User:A. di M./SoL Spanish

The speed of light is of great importance for telecommunications. For example, since the circumference of the Earth is 40,075 km (along the equator) and c is theoretically the fastest speed at which a piece of information can travel, the shortest time interval to connect to the other side of the terrestrial globe will be 0.067 s.

Actually the travel time is a little larger, partly because the speed of light is about 30% smaller in an optical fibre and there rarely exist straight trajectories in global communications; furthermore delays are produced when the signal passes through electric switches or signal generators. In 2004, a typical lag of signal reception from Australia or Japan to the US was 0.18 s. Additionally, the speed of light affects the design of wireless communications.

The finite speed of light became apparent to the whole world, in the control of communications between the control centre in Houston and Neil Armstrong when he became the first man who set his foot on the Moon: after each question, Houston had to wait about 3 s for the return of a answer even when the astronauts reply immediately.

In a similar way, instant remote control of a spaceship is impossible because the time elapsed, for example, for the terrestrial controls to detect any problem, plus the time necessary for the ship to receive the response, can be several hours.

The speed of light can also be relevant on short distance. In supercomputers the speed of light imposes a limit to the speed at which data can be sent between processors. If a processor operates at 1 GHz, the signal can only travel at most 300 mm in a single cycle. Therefore, the processors must be placed close to each other to minimize communication delays. If clock rates continue to increase, the speed of light will eventually become a limiting factor in the internal design of individual chips.