Could someone explain to me, in laymen’s terms, what is so unusual and irregular about the orbit of Mercury, that allowed it to be better explained by General Relativity than classical mechanics?
darkmatter4brains":2qcrcp87 said:I only ask because in one sense we do know how gravity effects gravity, in that gravitation actually couples to itself. This is the source of the non-linear nature of Einstein's Field Equations. It's also the distinct difference betweeb EnM (linear) and Gravity (non-linear), in that two photons cannot exchange a photon, but two gravitons can exchange a graviton, i.e. they're coupled.
In fact, if gravity did not couple to itself, the inertial mass and gravitational mass would not always be equivalent. In other words, we would have to throw out Einstein's equivalence principle.
Pieces of the Stress-Energy Tensor
T_tt Measures how much mass there is at a point—how much density
T_xt , T_yt and T_zt Measures how fast the matter is moving—its momentum
T_xx , T_yy and T_zz Measures the pressure in each of the three directions
T_xy , T_xz and T_yz Measures the stresses in the matter
As we see from the table, things like stress, pressure, and momentum come into Einstein's equations. That is, stress, pressure, and momentum all have some effect on the warping of spacetime. This is related to Einstein's most famous equation, E=mc2, which shows the mass/energy equivalence.
This is also why there are theories/ideas that gravity may have supplied a repulsive force shortly after the big bang. Assuming space was filled with a negative pressure, gravity then becomes a repulsive force. See Brian Green's Book for a good explanation on this.