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Just a thought about gravity.

  1. Mar 30, 2006 #1
    Is gravity a force? Does it exist?
    If gravity is a force, according to 1st law of thermodynamics, where does gravity gets its energy from?
    If it exist, it violates the cosmo speed limit. Just imagine that the sun were to just vanish one day, we on earth would logically experience 8 more minutes of sun before everything goes dark!(as the light from the sun takes approx. 8 minutes to reach us) BUTinstead earth would "float" away into the darkness of space as there is no more "gravitational pull" from the sun. So this means gravity travels faster than light.

    But it can be said that gravity exist as the earth orbits at 90 degrees around the sun, and using mathematics calculation, it does not require any energy for this movement!

    But i can't think of any explanation that how it can travel faster than light!!!
     
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  3. Mar 30, 2006 #2

    Pengwuino

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    Why do you think it travels at faster then c?
     
  4. Mar 30, 2006 #3
    If you let go of something you're holding, it would drop instantly. If the sun "let go" of earth, we would float away.
     
  5. Mar 30, 2006 #4

    Pengwuino

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    But where are you getting the idea that gravity travels faster then the speed of light?
     
  6. Mar 30, 2006 #5
    so let me ask you, what speed does gravity travel at? My explanation here is trying to say that if the sun vanishes, we will float away immediately! before light from the sun reaches us. In other words, gravity is faster than light.
     
  7. Mar 30, 2006 #6
    Einstein's theory of relativity tells us we will experience 8 more minutes of gravitation after the dissappearing of the sun. The speed of gravity is c.
     
  8. Mar 30, 2006 #7
    wow is that true? If it is then i think gravity does exist! But there is expansion theory which states that gravity can't exist. Instead it says all atoms in our universe is expanding at a universal rate of expansion!
     
  9. Mar 30, 2006 #8
    Gravity clearly exists. What exactly is the nature of gravity and how it behaviours is a more difficult question.
    You throw a ball fast enough into the sky (6km/s or more) and it'll never came back down to Earth. Does this mean gravity doesn't exist? Nope.
    As Gladi8or2 mentions, you're working with the Newtonian notions of gravity. Relativity gives the speed of gravity to be that of light.
    That is because the object you're holding is already within a constant gravitational field, it already feels the pull of gravity. You have to change the gravitational field and then see how long it takes objects a distance away to notice the change in order to work out how fast gravity moves.

    Your example is like me saying 'When I open the curtains of my room in the morning, light from the Sun immediately comes into the room. Therefore light gets from the Sun to my curtains instantly.' Can you see why this is false reasoning?
    It's not gravity which is an energy, it's a force. It's object which work against (or with) the force of gravity which have energy, since energy is (in one definition) force times the distance moved against that force.
     
  10. Mar 30, 2006 #9
    Here....i think that gravity is possible but trying to weigh the possibilities!
     
  11. Mar 30, 2006 #10
    if we drop a ball of lead and feather they will both fall at the same time right? But both the ball and feather has mass means it has certain amount of gravity pull. So each of them will have a different pull against the earth. But in expansion theory, the earth expands towards the object at the same speed. It is true that an astronaut in space feels the same as falling down from world trade center. This means in expansion theory all objects are floating. But we can stand on earth as the earth is expanding towards us and pushing us up. This idea is actually quite weird but i don't think we should totally forget about it.
     
  12. Mar 30, 2006 #11

    Mk

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    Righty-o, if you were dealing in a vacuum on the ground.

    I don't think thats right.

    Well the objects don't look like they're floating? Like the Earth. What is it sitting on? Its floating. An asteroid, what is it sitting on? Its floating!

    Isn't the expansion caused by the inflation of spacetime, not the stars and galaxies actually moving?
     
    Last edited: Mar 30, 2006
  13. Mar 30, 2006 #12
    actually i don't get what you mean.
    if everything expands at the same rate, you will not be able to tell the difference.

    but this is what the astronauts claim.
     
  14. Mar 30, 2006 #13

    vanesch

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    Congrats :biggrin:


    You are of course correct that if "everything is expanding at a universal rate" that this would simply come down to a continuous change of the definition of the unit of length, and not change anything.

    The "universal rate of expansion" in cosmology is of course only valid on large, cosmological scales. The moon is not receding from earth, your ruler is not expanding, the solar system is not expanding because of that. It is only when you are on such large scales that you can consider space "more or less uniformly filled with matter" that the notion of universal rate of expansion makes sense.
     
  15. Mar 30, 2006 #14

    vanesch

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    Upon reading this, I'm becoming more and more convinced that you have misunderstood completely the relationship between gravity and cosmological expansion. The cosmological expansion has NOTHING to do with an astronaute or a feather falling to earth!
     
  16. Mar 30, 2006 #15

    vanesch

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    In fact, Einstein's theory of relativity forbids you to consider the disappearing of the sun. A bit like charge conservation is dictated by the Maxwell equations, and hence you CANNOT ANSWER the question: "what would the electromagnetic field do if a charge suddenly disappeared ?" because according to the Maxwell equations, the charge CANNOT disappear.
     
  17. Mar 30, 2006 #16

    russ_watters

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    Gravity is a force. Energy is energy. They are not the same thing and force does not require energy.
    Again, why would energy be needed?
     
  18. Mar 30, 2006 #17

    if everything is expanding at the same rate....you cannot tell any difference
     
  19. Mar 30, 2006 #18

    Janus

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    I think it is pretty safe to forget all about it. A lead ball and feather will fall at the same rate if dropped from the same distance, but if you drop two objects from different heights(even the same object), they will not fall at the same rate. (which they would have to in order for gravity to be caused by the Earth expanding) Also, the moon has 1/6 the surface gravity of the Earth, thus it would have to expand at a slower rate, meaning that the relative sizes between the Earth and Moon would change with time.

    The facts we observe about gravity just do not mesh with an Earth expanding theory.
     
  20. Mar 30, 2006 #19
    BINGO! :rofl:
     
  21. Mar 30, 2006 #20

    DaveC426913

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    You are confusing two phenomena that are unrelated. Gravity and the expansion of the universe have nothing to do with each other.
     
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