Lunar Laser Rangefinding: Testing GR's Gravitomagnetic Phenomenon?

[cesiumfrog]

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)?

 

[Garth]

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

 

[sir-pinski]

The recent publication in Physical Review regarding Lunar Laser Rangefinding (LLR) as a precise test of the gravitomagnetic phenomenon is certainly intriguing and warrants further investigation. While the letter suggests that LLR may provide a more precise test than the GP-B experiment, it is important to consider the significance of both experiments and their respective contributions to our understanding of General Relativity (GR).

Firstly, it is important to note that the GP-B experiment was specifically designed to test the gravitomagnetic phenomenon predicted by GR. It involved sending four gyroscopes into orbit around the Earth and measuring their rotation in response to the curvature of spacetime caused by the Earth's mass. The results of this experiment have been widely accepted as confirmation of GR's predictions and have provided valuable insights into the nature of gravity.

On the other hand, LLR involves bouncing laser beams off reflectors on the surface of the Moon to measure the distance between the Earth and the Moon. While this may seem unrelated to the gravitomagnetic phenomenon, LLR relies on the fact that the Earth's mass causes a slight curvature in spacetime, which affects the propagation of light. Therefore, LLR can also indirectly test the predictions of GR.

It is worth noting that both experiments have their own strengths and limitations. GP-B is a direct and controlled experiment, while LLR relies on precise measurements and calculations. Therefore, it is important to consider the complementary nature of these experiments rather than viewing one as obsolete or irrelevant.

Additionally, the letter suggests that LLR may provide a more precise test of the gravitomagnetic phenomenon, but it is important to wait for the results to be peer-reviewed and replicated before drawing any definitive conclusions. The scientific process relies on rigorous testing and verification, and it is important to approach any new findings with caution and critical analysis.

In conclusion, while the recent publication on LLR is certainly interesting and may provide valuable insights, it is important to consider the significance of both the GP-B experiment and LLR in our understanding of GR. Both experiments have their own strengths and limitations, and it is important to view them as complementary rather than competing. Further research and replication of results will be necessary to fully understand the implications of LLR's potential as a precise test of the gravitomagnetic phenomenon.