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Weakness of Gravity

  1. May 31, 2006 #1


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    I saw a movie "The Elegant Universe" by Brian Greene. It was said that the reason for gravitational force being weak as compared to the other forces is that it is confined mostly to higher dimensions and that we feel only the effects of it in the known dimensions.

    My question is, why should we not just accept that gravity is a weak force by nature. Why do we have to set out explaining that it is not a weak force ? Does String Theory say that gravity is indeed comparable in strength to the Nuclear and EM Forces ? Do we really need to prove that the forces are comparable in strength in order to obtain a unification theory?

    Please educate me if I have got this understanding wrong.
  2. jcsd
  3. May 31, 2006 #2


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    "Does String Theory say that gravity is indeed comparable in strength to the Nuclear and EM Forces ?"

    Not string theory, but GUT theory does, yes. Or at leaswt, shows how they broke out into 3.

    "...why should we not just accept..."

    Because acceptance is not what we're looking for. If the forces are different strengths then we want an explanation of how they developed that way. Remember, they all had the same birth: the BB. So why we had 4 forces fall out of the BB needs to be understood. It could be understood if they all happened to be different, but we still need to uncover the mechanism that made them.

    If gravity had the same strength as the other forces, it would be much easier to show that they are "four sides of the same coin".
  4. May 31, 2006 #3
    Hi Dave,

    There are other mechanisms conceivable which keep gravity as a weak force but nevertheless unify it with eg. electromagnetism. An example (which does not work :wink:, but that is not important for now :smile: ) of such toy theory would be that EM is the first order interaction term (linear in the charge current Q), while gravity consists of the Q^2 terms. For this to work one would have to imagine that the neutron consist of charge - e and + e such that Q^2 for the neutron really is 2 e^2. Like I said, this is naive, but a better model (and mass/(charge + topological invariants?) formula) might do the trick.


    Last edited: May 31, 2006
  5. May 31, 2006 #4


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    the issue people usually express as "why is gravity so weak" was explored in a series of 3 intuitive idea-articles by Frank Wilczek published in PHYSICS TODAY circa 2003 and 2004, maybe someone can get links.

    these articles are written for non-specialist audience in informal, evocative language----but also have equations. they impress me as bold and having a lot of insight. also could be wrong of course

    the series is called SCALING MOUNT PLANCK

    and the first one is called "Scaling mount planck: a view from the bottom"=====they all have jazzy names like that

    I am relying on imprecise memory of the gist. I think he "pulls a fast one" on the reader and inverts the question and says that the real problem is not why is gravity so weak by WHY IS THE PROTON SO LIGHT? he shows that these are two sides of the same problem coin---equivalent questions

    and then he proceeds to try to ANSWER the new question and delves into QCD.

    the key number which he is trying to explain, in these 3 remarkable essays, is the number 13 quintillion
    13 x 1018

    that number is the Planck mass expressed in proton masses. (that number being big says that the proton mass is small in planck terms)

    this number 13E18 appears in several situations. if you can explain why that number then you automatically have also explained "the weakness of gravity"

    if anyone wants to follow up---do a search for Wilczek and "mount planck"----I mention this only in a "librarian" capacity. my personal view is not the same as Wilczek but I think his view is part of the picture

    Oh, Frank Wilczek does not "extra dimensions" to explain stuff :smile:, he is not Brian Greene but as a Nobel laureate I guess you could say he's almost as good.
    Last edited: May 31, 2006
  6. May 31, 2006 #5


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    We know of just four forces; three of them are of comparable strength, but the fourth, gravity, is way, way weaker. Doesn't that at least nudge your curiosity bone? If we just accepted every oddity of nature we'd never learn anything.
  7. May 31, 2006 #6


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    I will chime in, perhaps unnecessarily.

    the whole of the theory of atoms, and the periodic table of elements, and chemical bonds, and the solid state theory of how transistors work and lasers and how light is reflected and transmitted and interacts with atoms and stuff-----all that modeling and calculation that people do---is based on the famous number 137

    or 137.036...

    Feynman said any physicist who was any good should write that number on a piece of paper and put it up on his study wall to remind him to ask WHY IS THAT NUMBER WHAT IT IS.

    the number 13 quintillion (which you can write out more accurately if you want) is ANALOGOUS to that. It is ALSO one of nature's numbers.

    if somebody does physics in the Andromeda galaxy, or anywhere, he will also find the number 137 and the number 13 quintillion as proportions built into nature

    he may not use decimal notation to write down those numbers but he will find those numbers. and if he is smart he will ask why they are what they are.

    it happens that 137 is not big, and that 13 quintillion is big. but maybe the difference in size does not have to distract us from the fundamental puzzle which is how did those numbers get decided on? what determined them, and when in history did it happen? what causes them?

    so congratulations to this guy HMS, whoever he is, because he is wondering about something basic-----a good question is one you can go on wondering about a long time and not find anyone who knows the answer
  8. May 31, 2006 #7
    As an aside, but in referance to the last posting by Marcus, I found this statement:

    "It is important to know the numerical values of the fundamental constants with high accuracy for at least two reasons. First, the quantitative predictions of the basic theories of physics depend on the numerical values of the constants that appear in the theories. An accurate knowledge of their values is therefore essential if man hopes to achieve an accurate quantitative description of the physical universe. Second, and more important, the careful study of the numerical values of these constants, as determined from various experiments in the different fields of physics, can in turn test the overall consistency and correctness of the basic theories of physics themselves."

    Source: http://physics.nist.gov/cuu/Constants/introduction.html
  9. May 31, 2006 #8


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    the quote of Frank Wilczek is restated in


    and is pretty concise:

    http://www.physicstoday.org/pt/vol-54/iss-6/p12.html [Broken] (June 2001 Physics Today)

    the wikipeida article goes on to say explicitly:

    BTW, marcus, i am not sure that the aliens in the Andromeda galaxy will be thinking of 137 or 1/137, but perhaps the number they'll be thinking about is 0.30282212 which is [itex] \sqrt{4 \pi \alpha} [/itex] and, if you choose units so that [itex] \epsilon_0 = 1 [/itex] and [itex] \mu_0 = 1 [/itex] resulting in nondimensionalized or natural versions of Maxwell's Equations, then [itex] \sqrt{4 \pi \alpha} [/itex] is the magnitude of charge, measured in these natural units, that nature has bestowed upon fundamental particles like electrons, protons, and positrons.

    so while Feynman said that any physicist who was any good should write 137.03599911 on a piece of paper and put it up on his study wall to remind him to ask "why is that number what it is?", i think that the number they should put on their wall is 0.30282212 which is even closer, in orders of magnitude, to unity. and the Fine-structure constant comes out of this as a consequence.
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  10. May 31, 2006 #9


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    Agreed. Don't get me wrong, I'm not saying it needs to be (all the same), I'm just saying, if it were (all the same), we'll find it easy to unify them all. If they are different, we still need to understand what created the forces, we just have less clues to go on.

    Besides, ultimately, what we're looking for is symmetry in the universe: under what conditions (such as during the Big Bang) can we look at something (or many things) from any conceivable angle, and it still looks the same?
  11. May 31, 2006 #10
    Gently speaking, and without evidence, I would posit that the "Big Bang" is inherently asymetric by virtue of point-locality.
  12. Jun 1, 2006 #11


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    The problem I see with gravity leaking into extra dimensions is it should be badly behaved in our 4d universe compared to non-leaking fields - like EM - resulting in a 'not quite inverse square law of gravity'.
  13. Jun 1, 2006 #12
    Gravity isn't weak, it's just misunderstood.
  14. Jun 4, 2006 #13
    Gravity totally rules.
  15. Jun 6, 2006 #14

    Hans de Vries

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    Gravity as a second order effect is currently my best guess as well.
    For me it sure does look so.

    Gravity is different from the rest in that you need to multiply instead of
    add/subtract to determine the force. You can add/subtract because
    "charges" are quantized.

    A non-linearity in two fields [itex]\phi_1[/itex] and [itex]\phi_2[/itex] would have a second order term
    [itex]\phi_1\phi_2[/itex] which is a mass product. Assuming that nature is not better than
    100.00000000000000000000000000000000000000 % linear, (40 zeroes),
    there must occur "gravity like" interaction, that is, terms containing

    There is a good argument that, if the two fields are fermions, no such
    interaction is possible because the "dimension" of that interaction is
    9/2 > 4 and anything greater than four leads to a non-renormizable
    theory. However I would guess that with a coupling constant so
    extremely small you don't need to worry about the higher order terms...

    I wonder where this would leave [itex]\phi^3[/itex] and [itex]\phi^4[/itex] theories since these would result in
    interactions like [itex]\phi_1^2\phi_2,\ \phi_1\phi_2^2,\ \phi_1^2\phi_2^2\ [/itex] and [itex]\phi_1^3\phi_2,\ \phi_1\phi_2^3,\ \phi_1^3\phi_2^3\ [/itex] interactions respectively
    where some of these might have a repulsive nature. (dark energy like)

    Regards, Hans
    Last edited: Jun 6, 2006
  16. Jun 6, 2006 #15


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    Word. Totally.
  17. Jun 7, 2006 #16


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    A vanishingly small coupling constant sounds vaguely familiar.
  18. Jun 13, 2006 #17


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    This is the idea, yes.

    If there are extra dimensions, and they are very, very small, then gravity will behave well on all but the tiniest scales (scales on the order of the dimensions in question). At those smallest scales only, if we notice that gravity does not follow the inverse square law, then a good explanation would be extra, curled-up dimensions.

    The upshot here, is that gravity could then, when all summed up be the same magnitude as the other forces, but we only expereience a tiny portion of it in our three macro-scale dimensions.
  19. Jun 14, 2006 #18


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    That is one of the most refreshing and sensible ST predictions I've heard in a long time. I hope the LHC crew takes this into account.
  20. Jun 14, 2006 #19


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    But, I frankly doubt the LHC will do much to alter physics as we know it. I think the money would have been better spent improving cosmic ray surveys. Nature's accelerator is far more energetic.
  21. Jan 18, 2011 #20
    I don't agree that gravity is weaker than Electro Magnetism. I just watched a science channel episode called "Masters of the Universe". It featured Steven Hawking and some others. In the episode a female professor put a paperclip on the ground then proceeded to pick it up with a small magnet, thus showing that the magnet was more powerful than gravity. It is my theory that gravity is just as strong as Electro Magnetism. It's just that the energy field of gravity is spread across a greater distance than the energy field of Electro Magnetism. I would suspect that for this reason EM could not due some of the feats that gravity can accomplish. For instance I suspect that if our sun was replaced by a giant EM and all our planets were replaced with steel balls, I would bet that mercury may be sucked into the sun, the next two planets may continue their orbit and the other planets would leave our solar system, the further away the faster.

    This is just a rough illustration. But my point is you cannot say that gravity is a weaker force than mangnetism because it can pick up a paperclip. It may be stronger on the shorter distance scale but I wonder how big a magnet would have to be to hold a metal pluto in orbit from the distance of the sun.

    The "magnet is stronger" argument would be like saying a human throwing a baseball further on the moon is stronger than a human throwing a baseball on the earth simply because the baseball went much further on the moon. The properties of the moon allow for the momentum of the baseball to travel much further than the baseball on the earth. Yet the energy exerted by both throwers was the same.

    Is a sprinter more powerful than a jogger. Yes if he's chasing a purse snatcher. No, if he's delivering a letter 5 miles away. We need to stand back and look at the bigger picture before saying that something stronger than something else.

    A magnet is more powerful close up whereas gravity is more constant and reaches much further. There forces could easily be equal when all measurements are taken into account.
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