Time servers are common apparatus in modern server rooms but time synchronisation has only become possible thanks to ideas of physicist of the last century and it is our these ideas of time that has made many of the technologies of the last few decades possible.
Time is one of the most difficult of concepts to understand. Until the last century it was thought that time was a constant but it wasn’t until the ideas of Einstein that we discovered time was relative.
Relative time was a consequence of Einstein’s most popular theory the ‘General Theory of Relativity’ and its famous equation E=MC2.
What Einstein discovered was that the speed of light was the only constant in the Universe (in a vacuum anyway) and that time will differ for different observers. Einstein’s equations demonstrated that the faster an observer travelled towards the speed of light the slower time would become.
He also discovered that time wasn’t a separate entity of out universe but was part of a four dimensional space-time and that the effects of gravity would warp this space time causing time to slow.
Many modern technologies such as satellite communication and navigation have to take these ideas into account otherwise satellites would fall out of orbit and it would be impossible to communicate across the globe.
Atomic clocks are so accurate they can lose less than a second in 400 million years but consideration to Einstein’s ideas have to be taken into account as atomic clocks based at sea level run slower that those at higher altitude because of the Earth’s gravity warping spacetime.
A universal time scale has been developed called UTC (Coordinated Universal Time) which is based on the time told by atomic clocks but compensates for the minute slowing of the Earth’s rotation (caused by the gravity of the Moon) by adding Leap Seconds every year to prevent day from creeping into night (albeit in a millennia or two).
Thanks to atomic clocks and UTC time computer networks all over the world can receive a UTC time source over the Internet, via a national radio transmission or through the GPS network. A NTP server (Network Time Protocol) can synchronise all devices on a network to that time.