# Speed of light |

## Definition of Light | | Automatic translation | | Updated June 01, 2013 |

The light is a wave phenomenon visible corresponding to a small range of electromagnetic vibrations that we are in the electromagnetic spectrum. The nature of light is the quantum mechanics to which it is both a wave and a particle. The light has a wavelength, which determines the color, such as the **red** light in the wavelength of 700 nm, **Orange** 650 nm, **Yellow** 600 nm, **Green** 550 nm, **Blue** 500 nm and **Violet** 450 nm. This window is chosen by the human eye to specialize. But the invisible light is spread over a larger electromagnetic field. Maxwell found that light is an electromagnetic wave and there is no reason to limit the wavelength thereof at the interval corresponding to the spectrum of visible light, all the spectrum is light. Since then, it was observed that the electromagnetic wavelengths vary between 10^{-16} m and several thousands of kilometers. Different windows of the electromagnetic spectrum are characterized by a wavelength, but also by a range of frequencies defined. | | For example, a radio wave is an electromagnetic wave whose frequency is lower than 3000 GHz, a wavelength greater than 0.1 mm. Frequency is the number of electromagnetic oscillations that pass through a given point in one second. She speaks with the unit of frequency is hertz. Over the wavelength is shorter, the frequency is high to very high frequencies. The frequency is inversely proportional to the wavelength. The speed of light known as c, is a physical constant, as in vacuum since the photons have zero mass. Electromagnetic radiation typically has a constant speed in a vacuum and "straight line", which is called **speed of light**. It is set at **299,792,458 m/s** ### Image: The electromagnetic spectrum includes all windows of light. This picture was taken of the Sun in the ultraviolet at a wavelength of 171 angstroms or 17.1 nm. Credits: SOHO spacecraft EIT instrument (Extreme ultraviolet Imaging Telescope). ESA / NASA | | |

## The speed of light | | | | |

In 1905, **Albert Einstein** (1879 - 1955), published a theory that changed our world representation. He discovered that time can flow more slowly and that the space can contract. In the 19th century it was thought that all waves need a medium to travel such as air or water to the sound. We imagined that the light waves traveling in the lee of the luminiferous ether. In the late 19th century, 2 physicists, **Albert Abraham Michelson** (1852-1931) and **Edward Williams Morley** (1838-1923) sought to determine the flow of the ether by measuring the speed of light between two directions perpendicular to two periods of the year. They expected to measure variations in the speed but the result was surprising, all rays of light had the same speed. This famous experiment Michelson won the Nobel Prize in Physics in **1907**. Whenever a technical progress allows more precision, the experiment is repeated and the result is always the same, the speed of light is the same in all directions. The speed of light has been fixed at **299 792 458 m / s** in 1983 by the International Bureau of Weights and Measures thereby give a precise definition to the meter. As the speed of light is the same in all directions, it contradicts the laws of Newtonian mechanics and Galilean relativity where speeds are added.
NB: "The meter is the length of the path traveled in a vacuum by light during 1 / 299792458 second." | | Galilean relativity says that the movement depends on the repository where it is measured, i.e. it is different depending on the movement of the observer. A ground observer who sees a plane, has a different perception of the speed of the aircraft that a pilot flying alongside in another plane. According to this theory, the man on the ground and the pilot of the aircraft should see rays of light travel at different speeds. Also, a plane flying at the speed of light could see that the light was doing there.
**A light that does not move! **Einstein could not conceive. For him, it was more likely that the light is always the same speed, so he became interested in literature of the Michelson-Morley. If the speed of light is always constant, two observers have different movements, must register for a photon, speed and distance the same. This is not what we observe.
**299 792 458 m / s = space / time** ### Image: Einstein will challenge and relativize the absolute nature of space and time. Speed equals distance divided by time but the speed is constant and therefore the other 2 of the equation, distance and time depend on the movement of the observer. | | |

## Space and time | | | | |

A laser beam that reflected on the Moon is 1.3 sec to 1.3 sec to go and return. The reciprocating movement of the laser beam travels between two mirrors can be considered a clock. A driver passing at high speed near the Earth and Moon observed a strange phenomenon, he sees the light will not move vertically but zigzag. From his point of view the beam travels a longer distance than if it were a casual observer on Earth. If we compare the two distances from the laser, that of the ground observer is shorter than that seen by the pilot. Because light can only have one speed it can not compensate for the difference. She needs more time. Thus the point of view of the pilot light of the pendulum swings more slowly, time passes more slowly. In the spaceship, time dilates. This **first discovery** of Einstein revolutionized our view of the world, the movement causes a slowing of time. Since this theory has been demonstrated in a practical manner with the atomic clocks that allow extremely accurate measurements. | | Two atomic clocks, one mounted on a plane and the other placed on land, get different measurements of time. The clock is slower airplane a few nanoseconds, but the difference is real. This small difference is the speed of the aircraft. All vehicles in motion are affected by time dilation. More the pilot goes quickly more time expands. But he does not notice anything, they see no time dilation, or the ship itself and shrink in the direction of motion. This is the **second discovery** of Einstein's strange, moving objects contract. After traveling for a year at the speed of light the pilot of the spaceship has aged a year. The land they have aged 20 years. He has traveled in the future. But this imaginary journey the driver did verify that when we multiply by 15 000, the speed of our spacecraft. New Horizons is the fastest land vehicle traveling in space. Off Jupiter, it has reached, by the effect of gravity assist, a speed of 75 000 km/h or 21 km/s. The Earth travels around the Sun at about 105 522 km/h or 29.3 km/s. | | ### Image: To the left of the image the observer on Earth sees the laser beam to move vertically and the pilot of the spaceship sees the move and so the zigzag travel a longer distance. |

## What becomes masse? | | | | |

More than a body moves more rapidly its mass increases. This is the **third discovery** of Einstein. It finds that mass is nothing but the energy in a particular form. This is the most important conclusion of his theory of relativity. This energy is calculated by multiplying the mass by the square of the speed of light (E = mc2). We now understand that the power of the energy contained in matter is enormous, even when a body is inert.
**E** is the energy expressed in joules, **m** is mass in kg and **c** is the speed of light in m / s. The energy corresponding to 1 kg of material is staggering, as it reached 9 × 10^{16} joules (1kw / h = 3,600,000 J). This corresponds to the energy produced by a nuclear reactor with a capacity of 1400 MW for two years. While this formula is regarded as a mere theoretical possibility, the atomic physicists find that protons and neutrons are bound together by an incredible force. This force explains how our sun and the enormous destructive power of the atomic bomb. E = mc2 is no longer a mere theory. Einstein redefined certain laws of nature, but his theory has limits, that is why it is called theory of relativity. Only when objects pass at high speed in a straight line near the observer, they shrink and clocks slow down. He lacked strength in his theory that it does not incorporate the influence of gravity when it all natural phenomena. For 10 years he fought with the theory of general relativity to finally find the solution in 1916. Time and space are not only modified by the movement but also by the material. The closer we get to the center of the Earth, over the clock slows down. | | At the seaside time flows more slowly at the summit of Everest. At 20 200 km above the time difference with the ground is sensitive to the point where it is taken into account by the GPS satellites. The material also influences the space, which has the effect of changing the length of objects as they approach the heavenly bodies. If we introduce a measuring rod through the center of the Earth to measure the diameter, it will indicate a value greater than the length of the diameter. Einstein concluded that space increases. It's hard to imagine that at the center of the Earth there is a larger volume than is seen from the outside as on the image below cons. That is why we simplify the representation of space on a flat two-dimensional curved by gravity. When the space increases the surface stretches and bends. This is the greatest achievement of Einstein, he just managed to set the **Gravity**. If we fall, it is because of the curvature of Spacetime. This is also the curvature of a trajectory requires the planets orbiting the Sun's gravitational well. The proof of the existence of this curvature came may 29, **1919** during a total eclipse of the Sun by the Moon. The stars behind the Sun is visible for a few minutes because passing near the Sun's light is deflected and moves along a curved path. This first observation of the structure of space-time, consistent with the theory, Einstein was a **triumph**. When a star collapses on itself late in life, it implodes under the enormous pressure of its own gravity. It curves space-time so intensively that no light escapes from the gravitational well and the star becomes invisible, this is called a **black hole.** | | ### Image: simplified representation of space on a flat two-dimensional curved by gravity. When the space increases the surface stretches and bends. The relativity theory belongs to a still broader than we do not yet understand. "It is absolutely possible that beyond what our senses perceive, lurk unsuspected worlds." Albert Einstein Noble Prize in 1921. |