5 ms·
> Ie if you saw a video of undisturbed objects only affected by gravity, you couldn't tell if the video was reversed. How does that work with things like black
by floxy 1y ago
> Ie if you saw a video of undisturbed objects only affected by gravity, you couldn't tell if the video was reversed.
How does that work with things like black holes? If you saw an apple spiral out of a black hole, wouldn't you suspect that you were watching a reversed video? Even if you take account the gravitational waves?
- kgwgk 1y agoIf you saw a comet coming from the sun, or a meteorite coming from the moon, etc. you would also find that suspicious.
- Gravityloss 1y agoComets are in elliptical orbits around the sun so literally half the time they're traveling away from the sun.
- kgwgk 1y agoSometimes they don’t make it: https://en.wikipedia.org/wiki/Sungrazing_comet https://en.wikipedia.org/wiki/Sungrazing_comet https://www.space.com/8478-comet-collision-sun-captured-3.html https://www.space.com/8478-comet-collision-sun-captured-3.ht...
- Gravityloss 1y agoYea and those happen when other forces than gravity come into play. When matter starts colliding.
- kgwgk 1y agoEvery object has been subject to forces other gravity at some point. And the point is that sometimes comets do indeed fall into the Sun. If you object to people calling that a comet that’s fine - we can use whatever name you want.
- Gravityloss 1y agoThe point is that the gravity interactions are time reversible. Not so with friction etc.
- kgwgk 1y agoSure, nothing in the laws of physics prevents a celestial body from distancing itself from the Sun or from the Moon. But it would like suspicious! Wouldn't you suspect that you were watching a reversed video?
- Gravityloss 1y agoYou don't understand my point. If we watch purely gravity interactions, the video can be reversed and you wouldn't be able to detect it. If a meteor crashes into the moon, there's other effects than gravity that makes the video not reversible. Ie it's not only gravity. That's the point. In other words, if a comet approached the moon at high speed, missed and slingshotted in another direction, it would be traveling away from the moon, but the video would be time reversible and you couldn't be able to detect it. Gravity only interaction.
- kgwgk 1y agoI think you don’t understand my point. Someone wrote “If you saw an apple spiral out of a black hole, wouldn't you suspect that you were watching a reversed video?” I replied “If you saw a comet coming from the sun, or a meteorite coming from the moon, etc. you would also find that suspicious.” I don’t know what part do you object to (if any). > If a meteor crashes into the moon, there's other effects than gravity that makes the video not reversible. If a meteor (or an apple) is still in a crashing trajectory when you stop recording there are no effects other than gravity. The video is reversible - it just looks weird when you play it in reverse (because the meteor seems to be coming from the Moon and the apple seems to be coming from the black hole if you try to imagine where they were before).
- 1y ago
- immibis 1y agoThat's the question of why time only goes forwards. It seems to be that the universe started in an extremely low-entropy state. It will go towards high entropy. In a high entropy state (e.g. heat death, or a static black hole), there's no meaningful difference between going forwards or backwards in time - if you reverse all the velocities of the particles, they still just whizz around randomly (in the heat death case) or the black hole stays a black hole.
- Gravityloss 1y agoClassical gravity doesn't work like that. An apple does not spiral into a black hole in space there. It's in an elliptical orbit. (A circle is a special ellipse.)