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Gravitons supposedley transfers gravity between two particles

  1. May 9, 2005 #1
    gravitons supposedley "transfers" gravity between two particles

    i know that gravitons supposedley "transfers" gravity between two particles, but can anyone tell me how gravitons work, and why they "transfer" gravity?
  2. jcsd
  3. May 9, 2005 #2


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    They SHOULD work just lilke any ohther "force" carriers.Like photons,gluons,Z or the charged bosons.They should enter the same type of Feynman diagrams like gluons,since they exhibit a nonabelian gauge symmetry...

  4. May 9, 2005 #3
    yah but i dont understand those either
  5. May 10, 2005 #4


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    Then there's nothing else to do,but start learning.Unofortunately,books on QFT seldom treat gravity,for good reasons i guess.

  6. May 10, 2005 #5
    Hello H.D.

    I don't understand either, but the best image I have retained (I will attribute it to Brian Greene and try to verify this later) is of two ice skaters standing several feet apart face to face. One throws a basketball to the other. The one who throws then slides backward on her ice skates from action/reaction. He, the one who catches, also slides backward from the impact energy of the ball. Then he throws the ball to her again, and so on, out to the furthest distance that they can still throw each other the ball.

    The attractive forces are explained by having the skaters do an about face and then supplying them with a number of basketballs sufficient so that when they throw them away from each other, the resultant force is sufficient to push the skaters together. This side of the analogy is even more in violation of known fact. How to account for the inverse square again, this time as the skaters rush together? Do they throw more and more balls as they get closer, to manage the acceleration? And where do they get all those basketballs anyway? Is some god up in the ceiling grid raining basketballs down on them?

    I see some problems with this analogy. For one thing, it does not account for the inverse square relationship. For another, the existance of the force carrior particle is inferred from the motions of the two reacting particles. No direct evidence of force carrying particles exists, afaik. Perhaps someone better informed will be able to correct me on this.

    Further, in regard to gravitons, the analogy attempts to explain the effects of mass by invoking the effects of mass....a circular form of reasoning which cannot be expected to provide much new infomation.

  7. May 10, 2005 #6
    That anology is very confusing, are the basketballs gravitons? and also, supposedley gravity never pushes, but the way u explain it it can, u just have to throw the basketballs in a different direction.
  8. May 10, 2005 #7
    What if [sound of inhaling intoxicating substances], the skaters were throwing the basketballs behind them (still facing each other). If they also threw them to both sides, the net effect would be no movement. They also throw them forwards, but less of them, or less hard. The net effect would be to bring the skaters closer together.

    I don't care about the gravity, I'm just going skating tonight and I have a load of basketballs I don't know what to do with and there's a girl there I really want to impress. If I stun her with a basketball she might stay still long enough for me to get to her cos I can't skate for the life of me.
  9. May 10, 2005 #8
    Wouldn't it be more effective if they used medicine balls?

    Just a thought.
  10. May 10, 2005 #9


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    Back to the original post, the place to start is quantum electrodynamics. A very brief introduction can be found here: http://hyperphysics.phy-astr.gsu.edu/hbase/forces/qed.html and a more complex one is in a short book titled QED by Feynman.

    QED explains how electromagnetism works through the exchange of photons.

    A photon is a massless, spin 1 particle. A graviton is a hypothetical massless spin 2 particle. Just consider those labels for a second.

    Photons act in a particular way described in QED. You can't understand any quantum mechanics until you understand QED which is the most familiar of the quantum theories, the most accurately tested and the most easily applied. You will not really understand the graviton until you understand QED. There is no way around it. The way a photon acts is quite similar to spin 1 particles associated with the weak force (W and Z) and the strong force (gluons), which are also spin 1 particles. All of these force carrying particles act quite differently from quarks, electrons and neutrions which are spin 1/2 particles.

    The characteristic of a particle that a photon is interested in is "charge". The characteristic of a particle that a gluon is interested in is called "color". The characteristic of a particle that gravity is interested in is called "mass".

    Spin 0 particles (the "Higgs Boson" is a hypothetical particle of this type), if they exist, do not interact with spin 1 particles. Photons are spin 1 particles. Therefore, since gravity does interact with photons, we know gravity cannot be a spin zero particle. Spin 2 particles can interact with both spin 1 particles and spin 1/2 particles. This makes them a candidate for the graviton.

    A gravitons behavior is both simplier and more complex than the photon. As a first approximation, one can think of a graviton as a photon operating under QED in a world where the are only positive charges (aka positive masses). If such a graviton existed, gravity would be Newtonian. But, we know that gravity is not Newtonian. The differences between the behavior of a spin 1 particle and a spin 2 particle is equivalent (or very nearly so) to the differences between Newtonian gravity and general relativity.
  11. May 10, 2005 #10
    Yah. Medicine balls. Much more effective, but dangerous when dropped from the ceiling of a gymnasium.

    Yes, the ball is supposed to be "like" the force carrying particle.

    I am going to make an unsupported statement of opinion and then watch to see if the physics community comes around to my point of view. Keep in mind that I am just a guy hiding in the woods and not a member of any powerful academic faculty. Here it is. Given sufficient energy and the technology to manipulate it, any particle consistant with the standard model of particle physics can be demonstrated.

    If it doesn't exist in nature, we will construct it in the laboratory.

    I think force carrying particles are like that. Is it a hardball particle or is it a temporary intersection of conflicting energetic fields? I don't care. The important thing is that we see objects move, and we want to know why they move. For particle physicists who insist that there must be contact between hardball particles for any change to occur, the idea of force carrying particles is an absolute necessity. On the other hand, for field theorists, the only thing necessary is that the rink (field) can be contorted to any desireable shape, cone or plane or saddle or trough or sombrero, whatever seems necessary to explain the motions of the hapless skaters (particles) who are flung about the rink (field) by icy (zero point energy?) gyrations.

    Come on. E=Mc^2. Matter (mass) and energy are the same thing. There isn't really any conflict between particle physics and field theory. Just two ways to look at the same thing. If you want to explain attractive or repulsive motion by means of gyrating skating rinks or by means of skaters armed with pea shooters, it is the same thing. There is no reason to get fixated on the existence or non-existence of medicine balls or sombreros. Those are just models.

    The op wants to know how gravitons work. We don't even know if gravitons exist in nature. I suppose we may actually create them in the laboratory, or at least in mathematics, even if they don't exist in nature.

    It is only a model. The rudder doesn't move and the flaps don't go up and down, but look, the intelligent designer put hinges on the wheels so you can raise and lower the landing gear. Cool.

    If you want to know more about airplanes than you can learn from Revelle, the plastic model maker, you have to study aeronautics. It is no use hanging around the airport bothering the pilots about why your flaps don't go up and down. But if you really are curious and want to learn to fly, airports are a great place to begin. Little airports, that is, where pilots still check the tire pressure. If you want to learn physics, I can't think of a better place to start than right here at PF, where lots of interested people are poking about the scrap yard.

    Now, ohwilleke, can you tell us what spin is, actually, and why it comes in such nice easy fractions? Why don't we see any spin 11/32 particles, for example?


  12. May 10, 2005 #11


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    Gah! Not today. I've got papers to grade. But, maybe tomorrow. If you're good.
  13. May 10, 2005 #12
    Yah, as above, it is only a model. However, I should like to point out that there is a model of gravity where in fact gravity does push. Or at least, gravity can be thought of as the push caused by mutually exclusive particles as they expand in space. Objects in free fall are not moving, just expanding, until they come into contact with another expanding object. Then the mutual exclusion and expansion of the two objects pushes them apart, causing the acceleration we call gravity. The expansion of the universe is thought of as occuring from every point in space. Note that no gravitons and no sombreros are needed in this model. Gravity and the expansion of the universe are thought of as identical.

    Be well,

  14. May 10, 2005 #13
    Great! I'll look forward to that, and promise to be as good as possible. Well, at least, as possible for me. My mother used to tell me to behave myself. Sure, I would say. Who else would I behave?


  15. May 10, 2005 #14
    A classical ball is always be made of on-shell particles. But in QFT the "ball" can be off-shell and the force may be attrative.

    I think whether the force is attrative or opposite can be determined by this, the method is to draw feynman diagrams and consider the momentum of the virtual particle(at least a 2to2 process). The same shape of diagram may leads to attrative or excludsive force, it depends on the angular distribution of momentum.
  16. May 11, 2005 #15
    Hi wangyi

    and welcome to the discussion.

    As to "on shell" and "off shell", I have seen this terminology before but I am not certain what it means. But then, I am not sure what a "2to2 process" is, either. It sounds stringy.

    Angular momentum is the energy of motion possessed by an object due to its spin......hmmmmm. So if we take an object and spin it, its angular momentum will be distributed somewhere in the space the spinning object occupies. If the object is a coin, the distribution will be different than if the object is a hollow sphere. Or a dumbell or a bolo or something like that.

    Ok. So if the spinning object is a hollow sphere, or shell, and if the momentum is located in the same region of space as is the shell, then if the shell hits a large flat stationary object, it will translate some of its angular momentum from the shell into linear motion along the flat surface, more or less, ignoring elasticity and bounce and so on. The direction and energy of linear motion will be determined by the direction and energy of the angular momentum.

    Then if the location of the momentum is off shell, I might think of the same hollow spinning sphere, but this time the angular momentum is due to some inner massive structure, like a lead weight inside a soccer ball. The lead weight, depending on where it is located in the ball and how springy the matrix that holds it is, would cause the ball to do some rather odd loops and twirls after hitting the large flat surface.

    But that probably isn't what you meant at all.

  17. May 11, 2005 #16


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    On shell means the relativistic energy squared [tex]p^2 - m^2[/tex] is positive (c=1 is assumed). When this is so, the particles is observable. When the relativistic energy is negative the particle is virtual.

    Yes, the rotating shape is coded into a number called the radius of gyration. Calculating the ROG for different shapes is a calculus problem. Richard, do be careful about distinguishing momentum from energy - it does make a difference.

    Unless the line of the hit happens to pass through the center of mass of the flat object, it will acquire some spin - some angular momentum - too. And corresponding angular momentum will be lost by the colliding object - it's another conserved quantity. Otherwise you are right.

    No, what you're describing here is something like inelastic collision. Off shell is a totallly quantum idea.
    Last edited: May 11, 2005
  18. May 11, 2005 #17
    Thank you selfAdjoint

    Someday when I have a few extra millenia I want to sit down and re-write all of physics in a way that will require all of the terminology to make perfect descriptive sense. Ah, me. So much to do. Actually I suspect that the terminology is part of a human genetic preference for social heirarchy. All the confusion and false paths that terminology may lead us to is part of an intelligent design intended to seperate the neophytes from the acolytes.

    However, I couldn't resist the opportunity to play with the idea of a sphere with an eccentric center of gravity. One might suppose all kinds of fields and particles to account for the fact that it always comes to rest with the same up-down axis, but none of that is necessary. It just has an off-center weight in it.

    Thanks for the pointer on momentum vrs energy. I guess if one is assuming a constant mass the two can be treated as equivalent. However that is a big assumption in the quantum realm. Thanks.

    Now I presume that this "shell" idea comes from the fact that the formula looks very much like the formula for a sphere, IIRC. I want to think about what kind of space this formula exists in. P is momentum and M is mass, right? I have to do chores but hope to return to this soon.



    ps the book is one of the chores.
  19. May 11, 2005 #18


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    One thing that always distinguishes momentum from energy even when they are numerically equal is that momentum is a vector, with a direction, and energy is just a scalar, a directionless number. So it's the difference between the amount of gas you have in the car, and the way you're driving.
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