hewj11 Posted July 1, 2010 Share Posted July 1, 2010 In reverse? The energy that the apple transferred to the ground and dissipated instead converges and transfers to the apple, tossing it up onto the table. Where, you'll notice, it stays. This will look odd, to say the least. But not because gravity is working differently - the tossed apple still follows a parabolic arc and falls back down as normal onto the table. It's odd because it is a spontaneous decrease in entropy, which is not time symmetrical. conservation of energy? Link to comment Share on other sites More sharing options...
Johanluus Posted July 1, 2010 Share Posted July 1, 2010 Time cannot run "backwards" but a set of equations can be time invarient.Surely there is a difference? Link to comment Share on other sites More sharing options...
Sisyphus Posted July 1, 2010 Share Posted July 1, 2010 conservation of energy? What about it? Link to comment Share on other sites More sharing options...
hewj11 Posted July 2, 2010 Share Posted July 2, 2010 What about it? seems no matter which way you run the event energy is conservered and time is irrelevant. Follows the parabolic arc that you mentioned Link to comment Share on other sites More sharing options...
eoinmac Posted July 5, 2010 Share Posted July 5, 2010 sisyphus- what do you mean by time symmetrical? Link to comment Share on other sites More sharing options...
Sisyphus Posted July 5, 2010 Share Posted July 5, 2010 seems no matter which way you run the event energy is conservered and time is irrelevant. Follows the parabolic arc that you mentioned Ok, yes. Energy is always conserved, regardless of the direction of time. sisyphus- what do you mean by time symmetrical? I mean that the entropy is always less in the past and greater in the future, i.e. not "symmetrical" around the present. So if you watch a tape in reverse, gravity will act the same, but entropy will decrease, showing bizarre events like an apple getting spontaneously tossed up onto a table by converging shock waves. Note that such an event is not actually impossible, since all the same laws are in play. It is just astronomically improbable. Entropy is all about probability. Link to comment Share on other sites More sharing options...
Lunkdunk Posted July 12, 2011 Share Posted July 12, 2011 What if the universe did not start from a singularity? Its' gravity would have been too great to allow any expansion. That is why I prefer M brane theory that the universe began with a collision of higher dimensions from a region of indefinite size to allow expansion. Black holes don't "explode" or expand like a big bang. It could appear that black holes expand. I want to point out a similarity between the expansion of the universe and the black holes. If you imagine a particle closer to the centre of the black hole this particle will have a larger acceleration towards the centre than you have. A particle further away from the centre will have a lower acceleration than you. Considering these particles, they will both appear to move away from you. They will seem to "expand" from your point of view. I'm not a physicist so there could be something fundamentally wrong with my assumptions. Some feedback would be nice Link to comment Share on other sites More sharing options...
csmyth3025 Posted July 12, 2011 Share Posted July 12, 2011 It could appear that black holes expand. I want to point out a similarity between the expansion of the universe and the black holes. If you imagine a particle closer to the center of the black hole this particle will have a larger acceleration toward the center than you have. A particle further away from the center will have a lower acceleration than you. Considering these particles, they will both appear to move away from you. They will seem to "expand" from your point of view. I'm not a physicist so there could be something fundamentally wrong with my assumptions. Some feedback would be nice A black hole is thought to have a central singularity toward which everything within the event horizon moves, regardless of their velocity relative to any other thing within the event horizon. The universe is currently thought to not have a center of any sort. Everything that's not gravitationally bound is moving away from everything else in proportion to the Hubble parameter. As far as I know, light from everything farther away from the black hole central singularity than the observer would be blue shifted and everything closer in would be red shifted. We don't observe this asymmetry in our universe. Chris Link to comment Share on other sites More sharing options...
Lunkdunk Posted July 12, 2011 Share Posted July 12, 2011 (edited) A black hole is thought to have a central singularity toward which everything within the event horizon moves, regardless of their velocity relative to any other thing within the event horizon. The universe is currently thought to not have a center of any sort. Everything that's not gravitationally bound is moving away from everything else in proportion to the Hubble parameter. As far as I know, light from everything farther away from the black hole central singularity than the observer would be blue shifted and everything closer in would be red shifted. We don't observe this asymmetry in our universe. Chris Yes, the center part is kind of strange in my example... (edit) but we are at the centre of the observable universe, it doesn't necessarily have to say anything. I don't agree with the blueshift though. -The light from a object closer to the centre than you will be redshifted because the object is accelerating faster than you. Distance will increase. -The light from a object farther away from the centre than you will be redshifted because you are accelerating faster than the object. Distance will also increase. Edited July 12, 2011 by Lunkdunk Link to comment Share on other sites More sharing options...
xxSilverPhinxx Posted July 12, 2011 Share Posted July 12, 2011 (edited) Time cannot run "backwards" but a set of equations can be time invarient.Surely there is a difference? This^ I think so too (layperson opinion, so maybe I'm confusing things here). Also, as for the reverse gravity example, isn't it looking a bit like the grandfather paradox? I'm assuming you're not talking about anti gravity here... Edited July 12, 2011 by xxSilverPhinxx Link to comment Share on other sites More sharing options...
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