In physics, special relativity (SR, also known as the special theory of relativity or STR) is the accepted physical theory of how measurements can differ (but are related) in different inertial frames of reference moving relative to one another. It was originally proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies". Measurements affected can include those of distance, time, energy, momentum, acceleration, and inertia.
Galileo Galilei had already postulated that all uniform motion is relative, and that there is no absolute and well-defined state of rest (no privileged reference frames), a principle now called Galileo's principle of relativity. Einstein extended this principle so that it accounted for the constant speed of light, a phenomenon that had been recently observed in the Michelson–Morley experiment. He also postulated that it holds for all the laws of physics, including both the laws of mechanics and of electrodynamics, whatever they may be.
The speed of light in vacuum, commonly denoted c, is a universal physical constant important in many areas of physics. Its value is exactly 299,792,458 metres per second, a figure that is exact because the length of the metre is defined from this constant and the international standard for time. This is approximately 186,282.4 miles per second, or about 671 million miles per hour. According to special relativity, c is the maximum speed at which all energy, matter, and information in the universe can travel. It is the speed at which all massless particles and associated fields (including electromagnetic radiation such as light) travel in vacuum. It is also the speed of gravity (i.e. of gravitational waves) predicted by current theories. Such particles and waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc2.
The speed at which light propagates through transparent materials, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c / v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 km/s; the refractive index of air for visible light is 1.000293, so the speed of light in air is 299,705 km/s or about 88 km/s slower than c.
An astronomical unit (abbreviated as au; other abbreviations that are sometimes used include ㍳, a.u. and ua) is a unit of length now defined as exactly 149,597,870,700 m (92,955,807.3 mi), or roughly the average Earth–Sun distance.
The astronomical unit was originally defined as the length of the semi-major axis of the Earth's elliptical orbit around the Sun.
Rømer's determination of the speed of light was the demonstration in 1676 that light has a finite speed, and so doesn't travel instantaneously. The discovery is usually attributed to Danish astronomer Ole Rømer (1644–1710), who was working at the Royal Observatory in Paris at the time.
Rømer estimated that light would take about 22 minutes to travel a distance equal to the diameter of Earth's orbit around the Sun: this is equivalent to about 220,000 kilometres per second in modern units, about 26% lower than the true value. While the exact details of Rømer's calculations have been lost, the error is probably due to an error in the orbital elements of Jupiter, leading Rømer to believe that Jupiter was closer to the Sun than is actually the case.
In journalism, a human interest story is a feature story that discusses a person or people in an emotional way. It presents people and their problems, concerns, or achievements in a way that brings about interest, sympathy or motivation in the reader or viewer.
Human interest stories may be "the story behind the story" about an event, organization, or otherwise faceless historical happening, such as about the life of an individual soldier during wartime, an interview with a survivor of a natural disaster, a random act of kindness or profile of someone known for a career achievement.