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Expansion, Gravity and the Electric Force

  1. Feb 10, 2015 #1
    The explanation as to why galaxies like the Milky Way and Andromeda are not expanding away from each other is that they are gravitationally bound, and the expansion at that small scale is not really apparent anyway. My question is purely hypothetical. What if you had a measuring stick that could extend to great length far enough where the expansion of space were significant. Would the atoms in the stick be electrically bound? In other words, does my measuring stick actually get longer because the space it is occupying is expanding, or does it 'slip' by the expansion because of the more predominant electrical forces holding the atoms together the same way gravitationally bound objects do at smaller scales? If the electrical force wins, and the measuring stick's length doesn't actually change, then what is it exactly that is expanding? If space itself is what is expanding, then it seems to me that a measuring stick occupying some space that is expanding should physically occupy more space (get longer), but this doesn't happen with gravitationally bound objects.
  2. jcsd
  3. Feb 10, 2015 #2


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    What is expanding is the distance between unbound objects. No, your long ruler would not stretch, as it is bound.

    What it is that there gets to be more of between unbound objects when they get farther apart can get to be a philosophical question, but not a scientific one. Google "metric expansion" for more discussion.
  4. Feb 10, 2015 #3


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    there is a fun "thought experiment" called the "tethered galaxy problem" that is a bit like this imaginary long ruler.

    If it's too long the ruler might bust---geometric expansion might win.
  5. Feb 10, 2015 #4


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    I agree with Phinds that the electrical forces, the crystal bonds, are strong and tend to keep material objects bound.
    But I imagine reeling out a steel cable with something massive at the far end, And after a long while the cable is so long that the mass at the far end is beginning to approach relativistic speeds (in the local space around it).

    In the local coordinates of the space around it, that mass is traveling towards me at speeds approaching c. What real physical cable is going to hold?

    The Hubble radius (about 14.4 billionLY) is the length of an ideal superstrong massless cable which if one end were anchored at CMB rest would have the other end moving at c relative to CMB
    Last edited: Feb 11, 2015
  6. Feb 13, 2015 #5
    I think that the notion, of rulers and clocks slipping through curved space-time, is vital to understanding GR

    For example, in Schwarzschild space-time around massive objects, your ruler and your clock stay the same... But space stretches and time compresses, near the object, so that your clock slips through more slices of constant time...

    With the result that distant observers experience your clock ticks to span more time...

    I.e. Looking like life in slow motion
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