**Discovery Science: Theory of Relativity – Space, time, and Mass**

*Mass also has a third quality: it is the cause or “charge” of gravity. Einstein explained how this gravitational mass relates to inertial mass in his general theory of relativity (1919), 14 years after the special theory of relativity.*

Einstein gave a radical answer to the question of the relationship between gravitational and inertial mass: they are identical! Since inertial mass is inseparably related to the changing nature of space and time (inertia increases as the length shortens), this is also the case for gravitational mass.

As a result, gravitation—the attraction between masses—is directly linked to space-time. Thus the fundamental statement of the general theory of relativity can be understood as follows: gravity is a distortion in space-time. The more massive a body is, the more it warps space and time, which is expressed in the accelerated movement of other nearby bodies.

**Einstein’s triumph **

Mathematically, general relativity is significantly more complicated than special relativity. Nevertheless, it was confirmed in a spectacular manner in 1919. A solar eclipse made it possible to show that stars in the vicinity of the sun appeared in slightly different positions, since their light was displaced by the sun’s gravity. The observed displacement agreed with Einstein’s predictions, and he became a celebrity overnight.

Few people truly understood the meaning of Einstein’s ideas-aside from 7 mc'” and “everything is relative”-but in combination with his striking personality, they made him world famous. Being Jewish and a pacifist as well as a modern scientist, however, he was targeted for persecution by the Nazis. In December 1932, he emigrated to the United States, not visiting Germany again until 1952.

**“You have reached your destination” {thanks to general relativity]**

Neither special nor general relativity is significant at the speeds achieved by cars or airplanes. Atomic nuclei in particle accelerators reach slightly relativistic speeds, but few experiments deal directly with these effects. However, one technology dependent on relativity has already become a fixture of modern life: satellite navigation with

systems such as GPS, GLONASS, or Galileo.

Microwave signals are sent to the system’s satellites, and their transit time is used to calculate the distance to the satellite and thus the device’s own position. This would be a simple trilateration calculation—a routine task in measurement technology-if the Earth’s mass did not produce a tiny distortion in space.

This alters the time taken by the signal so significantly that without corrections based on general relativity, accurate and modern GPS (Global Positioning System) equipment would be a mere pipe dream.

**THE SEARCH FOR GRAVITY WELLS**

From a mathematical point of view, Albert Einstein’s equations resemble more complex versions of the mathematician James C. Maxwell’s electrodynamic equations. In general relativity, moving disturbances of space and time- gravity waves—are possible.

These waves cause unimaginably small variations in length around a trillionth of a billionth of a meter (10 2′ m). A gravity well is the pull of gravity that a large body in space exerts.