The document outlines the historical development of the understanding of the speed of light, beginning with Ole Römer's calculations in 1676 and highlighting contributions from scientists like Christiaan Huygens and James Clerk Maxwell. It emphasizes that the speed of light is now regarded as a constant in a vacuum, measured with high precision over centuries. Additionally, the speed of light has been adopted as a standard for measuring length and time in the International System of Units.

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LIGHT SPEED CONSTANCY
Cosmic Adventure: 6.01

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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.

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But is this
speed a
constant ?

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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.

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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.

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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.

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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.

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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.

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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.

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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

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

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

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The speed of
light is the
most
constant
dimension in
the universe.
Albert Einstein. 1967.

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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.

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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).

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SPEED OF LIGHT & OBSERVATION
To be continued on: Cosmic Adventure 6.02