Gyroscopes comparable to those in existence today have been around since the early nineteenth century and a German named Bohnenberger (say that three times fast!). Jean Foucault made them famous with his demonstration that the earth is rotating on an axis. They have had many uses since then, mostly having to do with navigation systems. Gyroscopes utilize the fundamental principles of spinning bodies, or conservation of angular momentum (each full rotation is through an angle of 360 degrees, so it is called angular). The picture to the left shows an example of the kind of gyroscope used by Gravity Probe B. It is an incredibly smooth and very round sphere of fused quartz coated with a thin layer of niobium. It is enclosed inside a quartz housing that has three electrodes carrying tiny voltages that keep the sphere actually levitating as it spins. Cool! On earth this would take around 1,000 volts but in space the required voltage is only fractional. The gyroscope rotates at a speedy10,000 rpm, which is not so far-fetched considering the lack of air resistance in space. The two main properties of gyroscopes have to do with its most important ability: spin.

• Gyroscopic Inertia

  Inertia is a fancy word to say that things in motion will stay in motion unless acted upon by an outside force. Gyroscopic inertia refers to a spinning gyroscope that prefers to stay spinning once it is set in motion. It takes a much greater force to change the angle of the axis of a spinning object than a stationary one.

• Precession

  Spinning objects cannot fall over easily. Rather, they turn at right angles to applied forces. Motorcycle riders use this concept to make tight turns. They can lean way into the turn (towards the center of the circle the arc of their turn traces) and the bike will smoothly change directions without falling over. If the rider were to lean like that while sitting still she would just crash to the ground. That is how a force will change the spin angle of a gyroscope without stopping it from spinning. The gyroscope is much more stable because of this property.

Gravity Probe B is a "Relativity Gyroscope Satellite" that will be launched into a polar earth orbit at a 400 mile altitude. Four gyroscopes in this system should be able to measure how space and time are warped by the presence of our tiny earth. Yes, the earth's mass is insignificant in comparison to neutron stars, black holes, and even our own sun. It does, however, still distort the spacetime around it. There are two effects due to this distortion that Gravity Probe B proposes to look for:

• Frame-Dragging

  Not only is the earth a massive body, but it is also rotating. The prediction is that as it spins it actually pulls the fabric of spacetime around with it. Picture a bowling ball held in a flat piece of fabric. Now rotate the ball. The fabric will ripple and twist around the ball. That's what we imagine is happening between the earth and its warped spacetime. The application of the gyroscope is to see how the pull of earth's warpage of spacetime changes the angle of the gyroscope. In an entire year this should still be an angle of only 42 milliarc-seconds (an arc-second is 1/60 of one arc-minute which is 1/60 of one degree, so the magnitude here is 0.0001167 degrees). Gravity Probe B claims, however, that its precision is to 1% or better. Quite remarkable!

• The Geodetic Effect

  The direction in which the gyroscope is spinning will also be changed simply by its motion through the spacetime curvature caused by earth's mass. This angle, over one year, should be a total of 6,600 milliarc-seconds (0.0183 degrees).

Gravity Probe B is a physics experiment and not an astrophysics experiment because it does not rely on any stellar phenomena that may not be totally understood. Technology today is such that we can calibrate the orbit of this satellite exactly. Therefore proponents claim that the results to this experiment will be much more conclusive proof of General Relativity.

Gravity Probe B uses cutting edge, highly sophisticated technology to acheive the desired precison. Therefore a great deal of research has gone into this project and funding goes back all the way to the 1960s! There are six specific issues that must be addressed before launch:

1. Drift-free gyroscope
   There are, as always, non-relativistic effects that can change the spin angle of the gyroscope. It is necessary to get that down to less than 10^-11 degrees per hour.

2. Taking gyroscope readings
   How can you determing changes in spin that are less than 0.1 milliarc-seconds without disturbing the gyroscope? That's rather important.

3. Trustworthy guide star
   This one is the most difficult. The way they propose to detect changes in the spin angle of these gyroscopes is by referencing them to a guide star whose exact position and inertial motion through space is known. This is the one place that relies a less concrete discipline than physics. Astrometry is the science of measuring the positions of stars and relies on extrapolating our nearest measurement methods out to greater and greater distances which tends to decrease accuracy.

4. Stable reference
   Now that we have a trustworthy guide star it is necessary to accurately compare its position to the positions of the gyroscopes. This requires a telescope and mechanical equipment with which to make the comparisons.

5. Separation technique
   There are two effects being measured by these gyroscopes and so there needs to be a way to separate them out and figure out how much of the total change is due to which effect.

6. Credible calibration system
   The most important thing, always, is to ensure that one is certain that their measurements are really due to warped spacetime and nothing else. It is easy to detect such tiny changes that could imply relativistic effects so there must be a reliable way to prove what is being detected.

Gravity Probe A actually did exist, but it was intentionally in the air for just under two hours in 1976. This probe used atomic hydrogen clocks on earth and in space to test gravitational redshift. The spacecraft was shot up into the air to a maximum height of 6200 miles and then came back down to earth. The point was to create a significant change in gravity over a short time interval to measure the redshift of light due to gravitation. The mission was judged successful.