.

Hard to argue with that, isn't it?


(Image credit: Alexandre Sixdeniers, from a painting by Henry Scheffer.)

Meet Francois Arago, former Prime Minister of France (among many other things). Shortly after Poisson's prediction, Arago decided to put the theory to the test, and actually performed the experiment to look for the "theoretically absurd" spot.

What happens if, in fact, you perform this experiment yourself?

(Image credit: Thomas Bauer at Wellesley.)

Amazingly, the spot is real! If your theory is any good, scientifically, this is exactly what it will do. It will not only explain what's already been observed, it will allow you to apply it to new situations, and make testable predictions about what you can expect to find. The crazier the prediction, and the more successful the experiment, the more compelling the theory becomes.

And examples abound. In 1927, Georges Lemaitre predicted that the Universe would be expanding, based on his application of Einstein's theory of General Relativity to the Universe. Einstein's initial response was, "Your math is correct, but your physics is abominable."

(Image source unavailable.)

And yet, two years later, Edwin Hubble discovered, in fact, that the farther away a galaxy is from us, the faster it expands away from us. The only thing that was abominable was Einstein's inability to recognize just how powerful his theory actually was.

In the 1960s, the Big Bang Theory was very much in doubt. But it had a prediction: that there would be a very low-temperature, uniform background of radiation in the Universe, permeating all of space and appearing in every direction.

(Image credit: U.S. National Park Service.)

But in 1964, exactly that was discovered, by Arno Penzias and Bob Wilson. The Big Bang has – quite justifiably – gone largely unchallenged ever since.

And, although they're outside of my expertise, the same goes for evolution and natural selection in biology, as well as global warming in climate science. They're theories, to be sure, just as sound and compelling as the ones I've described above.

So the next time someone tells you that the Big Bang, Evolution or Global Warming is just a theory, you'll know what to do.

(Image source unavailable.)

You'll tell them, yes, it is the best theory.

In science, that's as close as we ever get to certainty and the truth. The better we understand and test it, the more compelling, valid, and powerful it gets. And when that happens, we can learn from it, find and identify if there's a problem, and try to deal with it.

But only if you listen to the right theory.

About the Author: The views and opinions expressed here are the opinions of theoretical astrophysicist, physics professor at Lewis and Clark College, and writer of the 2010 Physics Blog of the year, Ethan Siegel. And yes, the beard is real. (Photo credit: Chad Shahan.)

The views expressed are those of the author and are not necessarily those of Scientific American.