Gravity Probe B nameplate

Quartz ping-pong balls in space:
Checking Einstein's General Theory

An Earth satellite named Gravity Probe B is putting Albert Einstein's general theory of relativity to the test.

The satellite is measuring two parts of Einstein's predictions by assessing how the presence of Earth warps space and time, and how Earth's rotation drags space and time.

Building Gravity Probe B required fundamental breakthroughs in a variety of technologies to ensure this experiment could be performed.

What is the
General Theory of Relativity?
Space and time depart from the usual rules of geometry when they seem to be curved by the presence of a gravitational field, according to Albert Einstein.
Geodetic Effect. The presence of Earth changes space and time in what is known as the geodetic effect. Imagine it as something like holding a bedsheet by its four corners and placing a basketball in the center. The bedsheet will seem to wrap itself around the ball slightly, in a manner similar to the way Earth slightly warps space and time.

Frame Dragging. Gravity Probe B also is measuring frame dragging, which is the effect of Earth's rotation on space and time. Einstein had predicted that very large objects in space would distort time and space as they spin, like a tornado. Frame dragging has not been measured previously because the effect is so small that technology previously could not record it.

The Gravity Probe B experiment uses three key components: Gyroscopes. At the heart of the experiment inside Gravity Probe B are four gyroscopes. The gyroscopes in Gravity Probe B are not flywheels, but electrically supported spheres spinning in a vacuum. Scientists use them as a freely suspended flywheel to study Earth's rotation.

NASA artist concept of Gravity Probe B satellite in space
NASA artist's concept of
the Gravity Probe B satellite
Glass balls. At the center of each of the four gyroscopes is a jewel-like sphere of fused quartz the size of a ping-pong ball. NASA says the ultra-smooth spheres, coated with niobium, are the roundest objects ever made by man.

The spheres are enclosed in chambers to prevent disruption from sound waves. They are chilled to near absolute zero to prevent their molecular structures from creating disturbances. The gyroscopes are 30 million times more accurate than any gyroscope ever built.

What does it look like? Overall, the satellite payload is a 21-in.-long block of fused quartz holding the four gyroscopes and a proof mass, all bonded to a quartz telescope. That package is inside a 1,500 liter helium dewar to hold its temperature down to – 456.43 degrees Fahrenheit (1.8 Kelvin).

A dewar is like a big Thermos bottle in which detectors are kept cool at the extremely low temperatures where they operate most efficiently.

The gyroscopes' quartz balls become superconductors at the temperature of liquid helium. That allows the gyroscopes to be suspended electrically.

The gyroscopes spin at 10,000 revolutions a minute. Very sensitive magnetometers detect any changes in the gyroscope's spin axis.

Who's right. If Einstein's predictions were correct, the gyroscopes should detect small amounts of time and space missing from each orbit.

To measure each orbit closely, a tracking telescope is used to align the gyroscopes with a guide star. A magnetic-field measuring device records changes relative to the guide star.

Cutting-edge technology. Gravity Probe B is giving us a clearer picture of how gravity works in our physical world as well as a better understanding of the underlying structure of the Universe.

NASA describes the probe project as among the most thoroughly researched programs ever undertaken by the space agency.

The satellite experiment is calculating data for 16 months.

Satellite launch. Gravity Probe B is a project of NASA and the National Academy of Sciences manufactured by Stanford University and Lockheed-Martin.

The satellite was launched to a 404-mi.-high orbit on April 20, 2004, on a Delta 2 rocket from Vandenberg Air Force Base, on the central coast of California.

Learn more:
  • Gravity Probe B quick look    NASA JPL

  • Gravity Probe B relativity mission    NASA Marshall
  • Gravity Probe B weekly highlights    Stanford University

  • Gravity Probe B image gallery    Stanford University

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