Siliskk t1_j9nh507 wrote
Reply to comment by Delicious_Wrap4944 in Time dilation question by [deleted]
Another thing to note is the interstellar movie, where on the planet 1 hour is 7 years on earth. If a civilization lived on a planet like this but opposite, where 1 hour on earth is 7 years on their planet, or perhaps more, it could be very likely
apple-pie2020 t1_j9nkovy wrote
It’s been a while since I saw the movie. I believe the one hour on the other planet created a +7 year earth time for the astronauts because an hour on that planet plus the trip home at near light speed was what caused the time. It’s a function that accounted for the delayed trip home and the travel at near light speed
[deleted] OP t1_j9npflu wrote
I think you may be misunderstanding the movie...(or I'm misunderstanding you...) There was no lightspeed travel. It was a wormhole. The time dilation was solely due to the proximity to the blackhole.
apple-pie2020 t1_j9owp0d wrote
Ok. It’s probably me. It’s been a good five years since I saw it. I’ll rewatch it, it was entertaining
DudeWithAnAxeToGrind t1_j9nrtvc wrote
It was very over exaggerated in the movie. The effect is real. There's time dilation due to both special relativity (the speed of satellite) and general relativity (how deep in the gravity well the satellite is).
The time dilation in the movie is so extreme, that I'm not sure a planet could even exist so close to the event horizon. There is such thing as innermost stable orbit around black hole; I wouldn't be surprised if for such extreme time dilation, a planet would need to orbit closer than innermost stable orbit (which would be impossible).
The clocks are slowed down by orbital speed, and also slowed down by how "deep" in gravity well they are. This means anything in orbit will experience its clock slowed down depending how fast it moves. Further away, the slower it moves, the less the clock in the satellite is slowed down compared to the surface of the body it orbits. However, the further away the satellite is, higher in the gravity well it is, and faster its clock ticks compared to the clock on the surface of the body it orbits. In case of Earth, there is an orbit where these two effects cancel each other. See this graph https://en.wikipedia.org/wiki/Gravitational_time_dilation#/media/File:Orbit_times.svg
DudeWithAnAxeToGrind t1_j9q4l6z wrote
Here's where somebody actually did the math:
TL;DR For such extreme time dilation, the planet would need to orbit just outside of photon sphere. There are no stable orbits that close to the event horizon; the planet would either fall into the black hole, or it'd be flung out into space.
The photon sphere is a sphere around the black hole where gravity is so extreme, photons are orbiting black hole in circles.
The black hole would need to be supermassive. Because anything smaller (e.g. solar mass black holes), the tidal forces that close to the event horizon would be so large, they'd shred the planet into tiny pieces... Or basically anything else, such as spaceship or a human.
For entire solar system to be so deep in the gravity well to experience time dilation as extreme as in the movie, and not be either destroyed or stripped of its planets, the black hole would need to be many orders of magnitude larger than anything we have ever observed. From what we know, no such black hole can exist. There was simply not enough time since Big Bang for any to grow that large, and due to the expansion of universe, no black hole will ever be able to grow to such a large size.
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