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.
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.
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.
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.
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.
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.
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?
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.