A gravity recap

v. 7 n. 34

In view of a recent article, this seems a good time to return to the theme of the basic nature of gravity. [1] General relativity is all about gravity and largely absent from quantum mechanics; this dichotomy exposes its two faces. Reiterating from the above reference, gravity is commonly described by the general relativity expression

G_uv + Λ g_uv = GT_uv,

the proposed recent modification being

G_uv = -Λ_n(x) g_uv,

showing five changes.

• the cosmological term with lambda, Λ, was transposed from the space curvature side to the mass-energy side as an active driver
• the cosmological term is no longer constant
• the cosmological term is negative and opposite in sign and action to the stress-energy or original matter term GT_uv of traditional general relativity
• the traditional matter term is considered redundant to the expanded cosmological term and has been left out
• the cosmological term is quantized.

General relativity is popularly described as matter (the right side of the first equation) tells space (the left side) how to curve, and space tells matter how to move. But if there is no explicit matter (in the second equation), this seems to undermine the little jingle. Furthermore, whatever remains on the right side in the proposed expression has a repulsive rather than attractive effect. This implies that gravity is repulsive here; another upset with the removal of the traditional matter term. Since these questions have already been rationalized in several recent articles, it is time to focus on the negative sign of right-hand side of the proposed, and to try nailing down the basic nature of gravity as attractive or repulsive, indeed the basic nature of gravity itself.

What is gravity? Traditionally, it is something that keeps matter on earth from drifting away. Then it was something that keeps the planets revolving around the Sun. It seemed that matter tends to be attracted to other matter when sufficiently close together to begin with. With galaxies, this was not so straightforward; matter behaves strangely. Then it became clear that matter tended to be repelled from other matter when sufficiently far apart to begin with.

These are the observations. Aside from any theory, is gravity attractive or repulsive? Not so simple. But notice that the views of what gravity might be in the above sequence depends on scale. At some stage where matter goes from near to far apart there is a definite transition from attraction to repulsion; there is a hint of this at the galaxy scale (where "something else" is required to hold everything together). This transition occurs between the cluster-of-galaxies and the galactic supercluster scales -- and there is a full circle return to descriptions of Newtonian gravity as fundamentally repulsive at galactic superclusters and above, and phenomenologically attractive at clusters-of-galaxies and below. [2]

So, again, what is gravity? Gravity is still a nebulous term after all and should probably be dispensed with. Observations are what they are. It would be useful, though, to retain Einstein's likening of space to the ambient gravitational field. [3] Then basically everything is gravity or everything is space; take your choice. At the micro scale, tightly curved (vibrating standing-wave) space can be referred to as matter, and at the large scale, slightly curved space can be referred to as ... well ... space. So, is gravity attractive or repulsive? It all depends on the scale you look at it. And the question could be as meaningless as the term "gravity;" if it describes everything, cosmological to quantum, it describes nothing in particular.

About that "something else" holding galaxies together -- that depends on the answer chosen in the cover image if "earth" is replaced by "galaxy."

[1] (2) General relativity without sources, Part 1 | LinkedIn

[2] (2) An explanation of dark matter and dark energy from unmodified Newtonian gravity* | LinkedIn

[3] A. Einstein, Relativity, Crown, NY, 1961, p. 155.