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My understanding is that the gravitational pull of a black hole is so intense nothing can escape it, not even light. So how is it in "Interstellar" that the planets orbiting Gargantua can withstand the gravitational pull of the black hole when it appears the system's sun is being torn apart?

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  • I was having a discussion with a friend about gravity. How is it that the moon's gravity can affect billions of tons of water and cause high tides, yet we don't feel it? How is it the ocean is affected by the moon's gravity and we aren't? My friend explained that gravity is a "weak" force, but that doesn't make sense to me (go jump off a ten story building, and then tell me gravity is a weak force I replied) how can a weak force move tons of water, yet not affect us at all? – JamieTheBastard Nov 9 '16 at 14:53
  • I'm afraid that should rather be tackled by a good book on physics, though. The tides don't result from just being "pulled towards" the moon, but from the tidal gravitation resulting from the Earth moon system. But really, that goes beyond what we are discussing here. For a more fundamental understanding of how gravitation and orbital mechanics work, you won't get around a more intricate tutorial than a pop-science-fictional film. – Napoleon Wilson Nov 9 '16 at 14:56
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They ARE falling in. Just very slowly. Thanks to Inertia.

Their linear speed forward and their distance away from Gargantua allows them to reach a stable orbit around their primary gravity well. But each rotation around Gargantua shave a tiny bit of momentum as it pulls in on the planet. Their forward momentum allows it to resist being pulled in directly right now, but in a few billion to trillion years, they will get pulled into a decaying orbit, as the planets don't have a way to course correct (without external influences).

Add to that, the other planets will also exert a force on each other, and can cause stabilization issues, making this happen sooner.

The same is true for our Solar System. Influences of Planets on each other make predicting celestial body movement over long time frames hard. It's believed Mercury will eventually be pulled on too much by Jupiter causing it to collide with Venus or the Sun.

But we are talking astronomical time frames. The human race will be long dead or gone by the time this happens.

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Because that's not how black holes and orbital mechanics work.

It is true that a black hole has basically so much mass (thus gravitation) that at a certain point nothing can escape its gravitational pull, not even light. That's true. But this only happens at a certain distance to the black hole, the so-called event horizon (because events beyond this can never influence events outside of the black hole).

But to the "outside world" a black hole is really just a very heavy thing, like any other planet or star, just quite a bit heavier. So planets or any other celestial bodies can just orbit around it, like the Earth orbits around the sun. Afterall you could ask the same question about why the Earth doesn't get sucked into the sun, as the sun also has a huge gravitational pull on it. But the Earth moves on a stable orbit around it (it's basically in a continuous state of freefall). In the same way the planets that orbit Gargantua move on a stable orbit around it.

What you shouldn't do is disturb that planet's orbit too much or get too near to the black hole's event horizon, then you're doomed.


I don't know why you think "it appears the systems sun is being torn apart", though. That happens at no point in the film (in fact there isn't actually a sun to tear apart there). What you might be talking about is the slightly unusual appearance of the black hole, with this glowing halo around it. This isn't a system's sun that's torn apart, but rather the black hole's accretion disc, a ring of very hot and very fast stuff that's circling around the black hole. (And it looks quite a bit weirder due to how optics work under heavy gravitational influence.)

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    HOWEVER..... as material gets pulled into a black hole, it gives them more mass, and greater gravitational pull, so, eventually, their reach might increase if they were still actively pulling in available matter, right? – PoloHoleSet Nov 8 '16 at 15:35
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    @AndrewMattson maybe, but the amount of stuff that's sucked into it over time is actually so negligibly low that I doubt it will ever signficantly influence its event horizon or strenth at all. (Kip Thorne elaborates a little in his book on the movie of how few of the stuff that gets near the whole actually falls in, rather than being catapulted away, if I remember correctly). And the orbit is also somewhat stable, so after minor disturbances the planet just gets back on track again automatically, which I'm sure accounts for small changes in the hole's strength over time. – Napoleon Wilson Nov 8 '16 at 15:38
  • I was wondering about how much of it would basically get sucked in very quickly, leaving everything else beyond reach, or if it was something that would gradually grow and grow over vast, galactic time spans. Thanks for the info. Now I don't have to lie awake worrying about whether I'd eventually get sucked into the black hole in the middle of the Milky Way, thus ruining my plans to not ever die. – PoloHoleSet Nov 8 '16 at 15:43
  • @AndrewMattson Even the stuff that gets sucked into it probably does so over a very long time in spiralling paths (it's basically the stuff in the glowing accretion disc), unless you directly catapult yourself into it, like Cooper did. Most of the stuff arrives on rather skewed paths, though. But well, I'm by far no physicist either and I don't really have my general relativity formulas ready right now. ;-) – Napoleon Wilson Nov 8 '16 at 15:46
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    @JamieTheBastard A mini black hole suddenly appearing on Earth is one thing. It would be in a perfect postition to "devour" Earth and would gain a comparatively big mass that way. But a humonguous black hole (of about 100 million sun masses) in an orbital system swallowing one or the other ton of stuff now and then (provided it falls in and doesn't get catapulted away) is a whole different thing. – Napoleon Wilson Nov 9 '16 at 14:40

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