On Friday, March 9, 2018 at 7:28:58 PM UTC-6, agrays...@gmail.com wrote: > > > > On Friday, March 9, 2018 at 12:51:29 PM UTC-5, Lawrence Crowell wrote: >> >> >> >> I am not exactly sure why you are stuck on the idea that the advance of >> time causes motion. >> > > > *Because in the absence of a force causing non-geodesic motion, the > increase in time results in a change in spatial position since in the > equations of motion, spatial coordinates are not independent variables; > they depend on time IIUC. AG* >

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The metric defines the arc length and this is identified with the proper time. The interval s is then s = cτ, where τ is time measurement of this arc length. The metric defines the interval ds^2 = g_{ab}dx^adx^b as a fundamental invariant. In effect all this says is we are all moving forwards along this parametrized interval at the speed of light. What we call motion or dynamics is the relative change in a vector separating two test masses. The geodesic equation dU^a/ds = Γ^a_{bc}U^bU^c is a connection based equation of motion. It really only makes sense from the perspective of a distant observere far removed from a gravity field where spacetime is approximately flat. However, we can look at this according to a variation in this to define relative motion of two nearby masses. This means we have δdU^a/ds = δΓ^a_{bc}U^bU^c. where δU^a = V^a for V^a the spacetime vector separating the two test masses and δΓ^a_{bc} = R^a_{bcd}v^d, with V = dv/ds. We now have the geodesic deviation equation dV^a/ds = R^a_{bcd}U^bU^cv^d, which is the general equation for the relative dynamics between two masses. We can compare this to Newton's laws. For a weak gravity field the coordinate time and the proper time are close enough so that ∂t/∂s = V^t ≈ 1 and for V^r = dr/ds ≈ dr/dt, which means as well v^r = r. The Riemannian curvature component most relevant, or highest order in the speed of light is R^r_{trt} = GM/r^3 so that d^2r/ds^2 = R^r_{trt} = GM/r^3(∂t/∂s)^2r and with our approximations d^2r/ds^2 ≈ GM/r^2, which recovers Newton's second law of motion with gravitation. LC > > >> In one sense you can say that by being at rest in a frame one is moving >> at T = ct distance along a fourth dimension. But that is not really a cause >> for motion in spatial directions. >> >> LC >> > -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.