Related topics: cern · large hadron collider · protons · neutrinos · light

The direct observation of highly nonlinear plasma waves

Over the past few decades, physicists and engineers have been trying to create increasingly compact laser-plasma accelerators, a technology to study matter and particle interactions produced by interactions between ultrafast ...

Black hole winds are no longer as they used to be

During the first billion years of the universe, winds blown by supermassive black holes at the centers of galaxies were much more frequent and more powerful than those observed in today's galaxies, some 13 billion years later. ...

Light-infused particles go the distance in organic semiconductors

Polaritons offer the best of two very different worlds. These hybrid particles combine light and molecules of organic material, making them ideal vessels for energy transfer in organic semiconductors. They are compatible ...

Physicists embark on a hunt for a long-sought quantum glow

For "Star Wars" fans, the streaking stars seen from the cockpit of the Millennium Falcon as it jumps to hyperspace is a canonical image. But what would a pilot actually see if she could accelerate in an instant through the ...

Impact of black hole winds, radiation examined in new study

Black holes are regions of space where gravity is so strong that nothing can escape. New research is examining the radiation and winds emanating from black hole activity and shows how they may exert effects on nearby planets.

page 1 from 19

Speed of light

The term speed of light generally refers to a fundamental physical constant of spacetime that limits the rate of transfer of matter or information. The speed of light is the speed of not just visible light, but of all electromagnetic radiation in vacuum (also called free space), and usually is denoted by the symbol c. Speeds faster than that of light are encountered in physics but, in all such cases, no matter or information is transmitted faster than c. The speed of light also plays a role in general relativity, and is believed to be the speed of gravitational waves.

In SI units, the magnitude of the speed of light in vacuum is exactly 299,792,458 metres per second (m/s) because of the way the metre is defined. More about this topic is found below in Speed of light set by definition.

For many practical purposes, the speed of light is so great that it can be regarded to travel instantaneously. An exception is where long distances or precise time measurements are involved. For example, in the Global Positioning System (GPS), a GPS receiver measures its distance to satellites based on how long it takes for a radio signal to arrive from the satellite. In astronomy, distances are often measured in light-years, the distance light travels in a year.

The speed of light when it passes through a transparent or translucent material medium, like glass or air, is less than its speed in vacuum. The speed is inversely proportional to the refractive index of the medium. In specially-prepared media, the speed can be tiny, or even zero.

For many years the speed of light was the subject of speculation, some believing it to be infinite. The first effective measurements of the speed of light were made in the seventeenth century, and these were progressively refined until, in 1983, the speed of light in vacuum was fixed by definition.

This text uses material from Wikipedia, licensed under CC BY-SA