On Wednesday May 4 Stanford and NASA scientists reported confirmation of two predictions of Albert Einstein's general theory of relativity, concluding one of the space agency's longest-running projects.
Known as Gravity Probe B, the experiment used four ultra-precise gyroscopes housed in a satellite to measure two aspects of Einstein's theory about gravity. The first is the geodetic effect, or the warping of space and time around a gravitational body. The second is frame-dragging, which is the amount a spinning object pulls space and time with it as it rotates.
The geodetic effect is often modeled with the idea of a bowling ball warping a rubber sheet. Here, the fabric of space is warped by the masses that are present in it. The more massive objects warp space more. The Moon follows this warp of space as it orbits Earth.
The
Gravity Probe B experiment is simple in concept.
This animation illustrates the idea. Monitor the spin axis of ultra-precise gyroscopes by comparing their axis to the direction of a guide star.
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An excellent overview of the mission is linked here from Stanford University's Gravity Probe B web site. It describes the second relativistic effect of frame dragging caused by the spinning of the massive Earth in the fabric of space. Visualize the effect of slowly rotating the honey dipper in a jar of honey.
Gravity Probe B was able to measure both effects which occur at right angles to each other. The goal of the GP-B experiment is to measure the geodetic effect to an accuracy of ~0.01%, and to measure the frame-dragging effect, which has not previously been directly measured, to an accuracy ~1%.
This award winning animated sequence shows how the probe was able to make those measurements over the course of about a year of data collection starting in August 2004.
Physics advances experimentally in two ways:
1. Measuring known effects with higher accuracy
2. Investigating previously untested phenomena
The geodetic effect has previously been determined to ~1% in complex studies of the Earth-Moon system around the Sun. GP-B aims to measure it to ~0.01%. The frame-dragging effect, on the other hand, is so minuscule around a planet the size of our Earth that until GP-B, it has not been possible to measure this effect directly. However, the frame-dragging effect is of particular interest to physicists and cosmologists who study black holes, because like the air in a hurling tornado, the hurling space around a black hole has enormous destructive potential.
The importance of the measurements to the astronomy community and those who study black holes was expressed by Kip Thorne one of the world’s leading experts on black holes.
In this video, he made the following comments on the significance of the frame-dragging effect:
“The black dot in the center of the drawing in the video represents a black hole. It is surrounded by an accretion disc of gas, shown in yellow, that we believe is forced into the equatorial plane of the black hole by the dragging of spacetime in that vicinity. Jets of energy [blue light in the video clip drawing] shoot out in both directions along the spin axis produced by frame-dragging around the black hole. Furthermore, the interaction of the frame-dragging around black holes with magnetic fields is responsible for the enormous and destructive power generation that produces the jets of energy streaming out of these objects.”