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Newton's Equation

  1. Nov 3, 2005 #1
    F= G (mM)/r^2

    I'm interested in knowing why people are so obssessed with the gravitational constant G? It is obviously true that it is perfectly tuned so planets and solar system and life could form but why do we even ask this question to begin with and not others? Obviously if you know for a fact that this is the only formula that could explain the gravitational force between two objects, then yes that would be the right question. But why do we not entertain the possibility for the existance of other equations and this one being wrong? Is it just because we haven't found others yet or does this one have to be the one and only one? For example why do we have r squared in the bottom of the equation? why could that not be some other complex term such as r to the 1.9834983484344? which then in turn could account for that wierd G? Or even the equation itself could be fundamentally wrong in that it is missing another variable or something or maybe instead of multiplying the masses we should divide the larger one by the smaller one and multiply by some power of r and so on...

    Are we just happy to have this formula and therefore ignoring everything else just because so far this is the only one that works?

    am I wrong in saying: until we are convinced this is the only formula (THE right formula) to explain gravitational force we should avoid requiring an answer to the question "why is G so well tuned?"
  2. jcsd
  3. Nov 3, 2005 #2


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    But G isn't perfectly tuned. A different value of G wouldn't have that profound a effect on the formation of planets, the Solar system etc. It is just a proportionality constant that has the value it does because of the units we use. If we used different units, G would equal 1 and vanish from the equation.
  4. Nov 3, 2005 #3


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    I'm not at all sure I understand your question. I, at least, don't know anyone who is "obsessed" with G!

    As far as the r2 is concerned, the reason that is called "Newon's" equation is that Newton showed (and he had to invent calculus to do it!) that only a "1 over r2" formula would satisfy all of Kepler's laws of planetary motion.

    Actually, as is the case with all scientific formulas, that one is not perfect. It is now known that the planets do not perfectly match Kepler's laws and, even allowing for iteraction between the planets, Newton's equation does not perfectly reflect their motion. Einstein produced other equations that do a better job. Of course, he didn't just alter r slightly- he produced a tensor equation that is far different even in theoretical basis.
  5. Nov 3, 2005 #4


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    actually, Janus, a change in dimensionful constants like [itex] G [/itex] or [itex] c [/itex] or [itex] \hbar [/itex] or [itex] \epsilon_0 [/itex] is, in the words of Micheal Duff, "operationally meaningless".


    of course, that is absolutely right. now if the dimensionless number of Planck lengths per meter stick has changed significantly, we would know the difference. likewise, if the dimensionless number of Planck times per clock tick changes significantly, we would know the difference, but there is no way any change of a dimensionful "constant" made by some "god", would be noticed by us. if some change was noticed, it would always boil down to some dimensionless quantity or ratio of like dimensioned quantities that would have changed.
  6. Nov 3, 2005 #5
    read and listen to #4 on this page: http://www.bbc.co.uk/radio4/science/further5.shtml
    I also read it somewhere else that in fact G is precisely tuned and if it was more than 2% different than its current value either we would not have any stars or planets at all, or stars would be so compact they would burn all their mass so quickly which in result there wouldn't be enough time for evolution and life to take place.
    a lot of peolpe must be obsessed with G that's why they ask does there have to be a God to fine tune G for our universe. Some also came up with multiple deminsions and universes to increase the possibility for G to happen to be just the right number to allow for life, in our case. (listen to that audio from the link above)
    Are you saying if we changed G we would not feel anything different in the universe at all? would the gravitational forces between stars and planets not change?! would we not weigh either more or less on Earth?
    Last edited: Nov 3, 2005
  7. Nov 3, 2005 #6


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    We exist because physical laws are what they are, so it's no surprise whatsoever that they are suitable for life.
  8. Nov 3, 2005 #7


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    well, "we" don't change [itex] G [/itex] or even the constants like [itex] \alpha [/itex] http://en.wikipedia.org/wiki/Fine-structure_constant that we could notice a change.
    i am saying that we measure [itex] G [/itex] against our previous defined notions of what a meter, kilogram, and second are (or whatever units we choose to measure physical things with). now, i hope you do not object to the notion that the "strength of gravity" does not give a rat's ass about what set of units we choose to use. so then if we measure [itex] G [/itex] in terms of Planck Units http://en.wikipedia.org/wiki/Planck_units , then the value always comes out to be 1. so if God (or some "god-like being") turns the knob on [itex] G [/itex], we will still measure it to be 1 in terms of the Planck units. now if, for some reason, we measure it to be different, in terms of our meter sticks, cesium clocks, and kilogram prototype, then what changed is a dimensionless physical quantity such as how many Planck lengths there are in our meter.
    the same song-and-dance applies to the speed of light [itex] c [/itex] or any other dimensionful physical quantity.
    Last edited: Nov 3, 2005
  9. Nov 3, 2005 #8
    ok I think you're completely off track regarding what my question actually is.I think you're trying to explani how we measure units in physics and how to write them in terms of planks. Again I don't care whether G is 1 or something else. forget about planck's units... and let's get back to my question. let me pose the question in this way why is gravity as strong as it is? why not more or less?

    if the speed of lights was different we wouldn't notice that much of a change and life most likely would still evolve and planets would still form and so on, the same could be true for other similar fundamental contants, they wouldn't stop life if their values were different, but gravity would... so I guess I kind of answered my own question... any comments?
  10. Nov 3, 2005 #9


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    you can think that, but you'ld be wrong...
    this is precisely the question i was addressing. take a look at the Planck units article on wikipedia http://en.wikipedia.org/wiki/Planck_units and read the Frank Wilczek quote. oh hell, i'll just paste it in for you:

    ...We see that the question [posed] is not, "Why is gravity so feeble?" but rather, "Why is the proton's mass so small?" For in Natural (Planck) Units, the strength of gravity simply is what it is, a primary quantity, while the proton's mass is the tiny number [1/(13 quintillion)]... (June 2001 Physics Today) http://www.physicstoday.org/pt/vol-54/iss-6/p12.html [Broken]

    we have the history of how the meter, kilogram, and second were originally defined. they are quantities that are not too many orders of magnitude different from the dimensions or sizes of us (human beings). but physics doesn't give a rat's ass about human beings.

    the salient questions to ask (and answer, someday) is: why is it that there are about [itex] 10^{25} [/itex] Planck Lengths in the size of an atom? and why is it that there are about [itex] 10^{5} [/itex] atoms across the diameter of a biological cell? why is it that there are about [itex] 10^{5} [/itex] biological cells across the length of an organism such as us? and, effectively why is it that there are about [itex] 10^{44} [/itex] Planck Times in the time of a clock tick (or about a human heartbeat)? and why is it that there are about [itex] 10^{8} [/itex] Planck Masses in a mass we commonly call a kilogram (originally defined to be the mass of 1000 cubic centimeters of water)? if you answer those questions, you will answer why the speed of light is what it is (or perceived to be what it is perceived to be by us). why the gravitational constant and Planck's constant are what they are.

    let these concepts mull around in your cranium for a while. i think you'll get it eventually.
    Last edited by a moderator: May 2, 2017
  11. Nov 3, 2005 #10


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    i meant to address this one, too, but i got distracted with the G thing. the answer to the [itex] r^2 [/itex] thing has to do with the surface area of a sphere (which is [itex] 4 \pi r^2 [/itex]) and this concept we call "flux" or "flux density" (in this case gravitational flux) in which this other concept we call "field" or "field strength" is proportional to. (in fact, the most natural units which are almost the same definition of Planck units, would make the constant of proportionality between flux density and field strengh equal to 1, so there would be no difference between flux density and field strengh. these would be the same as Planck Units but with [itex] 4 \pi G = 1 [/itex] and [itex] \epsilon_0 = 1 [/itex].)

    check out the wikipedia articles on "inverse-square law", "flux", and "Gauss's Law" to get a more explicit explanation.
  12. Nov 4, 2005 #11


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    The r^2 reflects the fact that gravity spreads in 3 spatial dimentions. in 1D it would be r^0, in 2D r^1 in 3D r^2 ...
  13. Nov 4, 2005 #12
    A good theory is always based on the experiment. And luckily, the result of the experiments are always can be described exactly by math, such as by formulas or by geometry. It is funny that physics can be compatible with mathematics. And therefore we can predict some phenomenas by computations.
    F= G (mM)/r^2
    The only 2 can make a force effective for long distances, which has been proved in classical mechanics and quantum mechanics.
    The fact is that we often try to explain what we see, but sometimes we may not see the truth and then the theory need to be revised.
  14. Nov 4, 2005 #13


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    I'm pretty sure that there have been experiments to measure the exponent in the inverse square law. IIRC, they have put some very strict limits on it. I think some are cited at the beginning of "Gravitation" (Misner, Wheeler & Thorne).
  15. Nov 4, 2005 #14
    or we can believe the simplicity and beauty of our Universe :-))
    (and 2 is much more "pretty" number than 1.999874564864613513243)
  16. Nov 6, 2005 #15
    A Scientist , and more especially a physicist , seeks primarily to know how things work the way they do , while a Philosopher seeks to know why things work the way they do. Although these statements might sound semantically equivocal , in actual terms they are worlds apart , they represent two extremes . On the one hand there is the factual , empirical , tabulated experimentally proven result and on the other is the abstract , ethereal , undefined concept. To try to explain in mathematical terms why a certain line of poetry sounds better than another , makes no –sense , everybody with an opinion could be right and they could as equally be wrong , to try and put this kind of logic to work in explaining factual phenomenon is equally futile. It might be that Physicists are erring in this regard . This is just one interpretation of “suckstobeyou” comment. Again , take the recent experiments to determine “locality” or “non-locality” . The whole context of the experiment and as a result its meaning has been “altered” until the very experiment and its result has become meaningless. In the original experiment two quantum entangled particles were separated , it was known that if one of the particles had a spin up then the other would have a spin down , now, and here is the relevant part , the spin of one of the particles was changed AFTER they had undergone a space like separation. If the other spatially separated particle underwent an opposite change in spin , it would prove many statements such as the collapse of the wave function , the existence of faster than light interactions and as a direct consequence of this the whole subject of “causality”. The experiment has been carried out using SPDC (spontaneous parametric down conversion) to produce quantum entangled photons which , although of identical energy , frequency , wave-length etc., have opposite polarization , therefore if photon A of the entangled pair has a vertical polarization then it is taken for granted that Photon B will have a horizontal polarization. In fact the experiment has progressed from the purely experimental stage to one of practical application in the form of Quantum Encryption : http://www.newscientist.com/article.ns?id=dn4914 [/URL] BUT and this is the crucial point , the two entangled photons are separated and the polarization of one is [I]detected without [/I]changing its polarization at the spatially separated location. What does this prove ? Does it prove anything about locality or non-locality or the wave function or causality ? No. The single point that it proves is that quantum entangled photons possess opposite polarization at the moment of separation , nothing else. Why is this so? How can a subject which has been so hotly debated for more than half a century be suddenly dropped or obscured in such a blatant manner? The claim now is that the state of polarization of the entangled photons is held in abeyance [I] until[/I] the polarization of one of the photons is detected at which time the wave collapses . Notice that there is nothing here about the polarization being changed after the photons are spatially separated , so the experiment tells us nothing of whether spatially separated particles can in fact interact. This passes totally out of the realm of Science or Philosophy and enters an unknown area altogether. P.S. Use of capitals for emphasis only and should not be attributed as shouting.
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  17. Nov 6, 2005 #16


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    Are you perhaps trying to ask about the anthropic principle -- i.e. that the laws of physics must be tuned such that we can exist? If so, then nobody will be able to answer your question. Such discussions are more for philosophy than physics. However, this hasn't prevented such threads from popping up on PF, so if that is what you're after, I'd suggest a search of the forums.
  18. Nov 6, 2005 #17
    I feel that suckstobeyou's post is capable of many interpretations. To give a prominent example of this take the moon landings and the Voyager spacecraft , which were phenomenally successful despite the fact that no-one till today knows for sure exactly what gravity is or why it works in the way it does. And its still getting better , Japan has launched its Hayabusha space probe which will land on an asteroid pick up samples and return to earth.
  19. Nov 6, 2005 #18


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    I think that what suckstobeyou is trying to do is to use the old argument that the physical constants of the Universe where finelly tuned to allow human life, so an Intelligent Designer created the Cosmos in order to give origin to some beings who would worship Him/Her.
    For a good debunking of that theory, please read this article by Victor J. Stenger.
  20. Nov 6, 2005 #19
    prehaps suckstobeyou is thinking about chaos theory and complex systems and that the universe is a complex system sensitive to initial conditions.
  21. Nov 6, 2005 #20


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    but the only numbers (unversal constants) where this fine-tuning has any meaning, are the dimensionless numbers. not dimensionful numbers like [itex] G [/itex] or [itex] c [/itex] or [itex] \hbar [/itex] or [itex] \epsilon_0 [/itex]. those numbers have meaning only in the context of the system of physical units we measure things with (in which the universe does not care).

    there are legitimate (albeit speculative) discussions in theoretical physics (like string theory and branes, etc.) about the possibility that other universes may have appeared out of this thing called the "multiverse" in which the salient dimensionless universal constants could have been different, so different that matter could not have formed or some other difference (the amount of clustering of stars, galaxies, etc.) so that nothing like life as we understand it, could have formed. it's not pure philosophy, but it is pretty speculative.
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