Experiments conducted on Earth have shown the principle to be valid to a precision of 1 part in 10 trillion. But a space-based experiment called STEP (Satellite Test of the Equivalence Principle) could pose a much sterner challenge, boosting the accuracy of these measurements by a factor of 100,000. That kind of precision could be enough to show physicists just where Einstein’s theories start to miss the mark — assuming they do.
The Next STEP
STEP began in 1971 as a thesis project by then-graduate student Paul Worden, with Stanford physicist Francis Everitt serving on the thesis committee and then as the project’s chief scientist soon afterward. Everitt has devoted a half-century of his life to testing general relativity and was the principal investigator of Gravity Probe B, a NASA-funded satellite mission that studied, and eventually verified, another aspect of Einstein’s theory.
By going into space like Gravity Probe B, STEP could dramatically improve the precision of equivalence principle measurements. These measurements are hard to do on the ground owing to vibrations from street traffic, Earth tremors and other disturbances. Space offers a much calmer environment.
Another advantage relates to observation time, suggests Towson University physicist James Overduin, who has worked on STEP, intermittently, since 1999. If you drop balls of different size from the Leaning Tower of Pisa, for example, their free fall lasts just seconds. “But you can drop things in space, and they never stop falling,” Overduin says; they remain in orbit, constantly falling toward Earth. This allows an extended time — days or longer — to look for subtle effects.
The plan calls for using four pairs of “test masses” made of at least three different materials — such as beryllium, niobium and platinum-iridium — which would be kept in a vacuum and cooled to just a few kelvins, reducing temperature fluctuations that can degrade measurement accuracy. Materials are chosen to reflect the broadest possible range of chemical properties so that disparities in acceleration (detected by an onboard accelerometer) would be the easiest to spot. The point, again, is to make meticulous measurements that show whether objects of different composition fall at different rates.