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I'm not sure that I understand your question. Certainly all of our understanding of astronomical effects includes (and often depends on) the effects of interven

I'm not sure that I understand your question. Certainly all of our understanding of astronomical effects includes (and often depends on) the effects of intervening clouds of gas and dust: that hasn't been overlooked or omitted from any professional-level analysis. (My memory/guess is that in the presence of an absorptive medium, influences like light intensity will fall off as e^(-a*r)/r^2 rather than just 1/r^2. But the point is that we can model that absorptive term reasonably well, and the underlying 1/r^2 still results naturally from 3D space.) (Meanwhile, to a first approximation, intervening dense objects like planets or stars are rare enough to be negligible: even in a region like our solar system where such objects are most likely to be found, the available volume is overwhelmingly empty space.)

mikorym 8y ago
My question is whether gravity works like light, or whether you can just ignore the planets in between. When for example you calculate the effect of the sun on the earth but venus or mercury happen to be in a line of sight, does it influence the sun-earth calculation component? (Of course, the other planets have their own gravitational influence in line with r^2.)
- xenophonf 8y ago
  Are you indirectly asking whether planets can be dark matter candidates? If so, those are called MACHOs (massive compact halo objects). They have been studied via the gravitational microlensing effect. The presence of MACHOs can't account for most dark matter. https://arxiv.org/pdf/astro-ph/0001272.pdf https://arxiv.org/pdf/astro-ph/0001272.pdf https://arxiv.org/pdf/astro-ph/9904401.pdf https://arxiv.org/pdf/astro-ph/9904401.pdf When you see figures like "1E-7 to 1E-2 M⊙ mass range" (from the above paper's introduction), note that a Jupiter mass is about 1E-3 M⊙---one thousandth or 0.1% the mass of the Sun. Earth is 3E-6 M⊙, the Moon is 3.7E-8 M⊙, and Pluto is 6.6E-9 M⊙; or 0.00037%, 0.000004%, and 0.00000066% the mass of the Sun respectively. Compared to stars, planets comprise a tiny fraction of the mass of a galaxy. TL;DR - No, that's not how gravity works, but you can still ignore the planets in between because they're tiny.
  - ruds 8y ago
    I think the question is: given a 3-body system with objects A, B, and C, is C's gravitational effect on A altered in any way by the position of B, or are B's and C's effects on A independent? The reason for the question, I think, is because of an intuition built up around EM. In the system A B C B can block and otherwise interfere with C's EM radiation so that its effect on A is different depending on B's location.
    - fhars 8y ago
      No, if you want to think of gravity in analogy to light, you can see every mass as analog to a fully transparent light source. There is nothing in this universe that can absorb gravity.
      - Steuard 8y ago
        LIGO was able to detect gravitational waves, which I'm pretty sure must require non-zero transfer of energy in one form or another. (I'm hedging a little only because I know LIGO was measuring relative changes in metric distance, which might possibly not require absorbing energy... but just intuitively it's hard for me to fathom any transfer of information without a transfer of energy: it simply shouldn't be possible.) But 1) it's a remarkably small loss of energy, since the gravitational coupling to matter is so weak, and 2) LIGO is very specifically reacting to gravitational waves rather than to static (or quasi-static) gravitational fields, and the absorptive properties there will certainly be different (just as they are for electromagnetism).
        fhars 8y ago
        General relativity will probably add some effects like that, but then quantum electrodynamics adds light scattering off of light, so the analogy is still ok as far as analogies go.
      - mikorym 8y ago
        > fully transparent light source Thanks. So, in the analogy between gravity and light, you have to change all "massive bodies" to "fully transparent light sources" to make the analogy complete. A nontransparent light source would not translate as a (straightforward) massive body.
- antognini 8y ago
  To first order, gravity is indeed linear in the way you suggest, so you can just add up the effects of different planets separately. To see why this is the case, note that the Sun itself could be considered to be a collection of independent masses. If there were some non-linear gravitational effect, then the gravity of the separate components would be different from the gravity of them all together. However, the surface gravity can be measured across a wide range of masses and scales as you would predict from Newtonian gravity. That being said, general relativity predicts that there is in fact a non-linear scaling when gravitational fields are extremely strong. The reason for this is that the there is some energy associated with the gravitational field, and any energy produces its own gravitational field. So the gravitational field itself exerts its own gravitational field, leading to the non-linearity.
- Steuard 8y ago
  I suppose that gravitational waves may be ever so slightly attenuated as they pass through matter, but the coupling is so weak that we probably wouldn't notice. (Think of how hard LIGO had to search before they found the first black hole merger signal.) So even for waves, the gravitational effects of the sun would be all but unaffected by Mercury or Venus being in the way. For a static 1/r^2 field, intervening matter has very little effect at all. (There are non-linearities in principle, but for all but the most extreme configurations they're negligible in practice.) Certainly when the Newtonian approximation holds you can just add up the separate 1/r^2 effects of all the separate sources to find the overall effect. (This, by the way, is very similar to the behavior of electromagnetic fields. The waves are attenuated or blocked by intervening matter fairly easily, but it takes a pretty special situation to actually block the 1/r^2 effects of a static electric or magnetic field: you'd need to surround your system in a conducting "cage" that would polarize in response to an external electric field to cancel out the effects of that field inside, for example. That isn't possible for gravity, since there doesn't exist negative mass to do the screening.) Ultimately, the 1/r^2 rule really is intimately connected to the three dimensions of space in our universe. In theories with additional spatial dimensions, that rule changes. Many theories with extra dimensions follow the Kaluza-Klein model, where the extra dimensions are "curled up" very small (in the sense that if you were to travel a distance L in that extra dimension, you'd come back to where you started: like a little circle). In the case of one extra dimension like that, if you were to measure the behavior of gravity over distances much shorter than L, you'd find that the gravitational force fell off like 1/r^3, but if you measured its behavior over distances much longer than L you'd find the familiar 1/r^2. (And for that reason, we know that the L for any system like this must be very small: my memory is that direct gravity measurements can set a limit like L<1mm or maybe even L<1 micron, and indirect evidence from things like particle physics observations pushes the limit down to the nuclear scale or below: L<10^(-15) m or even much less. I probably ought to know the actual values of both of those limits off the top of my head, but it's been a while since I looked at it.)

mikorym 8y ago

My question is whether gravity works like light, or whether you can just ignore the planets in between. When for example you calculate the effect of the sun on the earth but venus or mercury happen to be in a line of sight, does it influence the sun-earth calculation component? (Of course, the other planets have their own gravitational influence in line with r^2.)

xenophonf 8y ago
Are you indirectly asking whether planets can be dark matter candidates? If so, those are called MACHOs (massive compact halo objects). They have been studied via the gravitational microlensing effect. The presence of MACHOs can't account for most dark matter. https://arxiv.org/pdf/astro-ph/0001272.pdf https://arxiv.org/pdf/astro-ph/0001272.pdf https://arxiv.org/pdf/astro-ph/9904401.pdf https://arxiv.org/pdf/astro-ph/9904401.pdf When you see figures like "1E-7 to 1E-2 M⊙ mass range" (from the above paper's introduction), note that a Jupiter mass is about 1E-3 M⊙---one thousandth or 0.1% the mass of the Sun. Earth is 3E-6 M⊙, the Moon is 3.7E-8 M⊙, and Pluto is 6.6E-9 M⊙; or 0.00037%, 0.000004%, and 0.00000066% the mass of the Sun respectively. Compared to stars, planets comprise a tiny fraction of the mass of a galaxy. TL;DR - No, that's not how gravity works, but you can still ignore the planets in between because they're tiny.
- ruds 8y ago
  I think the question is: given a 3-body system with objects A, B, and C, is C's gravitational effect on A altered in any way by the position of B, or are B's and C's effects on A independent? The reason for the question, I think, is because of an intuition built up around EM. In the system A B C B can block and otherwise interfere with C's EM radiation so that its effect on A is different depending on B's location.
  - fhars 8y ago
    No, if you want to think of gravity in analogy to light, you can see every mass as analog to a fully transparent light source. There is nothing in this universe that can absorb gravity.
    - Steuard 8y ago
      LIGO was able to detect gravitational waves, which I'm pretty sure must require non-zero transfer of energy in one form or another. (I'm hedging a little only because I know LIGO was measuring relative changes in metric distance, which might possibly not require absorbing energy... but just intuitively it's hard for me to fathom any transfer of information without a transfer of energy: it simply shouldn't be possible.) But 1) it's a remarkably small loss of energy, since the gravitational coupling to matter is so weak, and 2) LIGO is very specifically reacting to gravitational waves rather than to static (or quasi-static) gravitational fields, and the absorptive properties there will certainly be different (just as they are for electromagnetism).
      - fhars 8y ago
        General relativity will probably add some effects like that, but then quantum electrodynamics adds light scattering off of light, so the analogy is still ok as far as analogies go.
    - mikorym 8y ago
      > fully transparent light source Thanks. So, in the analogy between gravity and light, you have to change all "massive bodies" to "fully transparent light sources" to make the analogy complete. A nontransparent light source would not translate as a (straightforward) massive body.
antognini 8y ago
To first order, gravity is indeed linear in the way you suggest, so you can just add up the effects of different planets separately. To see why this is the case, note that the Sun itself could be considered to be a collection of independent masses. If there were some non-linear gravitational effect, then the gravity of the separate components would be different from the gravity of them all together. However, the surface gravity can be measured across a wide range of masses and scales as you would predict from Newtonian gravity. That being said, general relativity predicts that there is in fact a non-linear scaling when gravitational fields are extremely strong. The reason for this is that the there is some energy associated with the gravitational field, and any energy produces its own gravitational field. So the gravitational field itself exerts its own gravitational field, leading to the non-linearity.
Steuard 8y ago
I suppose that gravitational waves may be ever so slightly attenuated as they pass through matter, but the coupling is so weak that we probably wouldn't notice. (Think of how hard LIGO had to search before they found the first black hole merger signal.) So even for waves, the gravitational effects of the sun would be all but unaffected by Mercury or Venus being in the way. For a static 1/r^2 field, intervening matter has very little effect at all. (There are non-linearities in principle, but for all but the most extreme configurations they're negligible in practice.) Certainly when the Newtonian approximation holds you can just add up the separate 1/r^2 effects of all the separate sources to find the overall effect. (This, by the way, is very similar to the behavior of electromagnetic fields. The waves are attenuated or blocked by intervening matter fairly easily, but it takes a pretty special situation to actually block the 1/r^2 effects of a static electric or magnetic field: you'd need to surround your system in a conducting "cage" that would polarize in response to an external electric field to cancel out the effects of that field inside, for example. That isn't possible for gravity, since there doesn't exist negative mass to do the screening.) Ultimately, the 1/r^2 rule really is intimately connected to the three dimensions of space in our universe. In theories with additional spatial dimensions, that rule changes. Many theories with extra dimensions follow the Kaluza-Klein model, where the extra dimensions are "curled up" very small (in the sense that if you were to travel a distance L in that extra dimension, you'd come back to where you started: like a little circle). In the case of one extra dimension like that, if you were to measure the behavior of gravity over distances much shorter than L, you'd find that the gravitational force fell off like 1/r^3, but if you measured its behavior over distances much longer than L you'd find the familiar 1/r^2. (And for that reason, we know that the L for any system like this must be very small: my memory is that direct gravity measurements can set a limit like L<1mm or maybe even L<1 micron, and indirect evidence from things like particle physics observations pushes the limit down to the nuclear scale or below: L<10^(-15) m or even much less. I probably ought to know the actual values of both of those limits off the top of my head, but it's been a while since I looked at it.)