Speed of light

The speed of light in a vacuum is defined to be 299,792,458 metres per second (1,079,252,848.8 km/h, which is approximately 186,282.4 miles per second, or 670,616,629.38 miles per hour). The speed of light is denoted by the letter c, reputedly from the Latin celeritas, "speed", and also known as Einstein's constant. Note that this speed is a definition, not a measurement; in fact, the fundamental SI unit of distance, the metre, is defined in terms of the speed of light and the second. The speed of light through a transparent medium (that is, not in vacuum) is less than c; the ratio of c to this speed is called the refractive index of the medium. "Speed of light" is sometimes abbreviated SOL.

Communications

The speed of light is of relevance to communications. For example, given that the equatorial circumference of the Earth is 40,075 km and c, the theoretical shortest amount of time for a piece of information to travel half the globe is 0.067 second.

Related Topics:
Communications - Earth

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The actual transit time is longer, in part because the speed of light is slower by about 30% in an optical fibre and straight lines rarely occur in global communications situations, but also because delays are created when the signal passes through an electronic switch or signal regenerator. A typical time as of 2004 for an Australia or Japan to US computer-to-computer ping is 0.18 second. The speed of light additionally affects wireless communications design.

Related Topics:
Optical fibre - As of 2004 - Australia - Japan - US - Ping - Wireless

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The finite speed of light became quite apparent to everybody following the communication of Houston ground control and Neil Armstrong when he became the first man to set foot on the Moon: For every question, Houston had to wait nearly 3 seconds for the answer to arrive, and would have to do so even if the astronauts replied immediately. (See animation.)

Related Topics:
Houston - Ground control - Neil Armstrong - Became the first man - Moon - Second - See animation

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Similarly, instantaneous remote control of an interplanetary spacecraft is impossible, in the sense that the time it takes, for example, for the earth-based controllers to become aware of a problem, plus the time it takes for the spacecraft to receive their response, can be some hours.

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The speed of light can also be of concern on short distances. In supercomputers, the speed of light imposes a limit on how quickly data can be sent between processors. If a processor operates at 1 GHz, a signal can only travel a maximum of 300 mm in a single cycle. Processors must therefore be placed close to each other to minimise communication latencies. If clock frequencies continue to increase, the speed of light will eventually become a limiting factor for the internal design of single chips.

Related Topics:
Supercomputers - Processor - GHz - Chips

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