8 ms·

Wow, so the sun can spin! I never knew that. https://www.space.com/does-the-sun-rotate https://www.space.com/does-the-sun-rotate

DemocracyFTW2 3y ago
Almost nothing in the universe can not spin. Everything is rotating all the time.
- unsupp0rted 3y ago
  What's a known exception?
  - perlgeek 3y ago
    The Higgs Boson has spin 0. See https://en.wikipedia.org/wiki/Scalar_boson https://en.wikipedia.org/wiki/Scalar_boson
    - bqmjjx0kac 3y ago
      I thought quantum mechanical spin is completely unrelated to Newtonian angular momentum, but IANAP.
      - petsfed 3y ago
        Its not completely unrelated, but it is not clearly related. That is, spin contributes to overall angular momentum, but that contribution is much too large to be explained as the actual rotation of subatomic particles. So, like most quantum properties, our Newtonian intuitions are poor guides.
    - truculent 3y ago
      Is using quantum mechanical spin a fair comparison? Isn't it moving the goalposts a bit? > As the name suggests, spin was originally conceived as the rotation of a particle around some axis. Historically orbital angular momentum related to particle orbits. While the names based on mechanical models have survived, the physical explanation has not. Quantization fundamentally alters the character of both spin and orbital angular momentum. > The classical analog for quantum spin is a circulation of energy or momentum-density in the particle wave field: "spin is essential a wave property". https://en.wikipedia.org/wiki/Spin_(physics) https://en.wikipedia.org/wiki/Spin_(physics)
  - weberer 3y ago
    Any tidally locked satellite such as the moon.
    - zmgsabst 3y ago
      A tidally locked satellite rotates: one rotation per orbit.
      - this_was_posted 3y ago
        unless the body it rotates around rotates at the exact same speed around another body in the opposite direction
        jacquesm 3y ago
        Then it still rotates.
        throwaway290 3y ago
        It can only rotate relative to an arbitrary point, not in some objective absolute sense. Here it does not rotate relative to chosen point.
        jacquesm 3y ago
        Something about privileged reference frames...
        throwaway290 3y ago
        But what's the alternative, right?
  - princevegeta89 3y ago
    A Pulsar, that's not really spinning but oscillating back and forth, becoming a very highly precise instrument of time measurement as well: https://physicsworld.com/a/pulsar-timekeepers-measure-up-to-atomic-clocks/ https://physicsworld.com/a/pulsar-timekeepers-measure-up-to-...
    - ISL 3y ago
      Pulsars are dense compact spinning objects. From your link: "Pulsars are neutron stars that rotate at very high speeds and appear to emit radio pulses at extremely regular intervals. The pulses are actually all we see of a radio beam that is focused by the star’s magnetic field and swept around like a lighthouse beacon."
- vibrio 3y ago
  This stuns me for some reason. I was not explicitly aware of this. Why? Just left over momentum from something? Is there a bias in direction (overall or among subsets)?
  - noir_lord 3y ago
    Lots of reasons, stuff like "an unevenly spread cloud of something that collapses under it's own gravity" will spin. http://burro.case.edu/Academics/Astr221/SolarSys/Formation/starform.html http://burro.case.edu/Academics/Astr221/SolarSys/Formation/s...
    - throwawaylinux 3y ago
      Link suggests that is because the cloud is already rotating due to "gravitational shearing in the galaxy's disk", not because it is uneven.
      - raattgift 3y ago
        tl;dr parent and parent's link are not wrong, just very simple. Unfortunately it does not make explicit that "at least a little" does not mean the entirety of the rotation, or even its dominant component. In the context above gravitational shear is the first of the empirical https://en.wikipedia.org/wiki/Oort_constants https://en.wikipedia.org/wiki/Oort_constants (differential rotation shears the non-solid disc). Differential rotation means gas in the direction of the galactic centre drags a bit compared to gas further from the centre, effectively acting as a small torque on a large gas cloud. If the gas cloud has no net rotation at all, this torque will develop it. That's what I take the sentence you focus on to be trying to say. Unevenness is related to the Jeans instability, which describes the failure of internal gas pressure to prevent gravitational collapse. Chandrasekhar developed a more complicated treatment useful for when the angular momentum of the bulk gas cloud is non-negligible. Roughly, collisions within the gas cloud convert kinetic energy to light which carries energy out of the collapsing cloud. The cloud consequently cools and contracts gravitationally. However, still roughly, superimposing a net rotation on random motions within the gas prevents collapse in the direction perpendicular to the net rotation's spin axis. So contraction is mainly along the spin axis. Result: a disc, which is essentially what parent's link says. The coupling of the galaxy's spin to the solar system's spin is at best weak; the solar system's spin axis is tilted about 60 degrees from the galactic plane; the plane is perpendicular to the galaxy's spin axis, in line with the previous paragraph. Here's a diagram <https://i.stack.imgur.com/VIipS.jpg https://i.stack.imgur.com/VIipS.jpg>. The sun's path around the centre of the galaxy is a bit messy compared to a kinematically hot star (which will feel perturbations like the bar or spiral arms less). The sun tends to bob up and down relative to the midplane of the galactic disc. The Oort constants are local and position-dependent, so the solar system's migration -- and any migration of its precursor -- means wandering into regions with different gravitational shear. Questions that afaik are still open (but I would be happy for a galaxy or solar system dynamics person to correct me, particularly if the correction includes magnetics and chemodynamics/metallicity-dependent pressure!): has the solar system's spin axis tilt to the galactic disc evolved since formation? Accepting the Coatlicue hypothesis, was the remnant of the explosion of the heavy star that was the precursor of the solar system (and others) aligned with the star's rotation? Was that precursor star's rotational axis aligned with the galactic disc? The remnant was almost certainly not of uniform density. Alternatively accepting the Wolf-Rayet bubble Giant Molecular Cloud (GMC) nebular hypothesis, did the GMC fragmentize under galactic shear, and if so was "our" fragment's spin dominated by that initially, with spin perpendicular to the disc but evolving to the present tilt, or was our fragment's spin axis initially tilted close to the present approx. 60 degrees to the galactic disc where it has since remained? Finally, also afaik, the spins (more broadly the angular momentum vectors) of stars in the local bubble are essentially random. So repeat the questions in the preceding paragraph for each of those...
perlgeek 3y ago
Even black holes "rotate". Since black holes aren't really normal objects, what we mean by that is that the sum of all the matter that created black hole had an angular momentum. This also affects their behavior. See https://en.wikipedia.org/wiki/Rotating_black_hole https://en.wikipedia.org/wiki/Rotating_black_hole for more.
princevegeta89 3y ago
It is also the reason why the planets revolve around it
- deleted 3y ago
  [deleted]
voytec 3y ago
Spinning is so much cooler than not spinning!
- guerrilla 3y ago
  warmer*

DemocracyFTW2 3y ago

Almost nothing in the universe can not spin. Everything is rotating all the time.

unsupp0rted 3y ago
What's a known exception?
- perlgeek 3y ago
  The Higgs Boson has spin 0. See https://en.wikipedia.org/wiki/Scalar_boson https://en.wikipedia.org/wiki/Scalar_boson
  - bqmjjx0kac 3y ago
    I thought quantum mechanical spin is completely unrelated to Newtonian angular momentum, but IANAP.
    - petsfed 3y ago
      Its not completely unrelated, but it is not clearly related. That is, spin contributes to overall angular momentum, but that contribution is much too large to be explained as the actual rotation of subatomic particles. So, like most quantum properties, our Newtonian intuitions are poor guides.
  - truculent 3y ago
    Is using quantum mechanical spin a fair comparison? Isn't it moving the goalposts a bit? > As the name suggests, spin was originally conceived as the rotation of a particle around some axis. Historically orbital angular momentum related to particle orbits. While the names based on mechanical models have survived, the physical explanation has not. Quantization fundamentally alters the character of both spin and orbital angular momentum. > The classical analog for quantum spin is a circulation of energy or momentum-density in the particle wave field: "spin is essential a wave property". https://en.wikipedia.org/wiki/Spin_(physics) https://en.wikipedia.org/wiki/Spin_(physics)
- weberer 3y ago
  Any tidally locked satellite such as the moon.
  - zmgsabst 3y ago
    A tidally locked satellite rotates: one rotation per orbit.
    - this_was_posted 3y ago
      unless the body it rotates around rotates at the exact same speed around another body in the opposite direction
      - jacquesm 3y ago
        Then it still rotates.
        throwaway290 3y ago
        It can only rotate relative to an arbitrary point, not in some objective absolute sense. Here it does not rotate relative to chosen point.
        jacquesm 3y ago
        Something about privileged reference frames...
        throwaway290 3y ago
        But what's the alternative, right?
- princevegeta89 3y ago
  A Pulsar, that's not really spinning but oscillating back and forth, becoming a very highly precise instrument of time measurement as well: https://physicsworld.com/a/pulsar-timekeepers-measure-up-to-atomic-clocks/ https://physicsworld.com/a/pulsar-timekeepers-measure-up-to-...
  - ISL 3y ago
    Pulsars are dense compact spinning objects. From your link: "Pulsars are neutron stars that rotate at very high speeds and appear to emit radio pulses at extremely regular intervals. The pulses are actually all we see of a radio beam that is focused by the star’s magnetic field and swept around like a lighthouse beacon."
vibrio 3y ago
This stuns me for some reason. I was not explicitly aware of this. Why? Just left over momentum from something? Is there a bias in direction (overall or among subsets)?
- noir_lord 3y ago
  Lots of reasons, stuff like "an unevenly spread cloud of something that collapses under it's own gravity" will spin. http://burro.case.edu/Academics/Astr221/SolarSys/Formation/starform.html http://burro.case.edu/Academics/Astr221/SolarSys/Formation/s...
  - throwawaylinux 3y ago
    Link suggests that is because the cloud is already rotating due to "gravitational shearing in the galaxy's disk", not because it is uneven.
    - raattgift 3y ago
      tl;dr parent and parent's link are not wrong, just very simple. Unfortunately it does not make explicit that "at least a little" does not mean the entirety of the rotation, or even its dominant component. In the context above gravitational shear is the first of the empirical https://en.wikipedia.org/wiki/Oort_constants https://en.wikipedia.org/wiki/Oort_constants (differential rotation shears the non-solid disc). Differential rotation means gas in the direction of the galactic centre drags a bit compared to gas further from the centre, effectively acting as a small torque on a large gas cloud. If the gas cloud has no net rotation at all, this torque will develop it. That's what I take the sentence you focus on to be trying to say. Unevenness is related to the Jeans instability, which describes the failure of internal gas pressure to prevent gravitational collapse. Chandrasekhar developed a more complicated treatment useful for when the angular momentum of the bulk gas cloud is non-negligible. Roughly, collisions within the gas cloud convert kinetic energy to light which carries energy out of the collapsing cloud. The cloud consequently cools and contracts gravitationally. However, still roughly, superimposing a net rotation on random motions within the gas prevents collapse in the direction perpendicular to the net rotation's spin axis. So contraction is mainly along the spin axis. Result: a disc, which is essentially what parent's link says. The coupling of the galaxy's spin to the solar system's spin is at best weak; the solar system's spin axis is tilted about 60 degrees from the galactic plane; the plane is perpendicular to the galaxy's spin axis, in line with the previous paragraph. Here's a diagram <https://i.stack.imgur.com/VIipS.jpg https://i.stack.imgur.com/VIipS.jpg>. The sun's path around the centre of the galaxy is a bit messy compared to a kinematically hot star (which will feel perturbations like the bar or spiral arms less). The sun tends to bob up and down relative to the midplane of the galactic disc. The Oort constants are local and position-dependent, so the solar system's migration -- and any migration of its precursor -- means wandering into regions with different gravitational shear. Questions that afaik are still open (but I would be happy for a galaxy or solar system dynamics person to correct me, particularly if the correction includes magnetics and chemodynamics/metallicity-dependent pressure!): has the solar system's spin axis tilt to the galactic disc evolved since formation? Accepting the Coatlicue hypothesis, was the remnant of the explosion of the heavy star that was the precursor of the solar system (and others) aligned with the star's rotation? Was that precursor star's rotational axis aligned with the galactic disc? The remnant was almost certainly not of uniform density. Alternatively accepting the Wolf-Rayet bubble Giant Molecular Cloud (GMC) nebular hypothesis, did the GMC fragmentize under galactic shear, and if so was "our" fragment's spin dominated by that initially, with spin perpendicular to the disc but evolving to the present tilt, or was our fragment's spin axis initially tilted close to the present approx. 60 degrees to the galactic disc where it has since remained? Finally, also afaik, the spins (more broadly the angular momentum vectors) of stars in the local bubble are essentially random. So repeat the questions in the preceding paragraph for each of those...

perlgeek 3y ago

Even black holes "rotate". Since black holes aren't really normal objects, what we mean by that is that the sum of all the matter that created black hole had an angular momentum. This also affects their behavior. See https://en.wikipedia.org/wiki/Rotating_black_hole https://en.wikipedia.org/wiki/Rotating_black_hole for more.

princevegeta89 3y ago

It is also the reason why the planets revolve around it

deleted 3y ago
[deleted]

voytec 3y ago

Spinning is so much cooler than not spinning!

guerrilla 3y ago
warmer*