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The gravity [what causes it?]

  1. Jul 11, 2015 #1
    we all know gravity but i was wondering how is it caused ? like in a nebula what causes the particles to come together and form a star or any object in the cosmos
     
  2. jcsd
  3. Jul 11, 2015 #2
    The short answer is simply, no one knows, we have no clue as to how gravity works. The closest description of gravity is Einstein's second theory of relativity call The General Theory of Relativity. With this, it is described as mass causing the warping and moment of space itself. It does very well to make very accurate predictions but the actual mechanics are still being debated today. Some insist that space itself is either a tensile fabric or perhaps a fluid, while others insist it must be a force carrier such as the predicted Graviton in particle physics. How does gravity actually work? We really do not know yet.
     
  4. Jul 11, 2015 #3
    but what causes gravity ,means what causes the particles to come closer to each other
     
  5. Jul 11, 2015 #4
  6. Jul 12, 2015 #5

    Drakkith

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    Science an give you an explanation that tells you how something works, but the ultimate cause of something may never be known. You can always ask, "Well, why does X work the way it does", no matter how deep your explanation goes.
     
  7. Jul 12, 2015 #6
    how does earth's gravity work one way to cause things to remain on it's surface or in a sense stick to it and then also cause something to orbit it like the moon. why didn't earth's gravity pull the moon right to it's surface instead of keeping it in an orbit? or why didn't the moon's orbits get smaller and smaller until the moon was fully drawn by the earth's gravity to the earth's surface?
     
  8. Jul 12, 2015 #7

    jtbell

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    The moon falls (accelerates) toward the earth at the same rate that an apple would fall if we could hold it stationary at that distance from the earth and then let it go. The moon doesn't hit the earth because it's also moving sideways, so it continuously keeps missing the earth. Likewise if you could shoot a cannon ball at the right speed and if there were no air resistance, it would orbit the earth.

    yIM0S.jpg
     
  9. Jul 12, 2015 #8

    mfb

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    Our moon probably formed from debris of a massive collision between earth and another object - some fragments fell down to earth, some reached an orbit. The latter group formed the moon.
     
  10. Jul 12, 2015 #9

    ZapperZ

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    Actually, you need to be very careful with your wordings here. We know very well ".. how gravity works.."! That's how we managed to predict celestial/astronomical events.

    And General Relativity describes how mass affects spacetime, not just space.

    So we know quite well how it works. What we don't know yet is how gravity can be incorporated as part of a more unified quantum field theory, or if this is even possible.

    And other question on how mass can cause gravity falls under the same group of question of "why charge create electric field" or "what is the origin of quantum spins"... etc... etc.

    Zz.
     
  11. Jul 12, 2015 #10
    could it be that the moon is falling through space but the earth's gravity prevents it from doing so yet it is not strong enough to pull the moon into itself at that distance so gravity then makes it orbit instead???
     
  12. Jul 12, 2015 #11

    mfb

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    That does not make sense at all. What does "falling through space" and "preventing it from doing so" even mean?
     
  13. Jul 12, 2015 #12
    That's a bit harsh, these might not be the words you would choose, but I think it's quite a good description.
     
  14. Jul 12, 2015 #13

    Drakkith

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    Instead of asking, "Could it be X", where X is your own description, you're better off learning our current understanding of gravity. Once you do that you will be able to look back and attempt to answer your own question.
     
  15. Jul 12, 2015 #14
    perhaps i should have said traveling through space or going through space or wandering through space. i don't know, isn't traveling through space almost like the same thing as falling through space, not that it has to fall fast or wander fast? If gravity from a planet wasn't holding a moon in orbit wouldn't it just keep on going and going (until something's gravity catches it); If our planet got blown up and somehow or moon wasn't for example. Then wouldn't our moon travel through space almost like it were falling? if you got in a space suit and someone dropped you off somewhere in space do you think you would just remain in one place? I guess things wouldn't be able to fall in the direction we term down too far since they would be caught up in the rotation of the galaxy; but wouldn't that also have to do with gravity? i am just giving a hypothetical description about the behavior the moon would have without the influence of earth's gravity, or how gravity and this natural traveling if you will, of the moon counteract each other to create an orbit.
     
  16. Jul 12, 2015 #15

    Drakkith

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    There's a very simple way of explaining it. An object moves through space in a straight line unless subjected to a force. Gravity, being a force, causes it to move in a curved line instead. If you do the math, you can figure out what the shape of the curved line is. For the Moon, the shape corresponds to an oval, with the Earth at one focus. Since an oval is a closed shape (compared to a parabola or a hyperbola, which aren't) this means that the Moon is in a closed orbit. If the shape of the line were a parabola or hyperbola then the orbit would be open, and the object would pass nearby and then head off into space and never return.
     
  17. Jul 12, 2015 #16
    It would be better to say that the moon is obeying Newton's first law: [...] an object that is in motion will not change its velocity unless an external force acts upon it
    It would be better to say that the moon is obeying Newton's second law: [...] the net force applied to a body produces a proportional acceleration
     
  18. Jul 12, 2015 #17
    No, its an ellipse with the Earth at one focus. An oval is a different shape (that is not followed by any object in freefall).
     
  19. Jul 12, 2015 #18

    Drakkith

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    Ellipse! That's the word I couldn't remember!
     
  20. Jul 12, 2015 #19
    We don't know how it happens, but it definitely happens. no doubt about that.
    We can measure gravity and have clever theories predicting what it will do, and that has proven very useful for things like accelerating space probes with a minimum amount of propellant.
    WHY is there gravity?, Well you may a well ask why not?, but it's there.
     
  21. Jul 12, 2015 #20

    davenn

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    starting to have those "senior moments" Drak ? :wink:

    I thought Ol' Jim ( see sig.) and I were the only ones with that problem :smile:
     
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