jbergman said:
The problem with your statement is that the Milne congruence is not an isometry
That's not a problem, it's the point: I was showing that the Milne congruence is indeed not an isometry, because it doesn't satisfy Killing's equation--yet it's a transformation from Minkowski spacetime to Minkowski spacetime, so the metric of the manifold itself remains the same.
jbergman said:
##\mathscr{L}_U g## tells us how the metric changes when we take a small step on our manifold in the direction of the integral curves of ##U##.
But that's the point:
the metric doesn't change. The metric of Minkowski spacetime is the same everywhere. So the fact that ##\mathscr{L}_U g## is nonzero
cannot be telling us that the metric changes, because it doesn't change. So what
is it telling us? What
is changing along integral curves of U? That's the question.
jbergman said:
If it is not zero then we are moving in a direction that the metric changes, i.e. the distance between points changes.
"The distance between points changes" makes sense, yes. But "the metric changes" does
not make sense, because, as above, it doesn't change.
jbergman said:
Really, a Milne congruence is just a coordinate transformation
No, it isn't. It's a perfectly good congruence. The definition I gave is obviously a vector field; it's obviously not any kind of coordinate transformation.
jbergman said:
- The congruence is only onto a subset of Minkowski space.
Many congruences only cover a portion of a particular spacetime. That's not a problem, it's just a limitation that needs to be recognized when the congruence is defined. That's why I specified that the Milne congruence I gave is only defined on the "upper wedge" of Minkowski spacetime.
jbergman said:
- Typically when I can only parameterize a small part of the overall manifold, I consider that a coordinate chart.
This makes no sense; the vector field I gave, as noted above, is not a coordinate chart, it's a vector field. I don't see how your terminology here makes sense.
You can of course define a Milne coordinate chart on the same "wedge" of Minkowski spacetime, in which the vector field U I gave gives the timelike basis vector at each point. But to define a coordinate chart you have to do more than just give the timelike basis vector. A vector field is not the same as a coordinate chart.
jbergman said:
- We have to change the metric.
I did no such thing when I defined U. The Minkowski metric remains the same, as I have already noted several times.
You can of course, as noted above, define a Milne coordinate chart, in which the metric takes a different form--but it's still the metric of flat Minkowski spacetime, as you can easily verify by computing its Riemann tensor and seeing that it vanishes.
jbergman said:
- Again, I think it's more helpful to think that there is an intrinsic manifold ##M## and the consider the auto-isomorphisms of ##M##. Things like KVFs if we are interested in the smooth symmetries. Then something like the Milne coordinates is just a different coordinate representation.
I see what you mean here, but I still think your viewpoint has issues, as I noted above.