I was watching a Vsauce video and learned that any two objects, like two baseballs, are attracted to each other because of their mass, and the bigger the mass, the more gravity an object has. What does mass have to do with gravity, and what causes gravity? Why does something just attract other things around it?
In: Physics
So you know how electrons and protons have electrical charges? In a sense, something’s mass is its gravitational “charge.” It’s kind of just a way of saying “how strongly does this object interact with a gravitational field?” Now, proving that it is the same mass as the mass that shows up in Newton’s laws of motion and is associated with inertia and everything is actually a non-trivial thing and was one of the first things that Einstein had to prove for his equivalence principle to be valid.
Okay, so that’s kind of the classical story. If you want to talk about relativity, that says that energy densities curve spacetimes. It is actually pretty mathematically beautiful, since the only assumptions that it comes from is that you can have a curved spacetime and matter living in that spacetime. This is what we know from Einstein’s equation. Mass really is just a bunch of energy tied up in one particular place (from E=mc^2 ), so it is really good at creating large energy densities and therefore curving spacetime.
But where does gravity come in? Well, things like to travel in straight lines, or at least they like to take the shortest distance between two points. When your space is curved, this shortest path is also changed from a straight line (think about being trapped on the surface of a sphere: the shortest distance between two points is really a circle). When particles follow these paths in a curved space, the effect looks exactly the same as a gravitational field. So mass curves spacetime because of the energy densities and the curves spacetimes give the effects of gravity.
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