On Tue, Feb 18, 2014 at 1:28 PM, John Clark <johnkcl...@gmail.com> wrote:
> On Sun, Feb 16, 2014 at 12:54 PM, Edgar L. Owen <edgaro...@att.net> wrote: > >> >> >> You say that "You can tell if spacetime is curved or not by observing >>> if light moves in a straight line or not." and then you say that light does >>> NOT travel in a straight line in the accelerating elevator example you give. >>> >> >> > So, by your terminology, does that mean that the acceleration of the >> elevator IS curving space ? >> > > You should stop talking about "space", it's "4D spacetime"; but yes it's > curved, although if you were inside that sealed elevator you couldn't tell > if the curvature was caused by rockets accelerating the elevator in deep > space or if it was caused by the Earth's gravity. Acceleration is absolute > in that there is no need to look outside your reference frame to detect it, > but according to General Relativity there is no way to tell the difference > between it and being in a gravitational field. > You are simply incorrect here, John. There is no sense in which an observer in an accelerating elevator in the flat spacetime of special relativity could correctly conclude that spacetime has any "curvature"--the fact that light curves relative to a coordinate system where the elevator is at rest is completely irrelevant, since there's no principle of physics that says curved light paths imply curved spacetime. In fact the observer inside the elevator should have ways of measuring curvature if he can measure second-order effects, or if the size of the elevator is taken as non-infinitesimal, and in either case he could definitely conclude that spacetime was *not* curved within an elevator accelerating in flat SR spacetime. The equivalence principle only says there's no way to tell the difference between acceleration and gravity *if* you only look at a first-order approximation to the equations of physics in your region, and *if* your region is infinitesimally small. But in that case there's no way for you to measure spacetime curvature at all, so there's no valid reason for concluding that spacetime in your region is curved. Jesse -- 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 http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
