Freely Falling Inertial As Seen By Distant Inertial Observer

Click For Summary

Discussion Overview

The discussion revolves around the calculation of time and space dilation for an observer (B) falling toward a planet as perceived by a distant observer (A) in deep space. The scope includes concepts from special relativity (SR) and general relativity (GR) in the context of gravitational effects on spacetime.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant suggests that GR is necessary whenever gravity is involved, which is the case in this scenario.
  • Another participant argues that A's frame of reference (IRF) cannot be considered a global inertial frame due to the presence of gravity, indicating that GR is required for accurate calculations.
  • A later reply emphasizes that it does not make sense to refer to an "SR piece" in this context since the spacetime is not flat near the planet.
  • However, it is noted that far from the planet, in the vicinity of A, SR can be used as a good approximation because the spacetime is approximately flat, but this would not suffice for modeling B's motion as B approaches the planet.

Areas of Agreement / Disagreement

Participants generally agree that GR is necessary for calculations involving gravity, but there is some contention regarding the applicability of SR in different regions of spacetime.

Contextual Notes

The discussion highlights the limitations of using SR in non-flat spacetime and the dependency on the proximity to gravitational sources, but does not resolve the implications of these limitations on the calculations discussed.

1977ub
Messages
530
Reaction score
22
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?
 
Physics news on Phys.org
You need GR anytime gravity is involved which it is in this case.
 
1977ub said:
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.
 
Thanks. so basically it doesn't even make sense to speak of an "SR piece" since the spacetime is not flat there.
 
1977ub said:
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.
 

Similar threads

Graduate Does the EP imply that a free-falling observer follows a geodesic?
  • · Replies 3 ·
Replies
3
Views
1K
Undergrad Non-Inertial Relativistic Dynamics
  • · Replies 18 ·
Replies
18
Views
2K
Undergrad Inertial frames in changing gravitational fields
  • · Replies 66 ·
3
Replies
66
Views
6K
High School Free Fall in Curved Spacetime: Does Minkowski Spacetime Exist?
  • · Replies 21 ·
Replies
21
Views
3K
High School Time Dilation: Accelerating vs Inertial Frame
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Reference systems and apparent counter-explanations on Twin Paradox
  • · Replies 5 ·
Replies
5
Views
2K
High School My paraphrased explanation of the Twin Paradox
  • · Replies 24 ·
Replies
24
Views
5K
High School Eddington Exp vs Newton: Comparing Starlight Deflection
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Twin Paradox Explained: No Math Required
  • · Replies 21 ·
Replies
21
Views
3K
Graduate What Are the Limits of Classical Physics in Understanding Inertial Frames?
  • · Replies 4 ·
Replies
4
Views
3K