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Ever since Einstein published his GR theory in 1916, there have been numerous experimental tests to confirm its correctness--and has passed with flying colors.

NASA and Stanford have just announced that their Gravity Probe B activity has confirmed GR's predicted geodetic and frame-dragging effects. Are there any other facets of GR that need experimental verification?

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Sure there are. The theory has been tested within only a teeny tiny part of the range of its predictions. For example it predicts gravitational redshift in the range of 0% (no redshift) to 100% (black hole), but experiments to date have shown a maximum gravitational redshift less than 0.01%. It matters less how many tests of GR are done than how extensively those tests cover the range of what GR predicts.

While we have little experimental data to definitively show that GR is the correct theory of gravity, we do know that it leads to major problems for physics, like its breakdown at gravitational singularities, its incompatibility with quantum mechanics, and the black hole information loss paradox. A competing theory of gravity that is confirmed by all experimental tests of GR to date need not have any of those problems, indicating that a lot more testing of GR is warranted.

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I agree in general that there are plenty of areas that haven't been tested, especially in the strong-field regime, but I think it's worth pointing out that astrophysical observations have tested gravitational redshifts far larger than 0.01%. I'm thinking specifically of iron lines observed near black holes (e.g.,, although I think there are plenty of other examples. – Ted Bunn May 5 '11 at 12:57
finbot: "A competing theory of gravity that is confirmed by all experimental tests of GR to date need not have any of those problems, indicating that a lot more testing of GR is warranted." The problem is we don't have a viable competing theory, do we? GR has so far, passed all the tests for almost 100 years. Maybe it should be called "Einstein's Law of General Relativity." – Michael Luciuk May 5 '11 at 19:31
@Ted Bunn: Well I'm talking about confirmations of GR, not just an observed gravitational redshift. Black holes are found by assuming GR is valid (i.e. it may not be a black hole if GR is invalid). An observed gravitational redshift from that object wouldn't be a confirmation of GR since it couldn't validly be compared to GR's prediction. Otherwise a competing theory of gravity, that predicts differently (e.g. no black holes, different gravitational redshift predicted for the object) could be "confirmed" in the same way. – finbot May 6 '11 at 0:27
@Michael Luciuk: We do have competing theories; they are ignored for no good scientific reason. A simple tweak to the Schwarzschild metric can result in a metric that doesn't predict black holes but is confirmed by every experiment of that metric to date, which is the vast majority of tests of GR. The new metric wouldn't have any of GR's major problems--it'd be compatible with quantum mechanics for one thing, no string theory required. Occam's razor strongly favors it. GR has passed a lot of tests, but all in relatively extremely weak gravity and, for any via telescope, at low precision. – finbot May 6 '11 at 0:36
finbot: Please provide some references that illustrate GR alternative ideas compatible with quantum mechanics. I have difficulty believing that such well thought ideas would be ignored for no good scientific reason. – Michael Luciuk May 6 '11 at 1:27

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