Freely Falling Inertial As Seen By Distant Inertial Observer

[1977ub]

Apologies if this is in a FAQ somewhere.

A is out in deep space.

B is falling toward the planet.

Does A need both SR & GR to calculate B's time/space dilation as determine by A's IRF?

 

[ghwellsjr]

You need GR anytime gravity is involved which it is in this case.

 

[PeterDonis]

Does A need both SR & GR to calculate B's time/space dilation as determine by A's IRF?

What you are calling "A's IRF" is not actually a global inertial frame. Locally, near A, it will look like an inertial frame; but it will *not* look like an inertial frame closer to the planet. There is no such thing as a global inertial frame when gravity is present. So you will need GR, as ghwellsjr says.

 

[1977ub]

Thanks. so basically it doesn't even make sense to speak of an "SR piece" since the spacetime is not flat there.

 

[PeterDonis]

Thanks. so basically it doesn't even make sense to speak of an "SR piece" since the spacetime is not flat there.

Correct. Far enough away from the planet, in the vicinity of A, you can still use SR as a good approximation since the spacetime is approximately flat there. But that won't allow you to model B's motion since B does not remain far away from the planet.