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7.7 Defining the speed of light as an explicit constant.

7.6 Increased accuracy of c and redefinition of the metre and second.

3 Faster-than-light observations and experiments.

In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1 / 299 792 458 of a second. He explored the consequences of that postulate by deriving the theory of relativity and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism.Īfter centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299 792 458 m/s ( 983 571 056 ft/s 186 282.397 mi/s) with a measurement uncertainty of 4 parts per billion. In 1905, Albert Einstein postulated that the speed of light c with respect to any inertial frame is a constant and is independent of the motion of the light source. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed c appearing in his theory of electromagnetism. Ole Rømer first demonstrated in 1676 that light travels at a finite speed (non-instantaneously) by studying the apparent motion of Jupiter's moon Io. The speed of light can be used with time of flight measurements to measure large distances to high precision. The finite speed of light also ultimately limits the data transfer between the CPU and memory chips in computers. The light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, or vice versa.

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 ( 124 000 mi/s) the refractive index of air for visible light is about 1.0003, so the speed of light in air is about 90 km/s (56 mi/s) slower than c.įor many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects.

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). The speed at which light propagates through transparent materials, such as glass or air, is less than c similarly, the speed of electromagnetic waves in wire cables is slower than c. The expansion of the universe is understood to exceed the speed of light beyond a certain boundary. phase velocities of waves, the appearance of certain high-speed astronomical objects, and particular quantum effects). In some cases objects or waves may appear to travel faster than light (e.g. In the special and general theories of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence, E = mc 2. Particles with nonzero rest mass can approach c, but can never actually reach it, regardless of the frame of reference in which their speed is measured. Such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Massless particles and field perturbations such as gravitational waves also travel at this speed in vacuum.

The speed of light is sometimes referred to as lightspeed, especially in science fiction.Īll forms of electromagnetic radiation travel at the speed of light, not just visible light. A light-year is a distance unit, defined as the distance travelled by light in one Julian year. According to special relativity, c is the upper limit for the speed at which conventional matter, energy or any signal carrying information can travel through space. It is exact because, by international agreement, a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄ 299 792 458 second. Its exact value is defined as 299 792 458 metres per second (approximately 300 000 km/s or 186 000 mi/s). The speed of light in vacuum, commonly denoted c, is a universal physical constant that is important in many areas of physics.