On 4/9/2019 7:52 AM, agrayson2...@gmail.com wrote:
On Monday, April 8, 2019 at 11:16:25 PM UTC-6, agrays...@gmail.com wrote:
In GR, is there a distinction between coordinate systems and
frames of reference? AG
Here's the problem; there's a GR expert known to some members of this
list, who claims GR does NOT distinguish coordinate systems from
frames of reference. He also claims that given an arbitrary coordinate
system on a manifold, and any given point in space-time, it's possible
to find a transformation from the given coordinate system (and using
Einstein's Equivalence Principle), to another coordinate system which
is locally flat at the arbitrarily given point in space-time. This
implies that a test particle is in free fall at that point in
space-time. But how can changing labels on space-time points, change
the physical properties of a test particle at some arbitrarily chosen
point in space-time? I believe that such a transformation implies a
DIFFERENT frame of reference, in motion, possibly accelerated, from
the original frame or coordinate system. Am I correct? TIA, AG
You're right that a coordinate system is just a function for labeling
points and, while is may make the equations messy or simple, it doesn't
change the physics.?? If you have two different coordinate systems the
transformation between them may be arbitrarily complicated.?? But your
last sentence referring to motion as distinguishing a coordinate
transform from a reference frame seems to have slipped into a 3D
picture.?? In a 4D spacetime, block universe there's no difference
between an accelerated reference frame and one defined by coordinates
that are not geodesic.
Brent
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