Why is gravitation force always attractive?

Question

Why is the gravitation force always attractive? Is there a way to explain this other than 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 queston as why does mass make space-time always curve with concavity?

Answer 1

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

Answer 2

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

Answer 3

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.

Answer 4

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.

Answer 5

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 ...)