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

Communicating with a relativistic spacecraft gets pretty weird

Someday, in the not-too-distant future, humans may send robotic probes to explore nearby star systems. These robot explorers will likely take the form of lightsails and wafercraft (a la Breakthrough Starshot) that will rely ...

Was going to space a good idea?

In 1963, six years after the first satellite was launched, editors from the Encyclopedia Britannica posed a question to five eminent thinkers of the day: "Has man's conquest of space increased or diminished his stature?" ...

Research achieves photo-induced superconductivity on a chip

Researchers at the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) in Hamburg, Germany, have shown that a previously demonstrated ability to turn on superconductivity with a laser beam can be integrated ...

Is anything absolute with relativity?

The theory of relativity is at once simple and elegant but also maddeningly nonintuitive. There's no need to get into the full guts and glory of that theory here, but there is one feature of Einstein's work that takes center ...

Quantum interference of light: Anomalous phenomenon found

A counterintuitive facet of the physics of photon interference has been uncovered by three researchers of Université libre de Bruxelles, Belgium. In an article published this month in Nature Photonics, they have proposed ...

Curved spacetime in a quantum simulator

The theory of relativity works well when you want to explain cosmic-scale phenomena—such as the gravitational waves created when black holes collide. Quantum theory works well when describing particle-scale phenomena—such ...

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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.

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