CA 6.01 Light Speed Constancy

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LIGHT SPEED CONSTANCY

Cosmic Adventure: 6.01


Velocity of light in celestial scale

In 1676, Ole Römer (1644-1710) became the first scientist who calculated the speed of light (𝑐) by astronomical observations.

This discovery was the first conclusive proof that the speed of light was finite, putting an end to the reign of Aristotle’s idea of infinite light speed for nearly two thousand years.

From then on, the speed was further refined by other experiments and was deemed to be constant.

Ole Römer, 1644-1710 , Danish Astronomy known for his determination of the speed of light in 1676.


But is this

speed a

constant ?


Constant Speed with Waves

In the 1600s, the Dutch scientist Christiaan Huygens (1629-1895) proposed that light is a kind of wave propagating in a medium. The speed of light therefore depends on the nature of the medium. If the medium is homogeneous, the speed of light should be a constant.


Constant Speed with Particles

Sir Isaac Newton (1643-1727) believed that light is made up of particles instead of wave, and, as particles, once let loose in space, will assume a constant speed in a vacuum.


Energy Packets

In the twentieth century, Max Plank (1858-1947) postulated that light was emitted in discrete packets of energy like particles.

As particles, the speed of light should be a constant in a vacuum.


Electromagnetic Constant

In 1865, the British physicist James Clerk Maxwell (1831-1879) made one of the greatest discoveries in the history of science. His equations for electromagnetism had led to a constant speed 𝑐 of electromagnetic waves propagating through space and this constant speed is just the speed of light. This constituted the establishment of the constant light speed purely by theoretical work.


Successive Measurements

At the same time, the measurements of light speed have been incredibly precise over the last century, achieving more than a certainty of 99.7%.

It is a generally concluded that the speed of light has not changed at all.


Measurements through the Centuries

Date Scientist Country Experimental Speed Uncertainty Error

Method (108m/s)

1600 Galileo Italy Lanterns and shutters "Fast" ?

1676 Roemer France Moons of Jupiter 2.14 ? 28%

1729 Bradley England Aberration of Light 3.08 ? 2.70%

1849 Fizeau France Cog Wheel 3.14 ? 4.70%

1879 Michelson US Rotating mirror 2.9991 75000.0 400 in 106

Michelson US Rotating mirror 2.99798 22000.0 18 in 106

1950 Essen England Microwave cavity 2.997925 1000.0 0.1 in 106

1958 Froome England Interferometer 2.997925 100.0 0.1 in 106 1972 Evenson US Laser Method 2.997924 1.1 2 in 109

1974 Blaney England Laser Method 2.997924 0.6 3 in 109

1976 Woods England Laser Method 2.997924 0.2 3 in 109

1983 International 2.99792458 0.0 Exact

Data From History of the Speed of Light by Jennifer Deaton and Tina Patrick. 1996.


𝑐 = 299,792,458 𝑚 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑

= 186,000 𝑚𝑖𝑙𝑒𝑠 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑

≈ 300,000 𝑘𝑚/𝑠 ≈ 300,000,000 𝑚/𝑠

1 𝑠𝑒𝑐𝑜𝑛𝑑

299,792,458 𝑚

Speed of Light


Light Speed Constant

It has also been verified by numerous experiments that: light travels at a definite speed c in a vacuum . . .


. . . and this speed applies to the entire spectrum of light. [Wikipedia]


The speed of

light is the

most

constant

dimension in

the universe.

Albert Einstein. 1967.


Light Speed as Standard Length and Time

Since the speed of light is believed to be constant, the International System of Units decided to make it a standard of measuring length and time.

The committee selected two specific waves from an emission spectrum of caesium-133 to define the second and the meter.


c in Classical Physics

So in classical physics, the speed c has been regarded as an universal constant, ranked in the same category as the other great constants such as h (Planck’s constant) or e (electron charge).


SPEED OF LIGHT & OBSERVATION

To be continued on: Cosmic Adventure 6.02

CA 6.01 Light Speed Constancy

Science

Transcript of CA 6.01 Light Speed Constancy