A rotating black hole can generate huge amounts of energy for a sufficiently high-tech civilization. Outside the spherical event horizon lies an ellipsoidal region called the ergosphere – within this region spacetime is being dragged around the spinning black hole faster than light. In 1969 Roger Penrose showed that an object projected into the ergosphere can be made to escape with vastly increased energy while slightly slowing the black hole’s spin.
The dragging of space-time around a rotating mass is called frame-dragging (think honey around a twirled spoon) and is predicted by General Relativity. But is it real? A seven year NASA experiment called ‘Gravity Probe B’ has just reported in and it seems that, as usual, Einstein was right.
How do you measure the dragging of spacetime when there is no convenient black hole to hand? Gravity Probe B is a satellite in polar orbit around the earth. Inside an onboard Dewar flask, cooled by liquid helium, float four precision gyroscopes pointing to IM Pegasi, a binary star in the constellation Pegasus. At launch in 2004, the gyroscopes were the most nearly spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms. If one of these spheres were scaled to the size of the earth, its tallest bumps would be only 8 ft high.
In the absence of frame dragging, the gyroscopes would continue to point to IM Pegasi but General Relativity predicts that the twisted spacetime around Earth should cause the axes of the gyros to drift 0.041 arcseconds over a year. An arcsecond is only 1/3600th of a degree so to measure this angle reasonably well, the experiment needed a precision of 500 microarcseconds. It’s like measuring the thickness of a sheet of paper held edge-on 100 miles away.
“We measured a frame dragging effect of 0.039 plus or minus 0.007 arcseconds,” said Stanford University physicist Francis Everitt, principal investigator of the Gravity Probe B mission.
In fact, Gravity Probe B aimed to measure the frame-dragging effect to a precision of 1%. The 5% discrepancy between predicted and measured values indicates the technical difficulty of this experiment which was nearly cancelled a few years ago as sponsors despaired of ever extracting enough signal from the background noise.
In fact the legacy of Gravity Probe B may lie equally in technology-driven spin-offs. Francis Everitt recalls advice given to him by his thesis advisor and Nobel Laureate Patrick M.S. Blackett: “If you can’t think of what physics to do next, invent some new technology, and it will lead to new physics.”
“Well,” said Everitt, “we invented 13 new technologies for Gravity Probe B. Who knows where they will take us?”
This may all seem very esoteric science but consider this. When you throw a ball up in the air and catch it again, the ball has only travelled a few feet in space but in its one second of flight it has travelled the equivalent of 186,000 miles of time. It’s the curvature of time in your vicinity which is most of the reason the ball came back to you. It’s interesting that spacetime curvature is this obvious, while frame dragging is so subtle that its detection is state-of-the-art.