When Sir Isaac Newton published his Theory of Universal Gravitation, he noted that he could not propose a mechanism by which it worked. In 1784 Georges-Louis LeSage proposed such a mechanism, sometimes known as the kinetic theory of gravity. LeSage extended the speculations of Newton's friend and contemporary Nicolas Fatio de Duillier, who first suggested a similar explanation for gravity in 1690. Though the theory eventually fell from favour, it strongly influenced John Herapath's thinking in developing the kinetic theory of gases. In more recent years, it has regained some attention, principally on the "fringe" of mainstream astronomy.
LeSage's theory is one based, not on universal attraction of matter, as in Newton's theory, but founded instead on a fluid-based explanation where a light gas fills the entire universe. In this model the gas would normally provide a uniform force in all directions, leading to no gravity at all, except that massive objects block the motion of the gas. Objects near other massive objects thus experience a force towards them, because the gas that would normally be pushing from that direction is blocked. For this reason the theory is often referred to as push gravity, although within the physics community it is more widely referred to as particle gravity.
Objections to the theory
It was quickly pointed out that in order for this theory to work, the collisions between these gravitational particles and the rest of matter must be inelastic, that is, the particles must lose energy in this collision. Without this condition the net force on the object would be zero. However this raises another concern. If the particles lose energy, then the object they react with must gain energy in order to satisfy the conservation of energy.
If this energy were converted to heat, over a long enough period objects would grow hotter until they melted. A simple calculation shows that in order for the Earth to remain in orbit around the Sun, the energy transfer would result in the planet being much hotter than it already is.
Alternatively the absorbed energy may be added to the mass of particles. A simple calculation shows that to give the correct strength of gravity relative to the charge force, the mass of particles would have to increase at about the Hubble rate. Therefore this alternative predicts the cosmological redshift without any motion in the universe and deserves to be taken seriously.
Another problem was later brought up. It is a simpler argument but one that only became "obvious" with the introduction of mechanics based on a frame of reference. When the particles interact with the matter, they will be moving in a particular direction. If these particles move at any speed slower than instantaneous, this will lead to an apparent drag. Consider the gravitation of the Sun on the Earth. In the LeSage model the attraction of gravity is due to a slightly smaller number of particles on the "sun side" due to the Sun blocking some of them out. However this picture ignores motion. If you consider the same picture from a frame of reference where the Earth is motionless (consider the view from a camera looking at the Earth from the Sun), then it should be clear that the particles are not only traveling directly "out", but that they are, from the Earth's perspective, traveling to the side as well - opposite to the direction of the orbit. If the particles are to have any effect on matter at all, they must logically then drag the Earth, slowing it down.
An alternative to a particulate flux, a wave flux has also been considered. Hal Puthoff has claimed that a wave flux proportional to the fourth power of frequency would have no drag, but it seems that very few physicists take Puthoff's seriously.
One prediction of this theory is a deviation from the inverse-square law for sufficiently large mass because eventually most of the LeSage particles would be absorbed/scattered by the body and no greater screening could occur. Although the effect would be small, certain astronomical events, like planetary occultations, would result in measurable differences in orbits which we do not see.
A final consideration is that if the gas is truly universal, the effects would, over long distances, lead to a net zero force. This disagrees with observations of the large scale structure of the universe, where matter has "clumped" into very large scale structures that would require a long-range gravitational force.
Recent attempts at a revival
While these considerations would appear to be fairly decisive, the concept of push gravity has nevertheless found new support from a minority of scientists working outside the "mainstream". In order to deal with the concerns above, a number of mechanisms have been offered to avoid them.
For instance, the heating issue could be avoided if the energy is not turned, eventually, into heat. While this is certainly the common result of many energy exchanges, it has been suggested that the absorbed energy might, for example, be turned into new rest mass energy instead. Energy and momentum could be conserved in such a process if bodies (especially galaxies) were simultaneously slowing down in their motions. Likewise the "drag" problem is not an issue if the effect travels at speeds much greater than the speed of light. Normally this would be a problem, but it dovetails nicely into general concerns that such a speed does not exist and that Einstein's theory of relativity is wrong anyway. Thus various theories of luminiferous ether are used to support push gravity, which is in turn used to argue for the existence of luminiferous ether.
Hal Puthoff has claimed that if LeSages particulate flux is replaced with a wave flux proportional to the fourth power of frequency, no drag would result, but few if any other physicists seem to take this seriously.
- Pushing Gravity: New Perspectives on Le Sage's Theory of Gravitation Apeiron, 2002; ISBN 0968368972. (Read with scepticism.)Template:Relativity-stub