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Sun disappearing thought experiment

  1. Jun 23, 2010 #1
    I have viewed other threads regarding this thought experiment but have not seen any that answer my question.

    "If" the sun disappeared, the earth would just continue on in a straight line, I understand that. But what about our moon? Would it continue to orbit the earth as the earth travelled through space? Would it eventually smash into earth or go on its own merry way in some other direction? How does the sun's gravity affect the moon now?

    I found this on Wikipedia, but could someone elaborate on it?
    "Although if the gravitational attraction of our Sun could be "turned off", the Moon would continue to make one orbit about the Earth with its current sidereal period."
  2. jcsd
  3. Jun 23, 2010 #2


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    The sun's gravity can best be viewed as acting on the earth plus moon system as a whole. Therefore turning off the sun's gravity would leave the earth-moon system together, going is a straight line, although the other planets would have some gravitational effect.
  4. Jun 25, 2010 #3
    The planets and the Oort cloud have a center of gravity as a group. It would be much weaker without the sun, so they go spiralling out, I guess. Not in a straight line.
  5. Jun 25, 2010 #4
    Not in a straight line? What kind of path would they take then?
  6. Jun 25, 2010 #5
    I think that would depend greatly on the alignment of the planets relative to Earth at the time of the 'disappearance'.
  7. Jun 26, 2010 #6


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    The sun cannot 'disappear' so the point is moot.
  8. Jun 26, 2010 #7
    Well then don't read this thread. I like discussing "what if" scenarios, even if they are completely impossible. It's impossible for anyone to fly through a black hole, but people still discuss what would happen if they did. I visit this site looking for a better understanding of physics with help from people way more knowledgeable than myself. Sorry if my thought experiment offended anyone, I'll stop thinking from now on.
  9. Jun 26, 2010 #8


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    1] The Solar System sans Sun has a mass that is far too small to serve as a centre of gravity.
    2] Even if it were large enough, the masses are distributed throughout the SS, and will not act as a single point-source; they will act individually (i.e. some are to the left, some to the right, some ahead, some behind) to deflect a given planet's path (and the deflection will be very small).

    The upshot is: the planets will move off from their former orbits in a tangential and (for all intents and purposes) straight line. The straight line will have a tiny wobble from perfectly straight, based on the tiny masses of the other planets, depending on their proximity.
  10. Jun 26, 2010 #9


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    For this thought experiment to be valid, it is not necessary for the sun to violate the laws of physics.

    1] It could be moved away very quickly. That would muddy the otherwise clean trajectories but we can still remove that variable study the ideal case.
    2] The general principle can be scaled down to the point where we could effectively whisk the central body away. For example, the venerable weight-on-a-string-on-a-record-turntable. Cut the string and the weights follow straight lines.
  11. Jun 26, 2010 #10


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    No need to get all on-the-cross about it. We are pretty strict about upholding the laws of physiis here, even when what-iffing.

    Thought experiments are tricky things. You have to be careful to avoid violating the laws of physics or you end up in fantasyland.

    It's one thing to handwave a law that is not relevant to the thing you are trying to observe (example, a magically sturdy spaceship that could survive the g-forces so we could see what's going on), but it's another thing to violate the very law you're trying to ask your question about.

    This is what Chronos is warning you away from.

    In a literal sense it is meaningless to ask how a discontinuity in the gravitational field would affect trajectories - if that discontinuity can't exist.

    But we can have a rapid (yet still continuous) change in the gravitational field (by whisking the sun away at an arbitrarily fast speed), and now we observe a continuous (though rapid) change in planetary trajectories from elliptical to straight line.
  12. Jun 26, 2010 #11


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    The problem with asking "What happens if the Sun disappears?" and expecting an answer based on GR is that there's no solution of Einstein's equation that describes a disappearing Sun. It's OK to ask what would happen if you eat 1020 hamburgers and fart so hard that it throws the Earth out of orbit, but it doesn't make any sense to ask a question that includes an assumption that makes the theory we're supposed to use to answer the question logically inconsistent.

    It's fine to ask what Newton's theory says would happen, because Newton's theory isn't made inconsistent by a disappearing Sun.
  13. Jun 26, 2010 #12


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    OK but the OP's question isn't utterly dependent on the Sun violating any laws of physics.

    The OP is interested in orbital mechanics. So let's change the question to:

    What would happen to the paths of the planets if the sun were propelled out of the solar system (along its "z" polar axis) at an arbitrarily high acceleration? Let's ignore planetary motion along the z axis, just the motion in xy.
  14. Jun 27, 2010 #13


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    I like a laugher as much as the next person. Zoom the sun away and the planets try follow, and get totally screwed up orbits - a spiral effect. Most would probably collide, eventually. Making mass 'disappear' is not realistic. The planets, still obeying newtons laws, would tend to tangentially leave the 'solar system' as noted. Nothing astonishing about that. Trying to force GR to do something weird is the nature of my objection.
  15. Jun 27, 2010 #14


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    Except that this is what the OP wanted to know.
  16. Jun 28, 2010 #15
    Dave is right, I was just interested in the "orbital mechanics" of the moon-earth system without the gravitational well of the sun. What happens to the moons orbit about the earth when the suns gravity is gone? I know it will never happen. I just thought it's orbit might change enough to the point where it could impact the earth at some point. Perhaps it might impact the earth before all life on earth would freeze. I just wanted an educated guess as to what might happen.

    I love all you guys here at PF, I come here everyday, mostly to read, but every once in a while I'll have a question. And most of my questions are thought experiments. Sorry for my previous rant, won't happen again.
  17. Jun 28, 2010 #16
    I am no physicist, surving merely one year of university level tuition, so consider my next statement in that light.
    All masses have a centre of gravity, surely. do they not?
    If not can you clarify for me why they do not?
    If you meant something else, which I think is the case, could you amplify this?
  18. Jan 9, 2011 #17
    Hi. I just wanted to expand on this thought experiment. I think the clear answer to what happens if the Sun suddenly is removed from the system is: the orbiting objects are no longer affected by the Sun's gravity, inertia takes over, essentially planets continuing in a straight line...

    The real question here, though, is WHEN does this happen? Since theoretically nothing can travel faster than the speed of light, which takes approximately 8 minutes to travel from the Sun to Earth, WHEN does the affect of the Sun's gravity cease to act on the Earth?

    IF gravitons do exist, do they travel faster than photons, and if so, how much faster (i.e., if the disappearance of the Sun has an immediate effect on Earth's orbit, does this imply that gravitons have infinite velocity...)?

    I'd love to hear some explanations. Thanks.
  19. Jan 9, 2011 #18


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    Whether or not gravitons exist, changes in gravity propogate at the speed of light. So, 8 minutes.
  20. Jan 9, 2011 #19


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    I'm sorry; I never saw this post, so never responded.

    The planets of the solar system do indeed have a centre of mass and do have a tiny effect on each other, but the pull is quite small at these distances. Planets will perturb each other, but it is a vanishingly small effect relative to the headlong flights of the planets on their beeline trajectories out of the now defunct solar system.
  21. Jan 10, 2011 #20
    The sun disappears and its gravity goes with it. However, the angular momentum of the planets which once kept them from falling into the sun remains. Now that the string of gravity is gone we can safely assume that each celestial object will travel away from the direction it was once tethered to. That includes asteroid belts, Keiper belt, Ort Cloud and comets all drifting into interstellar space at different velocities. Here are the velocities involved for eight planets:



    Mercury 47.89

    Venus 35.03

    Earth 29.79

    Mars 24.13

    Jupiter 13.06

    Saturn 9.64

    Uranus 6.81

    Neptune 5.43
    Last edited: Jan 10, 2011
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