Einstein and Your Phone

Many cars have navigation screens built into the dashboard. Every smart phone has a GPS chip and a maps application.  

Ever think about what makes this whole GPS thing work?

It’s pretty amazing. 

First, there are 24 to 32 satellites in orbit around Earth. Each satellite carries an extremely precise atomic clock. The goal is for any GPS receiver to be able to “see” four satellites at one time - usually receivers can see nine satellites.

These satellites broadcast a lot of information but the parts we’re interested in are: “This is what time is it.” and “This is where I am.” The satellites broadcast this information all the time. 

There are millions of GPS receivers in cars, boats, planes, and phones. The receiver unit listens to four satellites and, by calculating the time delay of their signal and the satellite locations, figures out where the GPS receiver is located on the surface of Earth.

Your phone has access to digital maps. These are maps that are developed and maintained with math. They’re really giant math algorithms continually working to display the map on your screen. That work is called mathematically exact geovisualization.

And here’s the coolest part of the whole GPS system: Einstein’s theories of relativity. 

Because the satellites and their very precise clocks are traveling at 8,700 miles per hour, Einstein’s special theory of relativity tells us time, according to those clocks, is moving slower than time is passing here on Earth. So the clocks run 7 microseconds slow per day. 

But, just to complicate things further, because the satellites and their little precise clocks are 12,427 miles above Earth, Einstein’s special theory of relativity tells us that gravity bends time and space. As a result, the clocks run 45 microseconds fast per day.

The net difference is just 38 microseconds per day. It doesn’t sound like much, but if the GPS chips didn’t take Einstein's relativity into account, the system wouldn’t work at all.

And you thought Einstein’s general and special theories of relativity had nothing to do with your everyday life...