Okay, Einstein, we get it. You were right.

As we’ve reviewed over the past few posts, Einstein’s theory of relativity can be demonstrated by messing with the wavefunction of atoms, during an eclipse or when you are looking for super-far-away galaxies. In 1919, scientists held what I can only imagine was one of the most grand experiments ever – trying to observe the gravitational bending of light. The story surrounding it was so romantic: the expedition to South America, the ominous setting created by a full eclipse, searching for a signal from such distant objects, and of course, finding that the results were so overwhelmingly positive. I find this story breathtaking, so I hope you won’t mind me telling you another tale of confirming Einstein’s theory of relativity.

This one has to do with another arena that I particularly enjoy investigating: the solar system. The solar system has proved to be such a wonderful playground of discovery. It was where Galileo, Copernicus, and Kepler not only began to lay the ground work for modern day physics, but challenged the institutions that would have science silenced. The solar system is also where I did some of my own first experimenting – working out the mass of Jupiter based on the motion of its moons.

If you watch Mercury in the night sky – and you happen to catch it during the right time of year, you’ll notice a funny thing: MERCURY GOES BACKWARDS!! Or at least, it appears to do so. (The image to the right is actually of Mars, but the apparent retrograde motion is the same. It looks like the planets do a loop!)

For many hundreds of years, this motion of Mercury confounded astronomers. That’s partly because they believed that the Earth was the center of the solar system. When Copernicus generated the sun-centered model of the galaxy, Mercury’s retrograde motion made a little more sense. Like cars on a freeway (or, in Copernicus’ time I guess that would be horses on a dirt road? Or something?) the car you are in may at times appear to be stationary, while other cars appear to move in the same or opposite direction. Cars going slower than you appear to move backwards, while those moving faster appear to move forward.

So Mercury changes speed. It appeared to move forward because it travels faster than us, and then appears to move backwards because it moves slower than us. But WHY?

It wasn’t until Kepler drew up the theory of elliptical orbits that we began to understand that planets do not travel at the same speed all the time. His 2nd law states that they will sweep out equal areas in equal time: meaning that as they travel further away from the sun, they go slower. So Mercury would zoom past us as it traveled close to the sun, but appear to move backward when it reached the furthest part of it’s ellipse and goes slower.

BUT that still wasn’t enough to totally explain what was going on.

The planets don’t live in a vacuum. No, wait, they DO live in the vacuum of space. What I mean is, they don’t live unaffected by the things around them. The motion of a planet is affected by the gravitational pull of the sun as well as the pull of the other planets. Trying to predict the motion of Mercury required taking into account all of these effects, and yet, Newton’s prediction fell short. It was close – but it’s imprecision was one red flag that something was missing in the classical interpretation of the universe.

Einstein’s theory would eventually show that in addition to pulling two heavy objects toward each other, gravity warps the fabric of space time and has some effects which can’t be predicted by classical physics. In this warped space time, planets do not orbit in a fixed ellipse, but can progress, creating a “flower petal” motion of Mercury. The planet’s perihelion (the point where it is closest to the sun) changes, moving forward each time it moves.To totally predict this motion took Einstein’s theory of relativity.

Einstein’s theory was quickly shown to account much more precisely for the motion of Mercury – showing that every twelve million orbits, it’s perihelion makes a full 360 degree turn. Here’s a good site if you’ d like some more technical and in-depth explanation for how Special Relativity predicts Mercury’s motion.

We have covered how Einstein’s theory of relativity is correct, and how it has been proven in the past. I know some of you may be aware that the theory of relativity is not perfect. There are extreme places in the universe where we believe it breaks down. But those, I’m afraid, are for another blog post.

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