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Why is gravity so weak?

  1. Jun 22, 2006 #1
    If all four fundamental forces were once unified and equal, why is gravity so much weaker than the other three forces?
     
  2. jcsd
  3. Jun 22, 2006 #2

    nrqed

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    well, if I knew why I would have a good shot at a Nobel prize!

    Nobody really knows why. One idea is inspired by string theory/ brane models scenarios. In string theory, gravity is mediated by closed strings whereas the other forces are mediated by open strings. And it turns out that open strings have their ends attached to submanifolds (the so-called branes) whereas closed strings may propagte freely in all dimensions. If our universe is one of those branes, this would explain why gravity appears weaker: the force is "spreading" out in all dimensions and appears to us weaker. The other forces mediators are confined within our brane and do not "leak" out in all the dimensions.
     
  4. Jun 22, 2006 #3
    This is by a science fiction writer so shouldn't be considered serious physics, but it's "open your mind" interesting.

    http://www.npl.washington.edu/AV/altvw98.html

    Why is gravity so weak? Why are the color forces between quarks so strong? In the standard model of particle physics, why are there so many different energies at which distinct fundamental forces are supposed to "unify", and what determines these widely separated energies? The answers to these questions may be provided by extra dimensions curled into loops a millimeter around. In other words, our universe may be only a millimeter across, in directions we are not yet able to perceive. In this column we'll consider millimeter-size extra-dimensional loops and their implications...
     
  5. Jun 23, 2006 #4
    This may or may not be on the topic, but does anybody know that since Fermi Labs found that neutrinos have mass how much more of the universe's mass is now accounted for? Is the search for gravitons still taken as seriously?
     
  6. Jun 23, 2006 #5

    ZapperZ

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    And this is certainly off-topic, but give credit where credit is due. The first laboratory/facility that verified the existence of neutrino mass is Super Kamiokande in Japan, not Fermilab. Fermilab only recently got into the neutrino business with MINOS, that just announced their first set of results a couple of months ago.

    And no one is looking for gravitons right now. Gravitons and "gravity waves" as the ones being looked for with LIGO are not the same thing.

    Zz.
     
  7. Jun 23, 2006 #6
    Sure is some interesting stuff out there.

    http://www.psc.edu/science/Winicour/winicour.html

    "To clinch the case, scientists at Caltech and MIT, with funding from the National Science Foundation, are building LIGO, the Laser Interferometer Gravitational-Wave Observatory...
     
  8. Jun 23, 2006 #7

    ZapperZ

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    You'll notice that this is OLD. The Adelberg's group at U. of Washington has already verified gravity up to sub-micron scales with NO deviation in G. This implies that there are no "extra-dimensional loops" at the millimeter scale.

    Zz.
     
  9. Jun 23, 2006 #8

    George Jones

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    I'm not sure what happened to this

    http://cosmicvariance.com/2005/08/12/rumors-of-new-forces

    I'll try and take a closer look this afternoon.
     
  10. Jun 23, 2006 #9

    ZapperZ

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    They themselves are not sure of it, and we only have the report of their presentations at the APS April Meetings, which is where this was first revealed. So I would suggest we wait till such a thing is published.

    In any case, it is still NOT at the mm scale that all of these "extra" dimensions for gravity have been predicting.

    Zz.
     
  11. Jun 23, 2006 #10
    OK thanks Zapper.

    Techno-Raver: I'm not sure about this, but I picked up somewhere that Gravity is so much weaker than eg Electromagnetism because of a dimensional difference. Both can be considered as curvature or distortion of spacetime, but gravity is a gentle distortion of the spacetime in our familiar dimensions, while the Coulomb forces is a severe distortion in unfamiliar, smaller dimensions. These other dimensions are like 10^40 smaller than the ones we're used to, and the amount of distortion is the perceived "force" strength.

    http://www.wordwizz.com/pwrsof10.htm

    "This is a visual journey consisting of 42 images -- 42 powers of ten. At one end of the journey is the immensity of the known universe, 13.7 billion years old with a radius of at least 12 billion light years (and probably much larger). At the other end of the journey is a depiction of the three quarks within a proton.
     
    Last edited: Jun 23, 2006
  12. Jun 23, 2006 #11

    George Jones

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    :confused:

    It's my impression that theory/experiment has ruled out extra dimensions at the anything above the mm scale. All scales below this are still fair game.
     
  13. Jun 23, 2006 #12
    Yes string theory does have a good idea about why gravity is so weak you might want to watch these video's for more about string theory.


    In cosmology was gravity soppsed to become sepreate form the other feild froces first. Could this have somthing to do with the reason why gravity is so weak.
    EDIT: Also I just remberd somthing. That there's a theory called mond theory that says that gravity strength changes. I think it might of also been callled varible gravity.
     
    Last edited: Jun 23, 2006
  14. Jun 23, 2006 #13

    DM

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    Can anybody ever look for gravitons knowing they're virtual particles?

    Can anybody look for any virtual particles for that matter?
     
  15. Jun 23, 2006 #14

    rbj

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    personally, i would like to know in what sense do you mean that gravity is weak?

    since gravity (in the classical sense) acts on mass and, say, E&M acts on this property called electric charge, you cannot compare the two. it depends on how much mass you have on one hand and how much charge you have on the other. i don't think that gravity is any "weaker" than, say, E&M. the force of gravity between two Planck masses is equal to the force of E&M between two Planck charges.

    if, when it boils down to it, that you say that the attractive gravitational force between two protons (or pick your fundamental particle) alone in free space is far, far weaker than the repulsive electrostatic force between the same two protons, you're right, it is. and that is because the charge of the two protons is very nearly the natural unit of charge, but the masses of the two protons is far, far less than the natural unit of mass and that is why the gravitational force between them is neglegible.

    the real question to ask is: why are the masses of the fundamental particles so, so small?
     
  16. Jun 23, 2006 #15

    ZapperZ

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    How did you think we confirmed the existence of the W and Z particles that mediate the weak interactions, the gluons that mediate the strong interactions, etc... etc? Virtual photons are not verified? Let's throw out QED.

    Zz.
     
  17. Jun 24, 2006 #16

    DM

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    Actually, I don't know.

    I've simply read that virtual particles cannot be directly detected, which makes perfect sense to me. Whether there are practical ways to detect them or not is of great interest since theoretically it sounds very difficult.
     
    Last edited: Jun 24, 2006
  18. Jun 24, 2006 #17

    ZapperZ

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    They can't be directly dectected, but the effects of their existence have ramifications that are measureable. You do not get "free quarks" either, yet we have many predictions that have been verified experimentally based on the quark model.

    You should know by now that in physics, unless things are experimentally verified, we normally do not award Nobel Prizes for it, especially if it is theoretical work.

    Zz.
     
  19. Jun 24, 2006 #18

    DM

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    In which of course, I do.

    Your previous post perplexed me. It gave me the interpretation that someone actually directly detected virtual particles, or if not, that there were ways of detecting them and therefore enabling physicists to apply the same experimental procedure to detect gravitons.
     
    Last edited: Jun 24, 2006
  20. Jun 24, 2006 #19

    ZapperZ

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    What's perplexing about that?

    Zz.
     
  21. Jun 24, 2006 #20

    DM

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    You agreed that virtual particles cannot be directly detected.

    Are you now implying they can?

    Again, I'm left somewhat perplexed.
     
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