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Thread: Questions about gravity

  1. #1 Questions about gravity 
    Forum Cosmic Wizard icewendigo's Avatar
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    I've read somewhere that gravity travels at the speed of light.

    Has this been measured? If so doesnt anyone find it bizarre?

    If so it would mean if a the sun vanished (and all its mass) it would still have a gravitational effect until minutes later when the abscence-of-gravity wave would reach a planet (and it would straighten it path towards the exterior of the solar system), right?

    Are there Doppler effects of effects related to massive objects that are traveling fast?

    If gravity propagates, and can alter the trajectory of light, why cant gravity not alter the trajectory gravity?

    Does one feel more gravity when a planet is between himself and the sun(beyond the planet+sun), meaning that sum of the gravity-ishess travelling on either side of the mid body gets curved and reaches you? (I guess not otherwise black wholes would have so much gravity that even gravity would not escape )


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  3. #2  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by icewendigo View Post
    I've read somewhere that gravity travels at the speed of light.
    It does. Or at least, that is what is predicted by general relativity.

    Has this been measured?
    No. It would be really hard. Maybe if we are able to detect gravity waves from an event we can also observe.

    If so doesnt anyone find it bizarre?
    Not really. Not much more bizarre than most results of GR or quantum mechanics, anyway.

    If so it would mean if a the sun vanished (and all its mass) it would still have a gravitational effect until minutes later when the abscence-of-gravity wave would reach a planet (and it would straighten it path towards the exterior of the solar system), right?
    Right. Although the purists will probably say that as the scenario is impossible, it isn't reasonable to try and predict what will happen...

    Are there Doppler effects of effects related to massive objects that are traveling fast?
    Not that I am aware of (but that doesn't mean much).

    If gravity propagates, and can alter the trajectory of light, why cant gravity not alter the trajectory gravity?
    I believe it can; gravity does effect gravity. But the effect is really small.

    Does one feel more gravity when a planet is between himself and the sun(beyond the planet+sun), meaning that sum of the gravity-ishess travelling on either side of the mid body gets curved and reaches you?
    No.


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  4. #3  
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    Quote Originally Posted by icewendigo View Post
    I've read somewhere that gravity travels at the speed of light.

    Has this been measured? If so doesnt anyone find it bizarre?
    It hasn't been measured. The reason why people think that this is so is that gravity (according to Newton's guess) obeys an inverse square law - increase the distance between two bodies by a factor of 2 and the gravitational attraction reduces by a factor of 1/4 . When people began to translate fields into an interaction between particles (field theory), an inverse square law was found to be consistent with an exchange of particles which travel with the speed of light. The force between electrically charged particles (Coulomb's law) is also an inverse square law and the interacting particles are considered to be photons which, of course, travel with the speed of light.
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  5. #4  
    Moderator Moderator Markus Hanke's Avatar
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    Static gravitational fields act instantaneously; gravitational waves propagate at the speed of light.Gravitational forces will add up or cancel each other depending on exactly where the sources are in relation to the observer.The gravitational field contains energy and is therefore itself a source of gravitation; gravity acts on itself.
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  6. #5  
    Comet Dust Collector Moderator
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    Got any references to support your pronouncement of "fact"?
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  7. #6  
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    Quote Originally Posted by MeteorWayne View Post
    Got any references to support your pronouncement of "fact"?
    Have these contributions become completely spurious - who on this thread has used the word "fact"?
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  8. #7  
    Comet Dust Collector Moderator
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    See Marcus Hanke (post #4)

    While the word fact is not used, the statement is worded as if it was. There are a number of items in dispute about the whole statement.

    All I am doing is asking for some support for them.
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  9. #8  
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    I offer my apologies. I understood you to mean that someone had proclaimed that something was a matter of "fact", rather than an implication that there was a factual basis underpinning what they were writing.
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  10. #9  
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    I'll be more specific:

    "Static gravitational fields act instantaneously". From my understanding, this is not an indisputable fact.

    "The gravitational field contains energy and is therefore itself a source of gravitation; gravity acts on itself. "
    I'm not sure that's true, based on currently understood physics, so would like a deeper explanation, rather than a statement.

    Wayne
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  11. #10  
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    "Static gravitational fields act instantaneously". From my understanding, this is not an indisputable fact.

    "The gravitational field contains energy and is therefore itself a source of gravitation; gravity acts on itself. "
    I'm not sure that's true, based on currently understood physics, so would like a deeper explanation, rather than a statement.

    I agree with all of what you have written. I also didn't understand the post you were commenting on, but there might have been some explanation that I wasn't aware of.
    Last edited by JonG; November 28th, 2011 at 06:48 PM.
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  12. #11  
    Moderator Moderator Markus Hanke's Avatar
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    My apologies, you are of course right, I should have given references. It is just that I have been interested in and studied GR since childhood, so these facts seem elemental to me, but I realize that this is not the case for most people.
    My own original text books are unfortunately in German, so probably of no use to you. I can recommend Wikipedia for a basic treatment of these principles, under the following headers ( further references on the bottom of each page ) :
    - Speed of Gravity
    - Einstein Field Equations
    - Stress-Energy-Momentum Tensor
    - General Relativity
    - Riemann Geometry
    I am not sure how to explain the fact that gravity is coupled to itself in a purely physical way. Mathematically, Einstein's field equations provide the basic relationship between the curvature of a Riemann manifold and a an energy/mass field as denoted by the energy-momentum tensor. Unlike in other gauge field theories ( e.g. electromagnetism ) the relationship between the two is non-linear - the Einstein equation is a tensor equation, within which each tensor is expressed implicitly as a function of the Riemann metric tensor. Written out for each element of the 4-tensors, we get a system of second degree non-linear partial differential equations, which is then used to find the individual elements of the metric tensor g.
    The fact that this system is non-linear ( each element depends on the values of all other elements, if you wish to picture it so ) means in a physical sense that the gravitational field depends on itself, or acts on itself. This is different to electromagnetism - Maxwell's equations are linear, and the fields aren't coupled internally.
    Maybe another way to visualize this is to consider that any gravity field contains energy - any point within such a field has a non-zero gravitational potential. Since any form of energy constitutes a source of gravity as per GR...well, you get the idea.
    I hope I am making sense in explaining this ?
    I am not a physicist, but do have a fairly good grasp on differential geometry, so would be happy to answer any further questions. Likewise, if I made a major error in some point I will be more than happy to be corrected :-)
    JonG likes this.
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  13. #12  
    Moderator Moderator Markus Hanke's Avatar
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    I should also mention that the self-coupling effect is very small for weak gravitational fields, and can therefore often be disregarded if the gravity field is a weak one - as a result of this it is actually possible to linearize Einstein's equations ( ref. Wikipedia on "Einstein Field Equations" ) for weak-gravity scenarios by omitting higher order perturbation terms in the equation. The linearized version is easier to treat than then the original tensor equation, and often allows for exact solutions where none can be found in the non-linear "full" version.
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  14. #13  
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    Quote Originally Posted by Markus Hanke View Post
    The linearized version is easier to treat than then the original tensor equation, and often allows for exact solutions where none can be found in the non-linear "full" version.
    This refers to a matter that I have wondered about. I have a very old book - The Mathematical Theory of Relativity by Arthur Eddington which was first written in 1923. In it Eddington writes, "No solution of Einstein's equations has yet been found for a field with two singularities or particles." And then, further down the page: "The problem of two bodies on Einstein's theory remains an outstanding challenge to mathematicians - like the problem of three bodies on Newton's theory."

    Is it still the case that no exact solution has been found for two bodies ?
    Last edited by JonG; November 29th, 2011 at 08:05 AM. Reason: typo
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  15. #14  
    Moderator Moderator Markus Hanke's Avatar
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    JonG, what you are referring to is the so-called "Kepler Problem", which is the movement of two bodies gravitationally interacting with one another. Back in 1923 this would indeed have been an unsolved problem if put into the context of General Relativity.
    Today we know that the Kepler problem is described by a solution of Einstein's equation called the Schwarzschild Metric. It is exactly solvable if one of the masses is much greater than the other, and can be numerically solved using computers if both masses are roughly equal. You can refer to this page for further details :

    Two-body problem in general relativity - Wikipedia, the free encyclopedia

    As you can see the mathematical treatment under GR for this simple problem is not trivial, but to answer your question : yes, a solution has been found. Given enough computing power the problem can be solved to any required degree of approximation.
    I hope this helps - the maths behind GR can be quite intimidating
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  16. #15  
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    Thanx for the clarification Markus.
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  17. #16  
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    Quote Originally Posted by MeteorWayne View Post
    Thanx for the clarification Markus.
    No problem, glad to help :-)
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  18. #17 The Apparent Discovery 
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    Experiment are happening every moment,
    thats why it happen religion and science.

    Cause science must to supply research that affects everything to the knowledge.
    So gravity in observe to other close said and know moon and planets,
    have no other elements that can be compared to experience gravity.

    No gravity can be experienced in the planet here,
    but to produce like in space, the earth needs to be killed out of breed,
    crazy politics and no doubt abusive impositions at watchful eyes.

    Gravity is made angry science,mad observations to conclude the thought.
    Social groups to be broken for knowledge,to fall apart,a religion thought.
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  19. #18  
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    Quote Originally Posted by WDHellS View Post
    Experiment are happening every moment,
    thats why it happen religion and science.

    Cause science must to supply research that affects everything to the knowledge.
    So gravity in observe to other close said and know moon and planets,
    have no other elements that can be compared to experience gravity.

    No gravity can be experienced in the planet here,
    but to produce like in space, the earth needs to be killed out of breed,
    crazy politics and no doubt abusive impositions at watchful eyes.

    Gravity is made angry science,mad observations to conclude the thought.
    Social groups to be broken for knowledge,to fall apart,a religion thought.
    Are you a real person, or are you a computer program generating random word sequences? You haven't written a meaningful sentence yet.
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  20. #19  
    Quagma SpeedFreek's Avatar
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    Quote Originally Posted by MeteorWayne View Post
    I'll be more specific:

    "Static gravitational fields act instantaneously". From my understanding, this is not an indisputable fact.

    "The gravitational field contains energy and is therefore itself a source of gravitation; gravity acts on itself. "
    I'm not sure that's true, based on currently understood physics, so would like a deeper explanation, rather than a statement.

    Wayne
    I'm getting deja-vu . We discussed this issue here.
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