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

Testing the symmetry of space-time by means of atomic clocks

In his Special Theory of Relativity, Einstein formulated the hypothesis according to which the speed of light is always the same, no matter what the conditions are. It may, however, be possible that—according to theoretical ...

High-speed supernova reveals earliest moments of a dying star

An international team of scientists, including astronomers from the Universities of Leicester, Bath and Warwick, have found evidence for the existence of a 'hot cocoon' of material enveloping a relativistic jet escaping a ...

Electrically charged Higgs versus physicists: 1-0 until break

The last missing particle of the Standard Model, the Higgs boson, was discovered in 2012 in experiments at the Large Hadron Collider. Since then, searching for new, related particles has been underway. Predicted by various ...

Brief reflections from a plasma mirror

When a dense sheet of electrons is accelerated to almost the speed of light, it acts as a reflective surface. Such a 'plasma mirror' can be used to manipulate light. Now an international team of physicists from the Max Planck ...

Will we ever see a black hole?

In the shadowy regions of black holes two fundamental theories describing our world collide. Can these problems be resolved and do black holes really exist? First, we may have to see one and scientists are trying to do just ...

Slow light speeds up the microscopic world

A team of researchers from the University of St Andrews and the University of York has slowed down the speed of light in a process which could have major applications in fundamental science and medical diagnosis.

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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.

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