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Thread: At what scale does gravity disappear?....

  1. #1 At what scale does gravity disappear?.... 
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    What puzzles me about relativity and quantum theories is that we know mass ( quantity of matter ) bends space-time, which we experience as the force of gravity, and we also know that mass ( quantity of matter) is made up of atoms and molecules/energy,(mass perhaps endowed by the Higgs field) so why is it that gravity appears to get smaller and smaller as we head towards quantum levels?. Surely gravity should be increasing as we get nearer and nearer to the atomic nucleus, where the biggest spacetime curvature should happen due to the energy density of the nucleons?. What am I missing here?. I read a lot about the difficulty in harmonising these theories of the very small and very large but there must be some connection?...ie a quantum theory of gravity. At what scale does gravity no longer have any meaning or impact in quantum mechanics?....


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    Quote Originally Posted by FreeThinker View Post
    why is it that gravity appears to get smaller and smaller as we head towards quantum levels?
    Um,
    we know mass ( quantity of matter ) bends space-time, which we experience as the force of gravity
    The masses involved get smaller, thus gravity (being a relatively weak force anyway) becomes increasingly over-ridden by other forces - to the point of being largely irrelevant.


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    On macroscopic levels we are dealing with large masses ( planets, moons etc ), whereas on a microscopic level only very small masses are present ( particles ). Hence the difference.
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    My understanding is that gravity decreases by the square of the distance between 2 masses, so surely should INCREASE as the distance between the masses gets smaller and smaller. Take a neutron star for example, each neutron is still only contributing the same mass, but the gravity is enormous. I thought spacetime curvature was determined by energy density?....so my question still needs an answer, at what scale does gravity disappear eg 1 micron, a nm, picometre etc?
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    Quote Originally Posted by FreeThinker View Post
    Surely gravity should be increasing as we get nearer and nearer to the atomic nucleus, where the biggest spacetime curvature should happen due to the energy density of the nucleons?
    So, the thing to do when you come across one of these "shouldn't this be a significant factor" moments is to do some sums.

    It is easy to work out the strength of gravity at the surface of an atom or even a neutron (making some crude assumptions about what the "radius" of a neutron means). You should try it. I am almost certain that it will be really, really tiny.

    acceleration due to gravity = G * mass / radius^2

    For a lead atom, for example, this works out to be about 1/10000000000000000th of g, the acceleration due to gravity on the surface of the earth.

    I haven't worked it out yet, but I think that for a single neutron it will be even smaller.
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    Yes the force increases as you get closer: but how large is it to start with when the masses involved are so small?
    A neutron star's gravity is enormous because there's a huge number of neutrons, all contributing an infinitesimally small amount toward that total.


    AFAIK gravity doesn't "disappear" it simply gets swamped by other forces.
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    Let us consider a system consisting of one proton and one electron, at a mutual distance d.
    the gravitational force between them is, (Gravitation - Wikipédia)



    Where

    = mass of a proton
    = mass of an electron
    It comes


    Now let us compare this with the electrostatic attraction between our proton and electron. According to Coulomb's law (Electrostatics - Wikipedia, the free encyclopedia) we have


    Where
    = electric charge of an electon =


    If my calculator is right, it comes


    So, the electric force is some stronger than the gravity force.
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    Still no answer to my question!, for all these calculations.....At what scale does gravity disappear?. Clearly it exists at a metre scale, and likely a micrometre scale, but where is the crossover point when quantum theory takes over from relativity, that is what I am getting at.
    I understand that for example the strong force only exerts its influence within a very short range between nucleons, the electromagnetic force may be higher as shown, thank you, but this must only exert over the scale of electrons. In other words how far from a nucleon do I have to go before relativity takes over from quantum mechanics?....
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    Quote Originally Posted by FreeThinker View Post
    Still no answer to my question!
    No?
    Quote Originally Posted by Dywyddyr View Post
    AFAIK gravity doesn't "disappear" it simply gets swamped by other forces.
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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    Quote Originally Posted by FreeThinker View Post
    Still no answer to my question!, for all these calculations.....At what scale does gravity disappear?. Clearly it exists at a metre scale, and likely a micrometre scale, but where is the crossover point when quantum theory takes over from relativity, that is what I am getting at.
    As the explanations and calculations have shown, as far as we know gravity still operates at the smallest scales. However, it is outweighed (if you'll excuse the pun) by other forces.

    When we have a theory of quantum gravity that successfully combines GR and quantum theory, then we may have a different answer.
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    Quote Originally Posted by FreeThinker View Post
    At what scale does gravity no longer have any meaning or impact in quantum mechanics?....
    Our understanding breaks down at the Planck scale, so above that GR does a good job of explaining gravity.
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    The OPs question has always interested me. For the sake of conversation would you guys help me out here and clarify. Sorry if you have already covered this. I've always assumed that acceleration due to gravity would be much much higher at the atomic level ( and contrary to mainstream I even consider g to be the source of the strong force). Bare with me but I believe it comes down to density. I read somewhere that if the Earth had the same density of an average atom its radius would be 183 meters!! So my questions are what would g be at this new surface radius and how many times more would something weigh compared to Earths original surface radius of 6,378,100 meters). Maybe you are already saying this but wouldn't gravity be much stronger?
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    Quote Originally Posted by bill alsept View Post
    I've always assumed that acceleration due to gravity would be much much higher at the atomic level ( and contrary to mainstream I even consider g to be the source of the strong force).
    Nope. Really, really tiny. See post 5.

    Bare with me but I believe it comes down to density. I read somewhere that if the Earth had the same density of an average atom its radius would be 183 meters!! So my questions are what would g be at this new surface radius and how many times more would something weigh compared to Earths original surface radius of 6,378,100 meters).
    An object that size with the mass of the Earth would have a g of about 1.2 x 1010 m/s2; i.e. about a billion times greater than the earth.

    http://www.wolframalpha.com/input/?i=G+*+%28mass+of+earth%29+%2F+%28183+metres%29^2

    Note that if the Earth were a black hole, its radius would only be about 9mm!
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    Quote Originally Posted by Strange View Post

    An object that size with the mass of the Earth would have a g of about 1.2 x 1010 m/s2; i.e. about a billion times greater than the earth.
    Yes thats my point g would be extreme on a surface that small but 6,378,100 meters away from the center (the distance of Earths original surface) it would be one g like we normally experience. On the surface of an atoms nucleus the Acceleration due to gravity may be close to 1.2 x 1010 m/s2 if your calculation are right. Proportionally g seems to be much stronger on a small scale.
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    On the surface of an atoms nucleus the Acceleration due to gravity may be close to 1.2 x 1010 m/s2 if your calculation are right
    Only if you've packed the mass of the earth into that small area. If all you've got is the mass of a proton, in the volumn of a proton, the gravity is so weak as to be virtually undetectable.
    Its the way nature is!
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    To another universe, where the rules are simpler
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    Quote Originally Posted by bill alsept View Post
    Quote Originally Posted by Strange View Post

    An object that size with the mass of the Earth would have a g of about 1.2 x 1010 m/s2; i.e. about a billion times greater than the earth.
    Yes thats my point g would be extreme on a surface that small but 6,378,100 meters away from the center (the distance of Earths original surface) it would be one g like we normally experience. On the surface of an atoms nucleus the Acceleration due to gravity may be close to 1.2 x 1010 m/s2 if your calculation are right. Proportionally g seems to be much stronger on a small scale.
    Perhaps now we can begin to perceive the enormity of the nuclear "binding forces"? It has always been mentioned in textbooks that it is very difficult to "tear apart" atoms' nuclei. jocular
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    Quote Originally Posted by AlexG View Post
    On the surface of an atoms nucleus the Acceleration due to gravity may be close to 1.2 x 1010 m/s2 if your calculation are right
    Only if you've packed the mass of the earth into that small area. If all you've got is the mass of a proton, in the volumn of a proton, the gravity is so weak as to be virtually undetectable.
    Proportionally g would be far greater on the surface of a proton. I would think that in the equation g = GM/R2 that when we calculate a small mass such as a proton which has a lot of mass per volume do the math and divide it by a much smaller radius that we would find g to be a lot higher. I don't want to highjack this thread soI'm starting another one in New Hypotheses and new ideas.
    Last edited by bill alsept; February 20th, 2013 at 11:54 PM.
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  19. #18  
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    Quote Originally Posted by bill alsept View Post
    Quote Originally Posted by AlexG View Post
    On the surface of an atoms nucleus the Acceleration due to gravity may be close to 1.2 x 1010 m/s2 if your calculation are right
    Only if you've packed the mass of the earth into that small area. If all you've got is the mass of a proton, in the volumn of a proton, the gravity is so weak as to be virtually undetectable.
    Proportionally g would be far greater. I would think that in the equation g = GM/R2 that when we calculate a small mass such as a proton which has a lot of mass per volume and divide it by a much smaller radius your sum will be a lot higher.
    Let's see.

    mass of a proton is 1.6x10-27 kg
    radius of a proton is .87x 10-15 m
    G = 6.67x10-11 m3/kg/s2

    This gives 1.4x10-7 m/s2. That's AT the 'surface' of the proton. It falls off as the square of the distance.
    Its the way nature is!
    If you dont like it, go somewhere else....
    To another universe, where the rules are simpler
    Philosophically more pleasing, more psychologically easy
    Prof Richard Feynman (1979) .....

    Das ist nicht nur nicht richtig, es ist nicht einmal falsch!"
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    I see some more thinking going into this now. 2 further observations - A certain well known particle physicist reckons that the strong nuclear force is just a subatomic/nucleonic version of gravity, and this idea is supported by recent experiments at LHC where Lead atoms have been bombarded in such a way as to creat superheavy quark-gluon plasmas, which have densities beyond those of neutron stars and second only to black holes - maybe this was the background to the fears that the LHC might create a black hole which would swallow us all up?.

    A certain Prof Cox showed that gravity's effect on life tail off at the micron level and electromagnetic forces become dominant, such that no kind of ( carbon based) life can exist smaller than about 120 nm ( bacteria are about 1000 microns ) because electrochemisttry just cannot function below that level, but also on Earth life cannot get any bigger than 120 m ( trees ) because of the limiting effects of gravity on growth, support structure and water transport.

    The formulae used thus far make many assumptions, especially about atomic dimensions. As I see it gravity is a consequence of the DISPLACEMENT of space ( ie displacement of vacuum or dark energy ) thats my theory anyway, and is a compressive rather as much as an attractive force. The more space removed from an object the greater its gravity becomes, even though its mass remains the same, ie its energy density increases. Atoms are basically 99.999999999999% "empty space" ( but filled with dark energy?) so its no wonder gravity calculations show its quantum effect to be so small on that basis. However squeeze out the space though and it becomes the dominant force, overcoming electromagnetic pressure of electrons, and then the nucleonic forces, eventually to create a singularity. Just my thoughts .....
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    Quote Originally Posted by FreeThinker View Post
    A certain well known particle physicist reckons that the strong nuclear force is just a subatomic/nucleonic version of gravity
    Can you name this "well known particle physicist"?

    and this idea is supported by recent experiments at LHC where Lead atoms have been bombarded in such a way as to creat superheavy quark-gluon plasmas, which have densities beyond those of neutron stars and second only to black holes
    Can you explain how this supports the idea that the strong nuclear force is gravity?

    The formulae used thus far make many assumptions, especially about atomic dimensions.
    I think you will find those are measurements, not assumptions.

    The more space removed from an object the greater its gravity becomes, even though its mass remains the same
    Wrong.

    However squeeze out the space though and it becomes the dominant force, overcoming electromagnetic pressure of electrons, and then the nucleonic forces, eventually to create a singularity.
    Would you like to do the math (actually just simple arithmetic) to prove that? Or you can use the link in post #13 to do it for you.
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    Brassica oleracea Strange's Avatar
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    OK. I'll do some of your research for you...

    Nuclear Properties
    Most nuclei are approximately spherical. The average radius of a nucleus with A nucleons is , where m. ... The volume of the nucleus is directly proportional to the total number of nucleons. This suggests that all nuclei have nearly the same density.
    They then go on to calculate the density of nucleus (which is pretty high) based on: "The mass of a nucleus is A times the mass of a nucleon, m ~ 1.6*10-27 kg."

    So, if we take iron as a typical element, we get
    Which, as you can see, comes to about 2.8 x 10-7 m/s2 or about 1/100000000th of the gravity on the surface of the Earth.

    http://www.wolframalpha.com/input/?i=G+*+%2856+*++1.6*10^-27+kg%29+%2F+%2856^{1%2F3}+*+%281.2*10^{-15}%29+m%29^2
    Last edited by Strange; February 22nd, 2013 at 05:14 AM. Reason: oops. Missed cube root.
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    The well known particle physicist is Bob Lazar - thats me just being provocative about gravity a and b waves!.

    no seriously LHC data on gluon-quark plasmas indicate a mass per unit volume of 30 billion tonnes per cubic cm, way beyond the Chandrasakar limit for a white dwarf star.

    Why am I wrong about space displacement?. Relativity indicates gravity is the acceleration due to space-time distortion which we experience as a force, I am only extrapolating that idea to the space in atoms. Is the space in atoms any different to space in space, so to speak?.

    We cannot measure the size of subatomic particles/waves absolutely. All formulae are therefore approximations and any calculations have to be quoted with degrees of error. We assume certain constants, but these may not be absolute either. Applying newtonian physics calculations in the quantum arena is debatable.
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    Quote Originally Posted by FreeThinker View Post
    Why am I wrong about space displacement? Relativity indicates gravity is the acceleration due to space-time distortion which we experience as a force
    Which is exactly why you are wrong about "space displacement".

    , I am only extrapolating that idea to the space in atoms. Is the space in atoms any different to space in space, so to speak?.
    No. As said earlier, there is currently no reason to think that GR does not apply at the level of atoms as well. It is just insignificant. (As the numbers show. Repeatedly.)

    But what I was really saying was wrong is:
    The more space removed from an object the greater its gravity becomes, even though its mass remains the same
    The gravitational effect depends just on mass. (OK, in GR you also have to take into account energy, pressure, etc. but these are normally insignificant until you get to extreme situations.)

    We cannot measure the size of subatomic particles/waves absolutely. All formulae are therefore approximations and any calculations have to be quoted with degrees of error.
    Certainly. And if this was a scientific paper or something, I would take the time to find out the error bounds on these measurements. But I am reasonably confident that they are not out by factor of 100000000.

    We assume certain constants, but these may not be absolute either.
    STOP IT!

    We do not "assume" constants. They are measured and/or calculated. And, again, of course there are error bounds on these. But not so great to support a claim that gravitational force in an atom is greater than other forces.

    Applying newtonian physics calculations in the quantum arena is debatable.
    What evidence do you have for that? What alternative do you propose? We just make something up because Newton/GR "might not apply"? That is not how science works.
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    Quote Originally Posted by FreeThinker View Post
    The well known particle physicist is Bob Lazar
    Never heard of him ... <tap> <tap> <tap> ...

    OK. So he isn't a particle physicist. He doesn't appear to be a scientist of any sort. Depressingly, it sounds like yet another electronics engineer with odd views and personal theories.
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    A certain well known particle physicist reckons that the strong nuclear force is just a subatomic/nucleonic version of gravity
    Complete and utter nonsense. The strong force and gravity behave in completely different ways; most strikingly, the strong force increases with distance, whereas gravity weakens with distance. Furthermore, the strong force is mediated by massive particles ( gluons ), whereas gravity is not.
    And just for your info - Bob Lazar is not a "well known particle physicist", but a notorious crank with a very dubious academic record.

    and this idea is supported by recent experiments at LHC where Lead atoms have been bombarded in such a way as to creat superheavy quark-gluon plasmas
    How is this evidence that the strong force is a microscopic version of gravity ? Quite the opposite, actually !

    A certain Prof Cox showed that gravity's effect on life tail off at the micron level and electromagnetic forces become dominant
    Obviously, considering that they differ in magnitude by a factor of 1036 !

    The formulae used thus far make many assumptions, especially about atomic dimensions.
    No they don't. Anything that isn't predicted by the field theories themselves is empirically measured and verified.

    As I see it gravity is a consequence of the DISPLACEMENT of space
    This statement is devoid of any meaning.

    Applying newtonian physics calculations in the quantum arena is debatable.
    Nucleonic physics involve quantum field theories, which are decidedly non-Newtonian...
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    Quote Originally Posted by Strange View Post
    Depressingly, it sounds like yet another electronics engineer with odd views and personal theories.
    You're safe on this one Strange.
    Lazar is, as best can be determined, a janitor with delusions and a predilection for lying.
    "[Dywyddyr] makes a grumpy bastard like me seem like a happy go lucky scamp" - PhDemon
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    Lazar was my curve ball!....Sorry I dont want to appear ignorant, I am just trying to understand why gravity and relativity are not included in quantum theory. There seem to be several ideas, like quantum loop strings for example, but who do I believe?. This prompted my original question about the scale at which gravity disappears. The answers thus far indicate.

    1. Gravity force at a subatomic level is negligible
    2. Gravity still exists at a subatomic level but its effects are swamped by other forces
    3. Gravity depends purely on mass ( energy), whatever the scale
    4. Gravity cannot as yet be reconciled with the other 3 fundamental forces at a quantum level to define a unified theory.

    Thanks for helping me learn something!...
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    Quote Originally Posted by FreeThinker View Post
    1. Gravity force at a subatomic level is negligible
    Correct.

    2. Gravity still exists at a subatomic level but its effects are swamped by other forces
    Also correct.

    3. Gravity depends purely on mass ( energy), whatever the scale
    No, this is not correct. Gravity is linked to all types of energy, no just mass. This also includes momentum, stresses, electric charge etc etc. Crucially, this also includes the gravitational field itself - gravity, unlike the other forces, is self-coupling.

    4. Gravity cannot as yet be reconciled with the other 3 fundamental forces at a quantum level to define a unified theory.
    Correct.
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