Tests of General Relativity, What exactly does each test prove?

I'm trying to understand the meaning of each of the basic tests of General Relativity, and I thought this forum might be a good place to pose my questions.

1. Deflection of light by the sun
2. Gravitational red shift
3. Precession of Mercury's orbit

I understand #1. GR says that gravity bends light, so this is a direct test of it.

Is #2 based on the same exact principle? That is, since gravity affects light, therefore we get kind of a Doppler effect? Or is it more complicated then that?

Is #3 based entirely on the premise that gravity travels at the speed of light? Newton's laws predict a certain precession based on the instant effect of gravity from other planets. Does GR get a different (and more accurate) calculation solely due to the finite speed (and delayed effect) of gravity? Or is there some other aspect of GR that feeds into the calculations?


The point of my questions is, do these tests simply prove that A) gravity travels at the speed of light and B) light is bent by gravity? If so, then would any other theory that fit these two requirements find the same support in these tests?

Thanks.

The reason for gravitational redshift is time dilation. You can also think of it as light gaining or losing energy as it travels between different gravitational potentials.

The precession of Mercury's orbit occurs because when Mercury is closer to the Sun, it experiences more time dilation than when it is farther away. This causes a slight additional force on Mercury, causing its orbit to precess.

These observations do prove that gravity travels at the speed of light and that light is bent by gravity. My theory predicts the same effects, and yes, it also does find the same support in these observations.

All three tests are predictions of GR. Specifically, they test GR in the Weak Field Limit. There are other theories of gravity which reduce to the predictions of GR in the Weak Field Limit, so none of these tests proof that GR is the one, true theory, but they show that GR is more correct than Newton's theory.

1. Deflection of light by the sun is experimentally tricky. Other proofs along this path inclusing deflections of starlight by Jupiter and the obvious galactic-sized gravitational lens. Some Newtonian theories predict deflection of starlight, but in the opposite direction and by a different amount.

2. Redshift, is predicted by any thoery of gravity which has a principle of local position invariance (LPI). This says no "direction'' in spacetime is mathematically distinguished from any other.

3. Mercury's orbit is a prediction that Einsteinian orbits are not purely Keplerian. Not only is the speed of gravity finite, but there is gravitomagnetism which makes gravity a non-central force. The two effect almost cancel out and so Newton's theory is approximately correct.

There are many more tests of GR, and a history of theories which have been rejected experimentally. There are some theories which appear to be mathematically indistiguishable from GR, so these are lumped in with GR. There are some other theories which only approximate GR in the weak field limit, and to test these experimentally we plug them into a Parameterized Post-Newton (PPN) model of gravity and see how close their predictions come to reality. GR is the first theory to make the cut.

More to read about this here: (Follow the links to find some really nice sites.)
http://math.ucr.edu/home/baez/RelWWW/tests.html
http://forum.physorg.com/index.php?showtop...indpost&p=99550

Thanks for the great replies. I have lots to read...

I like this site on light deflection because it has pictures and numbers.
http://www.astro.ucla.edu/~wright/deflection-delay.html

cef?

Light's energy is conserved in General Relativity. So if it has more or less energy it started off that way.

Well, such animation illustrates clearly, the light speed isn't invariant from outer observer perspective....

I agree.
It's a known prediction of Newtonian dynamics that light should move FASTER near the sun, but Einstein predicts that there will be a delay (Shapiro Delay), which you can say (as Einstein and Pentcho Valev have) that light is moving slower, or you can say (as modern textbooks say) that gravitation slows time.

Locally, we expect light to be measured at the same speed because our clocks will be slowed by the same amount.

Just an observation... Does anyone take into consideration how light is actually measured by the eye? When we measure the speed of something should'nt we take into acount the ability of the measuring device? It seems to me that light does'nt move at all. How can something without mass.... move? What I've understood light to be is simply an empty space. A space that gets filled by matter, however small that matter is.

The Eye determines everything it see's by the speed that the objects are moving. If you doubt that, then you should watch a rotating black and white alternately painted disk when it's spinning. It will turn different colors even though the only two colors on the disk are black and white. The reason? the speed of change between what your eye perceives directly in front of it, causes the mind to return a color to the brain. The color itself is fictional. I does'nt actually exist. Your eye simply measures the difference between the speed of one moving particle and the other. all color is perceived this way. the maximum speed difference that your eye can see is white, the slowest speed difference your eye can see, is black. What your eye actually does when you see light, is to compare the slowest moving thing "Nothing", with the fastest moving thing "Gravity" The difference (depending on the content percentage) returns the color white to your brain. Light actually has no color - If you look between the moon and the sun you will see black. but yet when you look at the moon it is a bright whitish color. If the light itself were actually white, it would be white between the moon and the sun since the sun provides the light. Now, you see black between the moon and the sun because the speed of gravity that flows through your eye and the speed of gravity flowing through space is almost the same speed. therefor the speed difference is small and the resulting color sent to the brain is black because your viewing far more gravity than you are anything else. In essense light only does one thing, it's what allows you to see everything else. It actually does'nt exist. It is the space between. You can measure the speed of things you can't see by using light, but the smallest thing you can't see, will be the fastest thing that moves.... and that can only be determined by the size of the particle itself. Since your eye can only measure speed differences by comparing particles, that speed will be determined by the size of the particle that the eye can measure. Likewise the speed of light can only be measured by the size of the smallest particle in the device that is measuring it or by the speed that light appears to travel inside of the smallest particle. Since that particle is not able to be measured, because any device that could measure it would have to be made of smaller particles than the smallest particle (not possible) it must be calculated by other means. Using the interaction between light and gravity we can get an idea of how fast the gravity is moving, and thus get a grip on the size of the smallest particle being thrust from the center of the earth, but because we don't have access to the surface of the sun we really can't tell how fast light is traveling up there but we can make some assumptions if you consider the gravity of the sun and other such issues.

Omnivisone