In physics, the speed of light (usually denoted c) is a fundamental physical constant, the speed at which light and all electromagnetic radiation travel in a perfect vacuum, which is 299,792,458 metres per second (about 300,000 kilometres per second or 186,000 miles per second). This constant is significant in the understanding and study of relativity, spacetime, astronomy, space travel, and other fields.
For much of human history the nature of light, including whether it was instantaneous or simply travelled very quickly, was unknown. By the 11th century many scientists had suggested that light had a finite speed but it was not until the 17th century that Ole Ramer demonstrated this by observing small differences in the apparent orbital period of Jupiter's moon Io. Using these observations, Christiaan Huygens estimated the speed of light to be about 220,000 km per second. Since then, scientists have devised more sophisticated techniques to improve the precision of measurement. By the mid-20th century, the speed of light was known to be approximately 299,792 kilometres per second. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, c is fixed at exactly 299,792,458 metres per second.
According to Albert Einstein's special theory of relativity, c is an important constant connecting space and time in the unified structure of spacetime. As such, it defines the conversion between mass and energy and is an upper bound on the speed at which matter and information can travel. It is the speed of travel of all electromagnetic radiation in free space, and is believed to be the speed of gravitational waves. In an inertial frame of reference, light in vacuum always travels at c. When light passes through a transparent material, such as glass or air, its speed is reduced; the ratio between c and the speed at which light travels in a material is called the refractive index of the material.
Can The Speed Of Light Be Exceeded?
Scientists at CERN have admitted an experiment that appeared to show neutrino particles could travel faster than light was potentially flawed due to a faulty cable.
The findings appeared to upend the fundamental laws of physics.
Albert Einstein proposed 100 years ago that nothing could travel faster than the speed of light in a vacuum. It underpinned the Standard Model of physics, whuch explains the way the universe and everything in it works.
But in September 2011 workers at CERN, the world's largest physics lab, announced they had recorded subatomic particles travelling faster than the speed of light.
They recorded neutrinos travelling at 300,006 kilometres per second in a 450-mile underground tunnel between Switzerland and Italy. Light travels at 299,792 kilometres per second.
Antonio Ereditato, spokesman for the researchers, said at the time: “We have high confidence in our results. We have checked and rechecked for anything that could have distorted our measurements but we found nothing.”
Today CERN physicists have admitted a faulty connection between a GPS unit and a computer may be to blame. Professor Brian Cox, a particle physicist at the University of Manchester and television presenter, said at the time that, if the result were proven, it would be the most profound discovery in physics for a century.