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Although counter-intuitive, reversing time doesn't change the rules of gravity: the basic law of gravity just says that objects accelerate in a certain way depe

by Chinjut 12y ago

Although counter-intuitive, reversing time doesn't change the rules of gravity: the basic law of gravity just says that objects accelerate in a certain way dependent on their masses and distances. Because acceleration is the second derivative of position with respect to time, it is doubly negated, and thus unaffected, by reversing time. Reversing a planet's orbit produces another orbit just as valid under the law of gravity. And even at the scale of ordinary experience, gravity doesn't prohibit objects from rising (after all, in the canonical example of tossing a ball through a parabolic arc, there is both a rise and a fall); footage of a ball falling under the influence of gravity reverses into footage of a ball rising under the influence of gravity and vice versa, but never does footage compatible with the law of gravity reverse into footage incompatible with the law of gravity.

Roboprog 12y ago
True enough in free fall in a vacuum. (if one could find such a perfect vacuum) Apples don't usually leap off the ground into trees, though. Of course, that would be more of a heat to kinetic energy conversion to start that process, but I think that's more along the lines of what the original question really meant: why don't things spontaneously launch, rather than having a one way tendency to convert potential energy to kinetic energy to heat energy. It's hard to see gravity act in isolation: trace particles eventually slow an orbit in the same way that air friction deforms an idealized parabola/elipse by dragging on a ballistic projectile; tidal forces between earth and moon alter the moon's orbit.
- Chinjut 12y ago
  Sure. As you note, an apple would leap into the trees if the ground under it conspired to push it upwards. That the ground rarely does so (but the reverse, an apple falling and dispersing energy into the ground, happens frequently) is certainly an observation which merits explanation, but the explanation doesn't come from an asymmetry in the laws of gravity, as such. It's often said the explanation is found in something like the Second Law of Thermodynamics, which in turn is explained by the fact that at some moment, there was a state of extremely low entropy (the Big Bang); the "arrow of time" is then a manifestation of being to one or the other side of this particular low entropy moment. And the work discussed in the link (which I have no direct knowledge of) purports to show why some such low entropy moment should be expected to exist at all given the action of gravity over history. But my point was simply to note that you don't have to pre-incorporate an arrow of time to make sense of the laws of gravity, in response to the poster who found this a sticking point and expressed confusion over it.

by Chinjut 12y ago

Roboprog 12y ago

True enough in free fall in a vacuum. (if one could find such a perfect vacuum) Apples don't usually leap off the ground into trees, though. Of course, that would be more of a heat to kinetic energy conversion to start that process, but I think that's more along the lines of what the original question really meant: why don't things spontaneously launch, rather than having a one way tendency to convert potential energy to kinetic energy to heat energy. It's hard to see gravity act in isolation: trace particles eventually slow an orbit in the same way that air friction deforms an idealized parabola/elipse by dragging on a ballistic projectile; tidal forces between earth and moon alter the moon's orbit.

Chinjut 12y ago
Sure. As you note, an apple would leap into the trees if the ground under it conspired to push it upwards. That the ground rarely does so (but the reverse, an apple falling and dispersing energy into the ground, happens frequently) is certainly an observation which merits explanation, but the explanation doesn't come from an asymmetry in the laws of gravity, as such. It's often said the explanation is found in something like the Second Law of Thermodynamics, which in turn is explained by the fact that at some moment, there was a state of extremely low entropy (the Big Bang); the "arrow of time" is then a manifestation of being to one or the other side of this particular low entropy moment. And the work discussed in the link (which I have no direct knowledge of) purports to show why some such low entropy moment should be expected to exist at all given the action of gravity over history. But my point was simply to note that you don't have to pre-incorporate an arrow of time to make sense of the laws of gravity, in response to the poster who found this a sticking point and expressed confusion over it.