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Forces in the solar system

  1. Dec 31, 2015 #1
    I read somewhere that on the scale of the solar system, all forces except gravity are negligible. I can't think of any forces except gravity that would be acting on the planets... what are these other negligible forces?
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  3. Dec 31, 2015 #2


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    There are four fundamental forces in nature: gravitational, electromagnetic, weak nuclear and strong nuclear. Only the first one matters as far as planetary motion is concerned.
  4. Dec 31, 2015 #3
    The electromagnetic force isn't completely negligible, it's the reason that we have amazing auroras on Earth and a few other places. They're negligible in terms of their ability to affect planets or other large objects, but the solar wind is pushed around by the magnetic fields of some of the planets.
  5. Dec 31, 2015 #4
    I'd like to add dark energy which is a sort of anti gravity. The effects of this force can't be seen in solar systems or even in galaxies, but they do effect the universe and is one of the factors thought to be causing the expansion.
  6. Dec 31, 2015 #5


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    I never heard DE referred to as a "force" in the professional literature. That must be a misleading distortion you get in popular media accounts.
    It is not something you measure in pounds units, or Newtons (the metric unit of force).

    As far as we know "DE" is a technical jargon alias for the cosmological curvature constant Lambda (Λ) introduced by Einstein in 1917. It has units of inverse length squared, or equivalently inverse time squared.
    There is no convincing evidence that it is associated with an actual energy, so far all the data is consistent with its simply being a small constant inherent spacetime curvature.
    Λ ≈ 10-35 seconds-2

    If you want to check that this value is right put this into the google window and press return:
    3/(17.3 billion years)^2
    Google calculator will answer:
    "3 / ((17.3 billion years)^2) = 1.00656 × 10-35 s-2"
    which is as close as we know the value of the "dark energy" or cosmological curvature constant.
    17.3 billion years is the longterm Hubble time, according to most recent (Planck mission) results.
  7. Dec 31, 2015 #6
    To be frank neither have I, but from my reading, it's analogous to an energy(dark energy or vacuum energy). Plus it causes a change in space which I think is what forces do.

    But isn't spacetime curvature caused by forces like gravity? The ever changing(because it's only constant for the static universe senario) cosmological curvature constant tells us that space is expanding at an increasing rate, and forces cause acceleration. What would be causing this acceleration if not for a force? which we know can't be one of the fundamental forces. Although it's not set in stone yet, the accelerating expansion of the universe seems to be pretty good evidence of dark energy to me.

    It makes sense that it's related to spacetime curvature but I wouldn't say inherent. We could be going into the what came first the chicken or the egg.

    In terms of units I get the feeling it could be something that can be measured in Newtons but all we have are, units from measured observations.

    A little something from scholarpedia (because I cant find my original source)

    In the context of cosmology the cosmological constant is a homogeneous energy density that causes the expansion of the universe to accelerate. Originally proposed early in the development of general relativity in order to allow a static universe solution it was subsequently abandoned when the universe was found to be expanding. Now the cosmological constant is invoked to explain the observed acceleration of the expansion of the universe. The cosmological constant is the simplest realization of dark energy, which is the more generic name given to the unknown cause of the acceleration of the universe. Its existence is also predicted by quantum physics, where it enters as a form of vacuum energy, although the magnitude predicted by quantum theory does not match that observed in cosmology.
  8. Dec 31, 2015 #7


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    A lot of people seem to be stuck on the idea that the small constant curvature Λ is caused by an energy density. You can convert back and forth between curvature and energy density using the constant 8πG/c4.
    So you can pretend there is an energy density and it causes the curvature but AFAIK there is no evidence that there is such an energy density.

    In any case an energy density is not a FORCE. So best not to call Λ a force.
    For Einstein, when he put Λ in the equation in 1917, it was a CURVATURE, a reciprocal length squared or time squared.
    We now know that Λ≈10-35 second-2 There is no observational evidence that this has ever changed.

    "The ever changing(because it's only constant for the static universe senario) cosmological curvature constant tells us that space is expanding at an increasing rate, and forces cause acceleration. What would be causing this acceleration if not for a force?"

    No what you say about Λ changing is not true. As best we can measure it is constant it was, as far back as we can measure, the same curvature that it is today.
    This seems to be the simplest way to understand Λ. Just a constant of nature a residual amount of spacetime curvature inherent in spacetime. Other curvature caused by matter, energy etc. just comes in on top of the small underlying amount. Some day we may learn its cause, might be all or in part some kind of energy density, might be something else. For now all we know is the curvature. Don't need to make "hypotheses" about it, myths fantasies, mysteries etc.

    But please don't call it a "force" :eek:. that it definitely is not :oldbiggrin:
    Last edited: Jan 1, 2016
  9. Jan 1, 2016 #8


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    Light is one of the insignificant forces. It's actually not negligible to fine dust and gas. As objects get smaller, their mass decreases as an inverse cube, while the cross section they display to the Sun decreases as an inverse square.

    For smaller asteroids, the Yarkovsky Effect can influence their orbits over long periods of time, as can the Poynting-Robertson effect.

    Sometimes astronomers discover small objects on low-velocity Earth-crossing orbits. Often they turn out to be man-made space junk. They determine this because the objects do not follow the path they would be expected to follow if acting under the influence of gravity only. Light has a much easier time pushing around a hollow piece of metal than a solid rock.

    Comets outgas. This creates a force that causes them to deviate from a gravitational-only path.
  10. Jan 1, 2016 #9
    That's not a matter of scale. A Dyson sphere would be a counter example.
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