Kind of newbie question about gravity

[A.T.]

What I don't understand: how is the above compatible with "the ground meeting the body and not the body meeting the ground"?

As I already told you, the distinction between "ground meeting the body" and "body meeting the ground" doesn't have any physical significance. The only absolute (coordinate independent) fact is that they do meet. Their movement is just a question of which coordinates you choose to describe the meeting.

 

[Tiago]

Ok, it makes sense. It's basically an adaptation of Einstein's Special Theory of Relativity, right? It doesn't really matter if the ground hits the person or if the person hits the ground, it all depends on reference frame. Still, if a person jumps of a building and an external observer is on a spaceship in outter space watching both of them (the Earth and the falling person), what would he see? He would see them both moving towards each other, right? Just like he would see those two guys walking north and he would say they both met each other.

 

[A.T.]

Still, if a person jumps of a building and an external observer is on a spaceship in outter space watching both of them (the Earth and the falling person), what would he see?

Depends on how the spaceship moves relative to them. Also note that there is a difference between what an physical observer at some position sees, and what happens at distant positions according to some coordinates where he is at rest.

 

[Nugatory]

Ok, it makes sense. It's basically an adaptation of Einstein's Special Theory of Relativity, right?

You've got it. (Strictly speaking, it's not an adaptation of SR, it's a different application of the basic insight behind SR, but that's a quibble here).

 

[Tiago]

Knowing that it's off subject here, but there is something else that's been mindblowing me. Of course we know that gravity affects time and the presence of big masses such as planets will cause big distortions in spacetime which slows things down (if I'm floating in space, won't I be distorting spacetime for.. say.. an ant? Won't the ant gravitate towards me?).

Anyway. we know that velocity also affects time, and if I'm traveling very close to the speed of light, time would go slower for me than people on Earth. But only the speed of light is constant, everything else is relative to another frame of reference. So if I say the Earth is traveling at N km/h, that would have to be relative to something (perhaps the Sun). So, in order for me to be traveling at 99% of the speed of light, what reference frame am I using? Earth? I need to be going at the speed of the Earth, plus the 99% of the speed of light? And which velocity of Earth? Relative to what?

 

[Nugatory]

That's two questions. Best to start two new threads, as the question that started this one has been answered.