So you thought that all Einstein did was reinvent everything we thought we knew about space and time. Fool. In his spare time, Einstein wrote papers in support of another multidisciplinary badass. He studied Baer's Law, the law that governs the erosion of rivers, and gives us his version of its mechanics.
We know about Einstein's paper on Special Relativity, and we know about his paper on General Relativity. Many of us even know about his paper about chemistry and Brownian Motion. What's less well know is his brief stop over in geology. In 1926, he published a paper that examined Baer's Law. Ever heard of that? You probably haven't - for two different reasons. It doesn't have any practical effect on the world, and Karl Ernst von Baer was such a talented researcher in some areas that, like Einstein, his other achievements overshadowed his work in geology.
Von Baer made a lifelong study of embryology, uncovering the development of everything from birds to humans. He showed that, at different stages of development, embryos from different organisms can look very similar. Basically, he kick-started the field of embryology. But he spent some time studying geology as well, and came up with a theory about the erosion of rivers. Baer's Law is the river equivalent of the Coriolis Effect - which shows that objects moving over long distances are affected by the rotation of the Earth.
Imagine standing on the north pole and being able to throw a ball to the equator. A target is set out for you. But you are standing, basically still, while the target is zooming by so fast that it travels the entire circumference of the Earth in a day. It's zooming, from your perspective, to the left, so your ball seems to curve to the right. The trick of the Coriolis Effect is, when you trundle down to the equator, retrieve your ball, and throw it back up, it will still seem to zoom to the right. How can that be? As you look up towards the north pole from the equator, you are constantly in line with it. But technically you're not. You're being pulled to your right as a high rate of speed. The pole is sitting relatively still, like the center of a merry-go-round. The ball is being pulled, along with you, to the right at a high rate of speed, and as it goes towards the north pole, the ground slows down under it, making it seem to pull to the right, again. In the southern hemisphere, the same thing happens but in reverse. Flying objects are pulled to the left.
Baer—and Einstein—reasoned that this didn't just happen with things like air and thrown balls. It happened in the water, too. Rivers experienced this pull, or at least the water in rivers did. Plant your feet to either side of a river in the northern hemisphere, and look downstream, and the water will wash against the right bank more. In the southern hemisphere, it will wash against the left bank. This wear and tear will erode away the sediment on the banks, so in the southern hemisphere the left bank will get more wear, and in the northern hemisphere the right bank will. This was Baer's Law, but people didn't understand the mechanics of it. Einstein's paper argued that, as water rushed toward a river bank and was pushed away, it experiences an inward push to match the outward push of the water. This establishes a pressure gradient along the banks of the river. Along the bottom of rivers, the water experiences friction, which slows it down and lessens the outward push. The pressure gradient then sweeps the sediment from the sides to the bottom of the river. This, Einstein thought, was the mechanism that ate away at the side of the river, and built up the bottom.
If you did a survey of all the rivers in all the hemispheres, it's thought you'd see more erosion on the Baer-selected banks. Unfortunately, this is a tiny effect compared to things like river flow and the local geography, and the world hasn't gotten so low on scientific problems that a globe-wide survey of the rivers can be made. Fortunately, since two different people have written about in theory, we're not too busy ourselves trying to work out if it's right.