Equivalence principle question

[PeroK]

All the skydivers and the Earth have zero proper acceleration.

You mean the Earth in its orbit around the Sun?

 

[anuttarasammyak]

I have not included the Sun. But even including the Sun as gravitation does not cause proper acceleration I suppose the Earth keeps zero proper acceleration.

 

[Ibix]

But even including the Sun as gravitation does not cause proper acceleration I suppose the Earth keeps zero proper acceleration.

The Earth as a whole has zero proper acceleration on average, yes (...I'd better cover myself and say I'm handwaving a bit here because tidal gravity is a thing and I haven't completely formalised the averaging process). But the surface, and any region inside except at the very center, does have proper acceleration outwards from the center, just to be clear

 

[Dale]

All the skydivers and the Earth have zero proper acceleration.

You have to be careful here. The surface of the Earth has non-zero proper acceleration upwards, as does almost every part of the interior of the earth. Only the very center of the Earth has zero proper acceleration.

Edit: I see that @Ibix already made the same point, and better!

 

[PeterDonis]

Re: post #12

One skydiver has his own frame of reference and another skydiver has her own frame of reference. They do not coincide but explain the events in good accordance.

For an example,
He observes the Earth coming to him with some acceleration and in its backside her coming with more acceleration.
She observes the Earth coming to her with some acceleration and in its backside him coming with more acceleration.

This is wrong, because neither skydiver's frame covers a large enough region of spacetime to include the other skydiver (or even the whole Earth--only a small portion of the Earth's surface nearest to the skydiver is covered). So neither skydiver can attribute "more acceleration" to the other skydiver than to the nearest portion of the Earth's surface.

 

[anuttarasammyak]

But the surface, and any region inside except at the very center, does have proper acceleration outwards from the center, just to be clear

Thanks. Now I understand forces preventing every parts of the Earth free falling and keeping the shape of the Earth provide proper acceleration. For an example when I am standing, the floor push my feet to give proper acceleration g upward. Skydivers keep zero proper acceleration while falling but will start to get proper acceleration after unfortunate crash.

 

[mucker]

Thanks for all your help everyone. On another note can I shameless link to this for some advice on what physics degree to take? https://www.physicsforums.com/threads/advice-on-type-of-physics-degree-to-do.1005630/

thanks