GPS: Einstein’s Theory in Action

Every time you use your phone to find your location, you are relying on Albert Einstein’s theories of relativity. Without compensating for the effects of gravity and speed on time, GPS would be off by several kilometers within a single day.

Special Relativity: The Speed Factor

GPS satellites orbit the Earth at about 14,000 km/h. According to Special Relativity, because the satellites are moving so much faster than we are on the ground, their internal atomic clocks tick slightly slower—by about **7 microseconds** per day.

General Relativity: The Gravity Factor

At an altitude of 20,000 km, the Earth’s gravity is much weaker than it is at the surface. According to General Relativity, clocks in a weaker gravitational field tick faster. This effect is even stronger than the speed factor, causing the satellite clocks to gain about **45 microseconds** per day.

The Net Result

Subtracting the 7 microseconds (speed loss) from the 45 microseconds (gravity gain) leaves a net difference of **38 microseconds** per day. To your phone, which uses the time it takes for a signal to travel from space to calculate your position, 38 microseconds translates to an error of about 10 kilometers.

How it's Fixed

To solve this, the atomic clocks on GPS satellites are "detuned" before launch. They are programmed to tick slightly slower so that once they are in orbit, their time matches the time on Earth perfectly.

Conclusion

GPS is the ultimate proof that time is not absolute. On the Epoch Clock, we see a unified count, but that count is only possible because we have mastered the bizarre physics of relativity.