ARMAND FIZEAU (1819- 96)

1849 – France

‘The first successful experiment to determine the speed of light’



Prior to this experiment it was believed that light had an infinite speed – although in 1676 the Danish astronomer Ole Røemer (1644-1710) had used GALILEO’s 1610 discovery of the four largest moons of Jupiter to describe the way of measuring the speed of light by measuring the times at which the moons were eclipsed by Jupiter itself.
The timing of the eclipses is affected by whether the Earth is on the same side of the Sun as Jupiter, or on the opposite side. Rømer explained the differences in the eclipse timings as due to the extra time required for light from the moons to reach the Earth when it is on the opposite side of the Sun.

Diagram explaining how the speed of light may be determined from observation of the moons of Jupiter from the earth if the distance between the planets is known.

Using modern measurements, it is calculated that it takes light more than eight minutes, traveling at 300,000km per second, to reach us from the Sun, across half the diameter of the Earth’s orbit; so the maximum delay in observing an eclipse of one of the moons of Jupiter is twice that – more than a quarter of an hour.

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Fizeau carried out his experiment in Paris between the belvedere of a house at Montmartre and a hill at Suresnes – a distance of 8.67 kilometres.
He placed a rotating toothed wheel with 720 gaps at Montmartre and a mirror at Suresnes. When the wheel was at rest, light passed through one gap and was reflected. When the wheel was rotated slowly the light was completely eclipsed from the observer. When the wheel was turned rapidly the reflected light passed through the next gap. Fizeau observed this at a maximum speed of 25 revolutions per second. Therefore the time required by light to travel a distance of 8.67 × 2 kilometres was 1/25 × 1/720 of a second. This gave a speed of 312,320 kilometres per second (the correct value is 299,792 kilometres per second).

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ALBERT MICHELSON (1852-1931) EDWARD MORLEY (1838-1923)

1887 – USA

‘The aim of the experiment was to measure the effect of the Earth’s motion on the speed of light’

This celebrated experiment found no evidence of there being an effect.

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1905 – Switzerland

  1. ‘the relativity principle: All laws of science are the same in all frames of reference.
  2. constancy of the speed of light: The speed of light in a vacuüm is constant and is independent of the speed of the observer’
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The laws of physics are identical to different spectators, regardless of their position, as long as they are moving at a constant speed in relation to each other. Above all the speed of light is constant. Classical laws of mechanics seem to be obeyed in our normal lives because the speeds involved are insignificant.

Newton’s recipe for measuring the speed of a body moving through space involved simply timing it as it passed between two fixed points. This is based on the assumptions that time is flowing at the same rate for everyone – that there is such a thing as ‘absolute’ time, and that two observers would always agree on the distance between any two points in space.
The implications of this principle if the observers are moving at different speeds are bizarre and normal indicators of velocity such as distance and time become warped. Absolute space and time do not exist. The faster an object is moving the slower time moves. Objects appear to become shorter in the direction of travel. Mass increases as the speed of an object increases. Ultimately nothing may move faster than or equal to the speed of light because at that point it would have infinite mass, no length and time would stand still.

‘The energy (E) of a body equals its mass (m) times the speed of light (c) squared’

This equation shows that mass and energy are mutually convertible under certain conditions.

The mass-energy equation is a consequence of Einstein’s theory of special relativity and declares that only a small amount of atomic mass could unleash huge amounts of energy.

Two of his early papers described Brownian motion and the ‘photoelectric’ effect (employing PLANCK’s quantum theory and helping to confirm Planck’s ideas in the process).

1915 – Germany

‘Objects do not attract each other by exerting pull, but the presence of matter in space causes space to curve in such a manner that a gravitational field is set up. Gravity is the property of space itself’

From 1907 to 1915 Einstein developed his special theory into a general theory that included equating accelerating forces and gravitational forces. This implies light rays would be bent by gravitational attraction and electromagnetic radiation wavelengths would be increased under gravity. Moreover, mass and the resultant gravity, warps space and time, which would otherwise be ‘flat’, into curved paths that other masses (e.g. the moons of planets) caught within the field of the distortion follow. The predictions from special and general relativity were gradually proven by experimental evidence.

Einstein spent much of the rest of his life trying to devise a unified theory of electromagnetic, gravitational and nuclear fields.

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