Gravitational Wave Detection

Detection of variations in the deflection of infra red light caused by variations in the strength of gravitational field (‘Gravitational Waves'). Chris Hollebon BSc ARCS ©2009 Chris Hollebon, Weston-Super-Mare, Avon, UK. This paper may be freely reproduced in its entirety by any person or organisation provided the author is fully acknowledged [email protected] Abstract Fluctuations with a period of the order of tens of seconds to several minutes are detected using selectively frequencially filtered and pulsed infra red light, focused on a small solid state detector. The fluctuations are caused by periodic fluctuations in the strength of the gravitational field, predicted by Relativity, varying the amount by which the infra red light incident upon the detector is deflected, which in turn varies the outputs from the detector. Apparatus Broadband infra red light is produced by a hot resistor infra red source. The light is filtered by a narrow bandpass thin film filter, and by one of two gas cells, sequentially introduced in the path of the light. One of the gas cells contains Carbon Monoxide which absorbs infra red light at very selective frequencies. The other gas cell contains Nitrogen, which is virtually transparent to infra red light in the frequency range of interest. The light is focused into a parallel beam by a Calcium Fluoride lens. The beam travels a distance of 200 – 300mm, and is then focused onto a Lead Selenide (PbSe) solid state detector with a small active area (~ 1 – 2mm2) by a second Calcium Fluoride lens. The entire instrument is contained within a Nitrogen environment.

Figure 1 1

Observations The instrument’s output oscillates with a period of the order of tens of seconds to several minutes.

Figure 2 Explanation The Lead Selenide detector outputs a signal proportional to the total amount of infra red energy incident upon it at any given moment, and with this apparatus the detector outputs a sequence of a peak corresponding to the Nitrogen gas cell passing through the beam, and a smaller peak corresponding to the Carbon Monoxide gas cell passing through the beam.

Figure 3 The Infra Red light beam is deflected varying amounts as the Gravitational Field varies (‘Gravitational Waves’). The Magnitude of the deflection is independent of the light’s frequency, and is exaggerated in the following illustration.

Figure 4 2

The Lead Selenide detector is a total energy detector. Its output is proportional to the total energy incident upon its active area, i.e. the output is proportional to the area under the curves in the following illustration.

Figure 5 When the Infra Red light beam is deflected to the left or right, the peaks in the detector output reduce in magnitude as less energy is incident on the detector’s active area. However, the detector output peaks corresponding to the passage of the Nitrogen gas cell through the Infra Red light beam reduce in magnitude more than those corresponding to the passage of the Carbon Monoxide gas cell through the Infra Red light beam, as the total energy incident on the detector’s active area reduces by a greater amount for the passage of the Nitrogen gas cell than for the passage of the Carbon Monoxide cell, relative to the incident energy when the Infra Red light beam is centred on the detector.

Figure 6

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The Nitrogen (N2) and Carbon Monoxide (CO) peaks are measured at the times illustrated.

Figure 7

Figure 8 The measured peak levels are combined electronically such that: Instrument Output = VCO - VN2 + Constant Where Constant = VN2 (CENTRE) – VCO (CENTRE), i.e. the difference in the measured peak levels when the Infra Red light beam is centred on the detector. As the beam deflects left and right the instrument output fluctuates as observed.

Figure 9

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Conclusion The slow periodic fluctuations in the instrument output are caused by variations in the strength of gravitational field (‘Gravitational Waves').

Author’s note The apparatus described is commonly used to detect Carbon Monoxide gas, which causes the instrument output to change when Carbon Monoxide is introduced in the path of the Infra Red light beam. Whilst the author was working to develop an improved variant of the instrument by introducing the Lead Selenide detector to replace the more commonly used and less sensitive Pyroelectric detector, the oscillations (unwanted in a gas analyser) were observed. All efforts to eliminate them failed. The author then proposed that Gravitational Waves were the cause.

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