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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 thesehttps://www.physicsforums.com/showthread.php?t=8241" 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 by a moderator: Apr 22, 2017
  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.
     
  22. Jun 24, 2006 #21

    ZapperZ

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    You don't put a detector, and detect virtual particles. You can see what the theory says, especially on higher order interactions, and THEN, measure THAT. Why do you think QED can calculate the electron gyromagnetic ratio to such high degree of accuracy that agrees with experimental observation?

    When the theoretical predictions using such model works, then one tends to put a lot of weight on the validity of it. If you look at how physics is done, you'll notice that a lot of what you accept to be true works this way, where a series of consistent experimental observation agrees with what is predicted by the theory. Even the so-called "real" particles are verified this way - how do you think we detect neutrinos?

    Zz.
     
  23. Jun 25, 2006 #22
    Anyhow, gravity.

    If the "action at a distance" forces like gravity and electromagnetism can be considered as some kind of spatial distortion, think about a horizontal pole, in a gym. You're clinging on to it with you arms and legs wrapped around it. And it's a thousand feet long.

    Now I come along and bend the pole into a U. But the pole is so long you hardly notice that it's bent. The distortion is slight.

    Now we repeat with a pole that's only ten feet long. This time you really notice it. The distortion is huge.

    You can perhaps apply this analagy to gravity. The same distortion is over two different scales, and the distortion on the large scale feels locally weak.
     
    Last edited: Jun 25, 2006
  24. Jun 25, 2006 #23
    A little offtopic question:
    Are the other 3 feild froces the same sterngth and gravity is the only feild force that isn't the same stergth. Or are they differn't(which is probally the correct answear since there's a weak and a strong nuclear force)

    If they all have differn't stergths they why are physicst only seem to wonder why gravity is so weak?
     
  25. Jun 25, 2006 #24
    As I've research M Theory, I've read that in the 11th dimensional framework, gravity is leaking into the 11th dimension, losing its strength. The 11th dimension is said to be where parallel universes coexist, so that would imply that gravity is leaking out and being shared between universes.

    Theoretically...:yuck:
     
  26. Jun 25, 2006 #25
    I dunno. You should always becareful what you read on String Theory because it has ventured into the realm of Popular Science, and a lot of the times Popular Science will give a really wishy-washy view that can be misleading. What you read I remember reading about in Brian Greene's book, so it's probably right, but be careful about accepting views on String Theory, or just String Theory in general. (Remember, it's still just a theory!)
     
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