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## Main Question or Discussion Point

Hi all,

I'm starting to learn a bit of general relativity now, but I'm a bit confused as to the measurement of speed.

Let's say we use the Schwarzschild metric.

Now, I can parametrize the wordline of a certain object by giving a

parametrisation (t(tau),r(tau),theta(tau),phi(tau)), where tau is the proper time of

that object. Now, I would like to know how to measure the speed of the object;

that is, the magnitude of its 3-velocity. I can find the 4-velocity, but can I deduce in a meaningful way the speed, i.e. as observed by an observer 'at rest' in the frame

(of course, 'at rest' is a relative concept - I'm referring to the observer at infinity, where the Schwarzschild metric is more or less flat)?

From special relativity, I recall that you could write the 4-velocity

as (gamma,gamma*V) where gamma is the usual gamma factor and V is the three-velocity.

I'm looking for something analogous - if that's meaningful.

Thanks,

APhysicist

I'm starting to learn a bit of general relativity now, but I'm a bit confused as to the measurement of speed.

Let's say we use the Schwarzschild metric.

Now, I can parametrize the wordline of a certain object by giving a

parametrisation (t(tau),r(tau),theta(tau),phi(tau)), where tau is the proper time of

that object. Now, I would like to know how to measure the speed of the object;

that is, the magnitude of its 3-velocity. I can find the 4-velocity, but can I deduce in a meaningful way the speed, i.e. as observed by an observer 'at rest' in the frame

(of course, 'at rest' is a relative concept - I'm referring to the observer at infinity, where the Schwarzschild metric is more or less flat)?

From special relativity, I recall that you could write the 4-velocity

as (gamma,gamma*V) where gamma is the usual gamma factor and V is the three-velocity.

I'm looking for something analogous - if that's meaningful.

Thanks,

APhysicist