For those unfamiliar with scientific endeavour, the goal is to develop a theory about something. Then devise a means to test that theory by designing an experiment that can either confirm or refute the predictions made beforehand.
This same rigor is applied to all theories, and should a theory fail to match experiment then a rethink is mandated and the theory must be altered. So, in spite of the fact that Einstein's theory of general relativity is taught as virtual law at schools across the world, it still must stand up to the same scrutiny as any other.
With a theory as important as general relativity it is important to be thorough and precise. But therein lies the rub. While it is possible to do ground based experiments to explore the effects of gravity, many of the predictions made by Einstein's theory are scarcely measurable at this level.
Two of these effects are the geodetic effect, the warping of space time due to a massive body, and frame-dragging, which results from a spinning mass pulling space-time around it.
It was for the purpose of measuring these two effects that NASA launched Gravity Probe B in 2004. The orbiting laboratory used four extremely high precision gyroscopes to keep its eye focused on the star IM Pegasi while in polar orbit above the Earth.
If geodetic warping and frame-dragging effects were taking place then the gyroscopes would have to make slight, predicable shifts over time to keep the probe pointing true. Over the subsequent six years of operation researchers were able to determine that these effects, as predicted by relativity, are in fact real.
This is not the first time that predictions of general relativity have proven true, but what is unique about this discovery is the amount of precision that was required to even take the measurements and for how small an effect they were probing. So this certainly goes a long way in furthering our understanding of gravity and can hopefully pave the way for finding a more unified theory of the Universe.
Artist concept of Gravity Probe B orbiting the Earth to measure Earth's affect on space-time, a four-dimensional description of the universe including height, width, length, and time. Credit: NASA