When a Line Isn’t a Line or Who’s Line is it Anyway?
What is the shortest path between two points? I bet most of you said a line, and in a lot of circumstances you would be correct. The problem is that this is only true if you are using a flat space like a sheet of paper. When your space begins to curve, you need to become more creative. Let’s take the cities of New York and Tokyo as an example. The shortest distance between them would be a straight line going through the Earth, but that’s no help to planes that need to stay above the ground. So airlines need to figure out a more complex path to make the journey as efficient as possible. This path is called a great circle! For our New York to Tokyo flight you need to travel north almost past Alaska to travel on the great circle.
Here is a fun site that you can use to map great circles connecting airports around the world: The Great Circle Mapper
Now you might think that outer space would be an escape from these silly curved geometries, but you would be very wrong. Einstein’s theory of General Relativity showed us that space is very far from flat. Any object with mass will warp space much like a weight will warp a trampoline. The heavier the object, the greater the warping. This is the basic principle of gravity! One of the interesting effects of this curving of space is that light will behave like our airplane and always follow the shortest path between two points, which often isn’t a line. That is what our vortex table is meant to show. The marbles are trying to go from one side of the metal ball to the other. If the ground was flat, they could simply go right next to it, but in our curved fabric space, the shortest path is a nice even circle several inches from the metal ball.
In space, we have even more extreme cases. For example, the light from a star might be split going around an object like a black hole. Some of the light goes around to the left, some of the light goes around to the right. After navigating the black hole, the two beams of light might eventually converge when they get to the Earth. A telescope detecting these two beams would see two identical stars on either side of the black hole, one on the right one on the left. In reality, there is only one star behind the black hole, but the telescope doesn’t know any better. We call this gravitational lensing, just one of the mind-bending things that can happen in space. Here is a diagram to make things slightly less clear than mud. The gray stars are what you see, the black is what is real.