The discussion revolves around the concept of time in the context of the Big Bang and cosmological models. Participants explore the implications of relativity on time measurement and the various frames of reference used in cosmology.
Participants express differing views on the nature of time in cosmology, with some supporting the idea of absolute cosmological time in specific models, while others emphasize the relative nature of time and the appropriateness of various reference frames. The discussion remains unresolved regarding the implications of these differing perspectives.
Participants highlight limitations in understanding time due to the dependence on specific definitions and the complexities of general relativity, which may not provide a single frame of reference applicable to all scenarios.
komodekork said:When physicist talk about time after the big bang, what do they mean? Time is relative, so which frame of reference are they talking about?
Could anyone please explain?
komodekork said:When physicist talk about time after the big bang, what do they mean? Time is relative, so which frame of reference are they talking about?
Could anyone please explain?
cosmik debris said:... cosmological time in some theoretical spacetimes is like an absolute cosmological time. I know this contradicts relativity ...QUOTE]
Uh ... how is that? Relatively has to do with the relative motion of two bodies. How does time since the big bang have anything to do with that?
Thanks,
Paul
In general, there are no frames of reference in GR that cover all of space-time (like they do in SR). However, one can usually come up with coordinate systems that cover all of space-time. Cosmologists often use a coordinate system wherein the CMB is at rest. So, the amount of time you mentioned is the amount of proper time elapsed since the big bang in the http://en.wikipedia.org/wiki/Synchronous_coordinates" in which the CMB, ideally, is a purely spacelike hypersurface.komodekork said:Time is relative, so which frame of reference are they talking about?
No it doesn't. It has to do with reference frames. In general relativity, all reference frames are equally appropriate -- the Lorentz symmetry enjoyed by inertial observers is no longer a preferred symmetry. What cosmik is saying, however, is that some spacetimes possesses sufficient symmetry to define a global time variable (just like one does for inertial frames in special relativity). In these spacetimes, this time variable is a 'natural' choice to run a clock by. In our FRW universe, observers comoving with the expansion (at reast wrt the CMB) are in one such frame. We are (very closely) comoving observers, and so this is the clock used by modern cosmologists. But this time is still no better than any other -- one can choose whichever frame one wishes; some are simply more convenient than others. (To see what I mean by convenience, imagine using galaxy-centric coordinates to plan satellite trajectories around the Earth -- obviously one should use geocentric coordinates for such a feat.)phinds said:Uh ... how is that? Relatively has to do with the relative motion of two bodies. How does time since the big bang have anything to do with that?
bapowell said:No it doesn't. It has to do with reference frames. In general relativity, all reference frames are equally appropriate -- the Lorentz symmetry enjoyed by inertial observers is no longer a preferred symmetry. What cosmik is saying, however, is that some spacetimes possesses sufficient symmetry to define a global time variable (just like one does for inertial frames in special relativity). In these spacetimes, this time variable is a 'natural' choice to run a clock by. In our FRW universe, observers comoving with the expansion (at reast wrt the CMB) are in one such frame. We are (very closely) comoving observers, and so this is the clock used by modern cosmologists. But this time is still no better than any other -- one can choose whichever frame one wishes; some are simply more convenient than others. (To see what I mean by convenience, imagine using galaxy-centric coordinates to plan satellite trajectories around the Earth -- obviously one should use geocentric coordinates for such a feat.)