cianfa72
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Maybe a bit off topic: assuming a spacetime with Minkowski metric structure implies an invariant two-way speed of light ?
Ok, of course the time interval reading of a physical clock between the start and the end of the round-trip light journey is invariant since they are along the clock worldline. What about the total distance traveled in the light round-trip journey?Ibix said:Assuming you mean a flat spacetime, or only care about small enough regions that you can neglect curvature, yes.
this above refers to the "small frame around the worldline to which ##V^a## is tangent". The orthogonality condition for this "small frame" is w.r.t. the underlying Minkowski structure/pseudo-inner product assumed by hypothesis.Ibix said:To actually realize this measurement you would need to construct a small frame around the worldline to which ##V^a## is tangent, and the orthogonality requirement makes this Einstein synchronised. If you use a different synchronisation you can get a different speed.
Sorry, not sure to get your point. From last part above it seems that to recover the fact that light two-way speed is invariant with value ##c##, an inertial frame (albeit local) is actually implied.Ibix said:If you mean the coordinate velocity then you can pick things like extremely rapidly rotating coordinates, where light follows an arbitrarily tight spiral path, and get an arbitrarily slow speed. Over a sufficiently small region, of course, the curved coordinates are well approximated by the usual inertial coordinates and you recover ##c## as the two-way speed and whatever your synchronisation convention says for the one-way speed.
Yes. Over large distances in non-inertial coordinate systems you can get different values of the coordinate speed. The limit as you consider shorter and shorter distances is ##c##, however, since on a sufficiently small scale all coordinate systems can be approximated by a flat (possibly non-orthogonal) coordinate system.cianfa72 said:From last part above it seems that to recover the fact that light two-way speed is invariant with value , an inertial frame (albeit local) is actually implied.
Therefore the claim: the two-way speed of light is the invariant ##c## (defined as the ratio between 2 times the traveled distance and the round-trip as measured by a single clock) strictly holds true only in inertial frames.Ibix said:Yes. Over large distances in non-inertial coordinate systems you can get different values of the coordinate speed. The limit as you consider shorter and shorter distances is ##c##, however, since on a sufficiently small scale all coordinate systems can be approximated by a flat (possibly non-orthogonal) coordinate system.
To be more precise, I would add to the last bullet that the interpretation makes correspondence/maps the concept of curvature of a timelike worldline at any point along it (as given by a formula in the chart) to the proper acceleration measured by a (physical) accelerometer attached to the body traveling along the worldline.cianfa72 said:Said that, suppose to assign Minkowski metric structure to spacetime. In 2D this just means there exists a global reference frame (aka a global coordinate chart) such that the invariant ##ds^2## can be written as $$ds^2 = dt^2 - dx^2$$ To get a "mapping/correspondence" between the mathematical model and physics we need an interpretation. Such interpretation establishes that:
- ##ds=0## paths are paths the light travels on (they define geometrically light cones at any point/event)
- physical particles/bodies having mass follow timelike paths through spacetime
- physics clocks measure the spacetime lenght along their (timelike) worldlines
- acceleration measured by an accelerometer attached to a body (i.e. its proper acceleration) corresponds to an invariant expression in the mathematical model (i.e. has the same form regardless the chart chosen)
I think that's too strong. Rindler coordinates are invariant under boost too, so ought to produce an invariant two-way speed.cianfa72 said:Therefore the claim: the two-way speed of light is the invariant ##c## (defined as the ratio between 2 times the traveled distance and the round-trip as measured by a single clock) strictly holds true only in inertial frames.
In this context, I think, invariant just means that the ratio between 2 times the traveled distance and the round-trip travel time is always the same. In case of Rindler coordinates ##(\chi, \xi)## (over the underlying flat Minkowski spacetime) that means a light pulse from the coordinate origin ##(0,0)## to the event ##(\chi_0,\xi_0)## and back will take an amount of proper time ##\tau## -as measured by a physical clock in the origin- such that the ratio $$\frac {2\xi_0} {\tau}= c$$ doesn't change from an experiment run to the next.Ibix said:I think that's too strong. Rindler coordinates are invariant under boost too, so ought to produce an invariant two-way speed.
More generally, I would say that if you're using non-trivial coordinates you shouldn't rely on anything that isn't expressed in tensor form.
Do you refert to the transformation from a Rindler chart to another Rindler chart?Ibix said:I think that's too strong. Rindler coordinates are invariant under boost too, so ought to produce an invariant two-way speed.
Sorry, do you refer to both my previous posts #12 and #13 ?Ibix said:Yes
I believe what you said above also extends to GR as well (notions of null vector ##N^a##, four velocity ##V^a## defined on tangent space at any event are the same in SR as in GR). So one can take that ratio as definition of (relative) velocity and therefore such a ratio is, by definition, coordinate independent.Ibix said:It depends what you mean by the speed of light.
For any null vector ##N^a##, the inner product with any four velocity ##V^a## is ##N^aV_a## and the component orthogonal to ##V^a## is ##N^a-V^aN^bV_b## (assuming +---) which has magnitude ##\sqrt{|N^aN_a-2N^aV_aN^bV_b+V^aV_a(N^bV_b)^2|}=N^aV_a##. The velocity is the ratio of these and the result is obviously 1, and this is coordinate independent.