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Constraining the Discovery Space for Artificial Interstellar Signals

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Constraining the discovery space for artificial interstellar signals Ian S. Morrison A thesis in fulfilment of the requirements for the degree of Doctor of Philosophy School of Physics Faculty of Science July 2017 Constraining the discovery space for artificial interstellar signals THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Morrison First name: Ian Other name/s: Stuart Abbreviation for degree as given in the University calendar: 1890 Physics School: Physics Faculty: Science Title: Constraining the discovery space for artificial interstellar signals Abstract: After more than 50 years of searching the skies across the electromagnetic spectrum, no evidence has yet been found for the existence of extraterrestrial life, let alone a signature that could be attributed to an intelligent and technological extraterrestrial civilisation. Proposed explanations range from the non-existence of such putative civilisations, to the likelihood that the search has barely "scratched the surface" in terms of covering the entire discovery space. The present work is premised on the latter position, and its central thesis is that most searches conducted thus far have been disadvantaged by sub-optimal design. In general, they have (1) not targeted the most appropriate signal types; (2) not targeted the region of the electromagnetic spectrum that should be preferred for interstellar communications; and (3) not consistently concentrated on the regions of the sky that hold the highest likelihood for the emergence of extraterrestrial intelligence. The very nature of past searches provides a credible explanation for the null result to date. It is impossible to predict the specific technologies and communications methodologies likely to be adopted by extraterrestrial civilisations, particularly when one considers how much older and more technologically advanced than ourselves they may be. However, beginning with a single key assumption – that energy efficiency is a concern to those wishing to transmit signals across interstellar space – this work shows how the application of fundamental principles of astrophysics and information theory can lead to meaningful constraints on the discovery space for artificial interstellar signals. It is shown why the search for extraterrestrial intelligence should focus on intentionally transmitted wideband signals occupying the upper end of the microwave range of the radio spectrum - a region largely ignored by past searches. Furthermore, searches should concentrate on the inner Galaxy rather than our own solar neighbourhood. A new signal processing algorithm is introduced for the blind detection of wideband signals of a class that is attractive for interstellar communications, offering high detection sensitivity while making minimal assumptions about the precise signal format. The findings of this thesis suggest a range of new priorities and approaches to the search for extraterrestrial intelligence, aimed at increasing its chances of success. Declaration relating to disposition of project thesis/dissertation I hereby grant to the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or in part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all property rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstracts International (this is applicable to doctoral theses only). …………………………………………………………… ……………………………………..……………… ……….……………………...…….… Signature Witness Date The University recognises that there may be exceptional circumstances requiring restrictions on copying or conditions on use. Requests for restriction for a period of up to 2 years must be made in writing. Requests for a longer period of restriction may be considered in exceptional circumstances and require the approval of the Dean of Graduate Research. FOR OFFICE USE ONLY Date of completion of requirements for Award: ii COPYRIGHT STATEMENT ‘I hereby grant the University of New South Wales or its agents the right to archive and to make available my thesis or dissertation in whole or part in the University libraries in all forms of media, now or here after known, subject to the provisions of the Copyright Act 1968. I retain all proprietary rights, such as patent rights. I also retain the right to use in future works (such as articles or books) all or part of this thesis or dissertation. I also authorise University Microfilms to use the 350 word abstract of my thesis in Dissertation Abstract International (this is applicable to doctoral theses only). I have either used no substantial portions of copyright material in my thesis or I have obtained permission to use copyright material; where permission has not been granted I have applied/will apply for a partial restriction of the digital copy of my thesis or dissertation.' bwtmowin Date ……………………………………………..........1618117 ................. AUTHENTICITY STATEMENT ‘I certify that the Library deposit digital copy is a direct equivalent of the final officially approved version of my thesis. No emendation of content has occurred and if there are any minor variations in formatting, they are the result of the conversion to digital format.’ Signed …………………………………………..........hmtmrmn ................. Date ……………………………………………..........1618117 ................. Constraining the discovery space for artificial interstellar signals Originality Statement ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed …………………………………………….............. Date ……………………………………………….............. iii Constraining the discovery space for artificial interstellar signals Table of Contents Originality Statement .................................................................................................................... iii List of Abbreviations .................................................................................................................... vii Acknowledgements ..................................................................................................................... viii Publications and presentations arising from this work ............................................................... ix Chapter 1. Introduction .............................................................................................................. 1 1.1 Are there signals to find? .............................................................................................. 1 1.2 A new paradigm for SETI ............................................................................................... 3 1.3 Thesis scope and structure ........................................................................................... 7 Chapter 2. Artificial interstellar signals .................................................................................... 10 2.1 Eavesdropping versus intentional beacons ................................................................ 10 2.2 The narrowband assumption and its weaknesses ...................................................... 14 2.3 Isotropic versus directional beacons .......................................................................... 21 2.4 Wideband signalling for efficient communication of information ............................. 23 2.5 Discovery versus communications .............................................................................. 27 2.6 Challenges of wideband SETI ...................................................................................... 28 2.7 Blind detection ............................................................................................................ 31 2.8 Examples of wideband signal types suited to interstellar communications ............... 32 2.8.1 M-ary orthogonal modulation ............................................................................ 32 2.8.2 Spread-spectrum phase modulation .................................................................. 34 2.9 Data rates for interstellar communications ................................................................ 38 2.10 Detection methods for wideband SETI ....................................................................... 39 2.10.1 Matched filtering ................................................................................................ 40 2.10.2 Power spectral density ........................................................................................ 46 2.10.3 Energy detection ................................................................................................. 47 2.10.4 Statistical
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