Gravity Probe B: Geodetic effect within 1%


The NASA/Stanford University Gravity Probe B (GPB) team has made its first public announcement at the 2007 American Physical Society April Meeting, and they have announced that the geodetic effect predicted by general relativity has been confirmed to within 1%. Analysis on the `frame-dragging’ effect is ongoing, complete results are expected to be announced in December 2007.

I’ll be honest, though, and admit that I picked up the news reading an article in The Guardian. GPB is a storied collaboration between Stanford and NASA to test general relativity. If you think this seems a little `retro‘ compared to current `sexy‘ relativity experiments such as LIGO/LISA, then you’d be right: the GPB project becan nearly half a century ago, back when people funded physicis because of the atomic bomb and well before any of the technology to carry out such an experiment existed.

The Science in a Nutshell

A niobium GPB gyroscope, image from the GPB website.

At the heart of the GPB mission are spinning superconducting niobium spheres, engineered to be the `roundest objects’ ever made. To measure the `geodetic effect,’ the satellite-based experiment measured deviations in the spin axis due to the curvature of space around our planet. (Think of the precession of a gyroscope or spinning top.) The net effect is a precession of 6.6 arc-seconds per year.

The second effect, frame-dragging, is the `tugging’ of spacetime around by a rotating mass (in this case, the Earth). This is roughly like a `viscosity’ for spacetime. The general relativistic effect causes a gyroscope to tilt away from its orbital plane. Frame-dragging causes an additional effect at the order of 42 milliarc-seconds per year.

Image from the GPB website.

(If you want something silly to compare this against, the Earth rotates about its axis 360 degrees = 1,296,000 arcseconds per day.)


The project was first proposed by George Pugh in 1959. Think about the historical context: this was before the `space age’ had sent a man into orbit. In fact, Professor Everitt is the only surviving founding member of the GPB experiment. Since its inception GPB has produced 79 PhDs, 15 master’s degrees, and a small army of former-undergraduate researchers. The experiment spanned forty-seven years. That’s a scale that’s unheard of even for collider-based particle physicists. If you think waiting for the LHC is bad, imagine how Professor Everitt must have felt.

The project persevered through redesigns, the development of the necessary technology, wrangling for funding, and even eventually the question of relevance.

By the time the experiment was actually launched in 2003, many people doubted its experimental relevance. To quote one faculty member at the time, “If they see the GR effects then nobody will be surprised. If they don’t, then they’ll have made an error.” More recently, my college master, Sir Martin Rees (Astronomer Royal) is quoted in the Guardian saying that the GPB announcement would “fork no lightning.”

While GPB is the only experiment to directly probe the geodetic and frame-dragging effects, the past fifty years (50 years!) have brought new sources of indirect evidence (e.g. the fact that GPS works) for GR and at least two generations of scientists have since decided that the relevant question isn’t whether GR is correct, but where it breaks down (e.g. quantum gravity). Unfortunately, in many respects it seems that GPB was the experiment that persevered only to have the scientific frontier overtake it.

This, however, is not meant as a swipe at the project. The experiment is a tremendous engineering feat and has already spawned spin-off technologies.

I was a second year physics student when the GPB was launched and I still vividly remember the excitement as they broadcast the news feed in the Hewlett lecture hall (back then it was still called `TCSEQ’), as some of the undergraduates begged to have our electromagnetism course moved so we could join in. I graduated just after the retrieval of the satellite and the beginning of data analysis. In the relatively-near future, the HEPL buildings (including the building housing the GPB team and control center) will be demolished as part of Stanford’s long-term construction plan and I can’t help but feel that this is the end of a storied chapter for the Stanford Physics community.


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