5 ms·

I'm only a very small passively interested layperson here, but does this seem much faster than anyone else thought? I looked it up and a "galactic year" for ou

I'm only a very small passively interested layperson here, but does this seem much faster than anyone else thought? I looked it up and a "galactic year" for our sun is only 250 million years. Meaning our planet has been around the galaxy 18 times since its formation! That seems like so much faster than I would have thought!

pvg 9y ago
Fast or slow compared to what? If you think of it as 18 times in a third of the age of the whole universe it doesn't seem particularly peppy.
Filligree 9y ago
This is indeed fast. In fact, it's much faster than they should be spinning, going by the visible mass; a galaxy spinning at the speed ours is ought to tear itself apart. It doesn't, which is a lot of the reason why we think dark matter exists. Here's a video on the subject: https://www.youtube.com/watch?v=Dx1Wf84bC2M https://www.youtube.com/watch?v=Dx1Wf84bC2M
- kstrauser 9y ago
  As an alternative, could we also be wrong about the gravitational constant at a large scale, or perhaps it's not really exactly an inverse square? Or maybe there's another relatively weak force that's only apparent at a larger scale that maybe only drops off linearly so it's predominant at large distances?
  - jerf 9y ago
    The Google term for that if you want to read more about it is "MOND", MOdified Newtonian Dynamics. It's not the theory favored by the consensus, but it's not out of the fight yet.
    - gepoch 9y ago
      There's also TEVES which is MOND with relativity, since MOND doesn't predict any of the relativistic effects we observe constantly. The problem is, they still end up needing some dark matter before they work very well. I think every bright person assumes dark matter must be fake when they hear about it, but nobody has succeeded in getting rid of it. Dark matter plus the physics we know about is actually a really great fit to what astronomers see, and it has been hard to improve on. https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
      - jerf 9y ago
        Yes, I should say that I'm not necessarily advocating for it, just giving a hint as to where to look the ideas up. Personally I'm just fine with either. Particle physics makes it pretty clear IMHO that the idea of a weakly interacting massive particle is perfectly reasonable, because the particle zoo that we have is already rich far beyond "normal matter" and adding a few more isn't conceptually too difficult. (It'll break our theories, of course, but we kinda already know there must be something wrong with them, so that's not a huge shocker.) And the idea that physical laws may not be exactly perfectly the math we thought they were is also not that shocking to me, for all kinds of reasons, again not least of which is that we know something is wrong with our current theories.
    - kstrauser 9y ago
      Awesome. Thanks for the starting off point!
Romanulus 9y ago
... Also, don't the disks spin at the same speed/uniformly? Meaning closer to the axis things go slower than on the edges?
- deleted 9y ago
  [deleted]
- davidp 9y ago
  That's the apparent paradox in the galaxy rotation curve: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve
Scaevolus 9y ago
Density wave theory [1] posits that the "arms" of a spiral galaxy are more like stellar gravitational traffic jams than persistent objects-- stars pass through and clump up in arms, but the stars in each arm change through the eons. If you plot mass extinction events against when the solar system transits through the arms, eleven extinction events in the last 500 million years occurred while going passing through one of these high density regions. [2] [1]: https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory [2]: https://arxiv.org/abs/1309.4838 https://arxiv.org/abs/1309.4838
- tpurves 9y ago
  I find this concerning. When is the next time the sun will do this? are spiral arms a great filter, or effective semi-regular eliminator of advanced galactic life?

pvg 9y ago

Fast or slow compared to what? If you think of it as 18 times in a third of the age of the whole universe it doesn't seem particularly peppy.

Filligree 9y ago

This is indeed fast. In fact, it's much faster than they should be spinning, going by the visible mass; a galaxy spinning at the speed ours is ought to tear itself apart. It doesn't, which is a lot of the reason why we think dark matter exists. Here's a video on the subject: https://www.youtube.com/watch?v=Dx1Wf84bC2M https://www.youtube.com/watch?v=Dx1Wf84bC2M

kstrauser 9y ago
As an alternative, could we also be wrong about the gravitational constant at a large scale, or perhaps it's not really exactly an inverse square? Or maybe there's another relatively weak force that's only apparent at a larger scale that maybe only drops off linearly so it's predominant at large distances?
- jerf 9y ago
  The Google term for that if you want to read more about it is "MOND", MOdified Newtonian Dynamics. It's not the theory favored by the consensus, but it's not out of the fight yet.
  - gepoch 9y ago
    There's also TEVES which is MOND with relativity, since MOND doesn't predict any of the relativistic effects we observe constantly. The problem is, they still end up needing some dark matter before they work very well. I think every bright person assumes dark matter must be fake when they hear about it, but nobody has succeeded in getting rid of it. Dark matter plus the physics we know about is actually a really great fit to what astronomers see, and it has been hard to improve on. https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
    - jerf 9y ago
      Yes, I should say that I'm not necessarily advocating for it, just giving a hint as to where to look the ideas up. Personally I'm just fine with either. Particle physics makes it pretty clear IMHO that the idea of a weakly interacting massive particle is perfectly reasonable, because the particle zoo that we have is already rich far beyond "normal matter" and adding a few more isn't conceptually too difficult. (It'll break our theories, of course, but we kinda already know there must be something wrong with them, so that's not a huge shocker.) And the idea that physical laws may not be exactly perfectly the math we thought they were is also not that shocking to me, for all kinds of reasons, again not least of which is that we know something is wrong with our current theories.
  - kstrauser 9y ago
    Awesome. Thanks for the starting off point!

Romanulus 9y ago

... Also, don't the disks spin at the same speed/uniformly? Meaning closer to the axis things go slower than on the edges?

deleted 9y ago
[deleted]
davidp 9y ago
That's the apparent paradox in the galaxy rotation curve: https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve

Scaevolus 9y ago

Density wave theory [1] posits that the "arms" of a spiral galaxy are more like stellar gravitational traffic jams than persistent objects-- stars pass through and clump up in arms, but the stars in each arm change through the eons. If you plot mass extinction events against when the solar system transits through the arms, eleven extinction events in the last 500 million years occurred while going passing through one of these high density regions. [2] [1]: https://en.wikipedia.org/wiki/Density_wave_theory https://en.wikipedia.org/wiki/Density_wave_theory [2]: https://arxiv.org/abs/1309.4838 https://arxiv.org/abs/1309.4838

tpurves 9y ago
I find this concerning. When is the next time the sun will do this? are spiral arms a great filter, or effective semi-regular eliminator of advanced galactic life?