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Why Don't Planets Fall into the Sun?

  1. Apr 11, 2012 #1
    Why Don't Planets "Fall" into the Sun?

    Ok, guys, I am new to physics (just studying it on my own before taking it next year in school) and watched a bunch of videos online about general relativity and gravity.

    I'm sort of stuck, though, with why if general relativity explains gravity as the warping/curving of space-time itself that planets don't "fall" into or towards the sun?

    ...So like, if the space around the sun is "indented" the way of bowling ball resting on a trampoline would be, then shouldn't objects not only be "pulled" towards the center of that indentation...but literally like fall into the center. If you have that bowling ball resting in the center of the trampoline and causing an indentation and warping the shape of the trampoline, then if you like roll a tennis ball on the part of the trampoline where it's warped/indented, then that tennis ball "falls" into the center. ....But why wouldn't that happen with our planets and the sun too?
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  3. Apr 11, 2012 #2


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    Re: Why Don't Planets "Fall" into the Sun?

    The tennis ball slows down due to resistance (rolling, airdrag) and so it spirals down. The planets don't have any resistance in space.

    But that trampoline model is a completely flawed analogy of gravitation in general relativity anyway. You can use it for Newtons gravity where it represents gravitational potential:

    But for GR you have to include the time dimension:
  4. Apr 11, 2012 #3


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    Re: Why Don't Planets "Fall" into the Sun?

    This doesn't need Relativity and I think is much the same with or without it.

    Your way of thinking is just the mental block everyone has had that it took the genius of Newton to get past. The planets are falling towards the Sun. Roughly speaking, as they are going sideways at the same time the result is they don't get any nearer to it. The direction of their movement is almost (or exactly in the case of circular motion) perpendicular to the directions of their acceleration.

    Quite simple and at the same time quite difficult.

    If by retro rockets you stopped a planet still, then it would fall into the sun. If on your dimpled trampoline you start the ball rolling not towards the centre and the thing is frictionless, it will go round and round for ever, not into the centre.
  5. Sep 25, 2012 #4
    Re: Why Don't Planets "Fall" into the Sun?

    Ok, so to be sure of what you guys are saying it's friction that causes that ball to "fall" into the center? Because friction slows the tennis ball down and it loses energy...and can't keep it's path and falls towards the bowling ball at the center of the trampoline?

    Why isn't there air resistance friction in space or other types of friction?
  6. Sep 25, 2012 #5
    Re: Why Don't Planets "Fall" into the Sun?

    There could still be any type of friction in space. There's just not much up there to cause any friction. No air in space.
  7. Sep 25, 2012 #6
    Re: Why Don't Planets "Fall" into the Sun?

    I think part of the problem with the bowling ball analogy is that it only takes into account a single slice of the spatial extent of the universe. It would be more accurate, probably, to have a series of slices stacked on each other, but there'd be so many of these slices that it would look like the bowling ball was embedded within a solid structure, at which point it would become extremely difficult if not impossible to visualize the gravitational warping.
  8. Sep 25, 2012 #7
    Re: Why Don't Planets "Fall" into the Sun?

    I'm sorry, but this continually falling theory may satisfy all physicists in the world and satisfy their theories, but I'm not very comfortable with this analysis of the situation. I know, my opinion doesn't count, but I feel there is more to it.

    Something can not fall for billions of years with such negligible affect.
  9. Sep 25, 2012 #8


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    Re: Why Don't Planets "Fall" into the Sun?

    "Cant"??? On what basis do you make that statement? Do you actually know the values of the forces and energies involved? If you want a Physics argument then you need to argue with Physics - or invent a complete Physics of your own.
  10. Sep 25, 2012 #9
    Re: Why Don't Planets "Fall" into the Sun?

    So you're saying that the angle of the warp is more "flat" than we may think? It's not like a dramatic warping ...like falling into a pit at a steep angle, but more like a very very slight angle?

    But even then, things do tend to "fall" into any indentation still right? ...well maybe not, I dunno. Can there be some indentation sooooooooo slight that it doesn't affect anything resting along that indentation? I would think that things would still "fall" towards the center of some angle/indentation.

    But I could be wrong. I was just starting to think that the reason they don't fell from what other people said is because in space there's no friction and the tennis ball rolling around the edges of the trampoline would never lose its energy??? and never slow down to a point where to just falls down that angle or indentation.

    Maybe I'm more confused now? lol. :D
  11. Sep 25, 2012 #10
    Re: Why Don't Planets "Fall" into the Sun?

    Look, think of it this way. Did you know gravity can affect light too? How, if light is massless?

    Very simple, actually. Gravity is not a traditional force. It literally warps spacetime. Light travels the shortest distance it can between two points. Under the influence of gravity, that shortest distance takes the form of a curve. Thus, in a sense, light is still traveling in a straight line even if it is "curved".

    So too with planets around a central body. They're technically traveling in straight lines. It's just that their velocity vector is continuously changing so that it is always tangent to the path formed by an ellipse with one of its foci at the central body. It'd be easier to draw you a picture of this.

    An alternative view is more classical. In the classical view, circular motion is defined as a=v^2/r, where a is the acceleration towards the center of the circle, v is the velocity of the object tangent to the circle, and r is the distance from the center of the circle to any point on the circle. The function of this inward acceleration is solely to change the direction of the velocity vector; it does not alter the magnitude of the velocity vector because the magnitude of the velocity vector of an object in constant circular motion is constant - it doesn't change. There is no tangential acceleration, and so no energy mysteriously goes into the object.

    Convince yourself of either one of these explanations. It's the only way to go.
  12. Sep 25, 2012 #11
    Re: Why Don't Planets "Fall" into the Sun?

    You being comfortable or not with a theory has no impact on its validity. Me being uncomfortable with quantum mechanics doesn't make it false.

    Newtonian physics does the best job of explaining planetary motions to anyone who hasn't studied plenty of physics. Relativity seems like a bunch of crazy talk. I know it isn't but it seems that way for those of us who haven't studied it extensively.

    You might want to view a few videos on something called Centripetal Force and Newton's universal law of gravity if you want to understand planetary motion.
    Last edited: Sep 25, 2012
  13. Sep 25, 2012 #12
    Re: Why Don't Planets "Fall" into the Sun?

    If you prefer to base your reality on things that make you comfortable, then try religion instead of science.
  14. Sep 26, 2012 #13
    Re: Why Don't Planets "Fall" into the Sun?

    I think my problem is with the words, continually falling. These words ignore forces and counter forces that make things stable or semi stable.
    I do not see much differences between a planet getting stuck in an orbit, after continually falling, and a charged particle in a magnetic field continually spirals until falls into a stable orbit.

    I have recently seen a video by a Russian physicist who demonstrated path of the earth is actually spirals in space. Which is correct if you view the motion of earth from center of galaxy. It seems point of view may also play a part.

    No, I have no intention to invent new physics, physics will invent new physics, if necessary. I have trust in physics.

    Thanks God, we have only one model of microscopic world, quantum mechanics?
  15. Sep 26, 2012 #14


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    Re: Why Don't Planets "Fall" into the Sun?

    "Constantly falling" is a bit of shorthand which could be confusing. It's best understood how the argument goes.
    Stand on a mountaintop and fire a very high velocity shell horizontally. (Ignore air resistance at this level) It 'falls' to the ground eventually. But the Earth is curved so it drops to the ground at a point beyond (below) the horizon, viewed from the gun. Increase the launch velocity (still keeping the gun horizontal), the shell will land further away. It has been 'falling' all the time because it is being attracted to the centre of the Earth. If you launch it fast enough, it will get right round the Earth and hit you on the back of the head - having been 'falling' towards the centre all the time but just enough to keep it the same distance above the ground. If you had removed yourself and the gun in the 90,or so, minutes it would take to get round, it would just keep on going in 'an orbit. This same argument would apply how ever far up you launched the shell as long as it is fired at right angles to a radius and with the right speed. Even when it's a satellite, 40,000km away, it is still, in effect, constantly falling (but getting no closer).
    Most orbits are not perfectly circular so the word 'falling' is a bit inaccurate because, in an elliptical orbit, the shell would be climbing sometimes and falling at other times. It still never actually gets to the ground. In all cases, the experience if you were on board the shell would be the same as if you were in free fall- i.e. no apparent gravity / no feeling of weight.
  16. Sep 26, 2012 #15
    Re: Why Don't Planets "Fall" into the Sun?

    It all makes sense if you do the maths.

    The acceleration vector (which is parallel with the force vector since f=ma) is not in general tangential to an objects trajectory. For example if you throw a ball (neglecting air resistance) the ball follows a parabola while it's acceleration is always directly downwards.

    In non technical terms, an objects path is not in general in the direction of its acceleration.
    Using the word "fall" is very misleading.

    If you want to get into that muddy quagmire that is general relativity, objects move on geodesics determined by the metric tensor of the spacetime around them, if you solve these geodesics they will give you orbits as long as you have non zero angular momentum (actually it's a conserved quantity generated by the killing vector, but i suspect you have stopped reading).
  17. Sep 26, 2012 #16

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    Re: Why Don't Planets "Fall" into the Sun?

    No! "Falling" implies a force. With no forces you have a constant velocity. No extra forces are make orbits stable. The inverse square law nature of gravitation is all that is needed.

    Imagine a well-struck baseball. It follows an arc, first rising, then falling. During that time that the ball is rising, gravitation is reducing the upward component of the ball's velocity outfielder. Gravitation is pulling the ball downward throughout the entire flight of the ball. In a sense, the ball is "falling" even when it is rising. This is the meaning of "falling" you should think of when we say that an object in orbit is "constantly falling". With this meaning of the word "falling", "constantly falling" is a perfect description of orbits, including elliptical ones.

    BTW, the solar system is only marginally stable. Mercury is at the borderline; it might eventually be kicked out of orbit or collide with Venus thanks to perturbations by Jupiter. The solar system is also dynamically full. There is no room for an extra planet; if there was one it would be kicked out gravitationally. This may well have happened during the formation of the solar system. There are, for example, signs that the solar system originally had five gas giants, with one of them having been ejected when the system was still quite young.
  18. Sep 26, 2012 #17
    Re: Why Don't Planets "Fall" into the Sun?

    I'm pretty sure there is some effect. How negligible or measureable is another matter. Artificial satellites, for example, also are "continually falling" and in many instances that effect is not negligible. They usually need boosting or end up falling back to earth after a few months/ years/ decades.

    Planets are MANY orders of magnitude heavier, so it's not as easy to slow them down. Even then, they also have many orders of magnitude more distance to fall towards the sun. And probably many orders of magnitude less air friction. I'm not going to do the calculations, but if you're asking a few molecules of gas to cause 5973600000000000000000000 kg to slow down and fall 149598261 km (in the instance of earth), billions of years doesn't seem so far fetched.
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