Lunar Laser Rangefinding: Testing GR's Gravitomagnetic Phenomenon?

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SUMMARY

Lunar Laser Rangefinding (LLR) currently offers the most precise test of the gravitomagnetic phenomenon, surpassing the results expected from the Gravity Probe B (GP-B) experiment. A recent publication in Physical Review indicates that LLR is set to achieve significant improvements in range precision within a year of data collection. The discussion raises questions about the relevance of GP-B, given its reliance on complex lunar tidal theories and the degeneracy present in gravitational measurements. The outcome of this debate will be clarified within the year as new data emerges.

PREREQUISITES
  • Understanding of General Relativity (GR) principles
  • Familiarity with Lunar Laser Rangefinding (LLR) techniques
  • Knowledge of Gravity Probe B (GP-B) experiment methodologies
  • Basic grasp of gravitational theories and their observational tests
NEXT STEPS
  • Research the latest findings in Lunar Laser Rangefinding technology
  • Explore the implications of the Gravity Probe B experiment on gravitational theories
  • Study the semi-metric SCC theory and its differences from General Relativity
  • Investigate the role of lunar tides in gravitational measurements and their effects
USEFUL FOR

This discussion is beneficial for physicists, astrophysicists, and researchers interested in gravitational theory testing, as well as those involved in experimental physics and precision measurement techniques.

cesiumfrog
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A few weeks ago Physical Review published a Letter (an edited version of http://arxiv.org/abs/gr-qc/0702028" ) that says:
"At this time, [Lunar Laser Rangefinding] provides the most precise test of [the gravitomagnetic phenomenon,..] likely better than the ultimate result from the GP-B experiment."​

Furthermore, "new effort in LLR is poised to deliver order-of-magnitude improvements in range precision [..and] requires only about a year of new data collection[.., which means that] a significantly improved test of this phenomenon is not far away." The letter concludes it would have to be "a profound empirical clash" if GP-B doesn't also match the predictions of GR.

So, is GP-B already obsolete? Does that make GP-B irrelevent, or is its "directness" still important? Or is the letter just wrong (say, will gr-qc/0702120 pass peer-review)?
 
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This has been discussed here Gravity Probe B and here Alternative theories being tested by Gravity probe B.

The accuracy of their conclusion can be questioned as it depends on a very complicated theory of lunar tides etc. Also there is a degeneracy in the theory because the theory of the orbit of the Moon includes many variables, such as the drag reaction to raising tides on the Earth, not just the frame-dragging gravitomagnetic effect. The other factors affecting the outcome are precisely controlled in the GP-B experiment.

To illustrate this degeneracy take the semi-metric SCC theory in which the value of G that enters into the metric is 3/2 the Newtonian G as measured in a Cavendish type experiment. However, the value of the Robertson PPN parameter \gamma is only 1/3 in SCC instead of 1 in GR. The difference in these two from their GR values cancels out in the standard GR observational tests. This degeneracy is resolved in the GP-B geodetic precession measurement where the differences do not cancel out.

We will know this year whether Murphy, Nordtvedt and Turyshev were wrong!

Garth
 
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