Global Positioning System (GPS)
Nowadays, we rarely use paper maps to figure out directions; instead, we punch the destination address into a Global Positioning System (GPS) unit in the car or even our smartphone. The ability to precisely locate ourselves using GPS owes a great deal to the mental musings of Albert Einstein. In his attempts to understand gravity, he developed in 1916 a bizarre and unintuitive way of looking at space and time and their interaction with matter — his general theory of relativity.
The idea of warped space-time led to an observational victory in May 1919. Renowned British astronomer Sir Arthur Eddington guided an expedition to witness the deflection of starlight as it passed by the Sun during a total solar eclipse, thus proving that the Suns mass warps space.
But what about time?
Because space and time are interwoven, massive objects stretch the rate at which time passes near them. The closer you are to an object with mass, theory says, the slower time ticks along. The effect is minuscule: A day atop Mount Everest is about 30 millionths of a second shorter than a day at sea level. By the 1950s, though, physicists had developed a precise timekeeping device that made use of natural atomic oscillations. Invented purely to test Einstein’s relativity, this cesium atomic clock is accurate to within a billionth of a second. ^
In 1971, commercial airline jets carrying four cesium atomic clocks flew twice around the world — once eastward and once westward. Relativity theory (a combination of Einstein’s general and special theories) predicted that the clocks would differ from the ground-based standard by a few ten to a few hundred nanoseconds, depending on their direction. When the planes landed and researchers checked the clocks, they determined that Einstein was right.
After this experimental success, these atomic clocks were neither relegated to the storage room nor only stationed in research physics labs. Concurrent with the development of the atomic clock had been another technological struggle: the Space Race. Following the launch of Sputnik in 1957, scientists began to realize that satellites could be used as “artificial guide stars” for global positioning (mostly monitoring the activities of “the other guys”).
In 1978, researchers launched the first operational GPS satellites incorporating atomic clocks. Today, an armada of 30 satellites helps us find our place. Engineers programed the time-altering effects of relativity into these satellites because of their motions and altitudes.
Residents of the 21st century have all but given up reading maps because of their heavy reliance on a technology that practically owes its existence (and certainly its usefulness) to what must have seemed a “pointless curiosity” about the nature of gravity. Moreover, the thought experiments that led Einstein to postulate relativity were performed nearly a century before anything useful came of them.
In an age of microwave meals and instant access, it’s often hard to wait a year, much less a century, for a scientific idea to move into the real world, but relativity did just that. The current worldwide value of the GPS network is estimated to be nearly a half-billion dollars, and that says nothing of the value of the lives it has helped save.
1916 -> Albert Einstein publishes his general theory of relativity;
1919 -> Astronomers perform the first test of general relativity;
1971 -> Jets carrying cesium atomic clocks test how mass alters time;
1978 -> First GPS satellite launches;
1998 -> President Bill Clinton announces plans to make GPS a dual-use system: both military and civilian;
2011 -> GPS satellite “constellation” expanded.