ellis@spar.UUCP (Michael Ellis) (04/30/85)
>>> This would leave me with one of two conclusions; either >>> 1- the universe is infinitely old, or >>> 2- there was a cause which is not of this universe that started >>> the whole thing. Why not the third possibility: 3- the universe has finite age, but no first moment. It may be distasteful to some, but need not contradict the assumption that each moment has a predecessor. It's extremely difficult for the vast majority of humans (me included) to hypothesize the difficulties of such problems. Anyone who's ever checked out texts on general relativity is aware of how bizarre and unintuitive regions near black (or white, I suppose, in this case) holes can be. Following a timelike path backwards towards the initial discontinuity would be analogous to asymptotically approaching an unreachable point, assuming you could go backwards thru time in the first place. Can anyone out there who understands general relativity comment? Is there such a thing as a person who understands general relativity? -michael
gwyn@brl-tgr.ARPA (Doug Gwyn <gwyn>) (05/02/85)
> 3- the universe has finite age, but no first moment. Not possible in any meaningful sense. > Following a timelike path backwards towards the initial discontinuity > would be analogous to asymptotically approaching an unreachable point, If the "time" you are talking about is a parameter along your infinite path, then you are saying that it extends from -infinity. > assuming you could go backwards thru time in the first place. No need to "go" anywhere; we're talking about matters of description, not action. Please remember that "time" in relativity is a coordinate and does not have a global invariant meaning. In general there is no absolute "time" one can apply to the entire cosmos (although some cosmologies happen to have one). > Is there such a thing as a person who understands general relativity? Sure, why not?
john@x.UUCP (John Woods) (05/03/85)
> Why not the third possibility: > > 3- the universe has finite age, but no first moment. > > It may be distasteful to some, but need not contradict the assumption > that each moment has a predecessor. It's extremely difficult for > the vast majority of humans (me included) to hypothesize the > difficulties of such problems. Anyone who's ever checked out > texts on general relativity is aware of how bizarre and unintuitive > regions near black (or white, I suppose, in this case) holes can be. > > Following a timelike path backwards towards the initial discontinuity > would be analogous to asymptotically approaching an unreachable point, > assuming you could go backwards thru time in the first place. > > Can anyone out there who understands general relativity comment? > > Is there such a thing as a person who understands general relativity? > > -michael > I don't completely understand general relativity (having not taken the right Physics electives at MIT), but I have here a paper by Dr. Stephen W. Hawking of the University of Cambridge, who does understand general relativity ("and a whole lot more!"). The title of the paper is "The Edge of Spacetime". The paper talks mostly about Friedmann's model of the expanding universe, and how most of the ways of solving it come up with singularities at T=0; ie, an "edge". The current search for solutions not involving a singularity involve using quantum mechanics. I will quote the last three paragraphs, for your enjoyment; they don't answer the question, but they at least talk about it. "In the classical General Theory of Relativity, which does not incorporate the Uncertainty Principle, the initial state of the universe is a point of infinite density. It is very difficult to define what the boundary conditions of the universe should be at such a singularity. However, when quantum mechanics is taken into account, there is the possibility that the singularity may be smeared out and thus space and time together may forma closed four- dimensional surface without boundary or edge, like the surface of the Earth but with two extra dimensions. This would mean that the universe was completely self-contained: it would not have any exterior infinite region and it would not contain any singularities at which the Laws of Physics would break down. One could say that the boundary conditions of the universe are that it has no boundary. "It should be emphasized that this is simply a _proposal_ for the boundary conditions of the universe [emphasis in the original -jfw]. One cannot deduce them from some other principle but one can merely pick a reasonable set of boundary conditions, calculate what they predict for the present state of the universe and see which agrees with observations. The calculations are very difficult and have been carried out so far only in simple models with a high degree of symmetry. However, the results are very encouraging. They predict that the universe must have started out in a fairly smooth and uniform state. It would have undergone a period of what is called exponential or "inflationary" expansion during which its size would have increased by a very large factor but the density would have remained the same. The universe would then have become very hot and would have expanded to the state that we see it today, cooling as it expanded. It would be uniform and the same in every direction on very large scales but would contain local irregularities that would develop into stars and galaxies. "What happened at the beginning of the expansion of the universe? Did spacetime have an edge at the Big Bang? The answer is that if the boundary conditions of the universe are that it has no boundary, time ceases to be well-defined in the very early universe just as the direction "North" ceases to be well-defined at the North Pole of the Earth. Asking what happens before the Big Bang is like asking for a point one mile North of the North Pole. The quantity we measure as time had a beginning but that does not mean spacetime has an edge, just as the surface of the Earth does not have an edge at the North Pole." I don't know if I can distribute this paper; he handed out several copies when he visited CRDS last year, but I have no idea of his opinions of further distribution of his papers (he also handed out an interesting paper entitled "The Unification of Physics" on GUTs). His address, according to these papers, is S. W. Hawking University of Cambridge Department of Applied Mathematics and Theoretical Physics Silver Street, Cambridge, England, CB 3 9EW. Or, find any and every book by him and buy it -- I, at least, find Dr. Hawking's writing very enjoyable. -- John Woods, Charles River Data Systems, Framingham MA, (617) 626-1101 ...!decvax!frog!john, ...!mit-eddie!jfw, jfw%mit-ccc@MIT-XX.ARPA "MU" said the Sacred Chao...