Wednesday, May 4, 2011

Testing Einstein - NASA's Gravity Probe B confirms relativity

Space-time around the Earth and Gravity Probe B

We are living in exciting times.  The results of the NASA's Gravity Probe B experiment will one day be written up as one of the classic experiments of modern physics.

Gravity Probe B was launched in 2004 to test Einstein's theory of general relativity.  For the non-physicists in the crowd, this was the change to gravity that Einstein introduced.  Gravity was thought to be well understood by simple Newtonian physics.  You through a ball on Earth, it follows a ballistic trajectory explained very simply by college freshman level physics (neglecting atmospheric drag anyway).

Einstein introduced the idea that gravity can simply be described by the geometry of the situation.  It generalizes Newtonian gravity and special relativity (also Einstein's creation) into a geometric description of gravity based upon the idea of spacetime.  Very basically, the curvature of spacetime is directly related the energy and momentum of anything subject to the gravity.  It was revolutionary.  Still is I believe.

Gravity Probe B will test two different effects.
Gravity Probe B was launched to test the effects of Earth's movement, both around the Sun and spinning on its own axis, on the gravitational environment near Earth.  General relativity tells us there should be two different effects seen near Earth: Frame-dragging and Geodetic precesion.

The frame-dragging is due to the rotation of the Earth around its axis.  The geodetic precession is due to the mass of Earth moving through spacetime.

Through some very impressive engineering the Gravity Probe B satellite uses spinning gyroscopes to measure the two effects.  The team who developed the experiment needed to do invent and develop some serious equipment to perform this experiment.  They invented 13 new technologies for this experiment.

In addition, the precision of this experiment was amazing.  They need to be able to measure the drift of the gyroscopes to a precision of 0.0005 or 5/10000 of an arcsecond.  An arcsecond is 1/3600 degrees.  So, they needed a precision better than 1/7,000,000 of a degree.  Amazing!  Truly Amazing!

Some links of interest:
Gravity Probe B project page
Gravity Probe B wiki page 
NASA Press Release about results

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