We Have No Idea: A Guide to the Unknown Universe

by Jorge Cham

And, yes, because we can measure the mass both ways, and so far we have never observed one iota of difference between the gravitational and inertial masses of an object. Think about how weird that is. There’s no real intuitive reason why the two should be the same. One of them (inertial mass) is how resistant something is to being moved, and the other (gravitational mass) is how much it wants to be moved by gravity. You can do a simple experiment to confirm this. Drop two objects with different masses (like a cat and a llama) inside of a vacuum (so there is no air resistance) and you will see that they fall at the same speed. Why does that happen? If the gravitational mass of the llama is larger, then it gets pulled by a larger force from the Earth; but since the llama also has a larger inertial mass, it takes a larger force to get it moving. The two effects perfectly cancel out each other, and the cat and llama fall at the same speed. In our current formulation of physics, we don’t know why that is. We just assume it is the same. And this assumed equivalence is at the heart of Einstein’s general theory of relativity, which looks at gravity in a very different way. Rather than thinking of it as a force that acts on an arbitrary charge attached to particles and the energies that bind them, it describes gravity as the bending or distorting of space around both mass and energy. So in Einstein’s theory the connection is much more natural, but it still doesn’t tell us why it’s ...