# A question of timing

From our everyday experience, the flow of time is a constant that relentlessly moves forward at the same pace for everyone. If you tell  a friend you will meet them in an hour, you can both agree on how long that hour is.

In the early 20th Century, Albert Einstein shook the foundations of our understanding of the Universe by saying time was not a constant, fixed quantity and proposing two theories: Special Relativity and General Relativity.

## It’s all Relative

Special Relativity tells us that moving clocks run slower with respect to a stationary observer. This slowing of time becomes more obvious as something approaches the speed of light. Satellites do not travel anywhere near the speed of light, however they are travelling very fast compared to an observer on the ground, so the effect is measurable.

General Relativity says that the closer you are to a heavy mass, such as Earth, the slower time will move for you. This means that a clock on a satellite orbiting the Earth will run faster relative to one on the ground.

## Direct Opposition

These two theories have little consequence for our daily lives as we are never travelling at such high speeds with respect to each other and we’re all pretty much the same distance from the centre of the Earth. However for the network of 24 GPS satellites orbiting Earth, the effects of both Special and General Relativity become noticeable.

The time differences of GPS satellites due to relativity are measured on the scale of nanoseconds – one nanosecond being just one billionth of a second – however the difference is enough to be measured and enough to matter.

GPS satellites travel at approximately 8,700 mph (14,000 km/h) with respect to Earth. This means time runs 7,200 nanoseconds per day slower for a satellite relative to us on Earth as described by Special Relativity.

However, using General Relativity it is possible to calculate that time goes faster for a GPS satellite by 45,900 nanoseconds per day, due to the satellite being 19,000km above the Earth (therefore in weaker gravity). This means overall time runs 38,700 (45,900 – 7,200) nanoseconds faster per day for a GPS satellite relative to us stationary on Earth.

## Telling the Time

GPS satellites are constantly transmitting signals with their location and the time the signal was sent. Your GPS device doesn’t transmit anything, it just receives the signals sent by at least four different satellites.

Even though these signals travel at the speed of light, it takes a finite time for the signals to travel from satellite to receiver. By knowing this time and the speed of light, it is possible for your device to calculate the distance between it and the four satellites.

However, if the GPS satellites didn’t correct for the time difference due to relativity, then the signals sent to your device from the satellite would read a false time, your device would calculate the distance wrong and wouldn’t know where you were.

## Atomic Clocks

Atomic clocks are responsible for the meticulous time-keeping of the GPS satellites and they are accurate to within 2 nanoseconds. The atomic clocks on board GPS satellites use the element caesium and work by exposing the caesium atoms to microwave radiation. This causes the atoms to vibrate at their resonant frequency which is so predictable we can use it to tell the time.

Watch The Great Relativity Show, a whistlestop tour of Special Relativity

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