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Thread: Problem about the definition of gravity

  1. #1 Problem about the definition of gravity 
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    According to Newton’s law of gravity (F = GMm / r˛), the classical definition of gravity should be “the interaction between mass and mass”.

    What’s the modern definition of gravity? I heard some physicist / philosopher said that it’s “curvature of space - time”.
    I don’t know whether I should agree with them. My own consideration is as below:
    What cause the “curvature of space - time”? That’s the root of gravity. The “curvature of space - time” is just a method employing space and time to describe the existence of gravity.

    Any opinion is appreciated. Thank you.

    Liqiang Chen
    Oct 9, 2020


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    Quote Originally Posted by htam9876 View Post
    According to Newton’s law of gravity (F = GMm / r˛), the classical definition of gravity should be “the interaction between mass and mass”.

    What’s the modern definition of gravity? I heard some physicist / philosopher said that it’s “curvature of space - time”.
    I don’t know whether I should agree with them. My own consideration is as below:
    What cause the “curvature of space - time”? That’s the root of gravity. The “curvature of space - time” is just a method employing space and time to describe the existence of gravity.

    Any opinion is appreciated. Thank you.

    Liqiang Chen
    Oct 9, 2020
    Mass and Energy can be seen to cause the curvature of space-time apparently. Not only that (and I don't understand this) gravity itself causes curvature of space-time in a kind of feedback mechanism.

    Which makes the calculation of space-time curvature extremely time consuming.

    I agree with your assessment

    The “curvature of space - time” is just a method employing space and time to describe the existence of gravity.


    and I do not think it is controversial


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  4. #3  
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    Quote Originally Posted by htam9876 View Post
    What cause the “curvature of space - time”? That’s the root of gravity. The “curvature of space - time” is just a method employing space and time to describe the existence of gravity.
    No, General Relativity says gravity is the curvature of spacetime caused by mass and energy. Science does not seek explain why mass causes spacetime to curve, that is philosophy.

    Seems to me you are just trolling. In one thread you showed that you do not understand the first postulate of SR and when this was pointed out to you, you abandoned the thread.

    Then you showed that you do not understand mass-energy equivalency and when this was pointed out to you, you ran away again.

    Now we have a thread showing you don't even understand what science is, are you going to run away again?
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  5. #4  
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    @ Bufofrog:
    Do you mean you understand everything and only your opinion is correct and you represent the scientific community? How many Nobel prize do you win?
    I often encounter such guys as you who does not show his location troll others on web.
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  6. #5  
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    "Mass and Energy can be seen to cause the curvature of space-time apparently"
    @ geordief:
    It's possitive and meaningful suggestion. Thank you.
    But I not yet got the answer for my question in this thread "What’s the modern definition of gravity?"
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  7. #6  
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    Quote Originally Posted by Bufofrog View Post
    Quote Originally Posted by htam9876 View Post
    What cause the “curvature of space - time”? That’s the root of gravity. The “curvature of space - time” is just a method employing space and time to describe the existence of gravity.
    No, General Relativity says gravity is the curvature of spacetime caused by mass and energy. Science does not seek explain why mass causes spacetime to curve, that is philosophy.
    Aren't gravitons ( so far undetected) posited to be the mechanism whereby mass-energy causes space-time to curve?

    Along the same lines as photons with electro-magnetism...( I know that gravity doesn't work like electro-magnetism )
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  8. #7  
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    Quote Originally Posted by htam9876 View Post
    But I not yet got the answer for my question in this thread "What’s the modern definition of gravity?"
    By "modern" do you mean as in General Relativity?
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  9. #8  
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    @ geordief:

    The "modern" here I mean "different from Newton's classic"...
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    Quote Originally Posted by htam9876 View Post
    @ geordief:

    The "modern" here I mean "different from Newton's classic"...
    I only am familiar (to a small degree ) with General Relativity
    I would not be familiar with any other theories of gravitation that came after Newton


    I think General Relativity is extraordinarily successful and only fails or breaks down in situations like Black Holes and in the areas covered by Quantum Mechanics (with which I am even less familiar)

    Dark energy may be another example that GR does not explain but I have no understanding of that at all
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    Quote Originally Posted by htam9876 View Post
    @ Bufofrog:
    Do you mean you understand everything and only your opinion is correct and you represent the scientific community?
    No. I mean that I have some understanding of mainstream science. I challenged some of your ideas because they appear counter to the mainstream. You did not respond with any additional support for your idea, so I assume you no longer believe that.
    How many Nobel prize do you win?
    They don't give a Nobel for understanding mainstream science.
    I often encounter such guys as you who does not show his location troll others on web.
    I live in upstate New York. I didn't realize my location it was not in my profile thanks for pointing that out.
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  12. #11  
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    @Bufofrog:
    Are there any rabbits living in New York? I care this most...
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    Quote Originally Posted by htam9876 View Post
    According to Newton’s law of gravity (F = GMm / r˛), the classical definition of gravity should be “the interaction between mass and mass”.
    What cause the “curvature of space - time”? That’s the root of gravity. The “curvature of space - time” is just a method employing space and time to describe the existence of gravity.
    Yes, gravity is the combined effect of space and time in the presence of matter so gravity is curved spacetime. It is hard to imagine how either space or time could curve so I like to think of gravity as an area where time is slower and distances are shorter. Objects free to move in spacetime tend to move from areas where energy is high to areas where energy is low so they curve toward areas where time runs slower and distances are measured as shorter.

    Gravity can be thought of as negative energy since it takes energy to separate matter from matter. Energy and matter are two forms of positive energy and gravity is negative energy. Gravity is like a hole left in spacetime after the positive energy became separate from spacetime in the formation of matter and energy so gravity is an energy deficit. The entire universe may have a net energy of zero because it contains equal amounts of positive energy (energy and matter) and negative energy (gravity).
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    Thank you for exchange ideas, guys.
    Perhaps think in alternative angle might lead to break through:
    Definition is sole while method of description can be different. A vivid analogy is machinery drawing. Six views are needed to describe a machine part: the top view, the bottom view, the front view, the rear view, the left side view and the right side view. Any method of description is limited ability. Different angle / method of description can demonstrate different properties.

    Throw out a small stone, people can demonstrate the existence of gravity (in method of movement / dynamic);
    Tightening a small stone at one end of a rubber string, people can also demonstrate the existence of gravity (in method of static).
    Actually, no matter the objects move or not, gravity exists. So, if we put aside the element of time t, the method of description will be simpler.
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    Quote Originally Posted by htam9876 View Post
    Perhaps think in alternative angle might lead to break through
    I doubt it. Most of what you have written are misunderstandings of current theories.

    Throw out a small stone, people can demonstrate the existence of gravity (in method of movement / dynamic);
    Tightening a small stone at one end of a rubber string, people can also demonstrate the existence of gravity (in method of static).
    Actually, no matter the objects move or not, gravity exists. So, if we put aside the element of time t, the method of description will be simpler.
    Simpler and rather useless.
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    @ Bufofrog:
    Perhaps other guys would not doubt it.
    And what does my personal theory and alternative idea do with the current theory?
    Can the current theory explain everything in cosmos ?
    ...................................
    I sincerely wish those guys with positive attitude toward new research win an award in the future.
    Last edited by htam9876; October 13th, 2020 at 03:46 PM.
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  17. #16  
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    Quote Originally Posted by htam9876 View Post
    Perhaps other guys would not doubt it.
    They should since your ideas do not match observations and experimentation.
    And what does my personal theory and alternative idea do with the current theory?
    Nothing, except since your conjectures don't match observations and experiment then they are wrong.
    Can the current theory explain everything in cosmos ?
    No, but your conjectures don't explain anything.
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  18. #17  
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    Okay, I invite those guys with positive spirit of exploration go with me. Thank you.
    ……………………………………………….
    Assume a simple system: a planet and a spaceship, excluding all surrounding elements. The coordinate system for observer is built on the planet.
    Assume the spaceship moves to the planet freely from remote place ∞. The length of the static spaceship in remote place ∞ is L, the rest mass is m0 and the dynamic mass is m. The mass of the planet is M.

    When the spaceship moves freely to the position r away from the center of the planet, the velocity is v and the length is L’,
    Then, Ek = mc˛ - m0c˛ = GMm/r , γm0c˛﹣ m0 c ˛ = GMm/r
    So, 1 / γ = r m0c˛/( GMm + r m0c˛ )
    Let ˙ = 1 / γ, then, ˙ = r m0c˛/( GMm + r m0c˛ )
    L’ = L / γ = ˙ L
    When r →∞, ˙ →1, L’ → L; when r → 0, ˙ → 0, L’ → 0.
    If no gravity existing between the spaceship and the planet, when the spaceship moves freely to the planet in constant velocity, the length of the spaceship will not contract more and more seriously.
    So, the more and more serious contraction of the length of the spaceship during the course of the free approaching could be considered as the effect of gravity.

    Illustration:
    The length of the spaceship (L or L’) is just the specific demonstration of 3D physical space.
    Due to the existence of mass (or say gravity), the three dimension physical space is uneven in the radial direction of the planet; Determined by the master (the planet) and reflected by the objective (the spaceship). Or say, the three dimension physical space inflates in the radial direction of the planet. The further away from the planet, the more serious the inflation will be.
    ……………………………………………….
    (A vivid analogy is the density of air of the atmosphere of the earth, which is thicker at the bottom while thinner at the top.)
    …………………………………………….
    Gravitational redshift phenomenon:
    If light travels from the ground to the top of the Pizza Tower, its wave length will increase. A physicist’s explanation is “It is the energy of the photon that reduces in doing work against the gravitational field.”
    I don’t care too much whether his representation is sufficient enough. What I care is the alternative method of explanation:
    Due to the inflation of the three dimension physical space, the wave length of light increases when it travels from the ground to the top of the Pizza Tower.
    A vivid analogy is to put a wave length in a balloon. When the balloon inflates, the wave length will stretch.

    No contradiction.

    Liqiang Chen
    Oct 14, 2020
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  19. #18  
    KJW
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    Gravity, including artificial gravity, is caused by the time dilation associated with an accelerated frame of reference. The ordinary gravity with which we are familiar is not caused by distortion of the three-dimensional space... it is the result of what happens to the time-dimension only.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    @ KJW:
    Thank you for exchange of ideas.
    “it is the result of what happens to the time-dimension only.” Sounds the third method of description.
    Time is fast at the top of the Pizza Tower, while slow at the bottom. If light travels from the ground to the top of the Pizza Tower, its frequency will decrease and wave length will increase.
    No contradiction.
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  21. #20  
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    Quote Originally Posted by htam9876 View Post
    @ KJW:
    Thank you for exchange of ideas.
    “it is the result of what happens to the time-dimension only.” Sounds the third method of description.
    Time is fast at the top of the Pizza Tower, while slow at the bottom. If light travels from the ground to the top of the Pizza Tower, its frequency will decrease and wave length will increase.
    No contradiction.
    But there is a contradiction. You said: "Due to the inflation of the three dimension physical space...", whereas gravity has nothing to do with the three-dimensional physical space, and everything to do with the time-dimension.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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    Quote Originally Posted by KJW View Post
    Gravity, including artificial gravity, is caused by the time dilation associated with an accelerated frame of reference. The ordinary gravity with which we are familiar is not caused by distortion of the three-dimensional space... it is the result of what happens to the time-dimension only.
    As per usual ,pardon my ignorance but I was wondering whether that was not circular logic (I appreciate,I hope that GR does not depend on logic but that it is a theory amply and robustly verified by very many experimental results)

    Even so ,are you interpretating GR in this instance in a circular way?

    I think you are saying that Gravity is caused by time dilation ,which is caused by an accelerating frame...

    But ,unless you are just talking artificial gravity the accelerating frame is caused itself by gravity.

    I have surely got the wrong end of the stick...

    I think I have come across the "gravity is caused by time dilation" idea before.Is it at odds with the view I am expecting may one day be shown to be the case of it being mediated by so far undetected gravitons?
    ..
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  23. #22  
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    Quote Originally Posted by geordief View Post
    Quote Originally Posted by KJW View Post
    Gravity, including artificial gravity, is caused by the time dilation associated with an accelerated frame of reference. The ordinary gravity with which we are familiar is not caused by distortion of the three-dimensional space... it is the result of what happens to the time-dimension only.
    As per usual ,pardon my ignorance but I was wondering whether that was not circular logic (I appreciate,I hope that GR does not depend on logic but that it is a theory amply and robustly verified by very many experimental results)

    Even so ,are you interpretating GR in this instance in a circular way?

    I think you are saying that Gravity is caused by time dilation ,which is caused by an accelerating frame...

    But ,unless you are just talking artificial gravity the accelerating frame is caused itself by gravity.

    I have surely got the wrong end of the stick...

    I think I have come across the "gravity is caused by time dilation" idea before.Is it at odds with the view I am expecting may one day be shown to be the case of it being mediated by so far undetected gravitons?
    ..
    There's no circularity involved, but there is an equivalence between the time dilation and the accelerated frame of reference. And note that being in an accelerated frame of reference is indistinguishable from being in a gravitational field apart from the tidal effect due to spacetime curvature. The cause of true gravity is the nearby mass that produces the spacetime curvature both at the location of the mass itself and the surrounding spacetime.

    A further point is that spacetime curvature doesn't produce gravity by some inexplicable magic process. How spacetime curvature produces gravity is well-defined from a mathematical perspective. Although the three-dimensional space surrounding the earth is curved, it is not this curvature that produces earth's gravity. Indeed the curvature of the three-dimensional space surrounding the earth is too small to have any significant effect. The reason time dilation does have a significant effect is because distances in time are amplified by the speed of light.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  24. #23  
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    Quote Originally Posted by KJW View Post

    There's no circularity involved, but there is an equivalence between the time dilation and the accelerated frame of reference.
    And note that being in an accelerated frame of reference is indistinguishable from being in a gravitational field apart from the tidal effect due to spacetime curvature. The cause of true gravity is the nearby mass that

    produces the spacetime curvature both at the location of the mass itself and the surrounding spacetime.

    A further point is that spacetime curvature doesn't produce gravity by some inexplicable magic process. How spacetime curvature produces gravity is well-defined from a mathematical perspective. Although the three-dimensional space surrounding the earth is curved, it is not this curvature that produces earth's gravity. Indeed the curvature of the three-dimensional space surrounding the earth is too small to have any significant effect. The reason time dilation does have a significant effect is because distances in time are amplified by the speed of light.
    An equivalence?


    So not a causal relationship? Two sides of the same coin?

    Also:

    It is the one and the same spacetime curvature is it? (local and in the surrounding spacetime) No distinction?


    Edit:hope my bolding was acceptable
    Last edited by geordief; October 15th, 2020 at 01:53 PM.
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    “But there is a contradiction…whereas gravity has nothing to do with the three-dimensional physical space, and everything to do with the time-dimension.”
    I really don’t know why some people must love in contradiction rather than harmony.
    …………………………………….
    “It is hard to imagine how either space or time could curve so I like to think of gravity as an area where time is slower and distances are shorter.”
    Seems that Mr. bangstrom loves in harmony. Good guy.
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  26. #25  
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    Quote Originally Posted by geordief View Post
    An equivalence?


    So not a causal relationship? Two sides of the same coin?
    From a mathematical perspective, the notion of causality is not straightforward. For example, if A = B, does B cause A, does A cause B, or simply that A is B?

    The equivalence between time dilation and an accelerated frame of reference is mathematically more straightforward. Given the acceleration of a frame of reference, one can derive the time dilation. And given the time dilation, one can derive the acceleration of the frame of reference. In general, if A implies B and B implies A, then A and B are equivalent. Note that proving equivalence requires proving implication in both directions.


    Quote Originally Posted by geordief View Post
    It is the one and the same spacetime curvature is it? (local and in the surrounding spacetime) No distinction?
    The curvature associated with energy-momentum is mathematically the Einstein tensor field, whereas the curvature associated with pure gravitation is mathematically the Weyl tensor field. The Einstein tensor field induces the Weyl tensor field in the surrounding spacetime.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  27. #26  
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    Quote Originally Posted by htam9876 View Post
    “But there is a contradiction…whereas gravity has nothing to do with the three-dimensional physical space, and everything to do with the time-dimension.”
    I really don’t know why some people must love in contradiction rather than harmony.
    Because what you said was wrong.
    There are no paradoxes in relativity, just people's misunderstandings of it.
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  28. #27  
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    Quote Originally Posted by KJW View Post
    Quote Originally Posted by geordief View Post
    An equivalence?


    So not a causal relationship? Two sides of the same coin?
    From a mathematical perspec
    The curvature associated with energy-momentum is mathematically the Einstein tensor field, whereas the curvature associated with pure gravitation is mathematically the Weyl tensor field. The Einstein tensor field induces the Weyl tensor field in the surrounding spacetime.
    Way out of my depth ,so this follow up question may make no sense:

    There is no parallel (or useful analogy ) with the way changes in the magnetic field induce (right word?) changes in the electric field,and vice versa -is there?

    I suppose for that to be a "runner" changes in the Weyl tensor field would at least have to also induce changes in the Einstein tensor field....
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    "Because what you said was wrong."
    @ KJW:
    Best kind of discussion, but non sense.
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    Okay, I invite those guys who love in harmony go ahead with me. The spirit of science should be exploration never stop its feet. The next issue is of course about energy.
    ………………………………
    As I illustrate in the post ahead, “due to the existence of mass (or say gravity), the three dimension physical space is uneven.” The contraction or say in counter way the inflation of the three dimension physical space reflects the gravitational potential. The inflation of the three dimension physical space reflects the increase of gravitational potential.
    At this moment, I pull in another thing, the mass- space equation to join in the fun: M ∝ 1 / L’ (note: I am habitual to use upper case letters in this equation because the lower case of L looks like a “1”) illustrates that the contraction or say in counter way the inflation of the three dimension physical space reflects the dynamic mass, in turn, reflects the kinematic energy Ek = mc˛ - m0c˛. The inflation of the three dimension physical space reflects the decrease of the kinematic energy.
    For example, if you throw a ball from the ground to the top of the Pizza Tower, the inflation of the three dimension physical space (the relativistic size of the ball) reflects both the increase of gravitational potential and the decrease of the kinematic energy. That’s why the energy conserves (︱△U ︱= ︱△Ek︱ ).

    Liqiang Chen
    Oct 24, 2020
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