text in English
Understanding what Gravity is:
Time Expansion & Space Contraction
by Aba Cohen Persiano
Physics Department, Federal University of Minas Gerais-Brazil
Click here and see also
why the concepts of INERTIA and GRAVITY are UNIVOCAL
As I said in the EasyPhysics twitter´s sentence, “gravity of a given mass M comes from Time Expansions associated with transverse (to g) Space Contractions”. The whole effect corresponds to Energy Gradients (you can understand this as “tidal forces”) drawing m towards M, where m is a second mass set in such a deformed spacetime tissue. We can see below two distinct approaches where gravity (that is not a “force” but just an acceleration or a kind of “surfing wind”, according to Einstein´s General Theory of Relativity – GTR) is present:
To understand its mechanism you don´t have to know, a priori, anything about the GTR to get the thing. You just need to know (have heard) from the Special Theory of Relativity -STR- that space shrinks and time expands with increasing velocities, when a given phenomenon is described by an inertial observer.
At first I use a MATERIAL spinning disc to explain what follows: Suppose a situation where the disc´s external edge moves at a given constant angular velocity, e.g. at relativistic speed (v < ~c), but NOTE, this huge velocity is not necessary at all. An observer from outside the disc (see the figure below) observes, relative to his/her clock, a time expansion, proportional to gamma, and a space contraction of the disc’s perimeter, proportional to 1/gamma, where gamma is the Lorentz Factor [1/ square root (1 – v2/c2)]; no change is observed in the disc’s radius as it is not material or it does not move (or, if you are not yet convinced, assume that “the radius moves” transversally to v and, from the STR, space shrinks only in the direction parallel to v – so “r gets thinner” conserving its length). As the internal concentric circumferences rotate with the same angular speed (ω), the respective tangential velocities (v´ = ω.r´) are progressively smaller than v as you get closer to the center, where v´= 0. Being so, the internal circumferences shrink regressively less and less. The same happens to the centripetal accelerations (a = v2/r´ = ω2 r´) as they also drop with r, when described by the external observer.
The consequence of progressivelly shrunk circumferences and lethargic clocks along fixed radii is a bend in space-time as you can see in the figure above. An internal rotating observer lying still on the disc’s surface feels the action of his feet (i.e. the reaction to the centripetal force, that keeps him in rotation) pulling the “floor” radially outwards –consider this floor as a step fixed on the external circumference, to avoid the internal observer being spit out form the disc– keeping him in rotation. He designates this as the “force of gravity”. Actually it is a strugle between his feet and the floor reaction to keep its inertial body in the accelerated system. For the external observer, this is also gravity and is described by the GTR as a consequence of the time expansion and transverse (to g) space contraction. From the Newtonian (Classical Physics) point of view, such a “centrifugal force” is called “inertial force” and neither time-expansions nor space-contractions exist to justify its existence. In the GTR inertial-mass is synonym of gravitational-mass: The “inertial feeling” of m, opposing acceleration, is exactly what we call gravity, where time “flows” slowly.
In the second approach a given mass M distorts the space-time tissue with stronger time expansions and space contractions as you get closer to M (increase g): Similarly, the gravity created by M comes from the time expansion and the transverse (to g) space contraction (I see inertia -space contraction- as a kind of “belt fastening” to stick M into the Universe’s space-time tissue, like a stamp sticker fixes itself on the album’s tissue) that creates tidal forces (observe the dragging gradient) pulling (accelerating) m towards M.
Click here and see also why the concepts of inertia ana gravity are identical
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