# GP-B scoop

1. Mar 6, 2007

### cesiumfrog

A few weeks ago Physical Review published a Letter (an edited version of 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)?

2. Mar 7, 2007

### 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

Last edited: Mar 7, 2007