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The Speed of Dark

An Experiment from the Second Year Physics Lab

Recent work by researchers working for Lausten Transit established that the light and dark were in a continual battle for supremacy. It was established that dark was nocturnal and it was postulated by Lausten Transit that the speed of dark was variable. Considering the viscosity of dark as described by Lausten Transit it may be shown that dark must in fact have a constant speed, using quantum electrodynamics, QED. Here, researchers from The Mill (YSA's new research labs) present the theoretical basis for these conclusions as well as a practical method for calculating the speed of dark are demonstrated.

Many people (mostly second year students) think that the second year lab is a waste of time. Well, to quash these ugly rumours once and for all, we present an ingenious new method for measuring the speed of light with high accuracy, using no more equipment than that easily found lying around the lab.

Have you ever noticed that when you switch off a laser, there is a noticeable time delay before the spot of light actually disappears? Utilising this important observation, a laser was set up to project a beam of light down the length of the lab, and the time delay between switching the laser off and the disappearance of the red spot on the front wall was measured with a stopwatch. Ten observations of this time were made, shown below:

The mean time and standard error of the mean from these readings was 0.41±0.01 s, but since the readings of 0.53 s, 0.44 s, and 0.47 s are clearly anomalous [2], they were discarded. The mean of the remaining seven readings was 0.374±0.001 s, which is clearly a better result since the uncertainty is so much lower [3].

The distance from the exit aperture of the laser to the diffuse reflecting planar surface upon which the coherent light beam was observed [4] was carefully measured [5] and found to be 18.49±0.01 m. Applying careful thought to the peculiarities of the chosen geometry, it was realised that the photons of the light beam were actually travelling twice this distance - to the front wall and then back again to the eye after reflection. The total distance travelled by the light is therefore 36.98±0.02 m.

Dividing the distance travelled by the observed time lag, the deduced speed is 98.9±0.3 m/s. Thoughtful readers will have realised already that this is not actually the speed of light, per se, but the speed of dark, as the observations were made of the time lag between turning the laser off and observing the arrival of the dark at the front wall.

However, it can easily be shown by symmetry arguments that the speed of dark is related to the speed of light by

,

where α is the dimensionless atomic fine structure constant.

Performing the calculation, the resultant speed of light from this experiment is (2.998±0.009)×10⁸ m/s, which is consistent with the accepted value of 2.99792×10⁸ m/s. In fact, the agreement is good to the fifth significant figure, indicating that the uncertainty estimates used throughout the experiment were somewhat conservative.

themill@ysa.org.au

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The Researchers at The Mill are interested in hearing about your unpublished research or new and interesting theories. If you have something in which you think our team would be interested, send it to us by email or snail mail to your YSA Chapter. Prizes will be awarded to good contributions as judged by the senior researchers.

Designed and maintained by the ever-efficient staff at The Mill themill@ysa.org.au.
Copyright © 2000 YSA Sydney Inc. for The Young Scientists of Australia
Last Modified: Monday, 08-Sep-2003 10:24:09 EST