Speed of light [1 of 4] Concepts in Ancient Greece

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THE SPEED OF LIGHT

Iris – the goddess of light


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


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 Odysseyprevalent 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).


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.


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.


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 Odysseyand 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.


IN ANCIENT GREEK PHILOSOSPHYSpeed of Light


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


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.


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.


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.


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.

∞


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.”


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.


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.


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


Pure SpeculationThere 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


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.


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


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.


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.

Science

Speed of light [1 of 4] Concepts in Ancient Greece