blike Posted August 18, 2002 Share Posted August 18, 2002 This is also a paraphrased version of some paragraphs in the latest scientific american special edition on time. Do space and time exist independently of stars, galaxies and their other contents(substantivalism) or are they merely an artificial device to describe how physical objects are related(relationism)? As John D. Norton of the University of Pittsburgh said "Are they like a canvas onto which and artist paints; they exist whether or not the artist paints on them? Or are they akin to parenthood; there is no parenthood until there are parents and children". This brings up a long-neglected thought experiment of Einstein's. Consider an empty patch of spacetime. Outside this hole the distribution of matter fixes the geometry of spacetime, per the equations of relativity. Inside, however, general covariance lets spacetime take on any of a variety of shapes. In a sense, spacetime behavios like a canvas tent. The tent poles, which represent mater, force the canvas to assume a certain shape. But if you leave out a pole, creating the equivalent of a pole, part of the tent can sag, or bow out, or ripple unpredictably in the wind. The thought experiment poses a dilemme. If the continuum is a thing in its own right (as substantivalism holds), general relativity must be indeterministic--that is, its description of the world must contain an element of randomness. For the theory to be deterministic, spacetime must be a mere fiction(as relationism holds). At first glance, it looks like a victory for relationism. It helps that other theories, such as electromagnetism, are based on symmetries that resemble relationism. But relationism has its own troubles. It is the ultimate source of the problem of forzen time: space may more over time, but if its many shapes are all equivalent(general covariance: which holds that the laws of physics are the same for all observers. Two observers will perceive spacetime to have two different shapes, corresponding to their views of who is moving and what forces are acting. Each shape is a smoothly warped version of the other.), it never truly changes. Moreover, relationism clashes with the substantivalist underpinnings of quantum mechanics. If spacetime has no fixed meaning, how can you make observations as specific places and moments, as quantum mechanics seems to require. Link to comment Share on other sites More sharing options...
MiguelBladesman Posted June 26, 2003 Share Posted June 26, 2003 Blike, The definition of time is a long time puzzler, is it not? Link to comment Share on other sites More sharing options...
alt_f13 Posted August 21, 2003 Share Posted August 21, 2003 If space-time is molded around matter, and somehow we travelled beyond the Universes boundries into empty-space time, would we be creating spacetime as we moved? Would it be moving with us and therefor allow us to move faster than the speed of light? If so, what would we be moving in relation to? Spacetime streatches with accordance to the expansion of the universe, correct? What is causing the universe to expand then, matter moving or the space-time expansion? If space-time is non-existant where matter is not present, where would it begin when matter becomes present? I don't believe you can have an empty pocket of spacetime between two objects, because everything that would allow the objects to coexist with each other would force space-time to exist in a predictable manner between them, according to those objects. For example, light and gravity would need space-time in order to utilize it. Perhaps light (or any other object/force) travelling between objects is the only reason space-time exists between them. Link to comment Share on other sites More sharing options...
KHinfcube22 Posted August 21, 2003 Share Posted August 21, 2003 hmmm.....I think you overused the word space-time.......I made me fall asleep......I agree with blike though..... Link to comment Share on other sites More sharing options...
Loki Posted August 21, 2003 Share Posted August 21, 2003 To be honest, blike, a lot of that went straight over my head. I took a few details from the article in, but I can't say I really understand what I just read. Anyone want to help? It may be just because I'm getting tired and I'm listening to some music quite loud, but those may just be excuses I'm using to not feel so dumb. Link to comment Share on other sites More sharing options...
YT2095 Posted August 21, 2003 Share Posted August 21, 2003 Time, a unit of measurement used to signify change, so I guess if there were nothing to change (no matter). by logic alone, there would be no time. I beleive the same would apply to space. without a frame of reference there can be no distance. so I subscribe to the "relationism" viewpoint. Link to comment Share on other sites More sharing options...
Clown Posted August 21, 2003 Share Posted August 21, 2003 If space-time is molded around matter, and somehow we travelled beyond the Universes boundries into empty-space time, would we be creating spacetime as we moved? Would it be moving with us and therefor allow us to move faster than the speed of light? If so, what would we be moving in relation to? Spacetime does not owe it's existence to matter in the sense of particles, atoms, etc. GR can describe universes where no matter exists at all, but this still doesn't leave room for independent existence of space, because the gravitational field still exists. The motion of an object will at the very least, be in relation to the field. Spacetime streatches with accordance to the expansion of the universe, correct? What is causing the universe to expand then, matter moving or the space-time expansion? The empty space in between galaxies is expanding. Link to comment Share on other sites More sharing options...
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