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The GP-B experiment
Posted: April 17, 2004

According to Einstein's theory of general relativity (1916), all planets and stars reside in an invisible, intangible structure of spacetime. The Earth, like all masses and energy, affects local spacetime in two ways. Earth's presence warps or curves spacetime around it, and Earth's rotation drags or twists the local spacetime frame with it (called "frame-dragging").

"Seeing" Spacetime with Gyroscopes

How could one test Einstein's theory? How could one "see" this invisible structure and measure the shape and motion of this intangible spacetime? In 1960, Stanford University physicist Leonard Schiff and his colleagues were discussing the possible scientific benefits of creating a perfectly spherical gyroscope. Certainly, this perfect gyroscope could improve navigation for airplanes, rockets and satellites. But Schiff (and independently, George Pugh in the Pentagon) proposed something else - a way to "see" local spacetime.

Schiff and Pugh suggested that if a near-perfect spinning gyroscope could be placed in spacetime, above the Earth and its spin axis precisely monitored, the floating gyroscope could show them the shape and behavior of our invisible spacetime frame. The experiment would only work with a near-perfect gyroscope, because Einstein's predicted effect of the curvature of spacetime around a body the size of Earth is microscopically small.

Why a gyroscope? Gyroscopes, or any spinning object, remain oriented in the same direction as long as they are spinning, a property called rotational inertia. A common example of this inertia is a spinning top. It balances on its end while spinning, yet topples over when friction slows it down. While it spins, its rotational inertia keeps it pointed straight up, oriented in its original direction.

Accordingly, if a top were spinning in the near-vacuum of space, it would remain constantly oriented in its original direction, since there would be no forces to slow it down. Our Earth is a prime example of this. The Earth's axis is oriented 23.5 degrees from vertical, relative to the Sun. It has remained in this position due in part to its rotational inertia.

If a perfectly-spherical, spinning gyroscope floated above the Earth in spacetime, and it was protected from external forces that could re-orient it (e.g., gravity, solar radiation, atmospheric friction, magnetic fields, electrical charges), and all internal imbalances were removed (e.g., imperfect shape, unbalanced density, surface imperfections) it would remain pointing in its original direction. The only thing that could alter its spin orientation would be the structure of spacetime itself.

If the local spacetime in which the gyroscope was floating was curved or was twisting, the gyroscope's position would change to follow this curve or twist. If we could monitor this change in orientation, we could "see" the shape and behavior of spacetime itself. This is the mission of Gravity Probe B: to experimentally measure our local spacetime far more precisely than ever before.

The Mechanics of Gravity Probe B

The Gravity Probe B experimental design is as follows:

1. Place a satellite into polar orbit. Inside the satellite are four gyroscopes (GP-B uses four gyroscopes for redundancy) and a telescope.

2. Point the telescope at a distant star (called the "guide star"). GP-B aligns the telescope by turning the satellite because the telescope is fixed within the satellite.

3. Align the gyroscopes with the telescope so that when they are spinning, each spin axis also points directly at the guide star.

4. Spin up the gyroscopes and remove any external forces (pressure, heat, magnetic field, gravity, electrical charges) so the gyroscopes will spin unhindered, in a vacuum within the satellite, and free from any influence from the satellite itself.

5. Monitor the spin orientation of the gyroscopes over 1-2 years. Keep the telescope (and the satellite) fixed on the guide star and measure any angles that open up between the telescope's orientation and each gyroscope's spin axis. If local spacetime around the Earth is curved and frame-dragging occurs, the spin axis of the gyroscopes should slowly drift away from the starting point during this time, revealing the shape and motion of spacetime around the Earth.

According to Schiff's and Pugh's calculations, with the gyroscopes and satellite orbiting 400 miles (640 km) above the Earth's surface, the orientation of each gyroscope's spin axis should drift 6,614.4 milliarcseconds (or 6.6 arcseconds) per year in the orbital plane of the satellite, due to the curvature of Earth's local spacetime, and their spin axes should drift 40.9 milliarcseconds per year in a perpendicular plane (that is, the plane of Earth's rotation), due to the frame-dragging effect. In other words, Gravity Probe B intends to use gyroscopes and a telescope orbiting above Earth to measure two microscopic angles, each predicted to be a very tiny fraction of a single degree.

Redefining the Meaning of Precision

The central challenge of the Gravity Probe B mission is to build gyroscopes, a telescope, and a satellite that can precisely measure two minuscule angles - 6,614.4 milliarcseconds (6.6 arcseconds) and 40.9 milliarcseconds. Because these angles are so small, GP-B has very little margin for error. GP-B must measure the shape and motion of local spacetime to within 0.5 milliarcseconds.

These angles are almost too small to comprehend. To visualize GP-B's minuscule angular measurements, imagine pointing a pencil. Then, raise it very slightly, moving it just one-half milliarcsecond from its original position. This is like trying to move your pencil:

  • From one side to the other of a strand of hair twenty miles away.
  • From the top to the bottom of Lincoln's face on a penny 3,000 miles away.
  • From the foot to the head of a short astronaut standing on the Moon.

Alternatively, think of a round clock face. Each minute mark is six degrees apart. Within that space between the minute marks, there are 21,600 arcseconds, or 21.6 million milliarcseconds. Gravity Probe B must be able to measure an angle one-half as wide of one of those milliarcseconds - an angle nearly five-hundred million times smaller than the angle between minute marks on a clock face.