Einstein Right Again

Not necessarily that surprising that this point, but still it is always fun (and extremely important) to have experimental/observational evidence to back up theoretical expectations. Courtesy of today's APOD, which as evidence of this blog will show is my number 1 source for cool astronomy stuff, we have the results of Gravity Probe B (astronomers and physicists are terrible at naming things).

So what theory of Einstein's did Gravity Probe B confirm? It confirmed one of the predictions of General Relativity (GR) known as frame-dragging. I will admit, this one is a little bizarre and incredibly tough to test but I will do my best to explain. Now let's say you were a gyroscope. It turns out for gyroscopes once they are pointed in a direction they remain pointed in the same direction forever unless acted upon by a force. This is actually true for any spinning body, which is why gyroscopes are so useful for navigation. You always know where the gyroscope is pointing so you can figure out the way you are pointing by looking at which direction the gyroscope is.

Now let's say you were in space, orbiting the Earth. You had a gyroscope pointed at a particular star, let's say IM Pegasi. In regular Newtonian gravity you would expect that the gyroscope would continually point in the same direction as you orbited the planet, never changing. However, this is not true for GR. In GR a mass physically warps the fabric of space-time, creating a sort of divet. This in turn creates a small force which will perturb our gyroscope causing it to precess (geodetic precession in the image on APOD). Not only that, since the Earth is rotating it creates an additional force called frame-dragging. Frame-dragging causes an additional force which causes additional precession. Think of frame dragging as similar to magnetism. In electromagnetism if you rotate a charged particle you generate a magnetic field. The same is true for GR, instead the charge is the mass of the object. The mass effectively pulls space-time with it as it rotates creating a sort of spiral effect in the fabric of space-time.

Both of these precession effects are super small, but can be predicted from GR. Thus Gravity Probe B did the above experiment to detect these two precession effects. To do this experiment was a heroic effort in astronautical engineering and precision physics. First the team made 4 nearly perfect spheres which were spun up to create the gyroscopes. Then they had to make a near perfect laboratory in space in order to make sure they didn't add apply any force to the gyroscopes once they were spinning. This required them to build a drag-free space craft, exclude the Earth's magnetic field from the area where the gyroscopes were, and measure the gyroscope without touching it. Details on how they accomplished this can be found here.

The sum total of there effort culminated in the most exacting test of GR to date. The precession they measured was consistent with the rate predicted by GR. With this confirmation, we can be confident in applying GR corrections to our every day items such as GPS, atomic clocks, communications satellites, etc. It also gives numerical astrophysicists confidence in their predictions which use GR, such as black holes and other compact objects.