From: Dan Asimov <dasimov@earthlink.net> To: math-fun <math-fun@mailman.xmission.com> Sent: Friday, July 24, 2009 3:40:24 PM Subject: Re: [math-fun] dumb question about general relativity Re time reversal: The fact that in our neighborhood in spacetime we see stars radiating only in positive time -- and all the entropic consequences of this -- is usually attributed to initial conditions, not to any actual "law" of physics. (The Ehrenfest urn model is usually considered a good microcosm of thermodynamics, and shows time reversal symmetry.) It's just a guess, but it seems likely to me that we are aware of only a small or infinitesimal fraction of the universe, and that some other time and place have stars radiating in negative time. [What exactly does this mean? How do you experimentally determine whether your stars are radiating in positive or negative time? -- Gene] (Or is it possible that entropically opposite parts of the universe somehow always end up pinching off from each other by black holes?) --Dan