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Thread: Gravity

  1. #1 Gravity 
    Forum Freshman Patrick_94's Avatar
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    Why is gravity? Does everything have a force of gravity?


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  3. #2 Re: Gravity 
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    Quote Originally Posted by Patrick_94
    Why is gravity? Does everything have a force of gravity?
    For now the answer to the question "...Why is gravity..." rests with General Relativity and its description that gravity is a curvature of spacetime. Objects traveling inertially through spacetime (that is, coasting along - as opposed to being accelerated by a rocket) follow geodesics (curved paths) along these curvatures.

    Even two objects that are initially "standing still" relative to each other continue to travel through spacetime ( they're not moving through space relative to each other, but they are both moving forward through time - which is a component of spacetime). If left alone they will start moving towards each other because that's their "natural" path forward through spacetime.

    Because mass causes spacetime to curve, the short answer to your question is that yes, everything exerts a gravitational force.

    Chris


    It is difficult to say what is impossible, for the dream of yesterday is the hope of today and the reality of tomorrow.
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  4. #3 Re: Gravity 
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    Quote Originally Posted by CSMYTH3025
    Because mass causes spacetime to curve, the short answer to your question is that yes, everything exerts a gravitational force.

    Chris
    Chris, don't you mean that everything that has mass is affected by gravity and affects other bodies that have mass? Time, energy, and space don't exert gravitational pulls.

    Such an issue leads me to a question I've pondered for a long time. Black holes presumably have such great gravitational fields that not even light is fast enough to escape those fields (the escape velocity within the black hole's event horizon exceeds the speed of light). How can the gravitational pull of a black hole affect light which has no mass?

    I believe the answer to this question is that the space within the event horizon is curved to such a degree that the path light takes leads it right back to the black hole. In other words, the geodesic is a closed curve within the bounds of the event horizon. In less powerful gravitational fields, like that of the sun, light follows a geodesic that although curved, is still an open curve allowing light to escape the gravitational pull of the sun.

    Agree?

    Jagella
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  5. #4 Re: Gravity 
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    Quote Originally Posted by Jagella
    Time, energy, and space don't exert gravitational pulls.


    Jagella
    Wrong.

    Energy is one element in the stress-energy tensor of general relativity, and does indeed figure into spacetime curvature, which is gravity. Energy in relativity is on an equal footing with matter, , and at some levels the distinction is a bit blurry.

    The non-linearity of the Einstein equations of general relativity result in gravity, curvature of spacetime, actually contributing to curvature -- often stated as "gravity gravitates". So in a real sense "space" can exert a gravitational pull. There are non-trivial vacuum solutions to the Einstein field equations -- gravity in an empty universe.

    In GR time and space are not really distinct, only the amalgamation, spacetime is ultimately meaningful, and time is just one dimension of a local chart. That local coordinate can figure in to the curvature of spacetime, and in that sense time is also a player in gravitation.

    Of course, none of this is evident in Newtonian gravity.
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  6. #5 Re: Gravity 
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    Quote Originally Posted by DrRocket
    There are non-trivial vacuum solutions to the Einstein field equations -- gravity in an empty universe.
    How are we to test any claim about an empty universe?

    Jagella
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  7. #6 Re: Gravity 
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    Quote Originally Posted by DrRocket
    Energy is one element in the stress-energy tensor of general relativity, and does indeed figure into spacetime curvature, which is gravity. Energy in relativity is on an equal footing with matter, , and at some levels the distinction is a bit blurry.
    But does energy exert a gravitational pull? That's the issue I raised, and you claim I'm wrong. Please provide examples of light or heat causing a gravitational pull on some object.

    Quote Originally Posted by DrRocket
    The non-linearity of the Einstein equations of general relativity result in gravity, curvature of spacetime, actually contributing to curvature -- often stated as "gravity gravitates". So in a real sense "space" can exert a gravitational pull.
    Again, please provide examples of space exerting a gravitational pull on some object.

    Quote Originally Posted by DrRocket
    In GR time and space are not really distinct, only the amalgamation, spacetime is ultimately meaningful, and time is just one dimension of a local chart. That local coordinate can figure in to the curvature of spacetime, and in that sense time is also a player in gravitation.
    Once again, post examples of time exerting a gravitational pull on an object. Now that I want to see.

    Jagella
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  8. #7 Re: Gravity 
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    Quote Originally Posted by Jagella
    Quote Originally Posted by DrRocket
    Energy is one element in the stress-energy tensor of general relativity, and does indeed figure into spacetime curvature, which is gravity. Energy in relativity is on an equal footing with matter, , and at some levels the distinction is a bit blurry.
    But does energy exert a gravitational pull? That's the issue I raised, and you claim I'm wrong. Please provide examples of light or heat causing a gravitational pull on some object.

    Quote Originally Posted by DrRocket
    The non-linearity of the Einstein equations of general relativity result in gravity, curvature of spacetime, actually contributing to curvature -- often stated as "gravity gravitates". So in a real sense "space" can exert a gravitational pull.
    Again, please provide examples of space exerting a gravitational pull on some object.

    Quote Originally Posted by DrRocket
    In GR time and space are not really distinct, only the amalgamation, spacetime is ultimately meaningful, and time is just one dimension of a local chart. That local coordinate can figure in to the curvature of spacetime, and in that sense time is also a player in gravitation.
    Once again, post examples of time exerting a gravitational pull on an object. Now that I want to see.

    Jagella
    Go back again and read my post. Several times until you understand it.

    Your questions are, in context, nonsensical.
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  9. #8 Re: Gravity 
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    Quote Originally Posted by Patrick_94
    Why is gravity? Does everything have a force of gravity?
    like many thing , gravity is the picture back in time of partical that push the ather object forward
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  10. #9 Re: Gravity 
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    Quote Originally Posted by Water Nosfim
    Quote Originally Posted by Patrick_94
    Why is gravity? Does everything have a force of gravity?
    like many thing , gravity is the picture back in time of partical that push the ather object forward
    Your comment makes no sense to me. Can you explain what you mean?

    Chris
    It is difficult to say what is impossible, for the dream of yesterday is the hope of today and the reality of tomorrow.
    Robert H. Goddard - 1904
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  11. #10 Re: Gravity 
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    Quote Originally Posted by Water Nosfim
    like many thing , gravity is the picture back in time of partical that push the ather object forward
    I think I can agree with you on this one. A very clear, concise explanation!
    von Cremola (in the officers' mess today).
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  12. #11 Re: Gravity 
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    Quote Originally Posted by Patrick_94
    Does everything have a force of gravity?
    Yes, all objects with a mass exert a gravitational force upon other objects of mass:



    And, as DrRocket states- energy also has an affect upon space-time curvature in General Relativity, which is essentially gravity. Gravity is the weakest of all fundamental forces (10^39 times weaker than electromagnetism), however it's extent ranges to infinity- the further away from the source, the weaker the gravitational force but this force never actually decreases to zero; it is asymptotic, I suppose.
    "Nature doesn't care what we call it, she just does it anyway" - R. Feynman
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  13. #12 Re: Gravity 
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    Quote Originally Posted by x(x-y)
    And, as DrRocket states- energy also has an affect upon space-time curvature in General Relativity, which is essentially gravity.
    That may be true, but energy itself cannot create a gravitational pull--a fact that some members here don't understand.

    Jagella
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  14. #13 Re: Gravity 
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    Quote Originally Posted by Jagella
    Quote Originally Posted by x(x-y)
    And, as DrRocket states- energy also has an affect upon space-time curvature in General Relativity, which is essentially gravity.
    That may be true, but energy itself cannot create a gravitational pull--a fact that some members here don't understand.

    Jagella
    Wrong

    What is clear is that the member who does not understand gravity is you.
    You have a marked propensity for making positive statements that are completely wrong. Your ignorance of physics is a match for your profound ignorance of mathematics.

    http://en.wikipedia.org/wiki/Geon_(physics)

    Even in classical gravity, the mass associated with ordinary mattere lies primarily in the protons and neutrons of the constituent atoms, which lies primarily in the binding energy among the quarks that comprise them, not in the rest mass of the quarks themselves, so the mass terems in even Newtonian gravity are demonstrably due largely to what ought be called energy.

    More importantly the most important aspect of E=mc^2 is the equivalence of mass and energy. In general relativity mass and energy are treated on equal footing as mass/energy.

    You would do well to put a sock in it and learn some science, rather than prattling like a fool.
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  15. #14  
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    I'm sure you can easily convert a given energy to its mass equivalent, Jagella, ever hear of E=mc^2 ??? So yes energy of any kind warps space/time to produce a gravity field, and space, having a 'vacuum' energy, will also warp space/time.
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  16. #15  
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    Quote Originally Posted by MigL
    I'm sure you can easily convert a given energy to its mass equivalent, Jagella, ever hear of E=mc^2 ??? So yes energy of any kind warps space/time to produce a gravity field, and space, having a 'vacuum' energy, will also warp space/time.
    You have the right idea but you need to be a bit careful. Quantification of energy is dependent on a particular reference frame (think about ordinary kinetic energy). Curvature, the manifestation of gravity, is a tensor invariant, and so does not have such a dependence. This is all handled neatly by the stress-energy tensor that determines spacetime curvature. I know of no way to explain this in everday Newtonian language. It is much more subtle than taking and plugging it in to Newtonian gravity.
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  17. #16 Re: Gravity 
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    Quote Originally Posted by Jagella
    Quote Originally Posted by DrRocket
    Energy is one element in the stress-energy tensor of general relativity, and does indeed figure into spacetime curvature, which is gravity. Energy in relativity is on an equal footing with matter, , and at some levels the distinction is a bit blurry.
    But does energy exert a gravitational pull? That's the issue I raised, and you claim I'm wrong. Please provide examples of light or heat causing a gravitational pull on some object.
    Google the "Eddington Expedition".

    It's been proven that gravity exerts a force on light to change its direction. If Newton's Third Law about every force being opposed by an equal and opposite force is true, then the light must also be exerting a force back on the gravitational object that is pulling on it.

    Dr. Rocket will likely point out that the Third Law doesn't necessarily always apply, though, so clearly it's not as simple as I'm making it. I just thought I'd bring it up because I can see that you seem to like axiomatic reasoning processes, and it wouldn't make axiomatic sense for a beam of light to be acted upon by gravity and not act back.
    Some clocks are only right twice a day, but they are still right when they are right.
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  18. #17 Re: Gravity 
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    Quote Originally Posted by Jagella
    That may be true, but energy itself cannot create a gravitational pull--a fact that some members here don't understand.
    It does, I'm afraid. Einstein worked all this out over a hundred years ago.

    Maybe I can explain it in simple terms: you know the old cannonball-in-a-rubber-sheet analogy for gravity? Think of a photon as a little wave or "pulse" in the rubber sheet. Something like the dimple caused by the cannonball, but without the cannonball, and moving at c. You can use pair production to turn it into an electron and a positron. The electron has spin angular momentum and magnetic dipole moment, and you can diffract an electron, so think of it as a pulse going round and round in a tight little circle. Now you've got a dimple in the rubber sheet that isn't going anywhere. You call it matter, and it has a gravitational effect, but that effect was there even for the photon.

    Another way to think of this is to capture a photon in a mirror- box. It adds energy to the system, and increases its mass as per E=mc² . It increases the mass and so the gravitational effect, but all you've added is a photon. Here's an interesting article where you can read more about this: Light is Heavy by M B van der Mark and G W ’t Hooft.
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  19. #18 Re: Gravity 
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    Quote Originally Posted by Farsight
    Quote Originally Posted by Jagella
    That may be true, but energy itself cannot create a gravitational pull--a fact that some members here don't understand.
    It does, I'm afraid. Einstein worked all this out over a hundred years ago.

    Maybe I can explain it in simple terms: you know the old cannonball-in-a-rubber-sheet analogy for gravity? Think of a photon as a little wave or "pulse" in the rubber sheet. Something like the dimple caused by the cannonball, but without the cannonball, and moving at c. You can use pair production to turn it into an electron and a positron. The electron has spin angular momentum and magnetic dipole moment, and you can diffract an electron, so think of it as a pulse going round and round in a tight little circle. Now you've got a dimple in the rubber sheet that isn't going anywhere. You call it matter, and it has a gravitational effect, but that effect was there even for the photon.

    Another way to think of this is to capture a photon in a mirror- box. It adds energy to the system, and increases its mass as per E=mc² . It increases the mass and so the gravitational effect, but all you've added is a photon. Here's an interesting article where you can read more about this: Light is Heavy by M B van der Mark and G W ’t Hooft.

    Right, for all the wrong reasons.

    The electron field of QED is not spacetime.

    E=mc^2 is not immediately applicable as "m" represents relativistic mass, which is not invariant, while spacetime curvature is a tensor, an invariant quantity.

    Crank
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  20. #19  
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    Sure, the electron field of QED isn't spacetime, but that was a good explanation. Far better than anything you can give. And E=mc² is immediately applicable. For an observer in the centre of the "mass frame", ie where a body has no motion with respect to the observer, m is rest mass. See Does the Inertia of a Body Depend upon its Energy-Content? along with this:

    "The rest mass is defined as the mass that an object has when it is not moving (or when an inertial frame is chosen such that it is not moving). The term also applies to the invariant mass of systems when the system as a whole is not "moving" (has no net momentum). The rest and invariant masses are the smallest possible value of the mass of the object or system. They also are conserved quantities, so long as the system is closed.


    All: be wary of this guy. He's a mathematician pretending to be an expert physicist, and his knowledge is not nearly as comprehensive as he'd like you to think. He really hates it when somebody corrects him, and gets even more abusive than normal.
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  21. #20  
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    Quote Originally Posted by Farsight
    Sure, the electron field of QED isn't spacetime, but that was a good explanation. Far better than anything you can give. And E=mc² is immediately applicable. For an observer in the centre of the "mass frame", ie where a body has no motion with respect to the observer, m is rest mass. See Does the Inertia of a Body Depend upon its Energy-Content? along with this:

    "The rest mass is defined as the mass that an object has when it is not moving (or when an inertial frame is chosen such that it is not moving). The term also applies to the invariant mass of systems when the system as a whole is not "moving" (has no net momentum). The rest and invariant masses are the smallest possible value of the mass of the object or system. They also are conserved quantities, so long as the system is closed.


    All: be wary of this guy. He's a mathematician pretending to be an expert physicist, and his knowledge is not nearly as comprehensive as he'd like you to think. He really hates it when somebody corrects him, and gets even more abusive than normal.
    Crank

    Invariant mass is no more relavant to the discussion than is relativistic mass. BTW "invariant mass" is just relativistic mass measured in center-of-momentum coordinates, so, for instance it varies with temperature, a real effect. See my post in the thread on mass.

    But if you take an object and accelerate it to nearly light speed, in your reference frame, then "m", the relativistic mass of E=mc^2, becomes humongous, while gravity is unaffected.

    Yep, I'm a mathematician. A real one. No bones about it.

    Eugene Wigner was a chemical engineer. With a Nobel Prize in physics. John von Neumann was a mathematician, and an expert in quantum mechanics. Isaac Newton was a mathematician, and my academic ancestor.

    You are a delusional nut job pretending to be sane. No education in and no knowledge of real physics. Just a self-published book of nonsense and a television appearance on a British conspiracy show. Crank.
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  22. #21 Reasoning by insults... 
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    Posted: Fri Jun 17, 2011 6:52 pm Post subject: Re: Gravity
    Quote Originally Posted by DrRocket
    Quote Originally Posted by Farsight
    ... by M B van der Mark and G W ’t Hooft.
    ...
    Crank

    Posted: Fri Jun 17, 2011 8:59 pm Post subject:
    Quote Originally Posted by DrRocket
    Quote Originally Posted by Farsight
    Crank

    ...
    You are a delusional nut job pretending to be sane. No education in and no knowledge of real physics. Just a self-published book of nonsense and a television appearance on a British conspiracy show. Crank.
    Besides your addiction to civil war, are you able to do something scientific and useful ?

    Please do not hesitate to show us your own achievements in any scientific domain, the patents you have taken, the discoveries you did, the pupils you have educated, the commonly accepted mistakes you have corrected, the companies you have fruitfully counselled, that kind of things. Do you understand that ?
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  23. #22 Re: Reasoning by insults... 
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    Quote Originally Posted by J.C. Lavau
    Posted: Fri Jun 17, 2011 6:52 pm Post subject: Re: Gravity
    Quote Originally Posted by DrRocket
    Quote Originally Posted by Farsight
    ... by M B van der Mark and G W ’t Hooft.
    ...
    Crank

    Posted: Fri Jun 17, 2011 8:59 pm Post subject:
    Quote Originally Posted by DrRocket
    Quote Originally Posted by Farsight
    Crank

    ...
    You are a delusional nut job pretending to be sane. No education in and no knowledge of real physics. Just a self-published book of nonsense and a television appearance on a British conspiracy show. Crank.
    Besides your addiction to civil war, are you able to do something scientific and useful ?

    Please do not hesitate to show us your own achievements in any scientific domain, the patents you have taken, the discoveries you did, the pupils you have educated, the commonly accepted mistakes you have corrected, the companies you have fruitfully counselled, that kind of things. Do you understand that ?

    1. Quoting posts from one thread in a totally unrelated thread without noting that fact is the act of a troll. You are clearly a troll, as well as a crank.

    2. Yes, I do things that are scientific and useful.

    3. Clearly labeling you and Farsight for the cranks that you are is useful and necessary for the benefit of neophytes on the board.

    4. I have done most of the things that you mention. Please do not hesitate to go to hell.
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  24. #23  
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    Oh, oh. They're ganging up on you Doc !

    Let's see, there's two of them and one of you.
    It's not really fair.
    They're gonna have to get more help.
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  25. #24  
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    Quote Originally Posted by MigL
    Oh, oh. They're ganging up on you Doc !

    Let's see, there's two of them and one of you.
    It's not really fair.
    They're gonna have to get more help.
    Yeah.

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  26. #25 My modest suggestion... 
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by MigL
    Oh, oh. They're ganging up on you Doc !

    Let's see, there's two of them and one of you.
    It's not really fair.
    They're gonna have to get more help.
    Yeah.

    In order to be more and more and more and more scientific, with less and less and less and less toils nor discipline, I humbly suggest you to reason only by smilies, such as :



    That is my modest suggestion, for such big boys, playing in the sandbox...
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  27. #26 Re: My modest suggestion... 
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    Quote Originally Posted by J.C. Lavau
    Quote Originally Posted by DrRocket
    Quote Originally Posted by MigL
    Oh, oh. They're ganging up on you Doc !

    Let's see, there's two of them and one of you.
    It's not really fair.
    They're gonna have to get more help.
    Yeah.

    In order to be more and more and more and more scientific, with less and less and less and less toils nor reasoning, I humbly suggest you to reason only by smilies, such as :



    That is my modest suggestion, for big boys like those, playing in the sandbox...
    Sorry if my post used mathematics beyond your capability.
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  28. #27 Re: My modest suggestion... 
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    Real Logic -> <- Troll Logic

    or

    My response to your suggestion -> (Sorry. Just to be clear, that's me laughing at you, not with you.)
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  29. #28  
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    OK - I'm new and need answers to dumb questions - which is why decided to be "gottaBtold". I hope some of you can help and be patient.

    If gravity is just a result of mass occupying space, then why is it thought to be so weak and why would we think there is some basic property, like a "graviton" to be found?

    With regard to the gravity of black holes, wouldn't there be visible light trapped in orbit around one? I guess if photons were trapped in orbit, they wouldn't be visible from anywhere except within the same orbit? If we were in that orbit, would we see light that had been trapped there since the origin of the black hole? Perhaps the black hole is a huge recording device - carrying around light that has been trapped in orbit?
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