According to this excellent ScienceClic video, satellites don't fall straight towards the center of the Earth, but rather, take a curved path, due to the Earth rotating. A similar thing for black holes rotating.

But this seems...off to me, though @.@. Imagining the Earth was a completely uniform homogenous sphere (equally dense everywhere), then even if it's rotating, the physical object will look exactly the same from the outside. Unless you gave each individual atom its own unique name, it'd be impossible to discern whether a uniform sphere was rotating (at a constant angular velocity) or not from the outside, right?

Anyways, my main questions are

  1. Is this effect actually real?
  2. Is there a name for this effect in particular, so that I might read more into it?
  3. What exactly is the cause of this effect, in terms of the equations for general relativity? Does a mass rotating at a constant velocity really behave completely differently in terms of how it warps space-time?
  4. What about a mass moving in a straight line at a constant velocity? Does it also behave differently in terms of how it warps space-time?

1 Answer 1


False, rotation has nothing to do with the fact that you see points of different names. The sphere, uniform still has angular momentum which is measurable. Intuitively, polish perfectly an aluminium ball and let it rotate: if you touch it you will feel the drag on your skin. Answer time

  1. Yes, this effect is very much real.
  2. The name of the effect is frame dragging.
  3. The cause of the effect in GR is the literal rotational drag of the spacetime fabric. So yes, a rotating mass acts very differently.
  4. I don't understand what you mean by that
  • $\begingroup$ I guess OP in the point 4 wanted to ask this: Instead of sphere rotating you have infinitely long pole that is moving across it's longitudinal axis. You then place an object at some distance and observe the path it follows as it falls towards the pole due to gravity. Will the path of the object be a straight line i.e. a straight line forming a 90 degree angle with respect to the longitudinal axis of the pole? Or will the path of the object start curving more and more in the direction of the velocity of the pole as it gets closer and closer to the pole? It's an interesting question... $\endgroup$ Apr 13 at 23:10

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