From Wiki:
http://en.wikipedia.org/wiki/Speed_of_light
Measurement
To measure the speed of light, various methods can be used which involve observation of astronomical phenomena or experimental setups on Earth. The setups could use mechanical devices (e.g. toothed wheels), optics (e.g. beam splitters, lenses and mirrors), electro-optics (e.g. lasers), or electronics in conjunction with a cavity resonator.
[edit] Astronomical measurements
Due to the large scale and the vacuum of space observations in the solar system and in astronomy in general provide a natural setting for measuring the speed of light. The result of such a measurement usually appears as the time needed for light to transverse some reference distance in the solar system such as the radius of the Earth's orbit. Historically such measurements could be made fairly accurately, compared to how accurate the length of the reference distance is known in Earth-based units. As such, it is customary to express the results in astronomical units per day. An astronomical unit is approximately equal to the average distance between the Earth and the Sun.[Note 8] Since the most used reference length scale in modern experiments (the SI metre) is determined by the speed of light, the value of c is fixed when measured in metres per second. Measurements of c in astronomical units provides an independent alternative to measure c.
One such method was used by Ole Christensen Rømer to provide the first quantitative estimate of the speed of light.[72][73] When observing the periods of moons orbiting a distant planet these periods appear to be shorter when the Earth is approaching that planet than when the Earth is receding from it. This effect occurs because the Earth's movement causes the path travelled by light from the planet to Earth to shorten (or lengthen respectively). The observed change in period is the time needed by light to cover the difference in path length. Rømer observed this effect for Jupiter's innermost moon Io and deduced from it that light takes 22 minutes to cross the diameter of the Earth's orbit.
Aberration of light: light from a distant source will appear to a different location for a moving telescope due to the finite speed of light.Another method is to use the aberration of light, discovered and explained by James Bradley in the 18th century.[74] This effect results from the vector addition of the velocity of light arriving from a distant source (such as a star) and the velocity of its observer (see diagram on the left). A moving observer thus sees the light coming from a slightly different direction and consequently sees the source at a position shifted from its original position. Since the direction of the Earth's velocity changes continuously as the Earth orbits the Sun, this effect causes the apparent position of stars to move around. From the angular difference in the position of stars (maximally 20.5 arcseconds)[75] it is possible to express the speed of light in terms of the Earth's velocity around the Sun, which with the known length of a year can be easily converted in the time needed to travel from the Sun to Earth. In 1729, Bradley used this method to derive that light travelled 10,210 times faster than the Earth in its orbit (the modern figure is 10,066 times faster) or, equivalently, that it would take light 8 minutes 12 seconds to travel from the Sun to the Earth.[74]
Nowadays, the "light time for unit distance"—the inverse of c, expressed in seconds per astronomical unit—is measured by comparing the time for radio signals to reach different spacecraft in the Solar System, with their position calculated from the gravitational effects of the Sun and various planets. By combining many such measurements, a best fit value for the light time per unit distance is obtained. As of 2009[update], the best estimate, as approved by the International Astronomical Union (IAU), is:[76][77][78]
light time for unit distance: 499.004783836(10) s
c = 0.00200398880410(4) AU/s = 173.144632674(3) AU/day
The relative uncertainty in these measurements is 0.02 parts per billion (2 × 10−11), equivalent to the uncertainty in Earth-based measurements of length by interferometry.[79][80] Since the meter is defined to be the length travelled by light in a certain time interval, the measurement of the light time for unit distance can also be interpreted as measuring the length of an AU in meters.
[edit] Time of flight techniques
A method of measuring the speed of light is to measure the time needed for light to travel to a mirror at a known distance and back. This is the working principle behind the Fizeau–Foucault apparatus developed by Hippolyte Fizeau and Léon Foucault.
Diagram of the Fizeau apparatusThe setup as used by Fizeau consists of a beam of light directed at a mirror 8 kilometres (5 mi) away. On the way from the source to the mirror, the beam passes through a rotating cogwheel. At a certain rate of rotation, the beam passes through one gap on the way out and another on the way back, but at slightly higher or lower rates, the beam strikes a tooth and does not pass through the wheel. Knowing the distance between the wheel and the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light can be calculated.[81]
The method of Foucault replaces the cogwheel by a rotating mirror. Because the mirror keeps rotating while the light travels to the distant mirror and back, the light is reflected from the rotating mirror at a different angle on its way out than it is on its way back. From this difference in angle, the known speed of rotation and the distance to the distant mirror the speed of light may be calculated.[82]
Nowadays, using oscilloscopes with time resolutions of less than one nanosecond, the speed of light can be directly measured by timing the delay of a light pulse from a laser or an LED reflected from a mirror. This method is less precise (with errors of the order of 1%) than other modern techniques, but it is sometimes used as a laboratory experiment in college physics classes.[83][84][85]
From:http://www.physlink.com/Education/AskExperts/ae22.cfm
Ever since Roemer, there have been many different attempts by different scientists to more accurately measure the speed of light. Here is the brief summary of their names and the values they obtained:
Date ..Investigator....... Method Result.. ..(km/s) (Error)
1849 Fizeau .........Rotating toothed wheel ..313,000 (5000)
1850 Foucault .......Rotating mirror ..298,000 (2000)
1875 Cornu ...........Rotating mirror... 299,990 (200)
1880 Michelson .......Rotating mirror ..2990,910 (159)
1883 Newcomb .......Rotating mirror ..299,860 (30)
1928 Mittelstaedt .......Kerr cell shutter ..299,778 (10)
1932 Pease and Pearson .Rotating mirror ..299,774 (2)
1940 Huttel ..........Kerr cell shutter ...299,768 (10)
1951 Bergstrand .......Kerr cell shutter ..299,793.1 (0.3)
The units of energy are joules, or kg m^2/s^2. So the units on E=mc^2 work out - mc^2 is mass times a velocity squared, or kg (m/s)^2=kg m^2/s^2, same as energy. E=mc^3 would be impossible because the units on the right side - kg m^3/s^3 are different than those on the right side.
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