Testing Einstein: it’s all in the details now

A team of York physicists, led by Norbert Bartel, Distinguished Research Professor in the Department of Physics & Astronomy, has been taking part in the Gravity Probe B project to prove Einstein’s General Theory of Relativity. This article on the mathematical aspects of the project is one in a series of updates on York researchers’ role in this historic $700-million experiment, which began in 1959.

norbert bartelWhat is arguably the most difficult-ever experiment in space ended last week as the last wisp of helium coolant in Gravity Probe B dissipated, putting an end to the strange craft’s part in the mission to prove Einstein’s General Theory of Relativity.

Left: Norbert Bartel

“So far so good,” said York’s Norbert Bartel, lead scientist in Canada on the joint NASA-Stanford project. “The experiment has gone pretty close to flawlessly.” And that is excellent news in this, the World Year of Physics, so-named by UNESCO to commemorate the 100th anniversary of Einstein’s miraculous year, 1905, during which he published three of his most influential theories.

Despite the success of the GP-B mission so far, Bartel said he looks with some nervousness at the next 365 days. That’s when his research team of Michael Bietenholz, Ryan Ransom and graduate students Jerusha Lederman and Peter Luca will put the final touches on their portion of the data analysis before colleagues at both Harvard and Stanford universities factor their findings into the grand calculation that will prove Einstein’s theory about space-time.


Right: Albert Einstein

It is an expensive experiment conducted over vast distances with absolutely no room for error as terabytes of data are rendered down to achieve a result measured in fractions of a degree. According to Einstein’s predictions, the gyroscopes on board GP-B will have changed the direction of their spin since they were launched in April 2004. This is due to the curvature of space-time caused by the mass of the Earth (geodetic effect) and also due to the twisting and warping of space-time caused by the diurnal rotation of the Earth (frame-dragging effect).

Both effects are extremely small. The geodetic effect changes the direction of the gyroscopes’ spin by 6.6 arcseconds per year or 1/500 of a degree per year along the meridian of the Earth. Frame dragging changes the spin direction by 0.041 arcseconds or 1/100,000 of a degree per year. The changes are measured with a precision of better than 0.0005 arcseconds or 1/10,000,000 of a degree per year. In other words, if the result comes out to be off by as little as a few 10-millionths of a degree per year, then Einstein would be proven wrong – provided the measurements are correct. That’s the part that makes Bartel a bit nervous.

 gp-b diagram

Above: A diagram showing the relationship of the gyroscopes on board GP-B to the objects the probe fixed on to measure warpage of space-time around the Earth

Scientists at NASA and Stanford are looking for changes in the gyroscopes’ spin axes relative to a bright, nearby star, called IM Pegasi, which the York team has studied extensively over the past eight years. Bartel’s team is measuring the motion of IM Pegasi relative to three quasars located in the distant universe, billions of light years from Earth. These quasars served as the reference points to which the gyroscopes’ spin motion will be measured. The end result of the GP-B mission will only be obtained when the motion of the gyroscopes’ spin relative to IM Pegasi is corrected by the motion of IM Pegasi relative to the quasars.

The whole project is a double blind experiment – neither group is allowed to look over the shoulder of the other. It will all come down to a simple addition of two numbers, one provided by the NASA-Stanford team, the other by Bartel’s team and colleagues from the Harvard-Smithsonian Center for Astrophysics who also work on measuring IM Pegasi’s motion.

“We already have an approximate value for the motion of IM Pegasi and are now fine-tuning our analysis,” Bartel said. “In particular, we are focusing on all kinds of possibilities that could throw off our measurement, even if only by a small amount.”

 GP-B researchers at York
Above: From left, Lederman, Ransom, Luca and Bietenholz

Possible spurious errors on the measurement of IM Pegasi’s motion are being calculated by Ryan Ransom. Michael Bietenholz and graduate student Peter Luca are studying how accurately we can image the quasars to possibly further improve the measurements. Graduate student Jerusha Lederman is looking into how motionless the quasars really are and how well they serve as the final reference sources for the GP-B mission.

One year from now, team members must complete their analysis and be absolutely sure of the measurement. No corrections allowed. Then, both teams will put their numbers on the table and do one final calculation to determine if Einstein’s theory, the basis of all our thoughts on space-time for the past century, is right…or wrong.

For more information on Gravity Probe B and Testing Einstein’s Universe, the DVD produced by Bartel to explain the experiment, visit www.yorku.ca/bartel/AstronomyFilms/. Previous articles in this series were published in the April 21, 2005 and April 20, 2004 issues of YFile. See also the story in the December 2004 issue of YorkU magazine.