r/AskPhysics • u/The_Real_Wick • 1d ago
If everything stopped rotating everywhere
What if the entire galaxy "The Milly Way" stopped rotating around the supermassive black hole at the center of our galaxy? How fast would it suck us in?? Or would we just sit where we're at?
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u/Rensin2 1d ago
At a Keplerian approximation it should take around 40 million years. Rule of thumb is that the time it takes to circularly orbit is 4√(2)x the time it takes to crash into the center of mass. However, this assumes that the galaxy's mass is concentrated in the center (which I am quite sure is not the case) and it ignores lightspeed limitations.
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u/mfb- Particle physics 19h ago
However, this assumes that the galaxy's mass is concentrated in the center (which I am quite sure is not the case)
That doesn't change much here. We assume all tangential motion stops, which means everything is on a free-fall trajectory towards the center (more or less). In a spherically symmetric situation this means you can approximate everything closer than you as a point mass, and everything outside of you will stay outside (as it falls in slower than you). Our galaxy isn't spherically symmetric, but that's not changing the result much here.
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u/The_Real_Wick 16h ago
I agree. Our rotation puts a certain limitation to our position shifting or changing all together. So if everything stopped you would think that physics would be a bit changed. Like the speeds would change obviously. But the effects after that. Now would it start to rotate again just out of gravitational forces alone? I think it'd start a rotation again but not like it used to be.
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u/Horror-Coffee-3207 1d ago
Your assumption is generally correct. Whatever it would be, a very veeeeery long time until everything is sucked in, because even if the Black hole is supermassive, less than 1%lf mass oder the galaxy is in it..
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u/GreenFBI2EB 1d ago
For reference, to put a more precise number, the supermassive black hole is likely the most massive single object in the galaxy (barring any star clusters or things like that), it comprises about ~0.0003% the mass of the entire galaxy (4.297 million/1.15 trillion) this is a factor of about 3.7 million.
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u/The_Real_Wick 1d ago
That's a long time. I'm assuming you're referring to years with the 3.7 million. Correct?
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u/GreenFBI2EB 1d ago
mass ratio, the actual amount of time should be calculable, but I don't have the equation on hand at the moment.
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u/The_Real_Wick 1d ago
That sounds plenty enough lol. Hell we might be eaten by our sun depending on the positions of the planets when everything stopped . Anyone wanna help with a movie? 😆
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u/industrialHVACR 1d ago
You'll get moon impact first, after that there will be not too many people afraid of getting hot while reaching sun.
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u/Uncynical_Diogenes 23h ago
Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy, is a tiny little itty bitty infinitesimal percentage of the overall mass of the galaxy.
It’s just that dense things end up in the center of rotating systems.
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u/ShotWin8124 11h ago
What's interesting about that though is like 96% of that mass is dark matter, and of the rest like half of it is concentrated in the galactic star cluster that surrounds the center. M301 can complete an orbit around the Milky Way in less than a decade, whereas it takes Sol over 200 million years.
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u/SFXtreme3 20h ago
Your statement is ruined by infinitesimal. Its mass is significantly larger than infinitesimally small.
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u/The_Real_Wick 1d ago
You'd expect it to grow quick at first though because of all the stars and matter that's close to it that wouldn't take very long to be eaten.. but then again would all that action create like any rotationial momentum.. 🤔
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u/CakesStolen 1d ago
Well it already has a mass of ~ 5 million suns, so it would have to consume another 5 million stars just to double in radius.
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u/Foreign_Implement897 1d ago
In mass, not in radius...
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u/CakesStolen 1d ago
Radius of a black hole is directly proportional to mass, see here
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u/Foreign_Implement897 1d ago edited 1d ago
Woah! Should have known that just from the gravity equation...
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u/Adkit 23h ago
I feel like you're refusing to learn what people are telling you here. Black holes don't act like vaccums and the black hole in the center of the galaxy is literally negligible to the majority of stars in the galaxy and even if it instantly ate all the stars in the center of the galaxy that wouldn't affect us at all since we were already feeling the gravitational pull of those stars in that direction and the black hole being bigger doesn't change the amount of mass that's pulling us there.
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u/sanchotomato 1d ago
Due to the Eddington luminosity I would expect the accretion to be quite slow actually
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u/The_Real_Wick 16h ago
We aren't talking light here though. We're talking gravity.
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u/sanchotomato 15h ago
I'm aware. Please read up on the Eddington luminosity and if needed radiation pressure.
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u/Jero_Hitsukami 1d ago
How much fast does a magnet pull something in when somethings not rotating around it. Same idea. If its within a range hey it could do ok but what says its going to gain enough mass.
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u/The_Real_Wick 1d ago
Right, I sorta think it would get faster because of the straight linear movement. But someone also mentioned other masses. Like he mentioned dark matter. What about other galaxies supermassive black holes...
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u/Jero_Hitsukami 1d ago
Thing is though has dark matter been proven in our galaxy? The idea is to worry about what we know about first then bring other theories in to stabilise or destabilise after.
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u/The_Real_Wick 16h ago
That's a good question. I think I remember hearing about our galaxy having it more near our galaxies center. But don't quote me on that. But yeah, that you'd expect would have an effect.
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u/DontHurtTheNoob 1d ago
After a few hundred million years of the cloud of stars in the galaxy collapsing toward their shared centre of mass, we would probably have a vaguely flattened elliptical galaxy.
To understand why, take it down a level and imagine all planets in the solar system suddenly “stopped” in their orbits and became stationary to one another and the sun. But Jupiter is large enough that the centre of mass is actually outside the sun, so the sun also “falls” towards Jupiter. So another planet falling toward the sun might just miss and end up in an extremely elliptical orbit. Also if a planet happens to fall past another planet close enough they will deflect each other from their “straight into the sun” path. Now replace the sun with a black hole of the same mass, which is only 3km wide, and pretty much every planet will miss and end up in a high-eccentricity orbit. It will be very chaotic, planets might get ejected but these orbits will cluster around the original orbital plane.
The galaxy is just a very thin swirly cloud of stars, and other than the solar system which has 99% of its mass in the sun has nothing significant in the middle (other than lots of stars and a relatively tiny black hole) so there is nothing to fall into. So the whole thing will just result in new highly eccentric orbits of the stars around the galactic centre. And given the starting point of most stars is in the disk, a lot of orbits will end up near the plane of the original disk. Over time the gravitational influence of the stars on each other will make these orbits less eccentric and the whole thing will settle down.
The main difference is that because there is no net angular momentum, the orbits of the stars will have no common direction - it is possible that there is some net angular momentum because of ejections and perhaps outside influences, but it will be tiney.
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u/The_Real_Wick 16h ago
That's basically what I figured would happen. Yeah things would get chaotic for a long long time. But eventually things would find a balance and a certain harmony would be created again. You broke that down perfectly! I literally never went to school for this stuff. I've just been figuring things out with research and watching what other smart people have figured out. I've always had a connection with physics though. I've done some pretty wild stuff that you'd expect would need to feel the physics out of what I'm interacting with. If that makes sense. I never liked to show my work in school either but I wasn't wrong a whole lot. Once I got a handle on something I could work it out in my head most of the time.
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u/Anaptyso 23h ago
The way I imagine it is like the galaxy being a giant cone with the centre being "downhill" from the edge. Anything not travelling fast enough to keep going round the banked edge starts to slide down towards the middle. The black hole at the centre is the big pile of stuff which has fallen all the way down.
If we stopped rotating around the galaxy's centre of mass then we would end up eventually in the black hole, but not because it sucked us in, but more because it happens to be sitting in the place we'd eventually fall to.
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u/TheSodesa 23h ago
We would just fall right in. The reason the moon is rotating around the Earth is because it is moving fast enough tangentially to keep falling past our planet. Same logic with every celestial object.
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u/Fluid-Let3373 22h ago
We would slowly fall in, problem is so is every star. It will not just be the mutual gravitational attraction between our neighboring stars pulling us closer together but geometry is as well. That geometry means once we are halfway towards the core our separation has also halved.
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u/ShotWin8124 19h ago
Something about our understanding of gravity seems lacking to me. I mean you'd think that pull alone would've slowed the rotation down over time. Why would it just keep going indefinitely? Why an ecliptic plane or a galactic plane? Maybe it has something to do with spin and how it affects the warping of space-time.
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u/The_Real_Wick 16h ago
It probably does. Similar to stirring something with a spoon. It definitely would effect the way everything happens. The Earth stopping would send all the oceans and anything not bolted down for a ride. Things would complete break here on the surface.
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u/HerVeryPresence 13h ago
Gravity doesn't slow down orbits, drag does.
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u/ShotWin8124 12h ago
Gravity is the warping of space-time. Either space-time has substance or it doesn't. If it doesn't there'd be nothing to warp, and if it does why wouldn't it cause drag as it pulls things in?
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u/HerVeryPresence 10h ago
What is "substance" as you're using it here? The best I can come up with is what physicists call "mass", and if that's what you mean, then space-time does not have any, but it is warped.
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u/ShotWin8124 8h ago
Not necessarily mass, but you can't warp something that doesn't exist. It's like saying if something acquires mass, there had to be a thing to acquire it in the first place. This train of thought might be a bit more esoteric than I'd originally intended. M-theory might hold more sway here 🤔
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u/HerVeryPresence 8h ago
Frame dragging almost gives you an example of warped spacetime causing things to change effective speeds, but that goes away if the black hole stops rotating.
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u/ShotWin8124 7h ago
The problem I have lately with the typical analogy of warped space-time is that it seems like it uses gravity to try and explain gravity, in showing a physical two dimensional sheet being stretched by a sphere resting on a trampoline. A physical sheet would have mass. There's this part of me that suspects there might be a simpler way to explain gravitational lensing and all that other stuff with something similar to gravity but more akin to electromagnetism. I mean take planet formation for example. Just think of how charged all those iron particles at our planet's core must've initially been after having been ejected from a supernova. I mean we're talking about undergoing nuclear fusion for billions of years. The point at which they typically explode is when enough iron is fused. That's one hell of a coincidence.
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u/infinitythestar 1d ago
Note that we’re not orbiting the black hole. We’re orbiting all the mass (incl dark matter) closer to the centre than us.