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A Pop Quiz for Einstein The Gravity Probe B mission will test two important aspects of Einstein's theory of General Relativity.

May 24, 2000: Crystal balls rarely have anything to do with science, but a special set of four soon may provide an answer to one of the last, untested portions of Einstein's General Theory of Relativity. Rather than peering into a crystal ball, scientists will put four - each a manmade quartz gyroscope - in orbit on a year-long Relativity Mission measuring how they spin to see if the Earth's rotating mass distorts time and space. Scientists often compare the fabric of space to a rubber sheet, with the Earth as a marble denting the surface, thus curving the paths of passing objects. If Einstein is correct, then the rotation of a planet or star also twists the sheet and distorts time, just a little, in an effect called frame dragging that will slightly repoint the gyros. The gyros also will measure an effect called geodetic precession, a miniscule compression of space caused by the Earth's physical presence. Both the frame-dragging and geodetic precession effects are so small that they require near-perfection in the design and construction of the instrument, Gravity Probe B (or GP-B). Right: Under the glow of a green light, a scientist at Stanford University checks for traces of dust on the quartz block assembly that contains the four quartz gyroscopes at the heart of the Relativity Mission. Credit: Stanford Send this page to a friend! "We've tried very hard to design an absolutely perfect gyroscope," said Dr. Francis Everitt, the Principal Investigator at Stanford University. Even in an age of exquisite measurements, nothing is perfect. The GP-B gyros, though, are about as close as humans can get. The gyros and their support system are so precise that non-relativity effects will cause them to drift by no more than 1/3 milli-arc-second during a year. It's a number that Everitt knows well. He once took a loose hair from his scalp and measured it in the machine shop at Stanford University. At a distance of 32 km (20 miles), that hair would appear to be a half milli-arc-second wide. "That means the gyros can measure frame dragging to about 1 part in 150, and geodetic precession to about 1 part in 100,000," Everitt explained. But while he's confident about the accuracy and precision of the answer, he won't predict the answer itself.

The rubber-sheet analogy is the best way to envision how mass curves space and time; the Earth is like a marble denting the smooth surface (Jupiter would be like a bowling ball). Frame-dragging will act as if there's just a trace of friction between the Earth and the rubber sheet so that the Earth's rotation twists the sheet just a little in one direction, while geodetic precession pulls a little at a right angle. Credit: UAH

"Our view as experimentalists is not to make any predictions that would prejudge the results," Everitt said. "Let's go ahead and do the experiment as accurately as we know how." Whatever number comes out will be too miniscule to affect our daily lives, but may help scientists explain the incredible jets of mass and energy that spew out from the regions above the poles of spinning supermassive black holes at the hearts of certain galaxies. Right: An artist's representation of frame dragging around a black hole. Credit: Joe Bergeron and Sky & Telescope. "GP-B is a landmark experiment in fundamental physics," said Rex Geveden, the project manager at NASA's Marshall Space Flight Center, "and it's result is critically important to our understanding of how the universe works." Sign up for our EXPRESS SCIENCE NEWS delivery "Einstein developed two different theories of relativity," Everitt said. "His Special Theory of Relativity is well tested. But for the General Theory of Relativity - Einstein's Theory of Gravitation - the amount of testing is very limited. "We have the deeply paradoxical fact that this is the most elegant physical theory ever conceived - far more elegant than the Standard Model [which describes how particles and fundamental forces interact]- yet we know it cannot be more than an approximation because it leaves several fundamental questions unsolved." Measuring very precisely the magnitudes of frame dragging and geodetic effects is the double objective of the Relativity Mission. It will be carried out by GP-B, scheduled for launch in 2002, 87 years after Einstein completed work on his General Theory of Relativity. Such a long wait to test a fundamental theory is unusual in science. (Other predictions of General Relativity Theory have been tested and confirmed years ago). That's because while gravity is the glue that holds the universe together, it's also the weakest of the four fundamental forces. Thus, testing the frame-dragging and geodetic precession effects had to await the advance of elegant technologies precise enough to measure up to Einstein's elegant theory. (continues after sidebar)

Twin clocks proved Einstein right If this is GP-B, what was GP-A? It was a completely different test of gravity and relativity that NASA flew in 1976. Part of Einstein's special and general theories of relativity holds that changes in gravity and speed will alter the speed at which time flows. This involves the famous Paradox of the Twins. There once was a girl named Miss Bright, Who could travel much faster than light. She left one day, In a relative way. And returned the previous night. (Anon) In the paradox, one twin hops on a spaceship and accelerates almost to the speed of light. When he returns after what seems like a short trip to him, he is stunned to see that his twin brother is now an old man for whom decades have passed. That's because high speed has three odd effects. When an object is moving, its mass increases, length decreases (along the line of flight), and time expands. The changes are not noticed aboard the moving object, since everything changes together. A stationary observer will see it happening to the spaceship. And the traveler will perceive the stationary world as going faster. The effects are insignificant at low speeds. Near the speed of light - called relativistic speeds - they become ever greater. That's why you can't travel at the speed of light. Mass would be infinite, length would be zero, and everything would take forever. The first proof of the special relativistic mass increase was in an experiment with accelerated electrons by F. Bucherer in 1908, and of time dilation by Blackett and others in the early 1920s. Einstein predicted that gravity has the same effect. To test the theory, NASA used the Paradox of the Twins with hydrogen maser clocks that lose no more than 1 second every 3 billion years. One clock stayed on the ground, while the second was rocketed to 10,000 km (6,200 miles) where Earth's gravity has less than half the strength it has at sea level. As the probe slowed, stopped briefly at the apex, and started falling back, scientists could measure slight differences between the flight clock and the ground clock. To a precision of 70 parts per million, Einstein was proven right, again.

The technologies developed by Stanford, NASA/Marshall and Lockheed Martin (the prime contractor for GP-B), will put the four quartz gyroscopes in an orbiting isolation chamber for a ride smoother than glass to eliminate vibrations and effects that could swamp the two effects. The gyroscopes, the heart of the mission, will do basically the same thing as a child's spinning top. Left: Stanford University scientists insert one of the gyroscope housings into the quartz block assembly that will be the center of GP-B. The star tracker telescope is attached to the right edge of the picture. The glass is so smooth that molecular forces effectively weld the parts of the QBA together. No glue is needed. Credit: Stanford Long before Einstein, Isaac Newton discovered the law of inertia: a body in motion will stay in motion unless acted upon by an outside force. In the case of a spinning top or the GP-B gyroscopes, the spin axes should stay pointed in the same direction until acted upon by an outside force. The top on Earth soon starts wobbling and falls because of friction between its tip and the ground. The gyros aboard GP-B will be isolated from such effects so they experience only the one force that can reach inside the isolation chamber. Earth's own gravity and rotation should drag space and time just a little and thus pull the gyro spin axes slightly out of position. Because frame dragging is so small, imperfections in the gyros and the rest of the spacecraft have to be correspondingly smaller. Each gyro is made of a special kind of glass - fused quartz, made when quartz crystals are melted-- machined to a diameter of 3.81 cm (1.5 in), and then polished to within 40 atoms - less than a millionth of an inch - of a perfect sphere. That's equivalent to the Earth's surface being polished so round that mountain peaks and ocean trenches are within 5 meters (16 ft) of the same level. Only neutron stars are thought to be smoother, because gravity flattens everything on the surface. Each gyro, gently levitated in a vacuum, will spin at 9,000 rotations per minute inside an almost equally round quartz housing, all of it isolated inside a giant liquid helium-filled Thermos bottle.

From the gyros' point of view, the universe will virtually disappear. All that should affect them are the gravitational pulls of the Earth, Moon, and Sun (the rest of the solar system and universe are too small to worry about) and the frame-dragging and geodetic effects. Right: Lockheed engineers standing next to a full-scale engineering model of GP-B (far right) provide a sense of size for a spacecraft and mission that center on four 3.8 cm (1.5 in) diameter quartz balls. Credit: Stanford Unlike astrophysics missions that quickly return dramatic images, or space missions where observers see crystals growing before their eyes, the Relativity Mission will need 13 months of data-taking before it reaches its full accuracy. Although the scientists will know within a couple of months of launch whether the experiment is working properly, they will be very cautious and scrupulous before making definitive claims about the result. Even after the main data-taking is over, the team will gently tweak the spacecraft and change settings for another two or three months to see how those variations affect the readings from the gyroscopes. "Check and check again," is the motto of the GP-B team. To make assurance doubly sure, they and NASA have set up an independent science advisory committee as one more check in this rigorous examination of Albert Einstein himself. If the answers on GP-B are different from what is to be expected on Einstein's theory then a number of fundamental assumptions in physics are affected. But we won't know that until after the gyros each have spun more than 4.7 billion times, and after the scientists feel they have analyzed their data almost as many times. Editor's note: an earlier version of this story misstated that the gyros would have spun 4.7 trillion times. The correct number, cited above, is 4.7 billion.

Web Links Relativity Mission at Stanford University. National Academy Press report on GP-B has interesting background information. Gravity Probe A and possible follow-on missions are described at Harvard University. Lockheed Martin Co., the spacecraft prime contractor, has a press kit and a photo gallery .

Join our growing list of subscribers - sign up for our express news delivery and you will receive a mail message every time we post a new story!!! More Headlines For lesson plans and educational activities related to breaking science news, please visit Thursday's Classroom Author: Dave Dooling Production Editor: Dr. Tony Phillips Curator: Bryan Walls Media Relations: Steve Roy Responsible NASA official: Ron Koczor