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I Anthropic constraints on G

  1. May 21, 2017 #1
    Martin Rees talks about the strength of G in his book Just Six Numbers and how it couldn't be much stronger than it is for life to evolve. However he seems to hint that it could be weaker without much problem. Im wondering if anyone knowns any papers on the lower bound constraint for G in terms of anthropoics. How much lower could G be without it threatening life as we know it?
     
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  3. May 23, 2017 #2
    I don't think life would need gravity at all to form and evolve. Consider that most early organisms would be neutrally buoyant and suspended in a fluid. If everything including your insides are all neutrally buoyant,are you even aware of gravity?

    The lower bounds on gravity probably would have more to do with things unrelated to chemistry. Like for example, planets with less gravity than earth, cool and die much faster (look at Mars.)
     
  4. May 23, 2017 #3

    mfb

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    Without gravity there would be no stars and no structures at all. We would just have a thin cloud of hydrogen and helium atoms, probably with a few hydrogen molecules.

    A lower gravitational constant tends to make gravitationally bound objects larger. For the lifetime of stars there are several competing changes in different directions, hard to tell what would win. Gas giants would be more common compared to rock planets.
     
  5. May 23, 2017 #4
    Oh shoot. I misunderstood the thread. I thought this was a discussion on tiny objects with small g, not the gravitational constant. Disregard my answer entirely.
     
  6. May 23, 2017 #5
    Thanks MFb, I am wodnerign though how big the constraints are . for example I understood the cosmological constant could be about 5to 10 bigger without it really disrupting life. So is there a lower bound published in the literature for how much weaker gravity could be anthropically?
     
  7. May 23, 2017 #6

    mfb

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    I don't know specific limits.

    A general thing to keep in mind: These limits are typically for "life as we know it". Different parameters could lead to life-like structures that look completely different.
     
  8. May 24, 2017 #7
    I agree, just looking for published material on this questions. But cant seem to find any.
     
  9. May 26, 2017 #8

    timmdeeg

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    I think even this is hard to imagine. Assuming molecules to exist would require a cosmological model which is consistent with this assumption. It seems that a spacetime without attractive gravity would expand exponentially or would be flat. So, "reheating" with condensation of matter wouldn't happen.
     
  10. May 26, 2017 #9

    mfb

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    You should always have a few odd neutral hydrogen collisions that form molecules.
     
  11. May 26, 2017 #10

    timmdeeg

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    Do you have a cosmological model (without gravity) in mind according to which hydrogen atoms would be created?
     
    Last edited: May 26, 2017
  12. May 26, 2017 #11

    mfb

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    Ah, I guess the early universe would change as well and expand too fast. I was thinking about the universe later.
     
  13. May 27, 2017 #12
    This thread isn't really about a universe with no gravity but what are the (believed) anthropic constraints on how low G can go ?
     
  14. May 27, 2017 #13

    mfb

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    Well, we ruled out G=0.
     
  15. May 28, 2017 #14

    Haelfix

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    One has to be a little careful here. G is a dimensionful constant, and it is frequently set = 1 in Planck units. That of course doesn't mean its SI value is arbitrary, but you have to carefully consider what it is that we are allowed to vary (and what is kept fixed).

    There is another dimensionless value called the gravitational coupling constant that might be a better candidate to use, but note there is also a certain arbitrariness in how the mass scale is set.
    https://en.wikipedia.org/wiki/Gravitational_coupling_constant

    Anyway, having said that whatever candidate you choose (after being careful) can't be zero or negative for obvious reasons.
    As far as other constraints, I found a paper:
    arXiv:0807.3697
    where the author derives a criteria (inequality) for stars to have a stable burning configuration (equation 44) that involves G. This of course is a composite statement (it does not just depend on G, but also involves two other parameters), but you see that there is some wiggle room (you can imagine increasing G by a factor of 10^5 if you keep the other two parameters fixed to their current values).
     
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