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speed of light
Designated by the letter "c." In empty space it equals 299,790 km per second (186,282 miles per second). The speed of light has the same value independent of the relative velocity between source and observer, an experimental fact that makes sense only if relative motion changes the relationship between space and time intervals to keep the distance covered by light per unit time the same for all observers. The fact that space and time are interchangeable to keep the speed of light constant implies that, in some sense, space and time must be the same, despite our habit of measuring space in meters and time in seconds. But if time and space are similar to the extent that they can be converted one into the other, then a quantity is needed to convert the units – something measured in meters per second that can be used to multiply seconds of time to get meters of space. This universal conversion factor is the speed of light. The reason it is limited arises simply from the fact that a finite amount of space is equivalent to a finite amount of time.
The “speed of light” can also mean the speed at which light travels in a given medium. For example, light travels only two-thirds as quickly in glass as it does in a vacuum. If something, such as a subatomic particle, travels faster through a medium than light does, the result is a kind of electromagnetic shock wave known as Cerenkov radiation. However, there is no violation of the laws of physics, since the universal speed limit is how fast light travels in a vacuum.
The speed of light is also limits the maximum rate at which information can be transferred from one place to another. Although nonlocal effects in quantum mechanics, such as teleportation, may be instantaneous even ove great distances, they do not violate the special theory of relativity because no matter, energy, or information is moved faster than light.
Any particles of zero rest mass, such as photons, travel at the speed of light. Massive particles approach the speed of light when their energy is very large relative to their rest energy.
Related category
• SPACE AND TIME
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