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Why is the gravitational force always attractive? Is there another way to explain this without the curvature of space time?

PS: If the simple answer to this question is that mass makes space-time curve in a concave fashion, I can rephrase the question as why does mass make space-time always curve with concavity?

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Curving spacetime made of rubber is a very problematic pop-sci concept; don't try to think of it as an actual model. –  mbq Jun 24 '11 at 16:53
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thank you mbq. what will be the right way to think about this ? –  New Horizon Jun 24 '11 at 17:02

5 Answers 5

up vote 15 down vote accepted

Gravity is mediated by a spin two particle. Electromagnetism by spin 1.

Here is a link that answers your question:

even and odd spin do differ in that they require a product of charges with different signs to get attraction or repulsion:

spin even:

  • $q_1 q_2 > 0$: attractive
  • $q_1 q_2 < 0$: repulsive

spin odd:

  • $q_1 q_2 < 0$: attractive
  • $q_1 q_2 > 0$: repulsive

In the case of gravity, mediated by spin 2 particles, charge is mass, which is always positive. Thus, $q_1 q_2$ is always greater than zero, and gravity is always attractive. For spin 0 force mediators, however, there is no restriction on the charges and you can very well have repulsive forces. A better rephrasing of the question is: "Why do particles of odd spin generate repulsive forces between like charges, while particles of even spin generate attractive forces between like charges?"

Goes on to derive this

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If OP is just asking about Einstein gravity with no cosmological constant, I think this answer is spot on. –  Qmechanic Jun 24 '11 at 21:35
    
Interesting read, this however does not preclude the existence of "negative mass" particles/objects that would act repulsively. Gravity is only attractive because a)Like "charges" attract and b)only one "mass charge" observably exists. (a) is explained by theory, and (b) is still (AFAIK) an open question –  crasic Jun 25 '11 at 12:51
    
@crasic Yes, there are experiments trying to see whether antiprotons are repulsed by gravity. cdsweb.cern.ch/record/1037532/files/spsc-2007-017.pdf . I also asked a related question a while ago : physics.stackexchange.com/questions/5521/… –  anna v Jun 25 '11 at 13:19
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The OP asked a question about classical gravity, and the answer is that classical gravity need not be attractive; GR is completely agnostic about what matter fields you give it, and it allows matter that violates energy conditions. The quantum-mechanical answer depends on general QFT ideas that aren't embodied in any known theory of quantum gravity, and it doesn't answer the question of why mass should always be positive. In fact, we know that all energy conditions are violated, for classical and/or quantum-mechanical reasons: arxiv.org/abs/gr-qc/0205066 –  Ben Crowell Aug 6 '11 at 18:33

When I was a schoolboy, our teacher of physics asked once one of our brilliant students (a girl), something like: "What is the nature of gravity?". She thought for a moment and answered: "I do not know. And what is it? Our teacher replied: "If I had known..."

As long as gravity is not derived from other nature features, it should be considered as a fundamental law. We may study its properties but we may not ask such questions - just by definition of a fundamental law. It is as an axiom - it is given as such.

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I think that asking questions like this can lead to new discoveries and insight into the nature of our universe. Why should we neglect a question on the grounds that there isn't a good answer yet? –  TylerHG Oct 16 at 4:37

It is not true at all that gravitation is always attractive. In fact, if it were always attractive, universe would not be expanding at an accelerated rate right now and an inflationary period would not have ocurred.

currently the only known source of expansion components of gravity is the cosmological constant, which, incidentally, is precisely the physical quantity that our theories fail to predict by the largest amount: 120 or 60 orders of magnitude, depending on whom you ask

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I don't think your argument holds, the repulsive force driving the expansion can simply be greater than the attractive gravitational force. Similarly, if space itself is undergoing inflation then there is no need for the gravitational force to be anything other than positive. –  Chris McCauley Jun 25 '11 at 11:11
    
--------exactly –  lurscher Jun 25 '11 at 11:57

If gravity is entropic, as suggested recently by Verlinde and earlier by others, gravity might be expected to be mostly attractive.

My speculative imaginary view of this has been that if the evolution of quantum physical objects is not perfectly conservative, then there might be processes which convert energy between scales. One possibility is that fields at the scales that determine gravitational interactions would be converted to lower scale matter that is currently not detectable. The result would be both an inflow and an outflow from regions that contain large masses, but on different scales and with different effects, attractive for matter that interacts more with the inflow, repulsive for matter that interacts more with the outflow.

At the human scale I suppose we have to imagine that we are pressed down by a flow of something that isn't the same as matter, it's the gravitational field if you will, and of course not an aether, metaphorically being sucked into the earth as food for a fundamental entropic process. Whatever exhaust there is from this process interacts with us enough less to be essentially undetectable on anywhere close to human scales, but would be detectable on either very large or very small scales.

Like I say, speculative, and also only a very small part of a whole entropic process. I can take a few or even a lot of downvotes, but pretend for a moment we're shooting the breeze at coffee. I haven't followed the Verlinde and other entropic gravity literature at all closely, so I don't know whether something like this of model has been suggested in a mathematical form, which would be required for it to be publishable (although being published definitely wouldn't be enough to make this not speculative).

If you ask for explanations for the established mathematics of General Relativity, I think the only currently possible response is speculation. GR is more grounded in empirical principles than in models that could be taken to be explanatory (which I say without prejudice insofar as I take empirically supported principled theories to be preferable to more-or-less ad-hoc models, except for the elusive question of how to imagine effective new empirically supported principles). I note that lurscher doesn't address your request for explanation.

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Verlinde didn't add anything about entropic gravity that Ted Jacobson didn't said back in 1995 arxiv.org/abs/gr-qc/9504004, although compared with Jacobson exposition of the idea, Verlinde is almost completely incoherent, a true pain to read –  lurscher Jun 24 '11 at 19:18
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as Lubos has explained in his blog motls.blogspot.com/2010/01/… there are good constraints that a underlying connection between gravity and thermodynamics have to satisfy. Of course there is more to it than fluctuations breaking equivalence principle. Maybe fluctuations cannot contribute is some naive ways. In any case, there is no doubt that the relationship is a very intriguing one –  lurscher Jun 24 '11 at 19:31
    
Fair enough, lurscher. The Wikipedia page says what you said slightly less trenchantly. Because Verlinde engages with the question at the Newtonian level instead of in terms of GR, it's arguably more accessible despite being less coherent. Of course anything serious has to make contact with GR in the sense of correspondence. –  Peter Morgan Jun 24 '11 at 19:32
    
no intention to be trenchant, sorry if it turned out like that. I just don't like the guy taking credit for an idea brought upon 15 years earlier (to which he doesn't seem to have added anything) –  lurscher Jun 24 '11 at 19:34
    
@lurscher, Trenchant is too strong a word. No worries. One feature is that entropy is, like temperature to which it is thermodynamically dual, fundamentally a phase space observable. Quantum fluctuations, in contrast, are Lorentz invariant (by definition, because the vacuum state is defined to be Lorentz invariant). To me, gravity is unlikely to be about temperature and entropy, more likely to be about quantum fluctuations and the Lorentz invariant thermodynamic dual of quantum fluctuations (whatever that is). I think (local) Lorentz invariance is critical for GR, but no-one seems to get this. –  Peter Morgan Jun 24 '11 at 21:36

artwork:

dipole +- <--- some distance ---> +- dipole

two dipoles are always attractive (or a dipole and another charge). if they are like this +- ... -+ or -+ ...+- the dipoles will rotate and the configuration become attractive +- ... +- or -+ ... -+ .
they obey a 1/r³ relation.
if you can consider that inside the baryons (neutron, proton) can exist a configuration of dipoles you have an answer.
(read the book of book of Douglas Pinnow 'Our Resonant Universe'. It is a monography of a model of particles where this happens).
how to go from 1/r³ to 1/r² ? integrate along the path.
why ? explore the concept of polarizable vaccum.

the consensus is that gravitation is not electromagnetism but, in that way it is always attractive. and I like it.
but the order of magnitude of gravitation is $10^{-35}$ (more or less, by memory) of EM so how can it be EM ?
can you figure out two EM radiators in each dipole in opposition of phase (one the +, other the -) extremely near one of the other? yes the radiated EM field has to be extremely faint.

(The connection with EM is much more compelling, imo, that a connection with thermodynamics or other ...)

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