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Why is gravity always attractive?

  1. Aug 22, 2004 #1
    The question might sound a lil funny, but it popped up when I was preparing some work on the influence of electrostatic force & gravity...

    Though electrostatic force can be repulsive or attractive, gravity is always attractive....

    Then it occurred to me, that its because there are two types of charges but theres only one kind of mass..(is it right? or theres something more to it?)

    But every things looks to be in equilibrium, so there must be something that neutralises gravity (and thats the dark energy??)...

    But universe is expanding!!....Gravity is attractive force, dark energy is weak force, and electrosatic force isnt applicable here...Then why is the universe still expanding? Shouldnt it contract??
     
  2. jcsd
  3. Aug 22, 2004 #2
    Actually, in Special Relativity, treating space time as a perfect fluid, you find that the pressure of this fluid also is a gravitational source. If we write Einstein's Field Equations and include a positive cosmological constant, we end up with a non-zero energy density and pressure for the vaccuum. This requires the vaccuum to have a negative-pressure equation of state. One consequence of this is a large scale gravitational repulsion. I believe, this is one of the theories behind why the Universe is expanding.
     
  4. Aug 22, 2004 #3
    its called exotic matter, or negative mass. It would repell gravity, but this is just a theory.
     
  5. Aug 22, 2004 #4
    a nagative force? like a antimatter, there is anti-gravitational-force? :surprise:

    umm is the expanding of universe is casuse by the big band and other expolsions that repel the masses that been influenced.
     
  6. Aug 22, 2004 #5

    Integral

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    This is yet another Why question which is not addressed by physics. It is that way because that is the way it is. We have observed it to be so and we have developed Theories which can be utilized to make meaningful predictions on the behavior of mass and the associated gravitational field. Any speculation on why is outside of the realm of physics and outside of the realm of this forum.
     
  7. Aug 22, 2004 #6

    cepheid

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    I have a question about that though. I've been reading Greene's The Elegant Universe. In the first chapter, he gives a brief overview of what I believe is the standard model of particle physics. He goes over three families of "matter particles", as well as the "force particles". Now, in looking at the tables he provides, the question naturally arises...why do such and such particles exist, and why do their masses (as measured empirically) vary in such a seemingly random way? Before the advent of superstring theory, would not the answer to such questions be (as you have stated) "because that's the way it is. We have observed it to be so, and the Universe would not behave with the same laws if it were not so. The question of why it is so is beyond the scope of physics to answer." YET, if I'm understanding him correctly, superstring theory may hold an answer to such questions that we previously considered not worth asking, because it adds another, more fundamental level to the structure of matter. One in which the different properties of various particles, including their masses, can be attributed to various "harmonics" of vibration of these fundamental strings. If so, perhaps it is worth asking "why" questions after all? I realise I'm way out of my league here, but it was just a thought.
     
    Last edited: Aug 22, 2004
  8. Aug 22, 2004 #7
    Antimatter does not have anti-gravity. Gravity, sound, light, ect all propagate from antimatter the same way they do from matter. Anti-mass would have anti-gravity, but its all theory so i dont know and im not going to keep going because that kind of a discussion doesnt belong in the general physics section. The expanding of the universe is because of the big bang, but thats not the problem. The problem is that we have found the universe to not only be expanding, but accelerating outward, instead of slowing down from gravity. As Integral said, there have been many theories proposed for why this is happening, some say its from dark matter, some say gravity is repulsive at long distances, some say other things...but nothing is for sure, its all theory. Nothing in physics is for sure actually, its just that consistency with uncountable observations is what eventually makes us assume that a theory is right.
     
  9. Aug 23, 2004 #8

    Chronos

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    Gravity is not a gauge field.
     
  10. Aug 23, 2004 #9

    Integral

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    String theory, may, or may not, have deeper answers. That is a theory very early in the development stages. Any answers to be found there must be addressed within the frame work of that theory. We have a forum for that, feel free to ask these questions there. I assure you that they do have have an answer for you now.
     
  11. Aug 23, 2004 #10
    According to general relativity it is. That is especially clear in the tetrad formalism, in which there are vectors fields(collectively referred to as the tetrad), [itex]e^a_\mu[/itex] with the property that [itex]\eta_{ab}e^a_\mu e^b_\nu = g_{\mu \nu}[/itex], where η is the minkowski metric, and g is the metric tensor. A guage transformation is just a rotation of the tetrad at each point(which can change the metric, but only in the same way a coordinate transformation can, which according to GR has no physical effect).
     
  12. Aug 24, 2004 #11
    dark matter

    antimatter doesn't have negative gravity, but dark matter does. at least thats the current theory of the expanding universe.

    Invent a way to generate and capture dark matter and there will be your cheap access to space, among other things.
     
  13. Aug 24, 2004 #12
    perhaps gravity is repelling for some lifeforms who's future is our past.
     
  14. Aug 31, 2004 #13
    he he he - that's funny.

    Would make a good sci-fi book I thinks.
     
  15. Aug 31, 2004 #14

    pervect

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    Edit: spelling!!!!!

    There's a couple of things I have to disagree with here. The first is a matter of terminology. Dark mater is something that we use to explain the galactic rotation curves, not the expansion of the universe. So dark mater is basically some sort of gravitationally attractive mater that we don't see.

    Exotic matter or dark energy is what's hypothesized to be driving the expansion of the universe at an accelerating rate, not dark matter.

    The second point is that if we had a container of dark energy / exotic matter here on the earth, it would fall down, just like any other matter.

    This is surprising at first, but it results from Einstein's principle of equivalence. If an object has a negative gravitational mass, it has a negative inertial mass. So the exotic matter will be repelled by gravity, but accelerate in the opposite direction to the force, so it will fall downwards.

    However, a large quantity of dark energy/ exotic matter would repel both normal matter and exotic matter.

    This makes more sense when viewed in the context of GR. Normal matter generates a positive curvuature of space-time, exotic matter generates a negative curvature of space-time. However, small amounts of either sort of matter will folllow geodesic curves, thus the path followed by a small piece of normal matter will be the exact path followed by a small piece of exotic matter.
     
    Last edited: Aug 31, 2004
  16. Aug 31, 2004 #15

    Nereid

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    Just to add a little to what pervect said (hopefully to clarify, not to confuse).

    "Dark matter" is the name given to a hypothetical substance which behaves in a very consistent way, throughout the universe ... it acts like ordinary ('baryonic') matter, except that a) it does not seem to emit (or absorb) any electromagnetic radiation - not gammas, X-rays, UV, visible, IR, microwaves, radio, ... anything; b) it does not seem to undergo collisions (or, more accurately, to interact with other matter only through gravitation). There's a huge amount of observational data pointing to this 'dark matter', and its distribution has even been mapped out (via gravitational lensing) in some rich galaxy clusters. Many physicists expect that 'dark matter' will turn out to be exotic particles, e.g. axions, or supersymmetric sparticles; however, there is (today) no observational data with which to constrain any such 'post-Standard Model' theories. (There are other, more exotic, theories on what DM 'really is').

    "Dark energy" is rather a loose term - it sometimes means 'whatever is causing the apparent acceleration of the expansion of the universe', at other times just 'some kind of negative energy, but not Einstein's Cosmological Constant'. The observational data from which we conclude that the universe is expanding at an accelerating rate is relatively new, and (IMHO) not fully understood yet (e.g. have all systematic effects been properly characterised?). There are many theories on how to account for this acceleration; many go by the shorthand 'dark energy'; whatever it is, it is quite mysterious at the present time. :wink:
     
  17. Aug 31, 2004 #16

    Chronos

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    This is still rather controversial, imho. GR requires diffeomorphism invariance, which is a different beast from the local gauge invariance that describles the other 3 fundamental forces. I don't think we can have a complete gauge theory of gravity without a quantum theory of gravity [not to say it hasn't been attempted]. Perhaps then it will become evident why gravity, unlike the other fundamental forces, is always repulsive... perhaps not. At the moment, that is just the way it is and nobody really knows why. See link for more detailed discussion.
    http://philsci-archive.pitt.edu/archive/00000834/00/gr_gauge.pdf
     
  18. Sep 3, 2004 #17

    Mk

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    Another clarifing post:

    Anti-matter is "anti" because all of its particles have an opposite charge, electrons with a negetive charge's anti-particle is the positron with a negative charge. Everything about it is the same except for the charge.

    When anti-matter and matter collide it results in an "anti-matter-baryonic matter reaction." What happens when baryonic or anti matter collides with exotic matter?
     
  19. Sep 3, 2004 #18

    Chronos

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    Oops

    err, attractive :blushing:
     
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