What causes black holes and stars to spin?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by wegs, Jan 24, 2020.

  1. wegs Matter and Pixie Dust Valued Senior Member

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    https://www.scientificamerican.com/article/what-causes-objects-such/

    So, I have a few questions on this.

    1. In the article, it's mentioned that the space-time distortion is exceedingly weak near Earth, as compared to black holes. What does this mean?

    2. What would happen if the Earth gradually came to a halt, and stopped spinning? I know that life as we know it would be adversely affected, but would humans be able to survive it? (there is speculation from scientists, but what do you think?)

    3. If one planet stopping spinning, would it affect other planets? If Earth stopped spinning, would the rest of the universe be negatively affected? I guess what I'm asking is, does Earth ''need'' the spin of other planets, and vice versa?
     
    Last edited: Jan 24, 2020
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  3. Janus58 Valued Senior Member

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    1. It just simply mean that around the Earth you can get by assuming Newtonian physics and Euclidean geometry holds for pretty much most practical purposes. Near a black hole you can't, as the deviation between GR predictions and Newtonian predictions vary significantly.
    2. The periods of day and night would gradually get longer. The Earth's shape would gradually become more spherical. Any life would have had to have time to evolve enough in order to adapt to such conditions. Whether human could adapt enough in an open question.
    3. The only other body presently effected by the Earth's rotation in the Moon as the Earth slowly transfers angular momentum to it. The Earth's spin has no measurable effect on the rest of the Universe.
     
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  5. billvon Valued Senior Member

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    It means relatively speaking gravity is pretty weak near Earth, so you don't get strange effects like bending of light when you are observing light going near the Earth. (You get a little but it's usually not noticeable.)
    Excellent question. We'll divide this into two problems - transient and steady state.

    The transient problems are caused by the deceleration of the Earth. The oceans would slosh out of their basins, for example, and you'd get very strong atmospheric currents as the Earth slowed but the atmosphere did not. There would be a huge amount of stress on the Earth's tectonic plates as the various parts of the Earth (some fluid, some not) slowed down at different rates.

    However, if you postulated this being done over a VERY long time, or by a mysterious force that acted on every part of the Earth (including the atmosphere) equally, you could conceivably see little trauma to the Earth. You would always see the seismic stresses as the Earth relaxed from an oblate spheroid (caused by rotational stresses) to more of a sphere.

    The steady state problems would include:

    Heating and cooling. One side would get very hot; likely hot enough to boil water. The other side would get very cold. It might even be cold enough to start condensing out portions of the atmosphere (like CO2 and perhaps even nitrogen.) So you might see CO2 snow and nitrogen lakes on the dark side.

    Slowing of tides. You'd see one tide every two weeks instead of two tides a day. The Moon would now cause the only tide; there would be no solar component.

    Atmospheric circulation. There would be a constant cycle of air - air would sink in on the cold side, flow towards the hot side, where it would be heated, rise, and travel back to the cold side. You'd see very strong constant winds blowing from the cold side to the hot side (if you were on the surface.)

    The Earth _might_ be able to sustain life in a narrow band between the hot and cold sides. The winds would be relentless, though, and would make life difficult. And the atmosphere might have a different composition as parts of it condensed out, and as the water on the hot side boiled away. You'd likely get glaciers pushing through the "twilight" area where they would melt, flow back to the hot side, and boil to start the cycle all over again.

    Not appreciably. Nearby planets would see slightly different forces acting on their spins (and orbits) but it's likely in the noise. You'd see a small effect on the Moon.
     
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  7. foghorn Valued Senior Member

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    Any idea how the ocean currents would behave? Would they be only on the warm side? Perhaps a little ''overflow'' near the borders of warm and cold. I'm thinking something like the conveyor effect of the currents going to and from the poles.
     
  8. paddoboy Valued Senior Member

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    Your questions have been well answered but anyway here's my simpler answers I hope.....

    The answer to your first question is I believe best illustrated by a diagram......here is an image of spacetime warping due to the presence of a BH

    Please Register or Log in to view the hidden image!


    here is the warpage caused by the Earth/Moon system....

    Please Register or Log in to view the hidden image!



    Venus is probably a good analogy to that question. On Venus the rotational/spin rate is 243 Earth days, while its orbital period is 224 days, which makes a day on Venus longer then a year.
    The Earth's rotational/spin velocity is very gradually being slowed down due to tidal interactions with the Moon, while at the same time, the same interactions are seeing the Moon gradually increase its distance from Earth by a centermetre or two every year. This will continue for approx 3 billion years or so, until the Earth's rotational/spin period is equal to a Lunar month and the Earth will have one side eternally facing the Moon, just as the Moon now has one side eternally facing the Earth. By that time anyway, our Sun will probably have evolved into a red giant, and scorched all life on Earth and evaporated all Oceans, and possibly engulfing us at the same time...if we are still around.
    Other issues covered by Janus and Billvon are also valid with relation to your question.

    If Earth stopped spinning some very minimal gravitational effects on the smaller planets in our system maybe evident, but certainly zero on the universe beyond.
     
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  9. billvon Valued Senior Member

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    Depends on how the planet ends up - where the terminator is. If there's a lot of ocean spanning the two sides the currents would be interesting indeed. Wind would drive a lot, but a fair amount would be driven by the vapor conveyor moving water from the hot side to the cold side in the atmosphere. Salt gradients would go from low (near the cold side) to high (near the hot side) and that would drive much of the current. One thing that would NOT drive any current flow would be Coriolis forces - so that would be a bit of a change. (It's a minor but notable force now when water rises or falls under the ocean.)

    The _general_ surface circulation, driven by winds, would be towards the warm side.
     
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  10. paddoboy Valued Senior Member

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    Actually the title of the thread" What causes black holes and stars to spin?"has not yet been answered...
    The first stars were formed from collapsing hydrogen/Helium gas under gravity. This collapse had a twofold effect of starting the collapsing cloud to start spinning [much as a skater increases his/her spin when they pull there arms in tightly] and seeing the core of these accretion disks undergo huge increases in temperatures, until nuclear ignition started. These stars were relatively short lived [measured in the 10's of millions of years] and finally underwent supernova destruction spewing their guts into the universe.
    The later metal rich stars formed from this ejected matter as it started to collapse under gravity.
    BH's are formed from the left over of giant stars that were already spinning, and so conserved that angular momentum.
     
  11. Janus58 Valued Senior Member

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    The point being is that stars and then black holes start out as large clouds of dust and gas. Each of the individual gas molecules and dust particles have their own random velocities and thus angular momenta relative to the center of mass for the cloud. It would be unlikely in the extreme for the sum all those individual angular momenta to exactly cancel out, leaving the cloud with no net angular momentum at all to be preserved as the cloud collapses.
     
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  12. wegs Matter and Pixie Dust Valued Senior Member

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    Thank you to all who have replied - it makes much more sense, but leaves me with a few more questions on black holes.

    So, how can black holes spin so fast, if they've ''consumed'' so much? Seems like they'd spin slower. I guess I'm thinking of say a car that is weighed down by a lot of luggage - like REALLY weighed down. It wouldn't be able to travel as fast as if it didn't hold the luggage, so I'm not sure why that same principle doesn't apply to black holes? And, do black holes only exist because stars have come and gone? (died)
     
  13. paddoboy Valued Senior Member

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    'BH's are collapsed matter that reaches a density at what is known as the Schwarzchild radius, beyond which further collapse is compulsory. eg: If we could magically squeeze all the mass of the Earth beyond its Schwarzchild radius, it would become a BH.,,that would be around 2.5 cms. The spin is achieved from the spin of the star/s from whence it formed and collapsed from and also due to conservation of angular momentum, much as a skater spins faster when he or she pulls in his arms tightly by his or her side, so reducing his or her moment of inertia...
    With a BH this spin is pretty fast and any matter being pulled in would not have too much of a detrimental effect on that spin. Space probes etc, also gain momentum via gravity assists.eg: swinging a craft in a certain trajectory pass Jupiter, will see that craft subtract spin from Jupiter orbit around the Sun, relative to the Sun. The vast mass difference between the craft and Jupiter means that subtraction of Jupiter's orbit is not noticable. I hope that is clear enough. Plus of course, unlike your car analogy, there is near zero friction caused by any mass increase of a BH in space.
    This may describe it better...
    https://medium.com/@uncertainquark/...ecrafts-escaped-the-solar-system-d8aaa4a9273c
    So the mass being pulled into the BH would have a minimal effect, and of course BH's are never all purpose vacuum cleaners. If we look at the earth's case for example, with it being squeezed beyond its Schwarzchild radius and becoming a BH, would not affect any of the planetary orbits....All would carry on as they do now.
    Essentially yes, although there is and has been speculation that possible quantum microBH's may have been created at the BB. If this is the case, then some could have originated that way. But as far as I know that is still just speculative.
     
    Last edited: Jan 29, 2020
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  14. paddoboy Valued Senior Member

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