On 9 February 2014 04:36, Edgar L. Owen <edgaro...@att.net> wrote: > Brent, and Liz, > > We have to be careful in our choice of words here. > > It is quite clear that e.g. during relative motion of frames A and B, that > each sees the other's clock running slower. So the two frames DO NOT give > the same results here. > > However when one twin returns with a different clock time and STOPS both > twins agree on the resulting different clock times. > > Relativity says this is due to the acceleration of the traveling twin. But > my question is 'acceleration relative to WHAT?' > > The very notion of acceleration (including that of Newton's bucket) > assumes there is an absolute background space in some sense that > acceleration is relative to. >
I believe Newton's bucket is normally taken to rotate relative to distant masses - i.e. it obeys Mach's Principle. But imagine a bucket rotating in space, in a universe with dark energy pushing everything else away faster and faster. Eventually there is nothing within the bucket's event horizon, it's alone in an empty universe. Does the water in the bucket suddenly (or gradually) stop forming a paraboloid as mass disappears, and become flat? (I'd say not, but I could be wrong...) We're assuming no friction, so the bucket rotates until frame dragging slows it down, far into the future. Or we could make it a neutron star if we prefer. Does it stop being an oblate spheroid when the last mass vanishes over its event horizon? Hm. Interesting. I hadn't thought about this before, but it seems to me this particular thought experiment indicates that there *is* some background spacetime framework, because how can the bucket "know" that distant masses - that it can still "see" (I think) - have crossed its cosmic horizon? Maybe someone with a better grasp of cosmology can comment on this one, and the implications (if any). -- 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.
