Are My Calculations for Proper Time on a Rocket in General Relativity Correct?

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Discussion Overview

The discussion revolves around the calculations of proper time for a rocket in the context of General Relativity, specifically using the Schwarzschild solution. Participants explore the differences in proper time experienced by observers on the rocket and on the planet, as well as the implications of energy calculations in this scenario.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant presents calculations for proper time on both the rocket and the planet, using specific values for Schwarzschild radius, planet surface, and rocket velocity.
  • Another participant suggests an integral approach to calculate proper time for the rocket, indicating that it can be expressed in terms of energy without needing to know the apogee.
  • A third participant references a formula from Choquet-Bruhat's book, asserting its correctness but expressing confusion over the results obtained.
  • Further clarification is provided on expressing coordinate time in terms of energy, reiterating the integral approach without needing the apogee.
  • A participant expresses frustration over the lack of responses to their initial question, indicating a desire for further discussion or clarification.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the calculations or the application of the formulas. There are multiple approaches and interpretations presented, with some participants questioning the results and others providing alternative methods without resolving the discrepancies.

Contextual Notes

There are unresolved issues regarding the assumptions made in the calculations, particularly concerning the application of energy in the integrals and the interpretation of proper time in relation to coordinate time. The discussion highlights the complexity of the calculations and the potential for differing results based on the methods used.

Passionflower
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Assume we are on a non rotating planet and shoot a rocket straight with a given velocity (smaller than the escape velocity) and we want to calculate using the Schwarzschild solution the coordinate and proper time, both for an observer on the planet and rocket, between lift off and return to the planet.

Let's take a very basic (but of course unrealistic) example:

The Schwarzschild radius rs = 1
The surface of the planet rp = 2
The velocity of the rocket v = 0.5

Here are my results:
For the apogee of the rocket I calculate: r=3
For the coordinate time I calculate: 14.90142209
For the proper time of a clock on the planet surface I calculate: 10.53689661
For the proper time of a clock on the rocket I calculate: 11.29502440

The question in this topic is about the approach used in:
"General Relativity and the Einstein Equations"
Choquet-Bruhat - Oxford, 2009

In chapter 9, page 87 we have:

http://img42.imageshack.us/img42/9963/formulae.png

She uses the energy to calculate the proper time of a clock on the rocket. But I do not seem to get it to work.

If I calculate the Energy for the rocket for an observer at infinity I get: 0.8164965809

Since the rocket is free falling this energy must be a constant in the integral.

When solving this integral for the proper time on the rocket with this energy I get 9.125220141 which is less than what I calculate for the proper time on a clock on the surface of the planet.

Where do I go wrong or misunderstand?
 
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We can calculate the proper time for the rocket using:
\Large \int _{{\it r_i}}^{{\it r_o}}\!{\frac {1}{\sqrt {{\frac {{\it r_s}}{r}}-{<br /> \frac {{\it r_s}}{{\it r_0}}}}}}{dr}
We can also express it in terms of energy and then we do not need the apogee at all:
\Large \int _{{\it r_i}}^{{\it r_o}}{\frac {1}{\sqrt {{E}^{2}-1+{\frac {{\it r_s}}{r}}}}}{dr}
(rs is the Schwarzschild radius and r0 is the apogee of the rocket and ri and ro are the r-coordinate ranges)

After multiplying by two both give a result of 11.29502440
 
The formula in Choquet-Bruhat's book uses this principle:
\Large {\frac {dt}{d\tau}}=E \left( 1-{\frac {{\it r_s}}{r}} \right) ^{-1}<br />
Which is correct as it is widely referenced in the literature.

But somehow it does not add up.
 
Passionflower said:
We can also express it in terms of energy and then we do not need the apogee at all:
\Large \int _{{\it r_i}}^{{\it r_o}}{\frac {1}{\sqrt {{E}^{2}-1+{\frac {{\it r_s}}{r}}}}}{dr}
And coordinate time can also be expressed in terms of energy:
\Large \int _{{\it ri}}^{{\it ro}}E \left( 1-{\frac {{\it rs}}{r}} \right) ^{-1}{\frac {1}{\sqrt {{E}^{2}-1+{\frac {{\it rs}}{r}}}}}{dr}
 
No comments to my question?
 

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