Thanks for the all the informative discussion about gravity. What follows is my attempt to track the development and application of the concept. Criticisms, corrections etc. are welcome.
I need to add a disclaimer that, like many here, the more I look into the subject the more uneasy I feel about our current understanding of gravity. So I am interested in alternative explanations.
At present I tend to favour some type of connection between the aether (which is now known by pseudonyms such as dark energy, neutrino, dark matter, Higgs field and so on) and matter. A connection that provides a single source driver for cosmological motion. This entails a rethink of both gravity and conservation of angular and (if it actually exists for matter at cosmological scales), linear momentum. It may also require the inclusion of aether into the conservation of energy scene. Such impertinence probably puts me a ways out on the crackpot branch as far as big science is concerned, but anyway.
Apart from an aside, I haven't mentioned spin in this polemic. That's because gravity is not thought to play any role in this type of motion, it being due solely to primordial angular momentum and the conservation thereof which is balanced by self-adhesion of the mass. As I understand it, gravity only plays a part in the accretion of the body from the primordial disk.
Is gravity losing its grip on reality?
Everything in the universe appears to be moving relative to something else, and the majority of that motion appears to be curved, as in a planet moving in an orbit about a central point. Some people, Johannes Kepler for example, thought that this demonstrated that natural motion is circular. Others, like Rene Descartes favoured the idea that natural motion is rectilinear, i.e. that matter is naturally inclined to move in a straight line, only deviating from this natural straight path if some other external force is acting on it (which is usually the case).
It could be argued that this difference is just a semantic issue, that the forces that cause deviation from the straight and narrow are part of the natural order so that the rectilinear idea is an unreal ideal state as far as matter is concerned. But Descartes' idea got a lot of traction when Isaac Newton proposed that the observed curved motion could be attributed to the existence of two separate, very unequal and unrelated types of motion, firstly inertia or linear momentum which is a pre-existing state of motion and a second one which has its origin in mass itself and which acts on other matter causing the direction of the inertial component to constantly change.
The earlier concept of angular motion or momentum (nee impetus) was also recognized by Newton but only in the context of spinning bodies. He seems to have been the first to propose that it was conserved.
"A top, whose parts, by their cohesion, are perpetually drawn aside from rectilinear motions, does not cease its rotation otherwise than it is retarded by the air. ... greater bodies of the planets and comets, meeting with less resistance in more free spaces, preserve their motions both progressive and circular for a much longer time"
- Axioms; or Laws of Motion, Law I. in The Mathematical Principles of Natural Philosophy. (See
here)
This division of motion into two separate, unrelated, vastly unequal components was helped along by the new-at-the-time idea that curved motion could be represented by two motion components or vectors (h/t Robert Hooke et al.). Curved motion could be divided into two components, one tangential to the curved track and the other perpendicular to the curve.
Once two components have been identified then the way is open to assign two different origins to the motion. And that, via Newton, is what happened. He identified the perpendicular component as originating in the center of a mass and he went further with this line of thinking by proposing that separate masses are influenced by this component of motion, that they attract one other. The rectilinear component or vector was assumed to be inherent and lately it has been proposed that this inherent motion is a result of the event that set everything in motion, the proposed cosmological big bang. Note the if a curved motion has more than two components, as is the case for helical motion (which has three components) then the set of components is called a tensor, but still involves separating the complete motion into component parts.
To bolster his idea, Newton came up with a suitable expression that enabled the observed motion of a falling body, called g (little g), to be calculated from the mass of the earth and the distance between the center of the earth and the falling body. However, in order to obtain the correct result it is necessary to apply an adjustment factor (also known as a constant) to the mass/distance relationship. This adjustment factor is written as G (big G) and is called the universal gravitational constant.
Because this constant must possess dimensions , i.e. length, mass and time (the expression is not valid if the constant doesn't have the dimensions of length^3 *, mass^-1, time^-2) it is called a dimensioned constant. This is in contrast with the arguably more fundamental constants (such as pi, which is just a number), which are called dimensionless constants. The other thing to note about G is that when everyday units are used to describe the masses and distance involved, it has an extremely small value. G's value is hard to pin down precisely but depending on the units chosen for mass and time it is something like 0.00000000000676 m^3 kg^-1 t^-2.
Newton's expression can be written as the force acting between two masses in the form,
F = GMm/r^2,
M and m are the masses involved and r is the distance between their centers. Note that, unlike length, where the product of two lengths can give an area, by itself, the product of two masses does not yield useful information. The more intuitive combinations of M + m or M - m do not work. Note also that this form of the expression is similar to the expression for the force between two electrical charges (Coulomb's Law), F = Kq1q2/r^2 with K being the constant and q1 and q2 the charges.
From this expression the acceleration due to the force between the two bodies, called little g, can be derived. We do this by substituting mg for F so that the expression becomes,
mg = GMm/r^2,
and from there it is just a matter of using the simple mathematical expedient of removing the little m's from both sides of the expression to obtain
g = GM/r^2.
And if we plug in the right values for big G, M and r we get the right (observed) value for little g, i.e. about ten meters/second/second.
Q.E.D.
No one knows exactly how Newton came up with this expression.
About a century ago a new idea was proposed by Albert Einstein. This still involved the mass of the large body, but instead of the mass directly influencing motion it was here proposed that there was a more indirect effect with the mass now influencing spacetime causing it to curve so that any other masses would then follow the now curved shape of space (this curved track becomes the shortest distance between two points and is called the geodesic). The motion becomes a function of this curvature of spacetime rather than direct mutual attraction between the masses. This necessitated the designation of gravity as a pseudo force as opposed to a real force. In my opinion, this idea is significant because it includes spacetime into the mix as well as the masses themselves.
However recent observations of the cosmos have upset this apple-cart
and cast doubt on the relationship between gravity and mass. It has been found necessary to propose the existence of at least four times more mass than we can see in order to account for the observed motions of some galaxies. This affects both of the mass related causes discussed above. Because we cannot see this mass it is called dark matter. Unlike ordinary matter, dark matter, by definition, does not radiate energy. One could perhaps imagine dark matter to be like a very cold non-radioactive rock or gas that does not reflect or re-radiate incident (incoming) energy. Strange stuff, no?
It could also be that the current understanding of gravity may be returning anomalous results for the density of some of the components of the solar system. For example, although photographic evidence suggests a rocky composition, the latest estimate for the density of comet 67P is about 0.5 which is similar to the density of fluffy snow.
Others think that the hydrogen sun model is obsolete and that iron is its most abundant element but the density is calculated to be about 1.5 which doesn't allow for much iron content.
Given this state of affairs others have looked for a different origin for the seat of this strange phenomenon we identify as the force due to gravity. And given the similarity of the force expressions between two masses and two charges, some have proposed an alternative mechanism, that being the influence of the electron field. This idea still treats the perpendicular vector as a separate force but invokes the electron field rather than the proton (field?) which contains the majority of the thing we call mass. The neutron is a sort of hybrid of a proton and an electron so its mass is also proton related. Recall too that (at some scales?) the force associated with the electron field is about thirty-seven orders of magnitude greater than the gravity field (that is assuming that gravity does actually constitute a separate field) so their is plenty of scope for it to hide in the electron field.
It is also possible that gravity is mass related but operates differently at different scales, sometimes attracting, sometimes repelling. But nevertheless until we positively identify this extra hidden mass then the concept of mass related gravity has to be on shaky ground.
* Note that length cubed is the unit for volume. It may be worth looking into why G has a volume component in its dimensions.
