History Of The Speed Of Light

History of the speed of light

Star trails as a result of diurnal motion

As soon as you open your eyes the world around you floods in; every object seems to be reflecting light at the retina, giving a picture of the world at an instant. With light traveling at what seems infinitely fast, it is no shock that Aristotle and lots of other philosophers and scientists thought light travelled ‘without movement’. However, with a bit more thought given from the Greek philosophers towards the nature of light, a debate about its speed of travel ensued. This debate about the speed of light continued for thousands of years.

Different thinking

Names such as Euclid, Kepler and Descartes, were all sat in one corner believing that light travelled infinitely fast. In the other, Empedocles and Descartes, separated by almost two millennia, felt that light must travel at a finite, if extremely high, velocity.

The reasoning

Empedocle’s reasoning was elegant, pre-dating Aristotle by a century. He took into consideration the large distance from the Sun to Earth, and took to note that all that travels has to move from one point another. That light must be somewhere between the Sun and Earth after it leaves the Sun and before it reaches Earth. Which means it must travel with a finite velocity. This argument was dismissed by Aristotle who had the idea in mind that light in simply a presence, not something that moves between things. Without any scientific evidence behind ether, it is quite a challenge to settle on position by just thinking about it.

Testing the speed of light

Galileo put things to the test by setting out to measure the speed of light with just 2 lamps. He took control of one and sent his assistant a good distance away with another. When in position, Galileo opened a shutter on his lamp, realising the light. As soon as his assistant saw the flash, he then opened his shutter, and Galileo did his best to note down the delays between the opening of his shutter and his own making of his assistance response. After his good efforts he came to the conclusion that light must travel extremely rapidly, as he was unable to determine its speed.

However Galileo was able to put a ‘limit’ on the speed of light, saying it must be at least 10 times faster than the speed of sound. His reasoning, because if it had been any slower, he should have been able to measure the time delay. So it was not deemed a ‘no result’ on whether it was an inability to measure the speed of light, but revealed that light travels faster than his experiment could quantify.

The first person to perform an experimental determination on whether the speed of light was not infinite was done by Danish astronomer, Ole Romer. Back in 1676, Romer was attempting to find reasoning behind one of the biggest scientific and engineering challenges of the of the time, telling the time at sea. Finding an accurate clock out at sea was an essential to ensure sailors safe navigation across oceans, but mechanical clocks that worked around pendulums or springs were no good, as they could not handle being bounced around on the ocean waves, and would soon fall out of correct timing. To pinpoint position on each you need both the latitude and longitude. In the northern hemisphere, latitude is easy; the angle of the North Star (Polaris) above the horizon is your latitude. However in the southern Hemisphere, there is a different story as there is no star directly over the south pole, but with a little astronomical and trigonometry knowledge behind you it is still possible to determine your latitude with sufficient accuracy for safe navigation.

History of the speed of light

The Sun from Earth

Light and time

Longitude on the other hand is a lot more difficult because you can’t work it out by simply looking at the stars; you must know which time zone you are in. Greenwich in London is defined as zero degrees longitude; as you travel west from Greenwich across the Atlantic, your time zone shifts so that in New York it’s earlier in the day than in London. Conversely, as you travel east from Greenwich your time zone shifts so that in Moscow or Tokyo it’s later in the day than in London.

Your precise time zone wherever you are on the Earths surface is defined by the point at which the Sun passes an imaginary arc across the sky between the north and south points on your horizon, passing through celestial pole. Astronomers call this arc the Meridian. The point at which the Sun crosses The Meridian is also the point at which it reaches its highest position in the sky on any given day as it journeys from sunrise in the east to sunset in the west. We call this time noon. Earth rotates once on its axis every twenty-four hours – fifteen degrees every hour. This means two points on Earth’s surface that are separated by fifteen degrees of longitude will measure noon exactly one hour apart. So to determine your longitude, set a clock to read 12 o’clock when the Sun reaches the highest point in the sky at Greenwich. If it reads 2pm when the Sun reaches its highest point in the sky where you are, you are thirty degrees to the west of Greenwich. Easy, except that you need a very accurate clock that keeps time for weeks or months on end.