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Originally Posted by Nesti
Geez, this conversation is so confusing.
From my understanding, there is no such thing as the speed of gravity. Einstein showed (and someone correct if I'm wrong here), that it is the gravity field (ie the field of acceleration) which propagates outward at c...it does this because it is the metric values which travels outward at c, nothing else, setting up the field as its wake, and the field is maintained in the same manner (remove the origin and the metrics changes at c as it propagates outward). The expression of the field in any part (it's local magnitude and direction) being an attribute o both the field origin (and dissipating and the rate of 'one over 'r' squared), as well as the local energy momentum (the local energy and matter which creates its own field of acceleration and which merges in with the overall field, it too dissipating and the rate of 'one over 'r' squared)...ergo the concept of a 'Test Particle'.
The constancy of light (vacuum) becomes unworkable since spactime becomes sympathetic to light's constancy. We have NO background template, or benchmark, with which make comparison, and since the gravitation field itself is forever in dynamical change, how can we ever hope to make any successful measurement. Have just one photon in an empty field, and there is no field which exists!
Be aware of one of Einstein's most important description of spacetime, it went something like "dimensions are not a vessel in which space-time is contained, yet events are spatially separated"...again "Have just one photon in an empty field, and there is no field which exists!". So, even your measurements WILL affect spacetime and thus your own data and conclusions as you yourself impart a field influence.
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The issue is not the speed of gravity itself but whether changes to the field strength itself result in information of the change being transmitted at the speed of light. This information is conveyed in the form of gravitational waves.
According to GR gravitational waves are transmitted at c, by allowing the components of the metric tensor to vary with time. The mechanism is similiar to altering the strength of a magnetic quadrupole which results in electromagnetic radiation travelling at c.
Regards
Steven