starthaus said:
No, Dalespam and I have worked on deriving the proper (not coordinate) acceleration at apogee by using covariant derivatives. I used the same method as one of the ways to calculate proper acceleration in rotating frames.
For the limited context yes. For the general case , no.
This is the problem with cutting and pasting from websites instead of deriving.
Well, at least now you know how to derive the general formula for coordinate acceleration, correct?
You also know not to try to get the proper acceleration by multiplying the coordinate acceleration by \gamma^3, correct?
And you also know how to use covariant derivatives to define "proper acceleration" in the context of GR from now on, correct?
In the page 39 of the
http://books.google.com/books?id=sl...e fall vanishes&pg=PA39#v=onepage&q&f=false" "General relativity and the Einstein equations" by Yvonne Choquet-Bruhat, we can see that author says:
"...It is always possible to choose local coordinates such that Christoffel symbols vanish at that point; gravity and relative accelerations are then, at that point, exactly balanced. It is even possible to choose local coordinates along one given geodesic*: astronauts have made popular the fact that in free-fall one feels neither acceleration nor gravity; in a small enough neighborhood of a geodesic the relative accelerations of objects in free-fall are approximately zero. "
This is strongly confirmed in some other book, Papapetrou's Lectures on GR, p. 57, as he puts it this way:
"In a Riemannian space we can always make the Christoffel symbols vanish on a
given curve by an appropriate coordinate transformation. According to the principle
of equivalence this property of the Riemannian space should mean physically that the
sum of the inertial and the gravitational acceleration** can be made equal to zero on the
given curve. This is really the case, as we see at once if we consider a freely falling lift
or a freely moving non-rotating artificial satellite: An observer inside the satellite,
using the frame which is connected rigidly with the satellite - a comoving frame -,
will observe neither inertial non gravitational accelerations."
By all of this stuff we are to back up the idea that proper acceleration is zero along any geodesic and what you're taking as a cheap guess at the riddle "what is proper acceleration!?" leads to nowhere but to where you can possibly get stuck in the mud of nonsense beliefs. Mathematically, these quotes are hinting at the fact that in the geodesic equation,
\frac{d^2x^a}{ds^2}+\Gamma^{a}_{bc}\frac{dx^b}{ds}\frac{dx^c}{ds}=0
the first term \frac{d^2x^a}{ds^2} represents the
total acceleration due to both gravity and inertial forces, not as that nonsense in your article stands for "the proper acceleration". It is apparent that if \Gamma^{a}_{bc}=0, then this total acceleration vanishes, leading to the obvious fact that we don't have any kind of proper acceleration felt by the freely falling object because now the first term is simply the proper acceleration of freely falling object as in Minkowski spacetime. So applying a Fermi-like transformation or using Riemann normal coordinates to make the Christoffel symbols vanish is just a clue on the reduction of the total proper acceleration to the famous term d^2x^a/ds^2=0 in SR. Without this transformation, the two terms in geodesic equation have to cancel out each other in order for the proper acceleration to vanish.
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*This kind of coordinate system is called "Fermi normal coordinates" that gives the coordinate conditions by which the metric tensor is to be equivalent to \eta_{\mu\nu} along an entire geodesic. In fact, this coordinate system would make the local-flatness happen to exist along a geodesic instead of forcing us to believe the old cliche of local-flatness occurring only exactly at some given point and approximately in the vicinity of it (Riemann normal coordinates). For a complete discussion of this along any time-like geodesic see, for example, An advanced course in GR by E. Poisson, p. 14.
** Earlier on the page 56, Papapetrou makes it fully clear what role Christoffel symbols are exactly playing in the geodesic equations:
"More exactly, the Christoffel symbols will now describe, according to the principle of equivalence, the sum of the inertial and the gravitational acceleration."