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I am pretty confused by this article. In order to simulate gravity, you have to have already decided on a direction of time, right? You'd know if time was run

I am pretty confused by this article. In order to simulate gravity, you have to have already decided on a direction of time, right? You'd know if time was running backwards because gravity would push things apart rather than pull them together.

AnimalMuppet 12y ago
No. The thing is that gravity is a conservative force; that is, energy is conserved in a gravitational interaction (ignoring radiation of gravitational waves). Because of that, if I reverse the direction of time, gravity stays attractive, and the objects trace the exact same motion in reverse.
- throwaway7808 12y ago
  Energy is conserved in any iteraction. And gravity is not a force. Gravity is the curvature of space-time.
  - JoeAltmaier 12y ago
    Well, its semantics right? Gravity causes acceleration, so its a force? It can be treated mathematically as a curvature. It behaves differently in different frames, but so do electromagnetic forces.
    - sp332 12y ago
      If someone applies a force to you, you can feel it. If you're in free-fall, you can't feel anything. According to general relativity, there's no way to tell if you're stopped, moving in a straight line with constant velocity, or in free-fall in a gravitational field. They're all the same.
      - readerrrr 12y ago
        There is a distinction. A gravitational field has a center, so if you are in free-falling towards it you can determine that you are in it. Take two objects and place them apart to float stationary( the line they make should be orthogonal to the center of the field ). Even though they started completely stationary, they will slowly start to move towards each other, as an unknown force is acting upon them. In reality they are falling towards a common center. I guess this doesn't work on dimensionless points, so the your comment is correct as far as mathematics are concerned.
      - FiatLuxDave 12y ago
        Actually, you only feel it if the force is applied differentially (which most forces we experience are). If your body had a volumetrically uniform electrostatic charge, which was not high enough to have charge screening, and you were acted upon by an electrostatic force, you wouldn't feel anything either. It is just that 'mass charge' is always distributed evenly along with mass, while with electrostatic forces this is a special case. Forces can indeed be modeled as a curvature in space-time. However, there is a bit more to GR than just that, which is why it took Einstein years to go from SR to GR.
    - Synaesthesia 12y ago
      Yes I think it's semantics - different yet valid ways of looking at the same phenomenon.
  - AnimalMuppet 12y ago
    > Energy is conserved in any iteraction. True. But gravity is a conservative field (again, neglecting radiation), meaning that the energy that goes into raising something against gravity, you can get back by lowering it to the original position again. > And gravity is not a force. Gravity is the curvature of space-time. If we're not in "overly pedantic argumentation" mode, then I'd say, go climb some stairs. Feel that? That's a force. But if we are in "overly pedantic argumentation" mode, then yes, gravity is the curvature of space-time in General Relativity. That may not be the final word, though. What is gravity in Loop Quantum Gravity?
    - throwaway7808 12y ago
      Let me give you a clue. Are you in the gravitational field of the Sun? How does it compare with the gravitational field of the Earth? Is it stronger or weaker? Do you feel it? What's the trajectory of the earth in the curved space-time around the sun? When you'd say "go climb some stairs" what you feel are the stairs. And you should also feel like a bit of an idiot, in my humble opinion.
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.

AnimalMuppet 12y ago

No. The thing is that gravity is a conservative force; that is, energy is conserved in a gravitational interaction (ignoring radiation of gravitational waves). Because of that, if I reverse the direction of time, gravity stays attractive, and the objects trace the exact same motion in reverse.

throwaway7808 12y ago
Energy is conserved in any iteraction. And gravity is not a force. Gravity is the curvature of space-time.
- JoeAltmaier 12y ago
  Well, its semantics right? Gravity causes acceleration, so its a force? It can be treated mathematically as a curvature. It behaves differently in different frames, but so do electromagnetic forces.
  - sp332 12y ago
    If someone applies a force to you, you can feel it. If you're in free-fall, you can't feel anything. According to general relativity, there's no way to tell if you're stopped, moving in a straight line with constant velocity, or in free-fall in a gravitational field. They're all the same.
    - readerrrr 12y ago
      There is a distinction. A gravitational field has a center, so if you are in free-falling towards it you can determine that you are in it. Take two objects and place them apart to float stationary( the line they make should be orthogonal to the center of the field ). Even though they started completely stationary, they will slowly start to move towards each other, as an unknown force is acting upon them. In reality they are falling towards a common center. I guess this doesn't work on dimensionless points, so the your comment is correct as far as mathematics are concerned.
    - FiatLuxDave 12y ago
      Actually, you only feel it if the force is applied differentially (which most forces we experience are). If your body had a volumetrically uniform electrostatic charge, which was not high enough to have charge screening, and you were acted upon by an electrostatic force, you wouldn't feel anything either. It is just that 'mass charge' is always distributed evenly along with mass, while with electrostatic forces this is a special case. Forces can indeed be modeled as a curvature in space-time. However, there is a bit more to GR than just that, which is why it took Einstein years to go from SR to GR.
  - Synaesthesia 12y ago
    Yes I think it's semantics - different yet valid ways of looking at the same phenomenon.
- AnimalMuppet 12y ago
  > Energy is conserved in any iteraction. True. But gravity is a conservative field (again, neglecting radiation), meaning that the energy that goes into raising something against gravity, you can get back by lowering it to the original position again. > And gravity is not a force. Gravity is the curvature of space-time. If we're not in "overly pedantic argumentation" mode, then I'd say, go climb some stairs. Feel that? That's a force. But if we are in "overly pedantic argumentation" mode, then yes, gravity is the curvature of space-time in General Relativity. That may not be the final word, though. What is gravity in Loop Quantum Gravity?
  - throwaway7808 12y ago
    Let me give you a clue. Are you in the gravitational field of the Sun? How does it compare with the gravitational field of the Earth? Is it stronger or weaker? Do you feel it? What's the trajectory of the earth in the curved space-time around the sun? When you'd say "go climb some stairs" what you feel are the stairs. And you should also feel like a bit of an idiot, in my humble opinion.

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.