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One last thought experiment

  1. Jul 10, 2008 #1
    Ok please comment on this one.
    Let's suppose for our thought experiment you the reader were suddenly promoted to god-like status. You decided to take the our universe and remove all matter, but bearing in mind that all known laws of physics will still apply as they do now.
    You then create two earth-like planets; that is to say they are exactly the same size in volume and density.
    With your newly obtained god-like hands, you place these two planets 10 light seconds apart, or even 1000 light-years apart, it doesn't matter. Let's just say 10 light seconds since we are impatient here.
    You are able to then release your hold on these planets in such a delicate manner that you cause no angular movement in any direction. They are in all sense of the word "motionless".
    Question: What would happen? Would the two planets remain "motionless" for 10 seconds as the gravity waves from each planet propagate outward and eventually reach each other (as in General Relativity)?

    OK so after you've answered that question consider this:
    Same conditions. You are still god-like and remove all matter from the universe and create two planets again. This time, however, you place them so that the surfaces are touching.
    I realize there would be distortions to the shapes of the planets, but we can discard that for now. You then, with your god-like arms and hands, begin moving them away from each other at a significant speed, perhaps at 10,000 miles an hour or so.
    As you move these planets apart you occasionally check the gravitational attraction, and, according to Newton, you see the attraction is inversly proportional to the square of the distance, but it is there.
    You eventually move the two planets a distance that is let's say 1 light year away, or even 1000 light years away. You then stop the seperation and they become for all practical purposes, "motionless". You again take a gravity attraction reading.
    Then what? Would they instantly start moving towards each other? They are already "aware" that the other exists and the attration is inversly proportional to the square of the distance. But, they have been moving further and further away and the delay of the speed at which gravity can transmit it's information is becoming greater and greater. At some point (and I dare say even right away), the two planets and the gravity information is becoming more and more "outdate". One planet recieves gravity information from the other at a delay, and so it "thinks" it is actually closer. This would explain (perhaps) how galaxies can hold together. The gravity information the outer stars are receiving is "old" and the starts "think" they are closer to the center of the galaxy, and feel a gravity force that is closer to the center than it actually is (assuming the galaxy expanded (as is the whole universe) from some much closer collection in the center).
    So let's take that a step further, the whole universe is expanding and so "old" gravity information from all matter that was once much closer is still being transmitted (just like we see "old" light from distant galaxies (or even background radiation). Thus holding it together at a much greater rate than can be calculated from the known matter of the universe (thus the need for "dark matter").
    This might also explain the pioneer probe being closer to the sun than expected. It is recieving gravity information at a delay, even though it is further away, it still feels the sun's gravity from a time in the past, when it was closer to the sun.
  2. jcsd
  3. Jul 10, 2008 #2


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    Staff: Mentor

    Changes in a gravitational field propagate at the speed of light.
  4. Jul 10, 2008 #3
    Yes I understand that so I propose this:
    If I see a galaxy that is 1000 light-years away I can assume I'm seeing it's light and relative position to earth as it was 1000 years ago. So then why not assume that we are also experiencing it gravity from 1000 years ago relative to our position.

    How is the second experiment different than the first experiment?

    "Ah" you say "Well in the first experiment they started out apart, so gravity is transmitted at the speed of light according to General Relativity and so they will wait motionless until the wave catches up! In the second experiment they were next to each other so the speed of gravity was immediately felt as the wave went out".

    And I say "Gravity wave? Travelling at the speed of light? Oh so eventually there would be a delay just like there is a delay talking to astronauts on the moon? Or the 8 minute delay we experience from the light coming from the sun? Then what? How can you calculate the gravitational attraction over time?"
  5. Jul 11, 2008 #4


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    You work out how far they are away and how long it will take gravity to reach the two objects, then work out the gravitational attraction normaly.
  6. Jul 12, 2008 #5
    Gravitational delay? Sure.
    Since gravity travels at C, if our sun were, by some magic, to suddenly disappear the earth would continue to act as normal for 8 or so minutes.
  7. Jul 12, 2008 #6
    Actually I think I found the answer. This site discusses the topic of "gravity speed", which was the crux of my experimental question.

    [crackpot link deleted]

    Gravity doesn't travel at the speed of light. This can be proven by looking at the orbits of the planets, and the detection of "gravity maximum" during a solar eclipse; The maximum pull on the earth occurs before we see the eclipse, which means the earth is pulled to a point where the sun actually is, not where it was 8 minutes ago.
    Last edited by a moderator: Jul 13, 2008
  8. Jul 12, 2008 #7
    so if gravity travels much faster than light would it be possible in theory to transmit data through an artificially pulsed gravity wave, faster than the speed of light?
  9. Jul 13, 2008 #8


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    Last edited: Jul 13, 2008
  10. Jul 13, 2008 #9
    OK thank you!
    I've been reading alot recently and understand a little more on how it all works, and how General Relativity stipulates gravity propagate at light speed. The effect on orbits or two bodies moving apart depend on reference frames, so you can't simplistically calculate the delay in a straight foward way (like what happens in my thought experiments).
  11. Jul 14, 2008 #10
    Gravity comes from mass. Pulsed gravity would be achieved by a moving mass. We can't practically move a significant mass anywhere near the speed of light. Not to mention the signal would be so minuscule that it be virtually undetectable, and inundated with noise.
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