References




Comments

Post a Comment

Popular posts from this blog

Faster-than-light observations and experiments

Image
There are situations in which it may seem that matter, energy, or information travels at speeds greater than  c , but they do not. For example, as is discussed in the propagation of light in a medium section below, many wave velocities can exceed  c . For example, the phase velocity of X-rays through most glasses can routinely exceed c , but phase velocity does not determine the velocity at which waves convey information. If a laser beam is swept quickly across a distant object, the spot of light can move faster than  c , although the initial movement of the spot is delayed because of the time it takes light to get to the distant object at the speed  c . However, the only physical entities that are moving are the laser and its emitted light, which travels at the speed  c from the laser to the various positions of the spot. Similarly, a shadow projected onto a distant object can be made to move faster than  c , after a delay in time. In neither case does any matter, energy, or informat...
Read more

Speed of light

Image
The speed of light in vacuum, commonly denoted c , is a universal physical constant important in many areas of physics. Its exact value is defined as 299 792 458  metres per second (approximately 300 000  km/s, or 186 000  mi/s).Note It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of ​ 1⁄ 299 792 458 second.Note According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any information can travel through coordinate space. Though this speed is most commonly associated with light, it is also the speed at which all massless particles and field perturbations travel in vacuum, including electromagnetic radiation (of which light is a small range in the frequency spectrum) and gravitational waves. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonze...
Read more

Practical effects of finiteness

Image
The speed of light is of relevance to communications: the one-way and round-trip delay time are greater than zero. This applies from small to astronomical scales. On the other hand, some techniques depend on the finite speed of light, for example in distance measurements. Small scales In supercomputers, the speed of light imposes a limit on how quickly data can be sent between processors. If a processor operates at 1   gigahertz, a signal can travel only a maximum of about 30 centimetres (1 ft) in a single cycle. Processors must therefore be placed close to each other to minimize communication latencies; this can cause difficulty with cooling. If clock frequencies continue to increase, the speed of light will eventually become a limiting factor for the internal design of single chips. Large distances on Earth Given that the equatorial circumference of the Earth is about 40 075  km and that c is about 300 000  km/s , the theoretical shortest time for a piece of information to travel ...
Read more