Historical research guide · 0033 · 2011
Gravity Probe B: final results (frame-dragging)
Everitt et al.
In one minute
Gravity Probe B placed four cryogenic gyroscopes in polar orbit to measure two relativistic effects around Earth. Geodetic precession is the small change in an orbiting gyroscope’s orientation caused by Earth’s mass curving spacetime. Frame-dragging is an additional change caused by Earth’s rotation. In their final report, Everitt and the Stanford-led team combined results from the four gyroscopes: geodetic drift was −6,601.8 ± 18.3 milliarcseconds per year, versus −6,606.1 predicted; frame-dragging was −37.2 ± 7.2, versus −39.2 predicted. Launched on April 20, 2004, the spacecraft returned science data from August 28, 2004 through August 14, 2005. The experiment converted both effects into measured changes in spin direction.
Connection
Frame-dragging is the one effect in this part of the library that has already been measured in orbit: a rotating mass really does drag the space around it. Chapter 11 uses Gravity Probe B as the yardstick for how large a natural gravitomagnetic field is, and chapter 4 uses it as the existence proof that the metric is a thing you can act on.
From the source
Gravity Probe B tested two predictions of general relativity — the geodetic effect and frame-dragging — with cryogenic gyroscopes in Earth orbit; the satellite launched on 20 April 2004 and science data were collected from 28 August 2004 to 14 August 2005.Abstract
Analysis of the data from all four gyroscopes gives a geodetic drift rate of −6,601.8 ± 18.3 milliarcseconds per year, against the general-relativity prediction of −6,606.1 milliarcseconds per year.Abstract; final results
The same analysis gives a frame-dragging drift rate of −37.2 ± 7.2 milliarcseconds per year, against the general-relativity prediction of −39.2 milliarcseconds per year.Abstract; final results
Frame-dragging is therefore a measured property of spacetime around a rotating body, not only a theoretical prediction — the Earth carries the local inertial frames around with it as it turns.Abstract; conclusions
The natural effect is very small at planetary scale: the frame-dragging signal is a few tens of milliarcseconds per year, where one milliarcsecond is 4.848 × 10⁻⁹ radians.Abstract, definition of mas
The way in
https://arxiv.org/abs/1105.3456How to cite it
Everitt et al. (2011) Gravity Probe B: final results (frame-dragging) doi:10.1103/PhysRevLett.106.221101