Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B How exactly does gravity work?

  1. Feb 23, 2017 #1
    how does gravity actually work? i understand the accepted theory is that mass bends space-time, how ever, the more i think about it, the more stupid the theory becomes to me. its just seems a bit "primitive"......ill explain.

    if planets orbit around suns due to the sun bending space, then why do the planets and the suns collide together? if gravity is the bending of space then there must be a counter force that prevents two massive objects in orbit from colliding together.
    plus, if gravity really is the bending of space, then it would be impossible for a massive object to move along the curved space without an initial force present to make it move. there must be some type of force that acts on massive objects besides the bent space. think about it, even if a large mass does bend space, then there is still no reason why smaller masses would move in orbit around the larger mass....to me, it seems like the smaller mass should simply just sit still in the bent space unless there is a active force that actually makes it move...

    i like to think of it this way, imagine a astronaut in space has a flat sheet of metal with a large, round dent in its center, now imagine the astronaut placed a small ball on the part of the sheet that is bent, what do you think happens? the ball will not just magicly start moving along the bent sheet because there is no active force that causes it to move in the first place.....
  2. jcsd
  3. Feb 23, 2017 #2


    User Avatar
    Science Advisor
    Gold Member

  4. Feb 23, 2017 #3


    User Avatar

    Staff: Mentor

    It's due to the sun's mass bending space-time, not space. That's a really important distinction because...
    The planets don't collide with the sun because their paths through spacetime (called a "worldline") don't intersect the worldline of the sun.
    An object can sit still in space, but it can't sit still in spacetime. Even if you don't change your position in space, time is still passing for you so you are constantly moving forward in the time direction.
    The sheet is an example of curved space, not curved spacetime. You'll have to get that picture out of your mind or it will just keep confusing you. A.T.'s apple video is a much better way of thinking about it.
  5. Feb 23, 2017 #4


    User Avatar
    Science Advisor
    Gold Member

    You're confusing gravity with momentum. Body A will orbit body B if it was given some momentum relative to B in earlier interactions. In case of planetary systems and galaxies, this is provided by the virtue of conservation of momentum of an initially-rotating cloud of gas (so in terms of dynamics, gravity acting on uneven distribution of matter).
    It's exactly the same in general relativity as with Newtonian gravitation - if you placed a planet next to a star without giving it some momentum, it'd fall straight in. This in no way contradicts the Newtonian description of gravity as a force.

    Furthermore, gravity is the curvature of space-time, not space. Notice that that's what A.T.'s videos depict.
  6. Feb 23, 2017 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    This is, I believe, a big issue with the rubber sheet analogy. The only reason that objects follow the curve of a rubber sheet is because of the force of gravity. Take away the force of gravity and all bets are off.

    In the absence of a force, therefore, what would induce a particle to move with a particular acceleration?

    The answer lies in a formulation of mechanics using the Lagrangian principle. For example, in classical mechanics you can look at Newton's second law in two ways:

    a) A particle accelerates under a force in inverse proportion to its mass.

    b) A particle moves in order to minimise a thing called the Lagrangian (which involves its kinetic and potential energies).

    And, these two can be shown to be equivalent.

    Now, principle a) cannot be applied when gravity becomes spacetime curvature. But, the Lagrangian principle b) can be generalised. And, in fact, b) becomes:

    b*) A particle moves in such a way as to maximise the amount of (proper) time it experiences.

    You probably need to study some Relativity to appreciate what b*) really means. But, that is the principle that induces a particle to move under the influence of gravity.
  7. Feb 23, 2017 #6

    the first video is exactly what i needed to know, gravity now makes complete sense to me, thanks!

    its crazy to think about this concept, i now see that motion is a complete illusion, very interesting....

    i cant help to think that there must be a way we can use this concept and maybe convert the movement of time into energy or somehow manipulate space to travel through time itself.....a man can dream cant he??? lol
  8. Feb 23, 2017 #7

    after giving this some thought, i can not accept that time is the cause of this illusion of motion. time is a perception of the mind, i do not believe that time acts like a physical force, pushing objects through space in a strait direction. instead, doesnt it make a lot more since to say that the expansion of space with in our universe is the cause of the motion we perceive?
  9. Feb 23, 2017 #8


    User Avatar

    Staff: Mentor

    Motion is not an illusion and I'm not sure where you got the idea that it is. The only thing that is slightly surprising about motion (and it's not that surprising - Galileo discovered this in the 16th century so we've had the best part of five centuries to get used to it) is that motion is always relative. It makes no sense to talk about something moving at a particular speed (including the special case of speed equal to zero, which we call "at rest") without saying what that speed is relative to. I'm sitting in my chair typing this.... Am I at rest? Yes, according to the dog who is asleep on the floor next to me... but the chair, floor, me, dog are all attached to the surface of the rotating earth.... and that earth is orbiting the sun, which is moving through interstellar space. So is my speed zero 9relative to the surface of the earth? A few miles per second relative to the sun? Or something completely different relative to the center of the galaxy? Or something even more different relative to the Andromeda galaxy?

    No, and a moment's reflection should convince yourself of that. How could the expansion of the universe account for the way that the worldline of the free-falling apple and the surface of the earth converge in A.T.s video? Where exactly is the expansion taking place, how much expansion is needed and how does that compare with the known rate of expansion (google will find that number for you), and is that consistent with the observed motion of other objects in the vicinity?
    Last edited: Feb 23, 2017
  10. Feb 23, 2017 #9
    If you instead model gravity as a force that pulls the planets towards the sun, you still have this same issue to resolve.

    It does seem that way until you learn that Nature doesn't behave the way it seems it should. It behaves the way it behaves with no regard to what we humans think about it.

    What happens is that objects don't necessarily move in the direction of the force exerted on them.

    No, but what if the sheet is in motion? What if the sheet and the ball share the same motion?

    What people have concluded is that all that matters is the relative motion. Do the ball and the sheet move relative to each other? That's the issue.
  11. Feb 24, 2017 #10


    User Avatar
    Science Advisor
    Gold Member

    Watch it again. Initially it's advancing only through time, but due to the space-time geometry it deviates towards the spatial dimension, and starts advancing in space.

    Thats irrelevant here. The massive object just curves space-time such that locally straight worldliness deviate towards it. Play around with this applet to see how it works on wider scale:

    You will find more illustrations here:
  12. Feb 24, 2017 #11


    User Avatar
    Science Advisor

    In addition to A.T.'s comment, note that it's perfectly possible to have an object moving relative to you in flat spacetime. It's just not the case that the animation shows.

    And if you want to consider motion to be an illusion, may I suggest that the next time you stub your toe you remind yourself that motion is an illusion, so the collision and resulting pain must be illusions too.
  13. Feb 24, 2017 #12

    thanks! ill watched it again, how ever, it still seems to suggest that an object moving through time is what gives it the illusion of motion.

    and as for what you say is irrelevant, its actually not lol. you took that quote from something i was explaining to nugatory XD

    i was explaining my reasoning behind thinking that the expansion of space could cause the motion of planets orbiting a larger mass. just keep in mind that through out history, physics, science and our understanding of the universe keeps constantly changing, many times in history has our beliefs about physics and science turned out to be wrong, even though we were so sure of ourselves at the time.i just mean that its easy to confuse and misunderstand certain things that we think we know beyond a shadow of doubt. for instance, if a objects passage through time along the surface of bent space, it would be easy to think that if the expansion of space pushed an object at the same pace, you probaly wouldnt be able to tell the difference between time and expanding space being the initial force and/or cause of motion.
  14. Feb 24, 2017 #13


    User Avatar
    Science Advisor
    Gold Member

    No expansion is needed for this.
  15. Feb 24, 2017 #14
    well im glad you have it all figured out.
  16. Feb 24, 2017 #15
    People who study physics beyond the freshman level learn that lesson and carry it with them for the rest of their lives.

    Once you learn the lesson mentioned above you never, and I mean never, think you know something about Nature "beyond a shadow of a doubt".
    Last edited by a moderator: Feb 24, 2017
  17. Feb 24, 2017 #16


    Staff: Mentor

    Let's please stop the discussion about motion as an illusion. I have deleted several of the previous posts about it and will delete all subsequent posts about it.
  18. Feb 25, 2017 #17
    What the OP may not know is that it was proven experimentally many times that large objects in space indeed curve spacetime. And there are even practical uses for this phenomena, for example gravitational lensing is used in modern astronomy to see a far away galaxy behind another galaxy by exploiting spacetime bending:

    I am strong believer in GR and I admire Einstein's gravity model, however I struggle when thinking about how this works on small scale. For example imagine a 20kg rock floating in free space with no gravity around, can the rock cause enough bending to have any gravitational effect on small objects around it, it seems to me that gravity does not exist on small scale.
    Last edited: Feb 25, 2017
  19. Feb 25, 2017 #18


    User Avatar
    Science Advisor

    The Cavendish experiment to measure G works using the gravitational interaction between objects in that range. So yes, gravity works for small masses. Why wouldn't it?
  20. Feb 25, 2017 #19
    Lol I was just reading about the Cavendish experiment, so yes gravity works on small scale.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: How exactly does gravity work?
  1. How does gravity work? (Replies: 4)

  2. How does time work? (Replies: 6)