Speed of Light | Earliest Ideas

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.

Wikimedia Commons

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?

Wikimedia Commons

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.

Aristotle Disagrees

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.

Next Time…

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.


31 thoughts on “Speed of Light | Earliest Ideas

  1. I’m enjoying this subject, Jim. I’ve studied lots of science, but the history of science only incidentally. Galileo, for instance, was even more ahead of his time than I knew. Knowing some of what’s ahead just spices the anticipation! Keep a go’in’.


    • It has been one of my favorites. We often mistakenly assume humans got smarter in the more modern era. Well, I don’t think that is necessarily true. Humans have been smart all along. What has changed is some of the tools we get to use to enhance what new things we know.

      You might enjoy reading Galileo’s Daughter by Dava Sobel. Perhaps you already have. It is a wonderful book about him, his daughter, and the times. It is taken directly from letters between the two.


  2. Wow, those are some pretty mind-blowing examples of how fast light travels. They defiantly help me to wrap my head around a number which otherwise would be inconceivable to me.


    • I remember watching Apollo 11 during the Moon landing. I noticed the 3 second delays between Mission Control questions and the answers. It seemed so awkward. Then, I realized how far it was and what was going on.

      Seeing is a form of time travel. Very cool.

      Thanks for stopping by.


  3. I never really “got” the concept of a speed of light until I realized why it takes so long to hear thunder after a lightning flash. So quick in comparison. But they both have to get from there to here. No wonder Aristotle et al assumed it was instantaneous. And all the more remarkable others were able to work out that it had to travel. This is a great topic. I look forward to the next installment.


    • I first noticed the difference in what I saw and heard by watching Dad drive in steel posts from a quarter mile away. It was a sort of revelation.

      The next installment is coming along well. It is a case where the thing being studied led to a discovery in a very different direction.

      See you then.


  4. I’d say our human brains evolved in such a way that we have a hard time internalizing large numbers, whether they’re velocities (as in the speed of light), lengths (as in interstellar or intergalactic distances), or whatever. The same holds true for money. Take the US national debt, which is currently over $17.5 trillion. Very few of us have any real feel for such a large amount of money. For example, even ignoring the inevitable increase in the debt as well as the interest paid on the debt, if we could reduce just the current amount by a million dollars a minute, it would still take over 33 years to pay it all off.


  5. Jim, I’m so pleased to meet you. I have 4 questions, if you don’t mind answering. I had this curiosity as to what the term “when light is projected into a vacuum” means. Wiki says:
    “Maxwell described light as a very special kind of wave — one composed of electric and magnetic fields. The fields vibrate at right angles to the direction of movement of the wave, and at right angles to each other. Because light has both electric and magnetic fields, it’s also referred to as electromagnetic radiation. Electromagnetic radiation doesn’t need a medium to travel through, and, when it’s traveling in a vacuum, moves at 186,000 miles per second (300,000 kilometers per second). Scientists refer to this as the speed of light, one of the most important numbers in physics.”

    1)What does the term “when light is projected into a vacuum” mean?

    2)Another question I have is what is meant when it says “Electromagnetic radiation doesn’t need a medium to travel through”?

    3)When Einstein said: “According to the assumption to be contemplated here, when a light ray is spreading from a point, the energy is not distributed continuously over ever-increasing spaces, but consists of a finite number of energy quanta that are localized in points in space, move without dividing, and can be absorbed or generated only as a whole”. What did he exactly mean?

    4)In understanding the “Twin Paradox”, is the concept of time ‘distorted’ when traveling at the speed of light? Is time therefore ‘dwarfed’ by gravity? Does time go by faster in space?


    • I will try to answer your good questions. Let me know if you need more information.

      1) Light is able to travel through transparent materials like glass and water. It can also travel through the space when nothing is present – a vacuum. It travels fastest through vacuum because no other particles are there to slow the progress.

      2) A wave on a long rope travels upon the medium of the rope. Water waves need water as their medium of travel. Sound needs air. Earthquake waves travel through the medium of the ground. Light does not need anything for it to travel.

      3) When light travels from a source, it does so in small packets, or bundles, of energy. Each bundle, or packet, does not spread out and get less energetic. It is somewhat like spraying paint from a can. The paint comes out in tiny droplets, or bundles, or packets, that travel to their destination. They will spread apart from each other packet. But, the individual packets each don’t spread out.

      4) This one is more difficult to explain in a brief way. Assume you have a twin. Your twin is sent on a long space journey at 86% of the speed of light. You wait 10 years for their return. Upon arrival back home, you compare your ages. You aged 10 years. Your traveling twin aged only 5 years.

      I hope this helped answer your questions.


    • @ Maria F,

      I too have grappled with questions about the speed of light and relativity and would venture to add my 2 cents to Jim’s explanation of the twins paradox. It is of course, like Jim says, but it’s important to note that each twin in this purely hypothetical thought experiment would separately experience time at its normal rate, so it is not a case of aging slowing down for the traveling twin. It’s merely that their frames of reference now differ by 5 years. That in turn is because time and space are not separate from one another, they are two aspects of the same thing. Some call it “spacetime”.

      Also, I would add, barring some very unlikely change in fundamental knowledge of physics, like, say, wormholes in space, there is no chance of any person actually experiencing a twins paradox. This is because there isn’t a powerful enough energy source known that could accelerate a traveler close to the speed of light and, even if there were, the motion of the craft would convert particles in space to enormous and deadly radiation.


      • Thanks Jim. I understand the concept of “time and space are not separate from one another, they are two aspects of the same thing”, and I see how the ‘twins paradox’ could be so hypothetical, as you say, “the motion of the craft would convert particles in space to enormous and deadly radiation.” I’m just marvelled at the whole concept of gravity. I know gravity plays a part in this twins paradox theme because time is slowed down at earth, is that not?


      • Time is affected by both high speed and by the strength of gravity. Those two are explained in the Special Relativity and General Relativity theories respectively.


  6. Thanks for answering! Regarding #4, I’m still trying to grasp how Einstein was relating the concept of the speed of light to time. Wikipedia says about relativity: “Measurements of various quantities [light] are relative to the velocities of observers. In particular, space contracts and time dilates.” What is this time dilation about?


    • That means that moving clocks tick more slowly that the stationary clock of an observer. Time does not pass the same for all observers. To see the effect, speeds near c must be obtained. Otherwise, the effects are not visible to us.

      You are not the only person who has trouble grasping what Einstein was saying. 🙂 You are in good company.


  7. Thanks for the explanation. The idea of the ticking clock helps to understand. It is poignant to think that the whole notion of moving already defies gravity and its challenges. Now how is time measured when you’re in space? Is time then measured the same as when you’re on earth? I ask because in space one is no longer in the solar system. How then do humans measure time, without a 24 hour time frame based of the earth’s orbit? I know this is all so hypothetical.


      • I finally found the Wikipedia article that I read at some point. It’s under “Time Dilation”. “Gravitational time dilation is at play for ISS astronauts too, and it has the opposite effect of the relative velocity time dilation. To simplify, velocity and gravity each slow down time as they increase. Velocity has increased for the astronauts, slowing down their time, whereas gravity has decreased, speeding up time (the astronauts are experiencing less gravity than on Earth).” This is all so complex. I’m grateful you taught me the concept of “spacetime”, which I did not know.

        “Clocks on the Space Shuttle run slightly slower than reference clocks on Earth, while clocks on GPS and Galileo satellites run slightly faster. Such time dilation has been repeatedly demonstrated for instance by small disparities in atomic clocks on Earth and in space, even though both clocks work perfectly (it is not a mechanical malfunction). The laws of nature are such that time itself (i.e. spacetime) will bend due to differences in either gravity or velocity – each of which affects time in different ways”.

        I also found this:
        “Spacetimes are the arenas in which all physical events take place—an event is a point in spacetime specified by its time and place. For example, the motion of planets around the sun may be described in a particular type of spacetime, or the motion of light around a rotating star may be described in another type of spacetime. The basic elements of spacetime are events. In any given spacetime, an event is a unique position at a unique time. Because events are spacetime points, an example of an event in classical relativistic physics is (x,y,z,t), the location of an elementary (point-like) particle at a particular time. A spacetime itself can be viewed as the union of all events in the same way that a line is the union of all of its points, formally organized into a manifold, a space which can be described at small scales using coordinates systems. A spacetime is independent of any observer”.

        I suppose this kind of explains some of the curiosity I had about this matter.


      • I appreciate that you have taken the time to do some digging into a rather difficult and non-intuitive concept. Good for you.

        I hope I can plant some more seeds of interest for you in my posts.

        Have a good week… Jim


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