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What is Gravity?

  1. Apr 9, 2008 #1
    Can anyone tell me why my feet stay on the ground?
  2. jcsd
  3. Apr 9, 2008 #2
    If you hold a piece of paper so it is stretched out, then place something heavy in the middle you will see it sinks down. Now if you were to place something lighter on the paper it falls to where the heavier object is. That is exactly what happens in space, you are falling toward the heavier object... the earth.
  4. Apr 9, 2008 #3
    I understand that concept yet it still puzzles me the fundamental physics of why that happens. Doesnt string theory predict the Graviton particle as some kind of explanation?
  5. Apr 9, 2008 #4
    Nice try but that response,(not to be too unkind), seems as something you saw on TV. You cannot talk two dimensionally about a three dimensional space. That's too much of a stretch. Plus, that does not explain why this man/woman is attracted to earth, really.
  6. Apr 9, 2008 #5
    Actually, string theory is not completely accepted yet like let's say Newton's law of Gravity. So to talk about string theory as if it is completely solid intellectually is jumping the gun a little. Furthermore, no graviton has been detected as of yet.
  7. Apr 9, 2008 #6
    Thats true but what we can say is that we still dont have a complete understanding of gravity if new theories are still being proposed to this day such as the graviton. So in essence we can say that we are still perplexed about why man/woman is attracted to earth. What do you think about String theory, do you think that the graviton exsits? Even if it doesnt how does string theory propose that it keeps our feet to the ground?
  8. Apr 9, 2008 #7
    Well if you think of some clear demonstration of 4 dimensional model please, feel free to introduce it here. I find the 2d model very good demonstration even though its not accurate of course. I used it ever since I was 4 or 5.
  9. Apr 10, 2008 #8


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    that deflected rubber sheet analogy is just not anywhere close to sufficient to explain (i presume from the POV of General Relativity) why scooby's feet stay on the ground.

    gravity is one of four Fundamental Interactions, the other thee being EM (elecromagnetic interaction), weak nuclear interaction, and the strong nuclear interaction. i think that something called "the Standard Model" is a comprehensive theory that unifies the latter three interactions into a single physical theory. i dunno much about it. it has all sorts of particles with all sorts of properties.

    General Relativity is supposed to be a more modern theory of gravitation than the simpler Newtonian theory where gravity was governed by the Newtonian inverse-square law which acted instantaneously between interacting bodies without propagation delay. i think a main postulate of it has to do with a thought experiment:

    The acceleration of gravity on the Earth is about 9.8 m/s2. That's how fast you would fall if no ground were pushing up on your feet. From a Newtonian perspective, a net force of gravity with magnitude that is always proportional to your mass is pulling you down and without your feet on the ground, no other force is pushing up to counter it, so you accelerate downward at a rate of 9.8 m/s2 (that force which is proportional to your mass divided by your mass, so the acceleration of gravity is the constant of proportionality). Now, if you're just standing on the ground, you are not accelerating downward but the interaction of gravity is still there pulling you down, so something (the ground onto the soles of your shoes) must be pushing up by the same amount of force (proportional to your weight) to result in a net force of zero. Now, it turns out that if we put a box over you, so you cannot tell that you're standing on the surface of a planet, what you would sense, with the floor pushing up on your feet by an amount that is proportional to your mass, is the same as what you would sense if you were standing in the same box that was in a spaceship in weightless space and that spaceship was acclerating upward at the same rate of 9.8 m/s2. You could not tell the difference.

    Likewise, if you were unlucky enough to get in an elevator on the top of a skyscraper that is in desparate need of maintanence (or was sabatoged) and all the cables broke and, ignoring the air resistance in the shaft and other friction forces, that elevator fell toward the ground at a rate of 9.8 m/s2 with you in it. You would be free falling, too, and the box of the elevator just happens to be falling at the same rate and surronding you. Likewise, it turns out that you could not tell the difference between that apparent weightless state falling in the elevator from if your were floating around inside the same box in an unaccelerated spaceship in weightless space. You could not tell the difference.

    Now consider the two spaceship cases: If you were in the unaccelerated spaceship with a window that let in a beam of light, that beam of light would travel in a perfectly straight line. You're not accelerated which means you're in an inertial frame and Special Relativity says that any inertial frame is just as good as one at rest. Now consder the accelerating spaceship (toward the direction straight above your head), a beam of light coming in the window would actually be traveling in a straight line, but would appear to you in your accelerated frame of reference to be curving downward (as the trajectory of any constant velocity body would from the POV of an accelerated observer - you understand why this is the case, right?).

    So similarly to how SR equates any inertial (unaccelerated) frame of reference with being indistinguishable with one at rest (and says the laws of physics are the same), GR equates these two situations in space (the accelerated case, where the floor of the spaceship is pushing up on your feet, and the inertial case where you are floating in a box in an unaccelerated spaceship) as being indistinguishable from the two situations on Earth (respectively, standing in a box on the surface of the Earth with the floor pushing up on your feet, and the free-falling case where you seem to be floating inside the elevator box free-falling).

    So if they have equivalent laws of physics, if you're standing in that box on the ground, and a beam of light (or a slower object) comes in the window, it will appear to be curved downward just as it did in the "upwardly" accelerated spaceship. And if you're in this free-falling elevator box and a beam of light (or a slower object) comes in that window, it will appear to you as traveling in a straight line, just as it did in the unaccelerated spaceship (but you're in this elevator box that is accelerating downward in freefall)

    Whether it's that very fast beam of light (which doesn't bend too much in the two cases it appears to) or the slower object (someone threw a baseball through the window), that trajectory that either took is called a geodesic. In space that would appear to us as a straight line, if we were unaccelerated. On Earth it appears as a straight line if we were free falling. But if we're not falling, but standing on the surface of the Earth, that trajectory, whether it's a beam of light or an inertially flying baseball, that trajectory looks curved around and toward the Earth, where all this mass is. General Relativity says that gravity really is this curvature of "space-time" (whatever the hell that is) where this curvature is caused by this other mass, the Earth. The more mass, the more that inertially flying objects (and beams of light) appear to us as curved, even though, in a sense of this space-time, they are moving in "straight" paths (these "geodesics") from the POV of GR. So GR is telling us that gravity is this curving or bending of space-time caused by large amounts of mass in your locality.
  10. Apr 10, 2008 #9
    Does this mean then there is a relationship between the rate that time elapses and the curvature of space-time itself?
  11. Apr 10, 2008 #10
    I have never found this to be a satisfactory explanation. To fall down the curved space, you still need gravity! If you have a curved space like you are explaining, but without gravity, you would just stand still on your point on the sloped paper.

    Creating a "downhill" space has no effect in itself, unless there is a force that says objects will roll downhill. Which brings you back to square one.

  12. Apr 10, 2008 #11
    Me too. I have always believed that the "rubber sheet" picture is useless for explaining gravity, since it requires that gravity be a given for it to work.
  13. Apr 10, 2008 #12
    I think maybe something like gravity ether. If it is so that gravity affects on both objects at equal force. Maybe gravity is attribute of space and objects with mass somehow disrupt it.
    (Sorry if my english is very bad. I don't use english very much and my first post here.)
  14. Apr 11, 2008 #13
    I was just thinking about how gravity by special relatvity is a wave that travels at c, and correct me if im wrong but theories think that the graviton could possibly be part of this wave. what if mass not only curves space time but also creates ripples in space-time and this is what we see to be gravity? Everymodel i see is a flat space-time curved by a large mass, but in reality wouldnt it make more sense if space-time were to ripple, such as the surface of a pond. The question is which direction would the waves be travelling in, towards the mass or away form the mass. I'm thinking towards the mass and earth is travelling into this wave like a boat sailing into a wave on the ocean of course though the earth would simultaneoulsy be travelling perpendicular to the wave otherwise the earth wouldnt orbit the sun, the only kind of earthly comparison i can think of is that of a whirlpool. Let me know what you guys think!
    Last edited: Apr 11, 2008
  15. Apr 12, 2008 #14
    They say that object cannot move at speed c. Is it because of assuming the gravity waves to travel at c?
  16. Apr 12, 2008 #15


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    Any ripple would register as a change in G, newton's gravitational force constant.

    Or in other words, it would be unmeasureable in that way.
  17. Apr 12, 2008 #16
    A second is about 300,000,000 meters of light long.
    Acceleration = change in velocity over time
    If a lump of matter can alter space, why not time (Spacetime?)
    A gradient in the "flow of time" of one thirty millionth per second per second just might yield an acceleration of 10 meters per second per second.(1G.)

    This is almost certainly wrong, but I'd really like to know why
  18. Apr 12, 2008 #17


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    According to GR that is exactly what happens. These effects come from the concentration of mass, however, so it's not the gradient in time causing acceleration, rather both are caused by the presence of mass. In addition, for this reason, acceleration is indistinguishable from gravity.
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