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Comment by thegrossman | Hacker News Reader

Gravity is attractive even if you run physics backwards in time. To see that this is the case, consider the following scenario:

You've got a ball at rest 500 meters up, and you drop it. It falls downwards at 9.8 m/s^2. After 10 seconds, it's traveling 98 m/s towards the ground and has falled 490 meters (10 meters above the ground).

Now freeze time and reverse it!

What do you see if you reverse the "movie"? Well, the ball changes direction. It's still 10 meters above the ground, but is now heading upwards at 98 m/s. It continues up, but slows down at -9.8 m/s until it finally comes to a stop 500 meters up.

In other words, running the movie backwards looks like just like the normal laws of physics -- attractive gravity and all. Weird, huh?

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Thanks for the mind expansion. Never looked at it that way.

Say what? Your gedanken simply proves that you've assumed that gravity is attractive when time reverses.

If you place a ball on an inclined plane then it rolls downwards to a position of lower potential energy, (under a Newtonian consideration) the gravitational force acting to accelerate the ball.

If you reverse time, unless you know how gravity acts in reverse then how do you know that the ball will roll up the hill rather than stay in place or roll down.

You're saying gravity remains attractive, ergo that if you reverse time the ball still rolls down, the universe still expands, etc..

A ball will not roll uphill, even when time is reversed. Gravity really is time-reversal invariant. But when you reverse time, you do have to invert all velocities. So the inverse of your experiment of a ball rolling down an incline is a ball starting at the bottom of the incline with a certain velocity pointing upwards, that'll roll upwards until it comes to a stop, just as it would do in normal time.

I'm curious--if the ball rolling down an incline were instead a photon falling through an event horizon, would the time-reversal involve breaking various physical laws? (Does it fall into the "physics doesn't apply inside of a black hole" disclaimer?)

Time "slows down" asymptotically as you approach the event horizon, so you will never "see" anything cross it. https://secure.wikimedia.org/wikipedia/en/wiki/Event_horizon...

No, the physics are still valid in the event horizon. It's just the singularity at the exact centre of the black hole that is problematic.

Interestingly, if we ignore only the point at centre, it can be shown that an object that falls in a black hole, goes right through it and is ejected at the other side. All in finite proper time. This situation is non-physical because it takes an infinite amount of time for the photon to reach the event horizon. Even if we could reverse the photon inside the black hole, it would still take an infinite amount of time to crawl back out. So even in theory, black holes are still inescapable.

No, he's saying that if you "reverse the movie" of the ball accelerating down the inclined plane, then it will appear to decelerate as it rolls up the plane; looking at a ball slowing down as it rolls up a plane you can't tell if time is going forwards or backwards.

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