Stedman - Light Revolutions

LIGHT
REVOLUTIONS

Geoffrey Ernest Stedman

Copyright 2014 by Geoffrey Ernest Stedman. 511725

Library of Congress Control Number:2014904889

ISBN: Softcover 978-1-4931-3776-3

Hardcover 978-1-4931-3775-6

EBook 978-1-4931-3777-0

All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing from the copyright owner.

Caption to cover picture Cover Picture the UG2 ring laser at Cashmere New Zealand with Professor Ulli Schreiber, copyright to the Christchurch Press, Fairfax NZ.

Caption to frontispiece picture p 1. Lyttelton harbor is visible as a coastal indentation to an old volcanic crater the lava flows on the north (upward) flank separate the harbor from the city and the Cashmere Cavern is on this north flank of these lava flows. Acknowledgements to NASA.

Rev. date: 06/04/2015

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Contents

Small ring laser gyroscopes were developed in the decades surrounding the 1970s for avionics the principal advance being through advances in mirror quality. Photos of avionic gyros are not readily available since the technology involved is highly sensitive commercial information. One system is illustrated in

Figure 3, an aircraft-type ring-laser gyro

Such devices are the basis of avionic navigation systems and have areas of dm as opposed to the areas of up to over 300 m of the devices discussed here see chapter 7.

We describe in this book not only some amazing properties of light but a particular program of laser gyroscope development, which is aimed to measure local variations in Earth rotation, this has expanding rapidly several major members of a family or farm of earth rotation devices of Figure 3, were built initially at cashmere New Zealand, all of which were much larger than the avionic devices of figure 3. It is the stuff of science that our dreams can be inadequate, certainly mine were. The story I shall tell was completely unexpected and unplanned in my scientific career. I remember as a postgraduate student in the 1960s hearing lasers were described at scientific conferences as a solution in search of a problem. Today lasers are a multi-billion dollar business. Every supermarket checkout, every CD player and builders toolkits hold one.

My aim is to work though some concepts to do with the origins of a major experimental project for which I have been responsible for and to give an overview of its history from1970 to the present. The Canterbury ring laser project had the ambition of gaining new results on earth rotation by optical means. Its origins are explained as a case study of several of the issues outlined above. For example It depends on the Sagnac effect (chapter 2) to measure the rate of rotation. This has sparked un-necessary misguided debates over light travel partly because of the tendency for confusion and inconsistent notions of simultaneity in relativity this topic is explored in Chapter 4. (It is important for the reader gains some general appreciation of relativity and quantum mechanics two 19 th Century revolutions in physics) brief introductions are included here Chapters 5 6.

This book was originally conceived on the basis of an adult education course I gave at the University of Canterbury in 2000. The course discussed theories of light down the centuries and a full description on the ring laser project, what follows is a compressed account the but also other topics related to light and electromagnetism, one of these is the topic of Chapter 10 namely a discussion of hidden momentum, another briefly mentioned in chapter 6 is a lecture demonstration of a physics experiment proving that we cannot know as much as we would like to about the natural world, a full description is in .

This illustrates one of the odd-ball topics I touched on during this course. In particular this requires a discussion of topics like the vector potential of electromagnetic induction and momentum conservation in it. These also are off shoots of the theory of light and some aspects of this are poorly understood even by professional physicists and I think they deserve an airing. Even in a book mainly devoted mainly to our experimental project on ring lasers as rotation sensors. These are based on two counter-revolving beams of laser light. an understanding of the operation of an optical gyroscope (is given in Chapter 2 on the Sagnac effect).

Light revolutions

In Grecian days it was imagined that light was composed of particles that were each copies of the object being looked at.

The pioneering work of Isaac Newton was also based on the concept that light was beam of particles which obeyed his laws of mechanics, this model had some successes which in retrospect were very surprising (however Newtons approach also had some major problems (e.g. such interference effects known as Newtons Rings seen when a convex glass is rested on a flat glass plate, to accommodate which Newton devised some extensions to his picture of light inventing the idea of fits of reflection one which is today unnecessary.)

But in the 1800s conclusive evidence was found that light must also be recognized as a wave, as sound had been. This in turn raised fundamental questions. It was known that sound needed a medium like air to travel in, what was the equivalent for light. A medium dubbed the ether was postulated to carry the wave motion of light. Efforts to detect this medium were not successful and eventually the concept of the ether was abandoned in science.

In the 20 th century big advances in Physics were made in relativity and the quantum theories both of which have now received overwhelming experimental confirmation. The particle idea (though not the Greeks idea of copies) was found to have much truth and particles of light are called photons. This had been vindicated by the huge advances in electromagnetism and optical studies in the 19 th Century by Maxwell who following the astonishing experimental insight of Faraday showed that one can understand the properties of light particularly the wave properties in terms of oscillations in electromagnetic fields, the electric and magnetic fields in space. Although this fact is well known and is described in detail in many books there are some aspects of electromagnetism and optics which are very poorly known even today so I will detour into a popular if incomplete account of some of these at various stages. The conflict between models raised many major puzzles in Physics whose full understanding has taken the 20 th Century advances to elucidate.

Enormous effort was spent by many scientists on trying to resolve the apparent conflict between Wave and Particle models of light but the outcome was that both proved essential. That the character of light was finally understood to be neither wave nor particle on its own but more profound than both. The full understanding of this required grappling with a number of major mysteries whose full solution has required well over a century of dedicated effort by scientists. I cannot review all of this here but outline some relevant considerations as far as is possible, in a work of this size. Nothing rivals a full physics text book account and I do not offer that. I aim to illustrate though aspects of the physical problems relevant to our main application the ring laser project so that a general reader may understand something of the background reasons for undertaking this scientific project.