The Speed of Light series consist of five parts. Quick access links are here.
Part 1 | Earliest Ideas
Part 2 | The Eclipses of Io
Part 3 | Chopping Light Beams
Part 4 | Done With Mirrors
Part 5 | Michelson and Morley
In this post, I describe some of the earliest ideas people had about light and whether it had a measurable speed. In subsequent posts, I will describe some of the attempts throughout history to measure this incredible speed. I hope you will join me.
You Must Obey The Speed Limit
The universe has a speed limit. In a vacuum, the fastest anything can travel is 299,792,458 meters/second. This speed limit applies to all electro-magnetic E-M waves which includes radio, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.
So…How Fast Is That, Really?
The speed of light 299,792,458 m/s is a number that is hard to comprehend. It is too big. It needs to be put into some other contexts to make it more meaningful. Here are a few examples of what I mean. Light is a term used to refer to any E-M wave.
- Light goes about 1 foot in 1 billionth of a second. What you see across the room is a few nano-seconds old.
- It takes about 1.3 seconds for light or radio to get to the Moon. Hence delayed responses of Apollo astronauts in the 70’s.
- Light takes 8.3 minutes to reach us from the Sun. If it blew up, it would take 8.3 min. to see it happen.
- It takes about 35 minutes for a radio signal to reach Jupiter when we are closest to it.
- Signals from the Voyager 1 & 2 spacecraft take 16 and 14.5 hours respectively to reach Earth.
- Our nearest star neighbor, Proxima Centauri, is 4.2 light years away.
- At the speed of light, it would take over 25,000 years to reach the center of our Milky Way galaxy.
- Light takes about 2.5 million years to reach us from our nearest neighbor galaxy Andromeda.
- The farthest and oldest light we can see has traveled about 13 billion years to reach us.
Who First Thought About It?
History records those thoughts by Empedocles. He lived between about 492-432 BC in the area of Acragas on the south coast of Sicily. It was a region of fine Greek culture. Many Pythagoreans moved there when their homeland was attacked. He was described as a philosopher, poet, a seer, physicist, and a social reformer. According to Sarton, ‘a man of so much enthusiasm that he would easily be considered a charlatan by some people, or become a legendary hero in the eyes of others‘.
He is attributed with the concept of the four elements of nature: earth, air, fire, and water. He was a vegetarian. He claimed magical powers including the ability to raise people from the dead. He had an early simple version of Origin of Species in biology. It was not the same as that of Darwin.
His ideas about light and how it must travel are most pertinent here. He believed that it did require some time to go from one place to another. It did not travel instantly. In the words of Aristotle:
Empedocles says that the light from the Sun arrives first in the intervening space before it comes to the eye, or reaches the Earth. This might plausibly seem to be the case. For whatever is moved through space, is moved from one place to another; hence, there must be a corresponding interval of time also in which it is moved from the one place to the other. But 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.
Of course, he gave no value for the travel speed of light. He only assumed through logic and reason that it must not be instantaneous. Such was the tradition of Natural Philosophy. Experimentation did not come until much later in history.
The view held by Empedocles was not shared by Aristotle (384-322 BC). In the absence of evidence, he believed light to travel at infinite speed. In his words…’light is due to the presence of something, but it is not a movement‘. From his writings in On the Soul: Book II, he states that such an idea strains our powers to believe.
Empedocles was wrong in speaking of light as `travelling’ or being at a given moment between the earth and its envelope, its movement being unobservable to us; that view is contrary both to the clear evidence of argument and to the observed facts; if the distance traversed were short, the movement might have been unobservable, but where the distance is from extreme East to extreme West, the strain upon our powers of belief is too great.
The view of infinite speed of light was held by many and lasted until the scientific revolution in the seventeenth century. Francis Bacon (1561-1626), Johannes Kepler (1571-1630), and René Descartes (1596-1650) were believers.
Enter Galileo Galilei (1564-1642)
In some of his writings, Galileo used a dialogue between characters to discuss concepts and reason them through for the reader. Here is an example. Galileo believed light to have a finite speed. He felt it should be something measurable given the proper type of experiment.
Sagredo: I cannot believe that the action of light, however pure, can be without motion, and indeed the swiftest.
Salviati: But what and how great should we take the speed of light to be? Is it instantaneous perhaps, and momentary? Or does it require time, like other movements? Could we assure ourselves by experiment which it may be?
He proposed having two shuttered lanterns on hills widely separated by a few miles. The shutter of lantern A would open and close signaling lantern B to be opened. A person at lantern A would time the duration of the event to show it was not instantaneous.
Unfortunately, light travels so fast that results would be inconclusive. Galileo claimed to have tried it for a distance of about a mile. All he could conclude was that it was exceedingly fast, maybe 10x faster than sound.
In a following post, we will see one of the first ‘successful’ attempts to measure light’s speed. I hope you will stay in touch.