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Einstein's relativity and how not to get lost

Philippa Jefferies looks at how Einstein’s theories affect us every day through our GPS devices

We’ve all heard of Einstein’s theories of Relativity, even if only by name. They’re often associated with black holes and other immense objects in space and they dictate the movement of objects from our own planet to vast galaxies. However, the consequences of Special and General Relativity affects us more personally every day. A good example is the GPS on your phone!

GPS, the Global Positioning System, is now available on every modern phone and SATNAV. GPS can be an extremely accurate way of determined your location, however without taking into account relativistic effects your high-tech phone would give you a location a few miles from where you are actually stood – which would be rather inconvenient if you were in a hurry to get somewhere!

Einstein’s theory of Special Relativity describes how observers moving at different speeds view each other, and can be summarised as:

1. The laws of physics are the same for everyone (in all frames of reference),

2. The speed of light is always the same. Even is a person is travelling much faster than someone else, they both see light travelling at the same speed, resulting in them experiencing time differently.

General Relativity also effects the way you experience time. One of the main ideas of the theory is that unless you have an outside reference, you cannot tell if you are stationary or accelerating in any direction. This means if you are in a gravitational field, you can also be seen as accelerating which affects the rate of time. For this reason, clocks in higher orbit tick faster than those on Earth.

These effects need to be taken into account by any GPS system. GPS utilises satellites orbiting the Earth, which emit signals with information about their position and the time the signal was sent. Your device receives these signals and by using the differences in the times taken for the signals to reach it, your position can be determined using a method called trilateration. This involves using the distances from at least three satellites to the device and determined at what point all these match.

To give your position to within a few metres, the timings have to be extremely accurate (about 20 nanoseconds). Relativistic effects mean that the clocks on the satellites tick faster than those on Earth by about 38 microseconds per day. When you’re trying to get to the precision needed for accurate GPS location this is a significant difference, about 1000 times the accuracy needed. Very quickly the whole system would be completely useless.

Luckily scientists have been able to calculate the relativistic effects and correct for this, allowing you to still find your way. This is only one matter in which Einstein’s theories of Special and General Relativity affect us and it shows just how important it is to have a full understanding of their effects, Clearly, it’s not all about black holes!

From Issue 12

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