Repulsive Gravity

Everyone knows that gravity is attractive, right? Since Isaac Newton, the universal law of gravitation states that the force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Einstein's theory of relativity superseded this for extreme cases, but in most cases, agrees very well with it.

But suppose that gravity was really repulsive; i.e. instead of attracting bodies together, the basic gravitational force actually pushed? Let me explain before you write me off as a completely bonkers. Imagine that gravity consists of a homogeneous, isotropic flux of particles throughout the universe. Let's call them "gravitons", although they would be quite different from the gravitons that scientists are currently searching for, and have not yet found. My gravitons produce a small push on matter when they are absorbed, deflected, reflected, or otherwise interact with it.

Now consider a single object, say a planet, alone in the universe. The gravitons would push against the planet's surface, and some would be absorbed, deflected, or reduced in energy as they passed through. The result would be that anything resting on the surface would be pushed against it (i.e. "down") more than they would be pushed away from it ("up"). The reason is that the gravitons coming up through the planet would be fewer or weaker than those impinging on the surface. So anything on the surface would be pressed down toward the centre of the planet, just as if they were attracted to it.

Now consider adding a second object to this thought experiment, say a second planet some distance away. Each planet would absorb, redirect, or weaken some of the gravitons passing through, and some of those would be in the direction of the other planet. Thus, each planet would feel a push toward the other planet; that is, the push coming from the direction of the other planet would be slightly less than the push from the opposite direction. In this way, the two planets would seem to be "attracted" to each other, when in reality, they are merely being pushed closer together.

Assuming that the gravitons are absorbed, deflected, or weakened in proportion to the amount of mass in the body they are passing through, and assuming that ordinary objects like stars and planets absorb only a very small fraction of the impinging gravitons, then it can be shown that the resulting push behaves just like Newton's law of gravity; i.e. the differential push felt between the two planets, or any two bodies, would be proportional to the product of their masses, and the inverse square of the distance between them. In this way, gravitons could push objects, yet result in a net attraction between them.

This concept might be seen as a simple way to explain the apparent attractive action-at a-distance behaviour that puzzled physicists before Einstein. No need for action at a distance, the action is all in the gravitons pushing on the matter they encounter on their way through the universe. Given the same net result, repulsive gravity can account for orbiting moons and satellites, and most other gravitational effects we observe in the solar system and universe around us.

In principle, repulsive gravity could explain the accelerating expansion of the universe. If these gravitons push everything they encounter, then over very long distances, where the attraction effect of the differential push becomes negligible, then bodies would tend to be pushed further from each other, especially if there was some "boundary effect" for the universe due to its finite age and the speed of light limitation on distances travelled. This concept might also help explain the "dark matter" or "dark energy" problems that cosmologists struggle with. If gravitons interact only as a weak gravitational force, yet have even a tiny mass, they could significantly change cosmic theories and models.

Of course, the repulsive gravity concept runs into severe limitations and difficulties when examined more closely. Even picogram quantities of matter are subject to the force of gravity in apparently smooth ways, so the interaction of gravitons with mass would have to be very fine and continuous, implying a huge flux of very small gravitons. Moreover, the wide dynamic range of the force of gravity, from these barely perceptible levels to the huge forces associated with neutron starts or black holes, would require a huge flux and a very low level of interaction for normal sized masses and bodies. It seems unlikely that a graviton flux model could accommodate these extremes.

Worse would be the effect of absorbing or weakening the gravitons. For them to produce any force, they would need to carry some amount of energy, which would be reduced during these interactions. That energy dissipation would have to show up somehow, presumably by heating the bodies that absorb them in ways that do not agree with the measured energy balance for planets. It might be possible to get around this by saying that the gravitons are not absorbed, but merely deflected or reflected, thus losing no energy, yet still applying a push as they change their momentum. But that probably raises questions about resulting flux density non-isotropy for other nearby objects.

There are doubtless other effects of actual gravity (which have been very precisely measured), especially the relativistic ones, that probably cannot be accounted for in this approach. Thus, it seems unlikely that the concept of repulsive gravity will go beyond my few notes here. Indeed, perhaps in Newton's time, some unknown physicist thought of this and quickly abandoned the idea once he had thought through the mathematics and consequences. Oh well, so much for my idea, but it was still an interesting thought experiment.