Einstein’s General Theory of Relativity ( GR), originally published in a 1916 paper, predicted that as a result of a body’s mass and motion, space-time would be distorted in certain ways around that body as it moves. While other predictions of Einstein’s Special Theory of Relativity (SR), such as the speed of light (‘c’), had previously been confirmed, only indirect evidence of this GR prediction had been found (via space telescope observations).

But now, after nearly 47 years of planning, operating, monitoring and analyzing the data from a special satellite-probe known as Gravity Probe B (GP-B), NASA scientists are announcing definitive confirmation of Einstein’s famous theoretical prediction of the deformation of space-time due to a mass in motion.

In the Einsteinian view, Space and Time are relative quantities (not absolutes), bound together into a “space-time fabric”, and their properties vary depending upon the movement of matter “within” them. Imagine a sheet of thin, flexible material upon which is placed a weighty marble that has been set in motion. As the marble moves over the sheets surface it creates an indented path in the material, and, as it also spins while it is doing this, it also creates small twists in this indented path — leaving 4 dimensional “imprint” or pattern of this effect.

This wobbling in axial orientation is known as “frame dragging” and the general warping of space-time is referred to as the vortex.

That, boiled down, is the theory, and it predicts that these twists and “eddies” should be present around our Earth as it spins on its axis and orbits the Sun.

The SP-B probe is/was both a simple and brilliantly conceived mission to prove or disprove this theoretical prediction. With four exquisitely calibrated gyroscopes on-board, and given a fixed orientation to a star as a reference point, GP-B’s axis could either remain fixed, or, if space-time is indeed warped by the Earth’s motion, drift periodically as it traced the twists in space-time.

A spinning spherical gyroscope in Earth orbit
should wobble due to frame dragging.
Now, this difference in axial drift (relative to the reference star) is incredibly small – 0.041 arcseconds over a year (one arcsecond is 1/3600th of a degree). This tiny variance in angle to the reference point requires a super-precision of 0.0005 arcseconds. Also, the ping-pong ball sized globes of fused quartz and silicon that are the heart of the gyroscopes need to be super-spherical — varying from a ‘true sphere’ by only 40 atomic layers.

And there were several other neat techno-tricks here: to insure no outside interference from the measuring technology, any change in the gyroscope sphere’s axis orientation had to be measured without anything actually touching it.

What’s more, the probe had to be designed to maneuver about the Earth “drag free”, that is, with out disturbing these space-time “eddies” created by the planet’s orbit and spin. In fact, the mission’s goal was so challenging in complexity that some 13 new technologies and techniques needed to be invented to make these “simple” measurements possible (to learn more about the probe’s unique features, check out A Pocket of Near Perfection.

And so, just how precise (and difficult) are these measurements in layman’s terms? According to Francis Everitt, principal investigator of the probe mission, this is “like measuring the thickness of a sheet of paper held edge-on 100 miles away.”

One of the superspherical gyroscopes of Gravity ProbeB
Elegant and precise and soon for the science history books.

But GP-B goes beyond even Einsteins; GP-B’s results have helped astrophysicists confirm that black holes also spin and act like wobbling gyroscopes .

NASA funding, planning and building of the Gravity Probe B mission began in 1963. The gravity probe experiment is a joint mission of NASA and the Stanford University Physic Department, and to date, has engendered 86 Ph.D. theses, and has seen the participation of several hundred undergraduate and over 50 high-school students.

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