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We are trying to find the relative velocity in a Lemaitre observer's frame, when said observer passes an observer dropped from rest at Painleve radius [itex]r_0[/itex]. I have searched for related previous posts, found some, but none that clarified this particular situation.
Let the passing happens at [itex]0 < r <= r_0[/itex]. We used Painleve coordinates, since this reaches the "interior" of the black hole. Since the relativistic velocity addition does not seem to work for "inside" velocities, exceeding c, we have tried proper velocities and rapidity. We would appreciate feedback as to the validity of the approach (or where we went of the rails).
The Lemaitre observer measures static (shell) observers passing him at proper velocity
[tex]w_{Lem(r)} = dr/d\tau =\sqrt{r_s/r}[/tex]
where [itex]r_s = 2GM[/itex], the event horizon (Schwarzschild) radius.
His constant mechanical energy is [itex]E/m = 1[/itex]. The observer dropped from [itex]r_0[/itex] has a constant [itex]E_0/m = \sqrt{1-r_s/r_0}[/itex] and she measures static (shell) observers passing her at proper velocity
[tex]w_{drop(r)} = \sqrt{E_0^2-(1-r_s/r)} =\sqrt {{\frac {r_s}{r}}-{\frac {r_s}{r_0}}}[/tex]
Both [itex]w[/itex] are valid for any [itex]r \le r_0[/itex], except for [itex]r \le 0[/itex].
Now the question is: can we convert these two [itex]w[/itex] to rapidity [itex]\eta(w) = \sinh^{-1}{(w)}[/itex], subtract them for "relative rapidity" and then get a relative proper velocity by taking the [itex]\sinh[/itex] of that difference?
We have done this and there seems to be no "blow-ups" at or inside the horizon. This graph resulted:
https://www.physicsforums.com/attachment.php?attachmentid=65884&d=1390302266
The observers are dropped from R=2.5 and 1.1, with the bright red and green curves their respective (proper) speeds relative to the Lemaitre observer, at the instant his passing them. The light green and amber curves are their proper speeds on the Painleve chart.
Looks pretty, but is the method valid?
-J
Let the passing happens at [itex]0 < r <= r_0[/itex]. We used Painleve coordinates, since this reaches the "interior" of the black hole. Since the relativistic velocity addition does not seem to work for "inside" velocities, exceeding c, we have tried proper velocities and rapidity. We would appreciate feedback as to the validity of the approach (or where we went of the rails).
The Lemaitre observer measures static (shell) observers passing him at proper velocity
[tex]w_{Lem(r)} = dr/d\tau =\sqrt{r_s/r}[/tex]
where [itex]r_s = 2GM[/itex], the event horizon (Schwarzschild) radius.
His constant mechanical energy is [itex]E/m = 1[/itex]. The observer dropped from [itex]r_0[/itex] has a constant [itex]E_0/m = \sqrt{1-r_s/r_0}[/itex] and she measures static (shell) observers passing her at proper velocity
[tex]w_{drop(r)} = \sqrt{E_0^2-(1-r_s/r)} =\sqrt {{\frac {r_s}{r}}-{\frac {r_s}{r_0}}}[/tex]
Both [itex]w[/itex] are valid for any [itex]r \le r_0[/itex], except for [itex]r \le 0[/itex].
Now the question is: can we convert these two [itex]w[/itex] to rapidity [itex]\eta(w) = \sinh^{-1}{(w)}[/itex], subtract them for "relative rapidity" and then get a relative proper velocity by taking the [itex]\sinh[/itex] of that difference?
We have done this and there seems to be no "blow-ups" at or inside the horizon. This graph resulted:
https://www.physicsforums.com/attachment.php?attachmentid=65884&d=1390302266
The observers are dropped from R=2.5 and 1.1, with the bright red and green curves their respective (proper) speeds relative to the Lemaitre observer, at the instant his passing them. The light green and amber curves are their proper speeds on the Painleve chart.
Looks pretty, but is the method valid?
-J
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