# March 14: Happy Birthday Albert Einstein

Albert Einstein is quite possibly the most recognizable scientist of all times, and not just because of his illustrious hairdo either. His insight into physics has led to some revolutionary breakthroughs to which modern science owes respect. This month just happens to mark the 135th birthday of the famous physicist. Thus, we take a closer look at how this man affects your everyday life with his most noted contribution, the Theory of Relativity.

### Relativity

It is a commonplace to think of "relativity" as some foreign idea that has no effect on our lives. This is in fact far from the truth. Relativity, and by extension Einstein, has played an integral part in your life.

GPS, for example, would be impossible if we did not have a working knowledge of relativity. Originally used only for military navigation, GPS has revolutionized every form of traveling. In fact, you would be hard-pressed to find a new vehicle that did not come with a GPS unit installed. Hand-held GPS units are also available for consumers to use with older vehicles, hikes and bike rides. Even more astonishing, GPS is easily accessible to anyone with a smartphone nowadays.

So how does GPS work? The 24 satellites that determine your position have an internal atomic clock, which measures time to within a billionth of a second. Similar clocks are found in your GPS unit and the times between satellites and your GPS help determine your position within a few meters.

This level of accuracy requires that the clock ticks on devices on the planet's surface and the satellites be within 20-30 nanoseconds of each other. Sounds simple, right?

However, we must take into account that the satellites are constantly moving relative to observers on the Earth at roughly 14,000 km/hour. Because the satellites are moving relative to us, the clock ticks will be slower on the satellites in relation to the clock in your GPS unit (check out this YouTube video as to why this happens). This effect, called time dilation, was noticed by Einstein years ago. Without accounting for relativity, the clocks on the satellites will tick slower than the clocks on the planet's surface by 7 nanoseconds each day, or a 7,000 times greater difference than allowed to achieve proper precision.

There is another aspect to relativity that pertains to gravity. Einstein noticed that clocks in a low-gravity environment would tick faster than clocks on the planet’s surface. Over the course of one day, clocks on board the satellites will tick faster than their counterparts on Earth at a rate of 45 microseconds per day.

If we combine the two aspects of relativity, both the speed of the satellites and the low-gravity environment, we expect the atomic clocks in the satellites to tick 38 microseconds (45-7) faster than clocks on Earth.

While 38 microseconds does not seem like much, remember that the precision of the atomic clocks must be within 20-30 nanoseconds and 38 microseconds is equivalent to 38,000 nanoseconds. This is roughly 13,000 times greater than the maximum error allowed.

What does this mean? It means that if relativity is not properly accounted for, GPS would be inaccurate in as little as two minutes and by the end of one day our GPS would have us in a location 10 miles away.

To counteract relativity, the atomic clocks that are in orbit are slowed by 38 microseconds. GPS units also utilize an internal microcomputer to preform calculations that adjust for relativity in determining your location.

In short, we can thank Einstein for our precise GPS units in our phones and cars.

### References:

**What the Global Positioning System Tells Us about Relativity**

http://www.metaresearch.org/cosmology/gps-relativity.asp

**Einstein's Relativity and Everyday Life
**http://physicscentral.com/explore/writers/will.cfm

**Real-World Relativity: The GPS Navigation System**

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html

**GPS, Relativity, and
pop-Science Mythology**

http://www.alternativephysics.org/book/GPSmythology.htm