<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'> Didn't Hawking reverse his stance on baby universes? I'd like to think I'm fairly familiar with gravitational black holes on the macroscopic scale, but their relationship with quantum mechanics is a bit beyond me. Anyway, thanks for the reference, but my request was more of a skeptical approach to the statement, "I've heard from some theoretical physicists that if you could create a universe identical to ours in a laboratory, it would expand into the size of our universe without actually taking up any space in our universe!" (emphasis mine), made by Weeman in a previous post.I could be wrong here, but I highly doubt Hawking's description of baby universes via black holes was intended to describe universes that could support life, much less, stars and planets. <br />Posted by derekmcd</DIV></p><p>I'm not sure whether or not Hawking reversed his stance on baby universes or whetheer in fact he ever really took a stance at all. He has certainly changed his thoughts on a number of topics. What I got out of his essay was a speculation on a mechanism involving the evaporation of black holes via so-called Hawking radiation and a further speculation on other universes. In any case he raised the possibility of something containing matter, potentially quite a bit of matter in the case of a massive black hole, with that something being cut off from our universe. Now, whether or not that universe could support life, stars, planets etc. I would think is open to debate. It seems to me that all that is needed in principle is sufficient mass/energy. Given the potential size of black holes, I think one might make an argument that there could be enough. In any case this stuff is pure speculation. The real point is that the universe, by definition, is all that we have to work with and any separate universe is indeed completely separate and non-interacting with our own universe. If it interacts with our universe, then it is a part of it. Lacking interaction, the question of the existence of a separate universe is a rather moot question. If it were to exist in any sense of course it would up no space in our universe, since if it did it would be a part of our universe and not some other universe. As to whether or not it would be "the size of our", I do not see how one can even formulate such a question since one could not take a yardstick from our universe into another one. Size measurements in a universe are meaninful only within that universe -- as are other physical questions.</p><p>This is starting to get into philosophy and metaphysics with the regard to the question of what it means for something to exist. Let philosophers debate that one, along with the number of angels that can dance on the head of a pin. I don't remembe who said this, but the quote remains valid: " There is no position so silly that some philosopher has not taken it."</p><p>This sort of stuff is really out on the fringe and, while great fun to think about, is not settled science, if it is science at all. We don't have a theory of quantum gravity, and therefore we don't know how to analyze phenomena that depend in a serious way on both general relativity and quantum theory. There are real, serious questions to be answered that we just don't have the tools to address. Even in areas that some consider to be settled there are difficulties. For instance, for purposes of calculation, quantum electrodynamics is a great success. That success, however, depends on a step in the calculation called "renormalization". The renormalization procedure involves performing a perturbation calculation and then discarding some of the terms. Once those terms have been discarded the results have proven to be astoundingly close to experimental measurements. On the surface this is really great, and the physicists love it. The kicker, however, is that the terms that are discarded are INFINITE, and no one has been able to provide a rigorous mathematical treatment of the renormalization process -- or even to clearly define it in mathematical terms.</p><p>While physicists are generally comfortable (I think too comfortable) with the ad hoc procedure of renormalization, a great deal remains to be understood. </p><p>"There was a time when the newspapers said that only twelve men understood the theory of relativity. I do not believe that there ever was such a time. There might have been a time when only one man did, before he wrote his paper. But after people read the paper, a lot of people understood the theory of relativity in some way or other, certainly more than twelve. On the other hand, I can safely say that nobody understands quantum mechanics." -- Richard Feynman in The Character of Physical Law.<br /></p> <div class="Discussion_UserSignature"> </div>
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