The relativistic explanation of gravity is geometric, the motion of a body in a field of space-time distortion can be described as being at rest and travelling along a geodesic of that field, but why does it travel along that geodesic? What causes it to move?
Answer
You ask what motivates it to move so I wonder if you're puzzled why matter is in some way forced to move along a geodesic instead of just staying still. If so, it's because a geodesic is a trajectory in spacetime i.e. in time as well as space. Since nothing can avoid moving in time that means everything is moving in spacetime even when it appears to be stationary in space. We measure the rate of movement by the four velocity, and in fact the magnitude of the four velocity is always equal to $c$, the speed of light.
So given that everything is moving, the only question is what controls the trajectory in spacetime that the object follows. The geodesic is simply the path followed if no external forces act on the object. It's the general relativistic equivalent of moving in a straight line.
No comments:
Post a Comment