A brief history into the search for the speed of light. Its significance and problems.

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THE SPEED OF LIGHT
Iris – the goddess of light

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Extraordinary speed
Most of us are familiar with the
fact that light travels at a
tremendous speed about 3⤬1010
cm. per second. It can travel
round the world seven and a
half times in a second – faster
than superman!
Compared with the speed of
objects observed in our daily
lives, this figure is indeed
astronomical. That is why for all
practical purposes, the speed of
light is regarded as infinite.
However the ancient Greeks
seemed to have the right idea.
Seven and a half times round the
world in a second

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Idea of Light in Greek
Mythology
The idea that light has a finite speed was long
existent in the mind of the ancient Greeks. In fact,
the nature of light was already truthfully depicted
in the great ancient epics of Iliad and Odyssey
prevalent as early as about the 9th century BC.
Homer was traditionally regarded as the author of
both epics. As he was supposedly blind, he would
have known what light was either in the time
when his eyes could still see, or through the
descriptions by his fellow Greeks. Anyway, it is
widely recognized that these Homeric epics has
become the foundation of early Greek culture and
Homer’s idea on light should be representative of
the traditional concept at the time.
The blind poet Homer and his guide - Painting of by
William-Adolphe Bouguereau (1825–1905).

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Iris – the Goddess of Light
In the Homeric poems, light was personified
by Iris who is goddess of light and a
messenger of the gods. She carried messages
around Olympus, from gods to gods, and
from gods to men.
Iris appeared in ancient Greek vase painting
as a beautiful young woman with golden
wings to signify her fleetness. She is often
seen as a flying maiden carrying a herald's
wand (an kerykeion) in one hand, and
sometimes a water-pitcher (oinochoe jug) in
the other. The herald’s wand is the precursor
of the caduceus and the pitcher contained
nectar with which she served Zeus and Hera.

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Iris – the Goddess of
Rainbow
In Greek mythology, Iris is also
regarded as the goddess of the
rainbow.
In the Homeric poems, Iris did
not appear as the goddess of the
rainbow, but the rainbow itself is
called iris (Il xi. 27, xvii. 547).
Iris is able to change shapes.
When she delivers messages to
mortals, she would usually
assume the appearance of a
mortal known to the message
receiver.

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Iris replaced by Hermes
As Iris worked as the messenger for the gods and men, she needed great speed.
However Iris only appeared in the Iliad and her figures on Greek vases. In Odyssey
and in Roman times, she was replaced by Hermes. But one thing is for sure - the idea
that light has a finite speed has long been existent in the mind of the ancient Greeks.

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IN ANCIENT GREEK PHILOSOSPHY
Speed of Light

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The Four Elements
The idea of light with a finite speed
was finally shed its mythological
shroud when it was first supported
with philosophy by Empedocles of
Aragas (492-432 BC). He was best
known as the Greek philosophers
who advocated the idea of the four
classical primary elements - earth,
fire, water and air. According to him,
the world we live in, including
ourselves were made of these four
basic elements. He believed that
Aphrodite (Goddess of Love) made
the human eye out of the four
elements. She lit the fire in the eye
so that light would come out of the
eye and made sight possible.
Empedocles and his four cosmogonic elements

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Empedocles of Aragas
Empedocles later found that
sight is not that simple and
postulated that the images in
their various colors from the
world outside the body were
carried by the light from the
objects to the eyes. This light,
according to him, was also one
form of the four elements, that
is, it is a kind of matter in a
specific form under
transportation.
Since all material objects took
time to travel from one place to
another, light should be no
exception.

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Image in between
Aristotle (385-322 BC) thought
that if light took time to travel,
"any given time is divisible into
parts, so that we should assume
a time when the sun's ray was
not as yet seen but was still
travelling in the middle space ...
before it reaches the earth." De
sensu and De anima.
With finite speed, there are
bound to be images in between
the object and the observer. The
observer sees the image a
moment later. But this did not
appeal to Aristotle.

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Aristotle
This does not mean that Aristotle
was supportive of Empedocles’
idea of a finite speed, he was only
expressing the unlikely situation of
having light in between the object
and the observer when every
vision is so instantaneous. He is a
strong advocate for the infinite
speed of light. He quoted
Empedocles simply for the
purpose of criticism. He often did
this as his favorite way of putting
down his opponents to make way
for his arguments.

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Infinite Speed
The second main idea of a
different nature about the speed
of light came from Plato (429–
347 BC) and was augmented by
Aristotle (384 – 322 BC).
Aristotle favored the idea of an
infinite speed - that light
traversed space in no time at all.
Once light is emitted from the
source, its effect is to be felt by
the receiver instantaneously.
∞

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Lucretius – Infinite
Speed
Even if light is made up of
particles, these particles will
travel across space in not time.
Lucretius (ca.99-55 BC Roman
poet), furthered this idea in his
epic philosophical poem De
Rerum Natura (On the nature of
the Universe - 55 BC):
“The light and heat of the sun
are composed of minute atoms.
When shoved off, they lose no
time in shooting right across the
interspace of air in the direction
imparted by the shove.”

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Heron’s Night Vision
In the first century BC, Heron of Alexandria (c. 10-70 AD the greatest inventor and
experimenter of antiquity) believed in the theory that one could see because light was
emitted from the eye to the object. When the light bounced back to the eye, vision was
established. This was evident to him because when he closed his eyes at night time and
opened again, he could see the stars immediately. This only meant that light took no time in
travelling from his eyes to the distant star and back.

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Heron’s Night Vision
Heron thought in such a way
because he was already
prejudiced by the idea that light
rays were issued only when he
opened his eyes in the first
place; secondly he did not
realize that the light rays from
the stars were already there in
front of his eyes even before his
eyes were open. As soon as his
eyelids were lifted, the images
of the star entered immediately,
giving him the impression of an
instantaneous transfer of image.

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Heron’s Stone Throw
By pure reasoning, Heron further argued that one can find conviction in the analogy
of objects falling freely after release. When an object is thrown horizontally, it first
travels in a straight line and then drops to the ground (a, b, c). The harder is the
throw, the longer will its path stay horizontal. If the object is thrown with an infinite
velocity, it would keep on moving in a straight-line forever (d). Similarly, for light to
travel in a straight line, it must move with an infinite velocity. So, in his book
Catoptrica (Catoptrics) written on light propagation and reflection, and on the use of
mirrors, Heron gave his support to Aristotle’s idea based on such and similar
convictions. The infinite speed is apparent in ordinary life on earth. The speed of
light is too fast to be detected.
Heron’s infinite stone throw

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Pure Speculation
There were also other
schools of thinking on the
speed of light. However as a
general practice, these
ancient Greek philosophers
did not bother to verify their
ideas by observation or
experiment, particularly on
the issue of light speed. They
just entertained themselves
by pure reasoning or
intuitive guesswork and did
not bother to go to the
length of actually measuring
the speed of light.
We cannot say that this is a fault in their way of
thinking, for even if they wanted to, they had no
method or instrument capable of doing so. The
acceptability of the concept on the speed of light
then depended entirely on the soundness of
arguments and the prestige

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Dominance of Infinite
Speed Theories
With Aristotle’s influence and the
support of many renown
philosophers, the idea that light
travels at an infinite speed became
the dominate theory over two
thousand years. Although Galileo
knew something was not going
right with the idea, he also had no
way or instrument with enough
precision to tackle the tremendous
speed of light - not until Ole Romer
(1644-1710) came along with his
accidental discovery in the 17th
century, some two thousand years
after Aristotle.

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Medieval Age
The Middle Ages in European
history lasted from the fall of
the western Roman Empire
(circa A.D. 395) to the
Renaissance in the 14th or 15th
century.
During this period, the
civilisation of the Greeks and
Romans were replaced by
barbarism. Most of the ancient
teachings and their records
were lost or destroyed.
Fortunately some of them were
copied and preserved and
developed in the Muslim
countries. The ruins of Roman civilizations

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Revival of Greek
Learnings
From the thirteenth century onward these
writings came to see the light again in
Europe. They were recovered as rare copies
from forgotten corners in the attic or store
rooms. Some of them were also brought
back from the Muslim countries translated in
Latin.
After emerging from a thousand years of
darkness, the European civilisation was in a
badly retrograded shape. The ancient
knowledge and philosophies appeared so far
superior to theirs that they were treated
with almost superstitious reverence. The
teaching of Aristotle in particular became
the guiding light of the time.

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Authority of Aristotle’s
Teachings
The weight of Aristotle's
teachings were later further
enhanced by St. Thomas
Aquinas (1224-1274 B.C.) who
brought them in line with the
Bible, making them the answer
books to all scientific enquiries.
In the centuries to come
Aristotle's philosophy was
regarded as the ultimate truth
throughout Europe. When
Aristotle said that light travels at
infinite speed, no one else
would have thought otherwise.