lightspeeds: Historical Determinations of the Speed of Light

Description Format Details Author(s) Source References See Also

Description

Contains historical determinations of the speed of light from many studies from Fizeau's toothed wheel in 1849, to determinations using stabilized lasers in 1983. Methods, year of study, authors, information on mechanisms used, as well as other remarks are recorded. The estimated speed of light is recorded for each study as well as the authors' determination of the error of their measurement.

The data are of particular value since in 1974 the speed of light was defined to be 299,792.458 kilometres per second (in vacuo). The data therefore provide a rare case where the 'true value' is known.

Also, the values might be grouped by the different methods were used over time to estimate the speed of light. In this way, the data provide a useful case study to discuss methods of meta-analysis as well.

Format

A data frame with 81 rows and 11 variables

method

The general method used to determine the speed of light (see details for more on this).

year

Year in which the determination was made.

researcher_1

First researcher named as conducting the study (surname, or prefixed with initials if surname not unique).

researcher_2

Second researcher named, if any.

researcher_3

Third researcher named, if any.

researcher_4

Fourth researcher named, if any.

mechanism

Type of mechanism used (see details below).

mechanism_2

More detail on mechanism used (see details below).

remark

Remark on some detail of the study or method used.

speed

The determined speed of light in air in kilometres per second.

error

The error of the estimate (in km/second) as reported by the researchers (see details below).

The row order of the values follow their order of appearence in the paper given as reference below.

Details

See reference for details.

On the meaning of the error variable, from the reference: "This error is rarely a standard deviation. Nor is it based solely on measurements taken in the study. Instead, it is a number out together bt the researchers from a number of possible sources and is very subjective. It is not uncommon for subsequent researchers to examine in detail the results of a given study and to arrive at a different value of the error. Finally, physicists are accustomed to reporting the probable error which can be interpreted as approximately 0.6745 times the standard deviation of the estimate."

On the meaning of variables related to method, also from the reference:

Optical: These methods are based on having a light beam leave a source, strike a rotating mirror or pass through the spaces of a toothed wheel, travel some considerable distance to be reflected back (again striking the rotating mirror or passing through the spaces of a toothed wheel) to near the original soource. The speed of rotation must be just right and is used in the determination of the speed of light. Precision of estimation could ve increased by increasing the distance the light had to travel (from source to stationary mirror and back) or by increasing the speed of rotation.

Electrical: This method is introduced after the electromagnetic theory of light was developed. Light could now be thought of as electromagnetic radiation. As such measures of the speed of any electromagnetic radiation in vacuo would also be legitimate measures of the speed of light (in vacuo). Moreover the ratio of electrostatic to electromagnetic units of measurement of electrical quantities could be taken to be measurements of the speed of light.

Electro-optical: These are based largely on the same principle as the toothed wheel in optical methods which effectively use a mechanical shutter to switch light on and off. With the electro-optical methods, non-mechanical shutters are used to much more rapidly (and in a more finely controlled way) alternate the light and so increase the precision. The Kerr cell consists of two electrodes immersed in a liquid like nitrobenzene. When high voltage is applied to the electrodes, the polarity of light passing through the cell changes from planar to elliptical. Switching between high and low voltage effects the shutter. The quartz modulator passes sound waves through a crystal to change its refractive index. An acoustic frequency can be found to produce a diffraction grating for light passing through the crystal; double that frequency and the diffraction grating dissappears. Switching between the two frequencies produces the shutter effect.

Radio wave: Understanding light to be electromagetic radiation also means that radio waves can be used in place of visible light to make measurements of its speed. Radar sends a short pulse of high frequency radio waves from a source to a distant object and measures the time taken to receive the reflected wave from the distant object. Knowing the actual distance allows a determination of the speed of the radio wave (or light). The cavity resonator sends high frequency radio waves down a hollow cylinder sealed at both ends. The cylinder resonates if its length is a whole number multiple of the half-wavelength of the radio wave. The speed of light (in the medium within the cylinder or cavity) can be determined from the dimensions of the cylinder (cavity).

Geodetic: These are improvements on the Kerr cell technology to make it capable of measuring geodetic distances. The resulting (commercial) instrument was called a Geodimeter. With known distances these instruments could be turned around to be used to provide measures of the speed of light. The Tellurometer was another instrument invented to determine geodetic distances. The principal difference between it and the Geodimeter is that it used microwave radiation to carry the signal.

Spectroscopy: Bombarding molecules with electromagnetic radiation causes them to absorb enough energy to change various states. Bombarding molecules with microwave radiation changes their rotational state, with infra-red radiation their vibrational state. Quantum theory allows the measurement of these to changed states to be turned into a determination of the speed of light. Different studies bombarded different molecules.

Ultrasonic modulation: This method can be regarded as an improvement on the quartz modulator. Instead of acoustic waves on a crystal, a diffraction grating is produced with ultrasonic waves in a liquid. Turning the diffraction grating on and off produces the shutter.

Interferometry: A single source light bean is split in two. Each travels some distance, is reflected, and returns to the source. The two are made to travel different distances and the amount by which they are out of phase with one another upon return, together with the difference in distances travelled, can be turned into a measure of the speed of light. Instead of visible light, radio waves or micro waves were used.

Stabilized lasers: In interferometry there can be some uncertainty in the measure of the wavelengths used. With stabilized lasers using a technique called sub-Doppler saturated absorption spectroscopy it became possible to fix the frequency (and hence the wavelength) of some lasers within a very narrow range of the electromagnetic spectrum. Such lasers are called stabilized lasers and have nice short wavelengths (micrometres) that allow more precise measurements of the speed of light.

Author(s)

R.W. Oldford

Source

https://www.researchgate.net/publication/275521939_The_speed_of_light_A_case_study_in_empirical_problem_solving

References

R.W. Oldford 1994, 'The speed of light: A case study in empirical problem solving', Unpublished manuscript. <doi:10.13140>

See Also

michelson_1879


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