Postby tuppence » Mon Nov 29, 2004 04:03 pm
I'm going to cut and paste a few sections from the chapter on the history of research of the speed of light, from our book which is not yet quite ready to go to the publisher's. I hope this will help. From here on, everything is cut and paste:
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Galileo devised an experiment using lights, shutters, and telescopes to be set up on hills a number of miles apart. The Academia del Cimento of Florence set up Galileo’s experiment using a distance of about a mile. The conclusion they reached was that the speed of light was infinite.
...More was needed. Ole Roemer, born in Denmark on September 25, 1644, had begun studies in mathematics and astronomy at Copenhagen University in 1662. Ten years later, in 1672, he was appointed to the newly constructed observatory in Paris. In 1675, he discovered that the epicycloid was the best shape for teeth in gears and communicated to Huygens that such gears would be advisable in his clocks. This resulted in an improvement so significant that clocks of this caliber made the determination of longitude possible.
With such accurate clocks and the knowledge that Jupiter’s moons eclipsed regularly, it was now be theoretically possible to measure the speed of light, and determine whether or not this speed was really infinite. The eclipses provided a regular astronomical phenomenon that was visible from both a standard observatory and the place whose longitude was to be determined. The Paris Observatory was chosen as the standard.
....During the course of these observations, Roemer noticed something: as the earth drew away from Jupiter, the eclipse times of Io fell further and further behind schedule. However, once the furthest point in our orbit was passed, and the earth began to approach Jupiter, the eclipse times began to catch up again. How could the position of the earth in its orbit affect the time it took Io to go around Jupiter once and be eclipsed? It didn’t, said Roemer. What is happening is that the light carrying the information from the Jupiter-Io system is taking time to travel across the diameter of the earth’s orbit, so the eclipse information takes longer to get to the earth when it is at the furthest point in its orbit.
...It might be expected that with the above information Roemer would pronounce a definitive value for the speed of light (or “c”), but this was not his main purpose. His prime concern was to demonstrate that light was not transmitted instantaneously, but had instead a definite velocity, as evidenced by the observations. In this he eventually succeeded. The main factor that was unknown to Roemer, and that prevented accurate calculation of the speed of light was the radius of the earth’s orbit. Without that knowledge it was impossible to know exactly how far the light had traveled and thus also impossible to determine its speed. Today we know the radius of the earth’s orbit to be 1.4959787 x 108 Km, a value that is adopted in all the following calculations.
[note from tuppence -- even with these earliest observations, the length of a day or hour or minute on earth was not relevant.]
...James Bradley was born in 1693, and educated at Balliol College, Oxford. His astronomical instructor was one of the finest of that period in England, his uncle, the Rev. James Pound. In 1717, Edmond Halley ushered him into the scientific world and by 1721 Bradley had been appointed to the Savilian chair of astronomy at Oxford. He also lectured in experimental philosophy from 1729 until 1760. Upon the death of Halley in 1742, Bradley succeeded to his position as Astronomer Royal.
...Since the motion of the earth in its orbit is essentially constant, Bradley knew that this same relationship could be applied to the figures tumbling through his excited mind, provided he used the appropriate units. The aberration angle, K, he had measure from the stars. If the speed of light was c, equivalent to the rain speed, then as written above Kc = constant. On the accepted figures today the result is Kc = 6,144,402 where K is in arc-seconds and c in kilometers per second. Bradley concluded “that Light moves, or is propagated as far as from the Sun to the Earth in 8 minutes, 12 seconds.” Bradley had confirmed not only the Copernican model for the solar system, but also the hotly debated idea of a finite value for c. His discovery was announced on the first of January, 1729.
...The 63 aberration determinations from 1740 – 1930 listed in Table 4 were made with basically the same type of equipment with essentially the same error margins and substantially the same observational methods. The results from Pulkova Observatory are illustrated in Figure I. A least squares linear fit to all data gives a decay of 5.04 Km/s per year with a confidence of 96.1% that c has not been constant at 299,792.458 Km/s for the period covered by these Bradley-type determinations. These results suggest that the possibility of a decay in c should be examined further.
...The two methods of measuring the velocity of light, c, that have been considered to date have both been astronomical. However, back in 1638, in his ‘Discorsi’, Galileo suggested the basis of a terrestrial experiment over a number of miles using lanterns, shutters and telescopes to timed flashes of light. The Florentine Academy in 1667 tried the idea over a distance of one mile without any observable delay. Just nine years later the reason became apparent: Roemer’s value for c was so great in comparison to human reaction times in operating the lantern shutters that there was no hope of observing the finite travel time delay for c over one mile (or 1609.344 meters).
It was not until 1849 that the French physicist H. L. Fizeau overcame the problem in the following fashion. In the first place it was desirable to have as large a distance as practical involved, instead of just one mile. Fizeau used as his base-line the distance between two hills near Paris, Suresnes and Montmartre, measured as 8633 meters. As he had arrange to observe the returning beam of light, the total distance traveled was thus 17,266 meters. Though this distance was large in comparison with the single mile used with lanterns, it was the second shortest base-line ever used in this type of experiment.
In place of the shutters on lanterns, Fizeau used a rotating wheel with 720 teeth and driven by clockwork made by Froment. Light from an intense source was focused on the rim of the wheel, then made into a parallel beam by a telescope, and traversed the 8633 meters. There it was received by another telescope which focused the beam onto a concave mirror, sending the light back along the same path that it had just traveled. The returned beam was viewed between the teeth in the wheel. The system was focused with the wheel at rest and with the light shining between the gap in the teeth. The wheel was then rotated, automatically chopping the beam into a series of flashes like the lantern shutters.
...In 1874, the Council of the Paris Observatoire, headed by LeVerrier, who was the Observatory Director, and Fizeau, decided to ask Cornu to obtain a definitive value for c. The date was April 2, and the reason was that a transit of Venus was to occur on December 9th of that year. A value for c accurate to one part in a thousand would be needed by astronomers observing the event. Cornu complied with the request.
The sending telescope was mounted on the Paris Observatory and the light flashes sent to the tower of Montlhery where the collimator lens returned the chopped beam. The base-line was 22,910 meters. Four smoked aluminum wheels of 1/10 to 1/15 mm thickness were used. Three had pointed teeth numbering 144, 150, and 200 respectively. The fourth wheel of 40 mm diameter has 180 square teeth. The wheels could be rotated in either direction, which eliminated a number of errors.
The apparatus was powered by a weight-driven, friction-brake controlled device. An electric circuit automatically left a record of wheel rotation rtes on a chronograph sheet advancing 1.85 cm/s. A 1/20 second oscillator was used to subdivide the one second intervals of the observatory clock. Times were estimated to 0.001 second were claimed. The main difficulty in observation was the determination of the exact moment of total eclipse of the returned flash as the background is always slightly luminous. The speed of the wheel corresponding to the disappearance of the beam was noted, as was the speed for its re-appearance and a mathematical averaging procedure was adopted.
... Conclusions from Toothed-Wheel Experiments
Table 5 summarizes the above results by listing the fourteen values obtained by this method. A least squares linear fit to all these data points gives a light speed decline of 164 Km/s per year, while a fit to the most reliable valued, marked [*], gives a decline of 2.17 Km/s per year.
...In 1834, at 32 years of age, Sir Charles Wheatstone of England (1802-1875), after whom the electrical circuitry known as the Wheatstone Bridge is named, entered the discussion on the speed of light. He was the first to suggest the method that incorporated a rotating mirror for the measurement of c. Unfortunately for the history of England in the debate about the value for c, Sir Charles’ suggestion was not taken up by his countrymen. Instead, the French again led the way in pioneer experimentation, following which the lead came under American control.
Sir Charles’ suggestion regarding the rotating mirror was picked up four years later, in 1838, by the noted Parisian astronomer and physicist D.F.J. Arago (1786-1853). (Arago is mainly remembered today for his work on the interference of polarized light, which he investigated in 1811, and electromagnetism in which he worked with Ampere (1775-1836).) He also conducted experiments confirming diffraction that resulted from Resnel’s development of the wave theory of light.
...Foucault’s Rotating Mirror
...Later experimenters overcame ... problems in another way that allowed for a much longer light-path and a vastly increased distance EE’ between images. In Michelson’s work, the lens, L, was of much longer focal length and such that R and M were virtually conjugate foci of L. The source, S, was placed close to R, and with L of appropriate focus, the concave mirror, M, could be placed several miles away. When viewed through the micrometer eyepiece, there is the direct reflection from the revolving mirror and beside it the returned image at a distance dependant upon the rotation rate. For Foucault’s arrangement, the mirror reflected the light back to the eyepiece once every revolution, giving a flickering effect until a high enough rotation rte was achieved. Newcomb and Michelson used mirrors of four or more reflecting faces which also gave a brighter image.
...Newcomb’s Experiments
Newcomb utilized a square steel prism as his rotating mirror, which was 85 mm. long and 37.5 m. square. All four faces were nickel-plated and polished. The prism was rotated about its long axis inside a metal housing that had two open windows opposite each other. Rotation of the prism was effected by a stream of air directed through the windows against the 12 vanes in each of the two fan wheels rigidly attached to either end of the prism. It seems from Newcomb’s Figure 5 of Plate VI that four of the vanes were pointing in the direction of the corners of the prism on the lower wheel. However, the prism corners were midway between vanes for the upper wheel. Dorsey suggested that wheels of 13 vanes may have been better for symmetry and to overcome any potential problems caused by its absence.
A stiff frame carrying the observing telescope swung about an axis coincident with that of the rotating prism. At its other end were a pair of microscopes for reading the deflection. The radius of the arc over which it swung was 2.4 meters. The sending telescope was placed immediately above the observing ‘scope and had as its light source an adjustable slit illuminated by sunlight reflected from the heliostat. Light from the slit was directed to the upper half of one of the rotating prism’s faces. Following its return from the distant concave mirror, the light beam was reflected from the lower half of the same face into the observing telescope.
...All told, there were three distinct series of experiments. Michelson assisted Newcomb for part of the first series from June 28 until September 13, 1880. The series continued until April 15, 1881, by which time 150 experiments had been performed. Of these, Michelson had been involved with 99. The second series went from August 8, 1881, until September 24, with 39 experiments performed. The third series, in which Newcomb was assisted by Holcombe, extended from July 24, 1882, to September 5 of that year for a total of 66 experiments. This made a grand total of 255 experiments.
...Albert A. Michelson was an American physicist born in 1852. Prior to his death in 1931, he had been Professor of Physics at the University of Chicago where many famous experiments on the interference of light were done. He had been an instructor in physics and chemistry at the U.S. Naval Academy after he had graduated in 1873. His Superintendent questioned his “useless experiments” on light that were done while he was there. He continued his light velocity experiments during his ten years at the Case Institute of Technology, and his work was rewarded in 1907 with the Nobel Prize.
...After moving to the Case Institute, Michelson was prompted by Newcomb to continue his investigation into the value of c. This was carried out in 1882, essentially concurrent with Newcomb’s final series. Michelson used virtually the same equipment as that in his second series. The same micrometer, the same rotating mirror, lens, and air drive were used both times. The general optical arrangements were the same. The main differences were the path length of 624.546 meters (the old one was 605.4 meters) and the distant fixed mirror was slightly concave and fifteen inches in diameter compared with seven inches for the old. The same tape was used for measuring as was the previous calibration of the micrometer screw. The new cross-checks and comparisons indicated that all was satisfactory, even for Dorsey.
...With essentially the same equipment, therefore, Michelson obtained 299,802 Km/s for his results in 1925.6, and 299,798 Km/s for his results in 1926.5. This was the second time that two series by Michelson have shown a lower value for c on the second occasion with the same equipment. However, that is not all. Comparison between those two sets of series also shows a drop with time. In other words, four determinations, in two sets of two, show a consistent drop with time through the four, within each of the two related sets, and between the two sets. As noted previously, Michelson’s results alone indicate, on a least squares analysis, a decay of 1.86 Km/s per year over the 47 years of his c experimentation. These last two series suggest about 2 Km/s per year for the decay rate.
...Dorsey (p. 79) noted for Michelson’s work that “Although each series of determinations has yielded a value that differs from each of the others, Michelson has made no attempt in his reports, or elsewhere, so far as I know, to account for these differences.” This statement still holds even if Dorsey’s modified values for Michelson’s work are used. With a persistent drop in values for c by a single experimenter, as well as for all values by any particular method, one would imagine that the simplest explanation that Michelson or Dorsey could offer is that the physical quantity itself is dropping with time. De Brey suggested this, but Dorsey preferred to have his problem unsolved rather than accept that explanation.
...Because of the increasing accuracy and necessity for precise vacuum correction, Michelson decided tin 1929 to initiate what was to be his last final experiment. His collaborators were Pease and Pearson. The idea was that a one mile long pipe would be exhausted of its air and by repeated reflections from mirrors at either end a path length of some ten miles could be achieved in a fair vacuum. Pressure in the pipe varied from 0.5 to 5.5 millimeters of mercury. The arrangement was essentially that shown in Figure V. A carbon arc source was at S focused on an adjustable slit 0.075 mm wide and was reflected from a 32-faced rotating mirror, R. The mirror was a glass prism 0.25 inches long and 1.5 inches along the diagonals of its cross-section. All its angles were correct to one arc second and its surfaces to 0.1 wave.
Light was reflected from the upper half of face ‘a’ of the mirror through a glass window, W, that was 2 cm. thick, into the pipe and via the mirrors Q and N, onto the large optically flat mirrors M and p, 55.9 cm. in diameter. N was essentially a concave mirror 101.6 cm in diameter and of 15.02 m. focus that gave a parallel beam forming and image of the slit on M. After repeated reflections from M and P, the beam was returned through the window to strike the lower half of face ‘b’ of the rotating mirror, if it were at rest, and into the eye of the observer E. The distance from the rotating mirror to the eyepiece micrometer was 30 cm. In operation, the mirror was rotated at such a speed that face ‘a’ would move into the position occupied by face ‘b’ in the time that the light took to be reflected along the pipe and return. Depending on the adjustment of the four fixed mirrors, light could travel eight or ten miles before being returned to R.
The two distances required different rotation rates of the prism. For a distance of 7,999.87 meters, the rotation rate was about 585 revolutions per second, while 6,405.59 meters required about 730 revolutions per second. This rotation rate was again controlled and determined stroboscopically with a tuning fork compared with a free pendulum swinging in a heavy bronze box of constant temperature and low pressure. The pendulum was itself compared with a time-piece that was checked against time signals from Arlington, Virginia.
...Michelson made preparation for this c determination from 1929 until February 19, 1931, when the experiments actually began. The work was sponsored by the University of Chicago, the Mount Wilson Observatory, the Carnegie Corporation and the Rockefeller Foundation.
...Conclusions From Rotating Mirrors
The results of the rotating mirror experiments are summarized in Table 6. If the results rejected by the experimenters themselves are omitted along with Fourcault’s admittedly pioneer experiment which was “intended to ascertain the possibilities of the method,” then a least squares linear fit to the six data points gives a decay of 1.85 Km/s per year. The value of the correlation coefficient, r, equals -0.932, with a confidence interval of 99.6% in this decay correlation.
...The persistent downward trend in the measured value of c was noted by de Bray after Michelson’s 1924 series results became available. As a result, he wrote to the Editor of Nature on the 20th December, 1924, and to l’Astronomie in France on January 23rd, 1925, calling attention to the trend. In the latter case, he predicted a lower value for Michelson’s next determination, which was in the process of being prepared. In the event his prediction was justified. As a result of that circumstance, the Editor of Nature, having ignored his earlier calls, decided to publish de Bray’s next offering, which opened up the discussion in the scientific literature throughout the late twenties, the thirties and into the early forties. Again, as de Bray himself noted, the only values that go against this trend in Table 6 are those that the experimenters themselves have rejected. If the polygonal mirror technique is counted separately, there are now five methods that have demonstrated a decay in the speed of light over time.
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THAT, Jovaro, gives you a very brief indication of how the speed of light has been measured in the past. I hope you can now see why the argument you have been raising about leap seconds, etc., has no bearing on this at all.
born again Christian, non-denominational. Young universe creationist.
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