NASA will put Albert Einstein’s theory of gravity to the test today with a satellite scheduled to launch at 10 a.m. from Vandenberg Air Force Base.
The satellite, called Gravity Probe B, will measure the warping of space-time predicted by Einstein’s theory of general relativity. The probe, which has been in the making for over 40 years, is now a reality because of technology and engineering developed at NASA and Stanford University. For two years the probe will measure properties of space-time and will provide the most definitive evidence in testing general relativity.
The concept of space-time describes how physical distances in space are inextricably related to time. It treats time as a dimension, just as left, up and forward are dimensions. In space-time, an event’s location must include spatial and temporal information. Many people refer to the “fabric of space-time” because it is convenient to imagine all events in the universe occurring on an underlying sheet of fabric of space and time.
Einstein proposed that space-time is curved by mass and energy, which accounts for the familiar gravitational pull experienced by objects on Earth’s surface. According to Einstein, the objects are slipping down the curved sheet of space-time toward Earth. Imagine a bowling ball on a mattress: The depression caused by the ball will cause other objects on the mattress to fall toward it.
Measuring Curved Space-time
Gravity Probe B will use a device called a gyroscope to test whether space-time is really curved by mass. A gyroscope consists of an axis about which a mass spins. Because a moving object will continue moving unless a force acts upon it, the gyroscope’s axis will not move unless something acts upon it, like gravity or a curved space-time.
UCSB Physics Professsor James Hartle said that a gyroscope is like a toy top.
“Think of a top you might spin,” Hartle said. “If you put it in the gravitational field of the earth, its axis will move around in a circle. That’s called precession.”
A top wobbles, or precesses, because the Earth’s gravity pulls on it. The top stops spinning only because the force of friction opposes the motion, which is unrelated to the precession.
This precession is common in many applications of physics. The Earth itself is like a giant gyroscope, as are tiny subatomic particles. The reason that all spinning things are able to precess is because an external force acts on them. In the case of a spinning top, if the axis deviates even slightly from vertical, the earth’s gravity pulls unevenly on it, which causes the top to precess.
The gyroscopes in the gravity probe are made of quartz spheres 1.5 inches in diameter. They are claimed to be the most perfect spheres ever created, with surfaces accurate to within .5 microinches (12 nanometers). The spheres are supported by electric fields and rotate in a vacuum to virtually eliminate friction. The extreme precision is required because the magnitude of the effect being measured is extremely small. Any bumps or irregularities would affect the gyroscope’s ability to detect small forces.
Such precision does not come cheap. The entire project has cost NASA $700 million dollars and has suffered numerous setbacks and cost increases due to the complexity and novelty of the parts.
Objects in orbit, such as the gravity probe, are in a state of free fall. They do not feel gravitational acceleration like objects on Earth’s surface. Because of this, the gyroscopes on the orbiting gravity probe should not precess as they would on Earth’s surface.
On the other hand, if space-time really is warped, as predicted by Einstein, then the gyroscopes should precess a very small amount because of the warping caused by earth.
“These people aim at detecting precession specifically due to general relativity,” Hartle said. “In fact, their effort is to get the gyroscope as free of forces as possible.”
Any of these unexpected forces would cause the probe to be inaccurate. Hartle said these forces might be caused by random flying particles in space.
“That’s the real technological achievement – what’s called ‘drag free’,” Hartle said. “They want the gyro to be essentially falling freely, so there are no forces acting on it. It is just detecting what space-time is doing.”
The drag-free system means that the gyroscopes are surrounded by a protective housing which orbits in lock step with the gyroscopes. There is no connection between the housing and the gyroscopes, so very responsive sensors and thrusters are needed to guide the housing along with the free-falling gyroscopes.
If the gyroscopes do precess because of space-time curvature, the theory predicts a contribution from the Earth’s mass and another from the Earth’s rotation, causing two types of precession. Hartle said the first type, geodetic precession, is caused by the presence of the Earth’s mass.
“The geodetic precession is something that would happen whether the Earth was rotating or not,” Hartle said. “Just because the space-time outside the Earth is slightly curved.”
The second type of precession, frame dragging, is caused by the rotation of Earth pulling space-time along with it, Hartle said. Instead of just curving the local space-time, a rotating planet should also twist it.
If the probe measures the expected amount of precession, then substantially more credit would be given to Einstein’s theory.
The angular amount of precession that theory would predict can be accurately calculated, and it is extremely small. The contribution from frame dragging is 42 milliarc-seconds per year. The size of this angle is equivalent to the width of a human hair viewed from 10 miles away. The satellite will collect data for two years in order to get an accurate reading.
“I think by now people have had certain expectations, and it would be very surprising if it gave back a different [result], but it would also be very interesting,” Hartle said. “You would have to have a different theory of gravity. That’s how you do physics – propose a theory, check it, if it doesn’t check, change the theory.”
NASA spokesman Don Savage said that the designers of the probe are not making any assumptions.
“The scientists are not going to make any predictions,” Savage said. “However, they expect it to get very good data.”
The choice to launch the probe from Vandenberg was not a random one. Its position on the Earth is beneficial for satellites that will assume the type of orbit preferred for the gravity probe.
“That is the location from which we launch satellites that go into polar orbit,” Savage said. “It’s more advantageous for the satellite to travel north-south.”
A polar orbit is one in which the satellite passes over the north and south poles of the Earth.
It is not uncommon for Vandenberg to participate in science missions such as this one. Vandenberg spokesman Jack Hokenson said the base typically launches five scientific instruments a year.