Mercury's orbit is a precession (it's perihelion is a precession). It's elliptical, but the ellipse travels kinda like a Spirograph image. It's closest point of approach to the sun doesn't always appear in the same place.
All planets orbits precess due, primarily, to the pull of other planets. Newton predicts it. That's great. The problem is that the _amount_ of precession can't be predicted for Mercury. It's off by about 43 arc-sec/century.
A whole lot of bandaids were purposed, but further study shot them all down.
Along comes Einstein and General Relativity, and he was able to predict, without any adjustments, that the orbit of Mercury should precess by an extra 43 arc-sec/century. His unaltered predictions were _exactly_ on the money.
Why? Mercury is inside the region of spacetime that is disturbed by the Sun's massive gravity.
The most common that I'm aware of was that there was a certain amount of dust in between the sun and Mercury. Probes, of course, showed that not to be true. And it was a hack anyway since no one could figure the PRECISE amount of dust that would do it every time.
Then there's the Dicke Bulge. An assumption about the sun bulging out around its equator (solar oblateness). That only corrected 3 arc-sec/century though. And we need 43.
Then there was the hack add-on to that, the Brans-Dicke scalar-tensor theory. It had an adjustable variable. But those measurements were all over the board. The Solar Disk Sextant in 1996 pretty much shot that and agreed with GR.
For every serious examination of direct observation, GR proves out every time vis Mercury's orbit.
Thank you adrenalynn you’ve given me, in just two post, more of information that I wanted than I’ve been able to get from a half dozen books. But unfortunately I didn’t find what I was looking for, and I don’t know how to ask the next question without bringing out the nuts and crackpots.
This is an old post by me, which was orignally made partly in reference to the GR/Mercury thing, as well
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.
Basically, it's the fact that the gravitational field couples to itself that Mercury has a small General Relativistic contribution added to it's precession. Mercury precesses due to Newtonian effects as well, but it was the small bit leftover that Einstein was able to explain that made him famous.
All the other planets have the same thing happen - just to a negligible degree. Mercury is close enough to the Sun where the effect becomes pronounced enough to be more noticeable.
Looking at the stress-energy tensor really shows you how much more is involved in GR than in Newtonian picture of gravity.
Where mass was the source of the gravitational field in Newtonian mechanics, the stress-energy tensor is now the source in General Relativity.
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.