The issue of the speed of light and its properties has always been held as an issue of great interest and importance throughout history. However, it is only fairly recently where an accurate measurement has been acquired, largely due to lack of technology and lack of knowledge of the properties of light. Therefore, throughout history, there has been much controversy over the issue of the speed of light, as one shall see in this account.
Naturally, the earliest accounts about the speed of light date back to the time of the Greeks, where the issue was whether the speed of light was finite or infinite. Some, including Empedocles believed that the speed of light must be finite, as it was something in motion though others such as the infamous Aristotle, believed that the speed of light was infinite, who said that ‘light is due to the presence of something, but it is not movement’. One can observe that despite the hypotheses, the level of technology and knowledge was too basic to produce solid evidence, leaving the problem to be unresolved until 18th century. Indeed, Aristotle said himself that ‘the strain upon our powers of belief is too great’.
Perhaps due to Aristotle’s considerable reputation, most early Muslim philosophers agreed with his view, though this did not stop the opposing flow of ideas. Ibn al-Haytham published his Book of Optics in 1021, which indicated that the speed of light, backed by a huge array of experimentation. This shook the scientific community strongly, and caused many scientists to regard the issue with greater importance.
The physics of projectile motion greatly impacted our history. Steve Adams, Jonathan Allday (2000), Advanced Physics (first published), published in. Advanced Physics (English, Paperback) Steve Adams, Jonathan Allday Written by members of the Editorial Board of the Institute of Physics, Advanced Physics makes A-level physics accessible to all students, with Maths boxes throughout to support concept development.
Actual attempts at the measurement of the speed of light began in 1629 with Isaac Beeckman, who observed the flash from a cannon reflecting off a mile away. Needless to say, the experiment was an utter failure, as with many attempts due to the lack of equipment. In 1638, Galileo conducted an experiment using covered lanterns separated by one mile, attempting to see the time the light took to reach the observer one mile away from the lantern. This experiment was very controversial among the scientific community and heavily criticized as no delay between uncovering the lantern to reaching the observer existed.
The first real breakthrough about this concept was done by Ole Roemer in 1676, which was completely accidental, as with the case with many breakthroughs. While he was studying the time for Jupiter’s moon, Io to complete an eclipse, he noticed that his measurements varied considerably, becoming inaccurate, and then strangely becoming accurate again. More specifically, at some times, Io’s exit from the shadow would begin later than predicted, and vice versa.
After a series of observations, Roemer came to the conclusion that those inaccuracies was due to the varying distances between Earth and Jupiter during their respective orbits, therefore, varying the time light takes to travel to earth.
(Taken from: http://www.micro.magnet.fsu.edu/primer/lightandcolor/speedoflight.html)
Roemer’s work influenced later scientists, most notable of them being Newton, who showed his acceptance of finite speed in his book ‘Opticks’ in 1704. Newton’s influence and reputation as a scientist caused the concept of finite speed to be accepted among most scientists, possibly even increasing the amount of measurements, which was certainly what occurred during 18-19th century.
Another great breakthrough was conducted by James Bradley in 1728, producing the first ever quantitative experiment concerning the speed of light. He believed that he could use stellar aberrations to determine the speed of light. Using the knowledge that the degree of stellar aberration could be determined by the ratio of the Earth’s orbital speed and the speed of light, he produced an estimate of the speed of light, measuring the degree to be 1/200? and using the value of the Earth’s orbital speed known at the time. Hence, he produced an estimate of 298,000 km/s which is surprisingly accurate for his time, which effectively destroyed the argument of light possessing an infinite speed.
After that measurement, a series of breakthroughs followed, the most significant being Armand Fizeau’s experiment in 1849, being the first successful artificial quantitative measurement, not relying merely on naturally existing entities such as stars, as his predecessors had done.
Naturally, it was far more complex than the previous experiments but was far more accurate. It involved directing a beam of light several thousand metres away at a glass plate, reflecting it into a mirror which ultimately directs the beam through a rotating cogwheel to an observer. As the light could be blocked by the teeth in the rotating cogwheel, it passing through to reach the observer depended on the rate of rotation. Based on the knowledge of the rate of rotation, distance to the mirror, and the number of teeth on the cog, the speed of light was calculated to be 313,000 km/s. This process was later improved by Leon Foucault who replaced the cog with a rotating mirror as shown in the diagram:
Diagram depicting the Fizeau Foucault apparatus
(Taken: http://upload.wikimedia.org/wikipedia/en/4/4f/Speed_of_light_%28Fizeau%29.PNG )
This experiment of measuring light was held to be extremely accurate and efficient; therefore, many scientists merely developed the technique while keeping the foundation of the experiment the same in later measurements of light, most notable of those Albert Michelson, who dedicated his life in producing increasingly accurate measurements of the speed of light.
In 1878, Albert Michelson increased the accuracy of the technique by 20 times by increasing the distance and quality of the lens in the experiment. He also replaced the rotating mirror with a regular prism. Through this, he managed to formulate an equation for his experiment:
c=2dNf
where:
- c= the speed of light
- d= the distance between the concave mirror and prism
- N= the number of sides of the prism
- f= the lowest frequency of rotation to cause the light to reflect off a different face of the prism to reflect directly at the observer
This method was used at Mount Wilson and Mount San Antonio, as shown in the below diagram, achieving result of 299,909 km/s.
(Taken from: http://smartaxes.com/youngcosmos/v1_0/cx4_clip_image001.gif )
However, at that time, in addition to the speed of light, scientists were also intrigued about the nature of light, which led Michelson to also experiment with the concept of the theory of ether , which he believed in.
As a result, he teamed up with a fellow scientist, Edward Morley, and built an interferometer , to monitor the interferences they expected due to the ether. Although the experiment was a failure, and no interference was observed (as ether does not exist), Michelson used the device to produce an even more accurate measurement of the speed of light being 299,909 km/s. Due to this experiment, and the work of Einstein, the theory of ether was also disproved.
(Taken from http://www.micro.magnet.fsu.edu/primer/lightandcolor/speedoflight.html )
Today, as technology improves, the speed of light reaches a more and more precise value. In 1983, the value for the speed of light was standardized as 299,792,458 km/s, and this value is currently defined as a constant.
In conclusion, the measurements of the speed of light has progressed hugely, beginning from where it was merely debated as either finite or infinite to an actual numerical value. One can observe how this process of discovery was accelerated through the introduction of technology, additional knowledge, the urge to attain greater accuracy, and rare, revolutionary ideas, we aptly call breakthroughs.
References
1. http://www.micro.magnet.fsu.edu/primer/lightandcolor/speedoflight.html Back to cited text no. 1
2. http://en.wikipedia.org/wiki/Speed_of_light#Measurement_of_the_speed_of_light Back to cited text no. 2
3. http://www.discover.ac.uk/sciences/cgi-bin/search.pl?term1=Abu+Ali+Hasan+Ibn+al-Haitham&limit=0 Back to cited text no. 3
4. http://www.discover.ac.uk/sciences/cgi-bin/search.pl?term1=Abu+Ali+Hasan+Ibn+al-Haitham&limit=0 Back to cited text no. 4
5. http://en.wikipedia.org/wiki/Light Back to cited text no. 5
6. http://www.micro.magnet.fsu.edu/optics/timeline/people/alhazen.html Back to cited text no. 6
7. ‘Advanced Physics’, by Steve Adams, Jonathan Allday, page 260 Back to cited text no. 7
2. http://en.wikipedia.org/wiki/Speed_of_light#Measurement_of_the_speed_of_light Back to cited text no. 2
3. http://www.discover.ac.uk/sciences/cgi-bin/search.pl?term1=Abu+Ali+Hasan+Ibn+al-Haitham&limit=0 Back to cited text no. 3
4. http://www.discover.ac.uk/sciences/cgi-bin/search.pl?term1=Abu+Ali+Hasan+Ibn+al-Haitham&limit=0 Back to cited text no. 4
5. http://en.wikipedia.org/wiki/Light Back to cited text no. 5
6. http://www.micro.magnet.fsu.edu/optics/timeline/people/alhazen.html Back to cited text no. 6
7. ‘Advanced Physics’, by Steve Adams, Jonathan Allday, page 260 Back to cited text no. 7