4 ms·
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 velocit
by kmm 15y ago
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.
- evo 15y agoI'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?)
- sp332 15y agoTime "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#Interacting_with_an_event_horizon https://secure.wikimedia.org/wikipedia/en/wiki/Event_horizon...
- kmm 15y agoNo, 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.