The Search for Extraterrestrials: Intercepting Alien Signals (Springer Praxis Books Popular Astronomy)

167 Pages·2009·24.75 MB·English

by  Monte Ross

The Search for Extraterrestrials
InterceptingAlien Signals
Monte Ross
The Search for
Extraterrestrials
Intercepting Alien Signals
Published in associationwith
~ Praxis Publishing PR
Springer
Chichester, UK
Mr Monte Ross
Chairman
Laser Space Signal Observatory
Olivette
Missouri
USA
SPRINGER-PRAXISBOOKS IN POPULARASTRONOMY
SUBJECTADVISORYEDITOR: John Mason, M.B.E., B.Sc.,M.Sc.,Ph.D.
ISBN978-0-387-73453-8Springer Berlin Heidelberg NewYork
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Cover design: Jim Wilkie
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Printed in Germanyon acid-free paper
To Harriet,
our children
Ryia, Diane & Ethan,
and ourgrandchildren
Jake, April, Gabriella & Julia.
Contents
Illustrations ix
Author's preface xi
Acknowledgments xv
Part 1:The likelihood ofextraterrestrial intelligence
1. Vastdistancesand long travel times 3
2. Stars, their evolution and types 9
3. Planets and our Sun 19
4. The many constraints on life 25
5. Why would anyone transmit to us? 33
6. TheDrakeEquation and habitable planets 41
Part 2: The basics ofspace communication 57
7. Whereto look in the electromagnetic spectrum? 59
8. Receiver basics and how big isbig? 71
9. Noiseand limitations on sensitivity 81
Part 3: Programs in SETI 93
10. A brief history of SETI 95
11. Radio-frequencyand microwave SETI, including the Allen
TelescopeArray 99
12. Early optical SETI and the all-skyHarvardsystem 109
Part 4: Possibilities in SETI 125
13. ThePhotonStarproject 127
14. Keyissues for SETI 135
15. Other partsof the spectrum, decoding the data and forming
pictures 143
16. Futuresystems for intercepting alien lasers 153
Appendix 165
Bibliography 167
Index 169
Illustrations
1-1 The vastness of our galaxy 5
1-2 The number of Fthrough Kstars asafunction of distance 6
1-3 Using the parallax effect to determine distance 7
2-1 The number of starswithin a given distance from the Sun 10
2-2 The main sequence of the Hertzsprung-Russell diagram 12
2-3 The abundance of classM stars 15
2-4 Blackbody peak wavelength asafunction of temperature 15
2-5 Distances from Earth of the stars of the Big Dipper 16
2-6 The five great extinctions in Earth history 17
3-1 Planet occurrence asafunction of stellar metallicity 21
6-1 The Drake Equation for a low number of communicating civilizations 44
6-2 The Drake Equation for a high number of communicating civilizations 44
6-3 The habitable zone of astar 50
6-4 The Sun's place in the galaxy 52
6-5 A radius of 1,000 light-years isa small volume of our galaxy 53
7-1 Signal intensity falls off with the square of distance 59
7-2 The electromagnetic spectrum 61
7-3 Relationship of wavelength to frequency 62
7-4 Visible wavelengths 63
7-5 Transmitter and receiver directionality 65
7-6 Pointing errors 68
7-7 Beamwidth in terms of antenna diameter and wavelength 69
7-8 A typical radio-frequency antenna gain pattern 69
8-1 Comparisons of 'bigness' for a receiver 73
8-2 How a pulse detector usescoincidence to reject false alarms 75
8-3 A heterodyne receiving system 76
8-4 A direct detection receiver 78
9-1 Solar spectral radiance 82
9-2 An antenna of NASA's Deep Space Network 84
9-3 Thermal and quantum noise versus frequency 86
9-4 Poisson statistics for small numbers of photons 86
9-5 Probability of received energy for non-signal and signal-added cases 87
9-6 Threshold detection 88
9-7 Fraunhofer lines in the visible spectrum 89
9-8 Modulation methods 90
9-9 Pulsecode modulation 91
9-10 Polarization of electromagnetic waves 91
x The Search for Extraterrestrials
11-1 Amateur radio-telescope configuration 100
11-2 Project BETA's three-beam feed to eliminate interference 101
11-3 Overall structure of the Allen TelescopeArray 105
11-4 Allen TelescopeArray antennas 106
11-5 Blockdiagram of Allen TelescopeArray signal path 106
11-6 Allen TelescopeArray growth 107
12-1 Table from the Cyclops report, transmitter side 112
12-2 Chronological chart for optical SETI experiments and search 113
12-3 Direct detection and heterodyne (photomixing) detection 115
12-4 A large collector using segmented mirrors 117
12-5 Harvard's all-skyoptical SETI system 119
12-6 Photoelectron multiplier tube responseversuswavelength 120
12-7 Avalanche photodiodes responseversuswavelength 120
12-8 Point-ahead issues 122
12-9 Cloud-free line-of-sight probabilities 123
13-1 Conceptual diagram of the PhotonStar system 127
13-2 Receiver station hardware for the PhotonStar system 130
13-3 Collection areas for different large-diameter telescopes 131
13-4 Using small telescopesto attain equivalent areaof alarge telescope 131
13-5 Laser transmitter requirements for alink at 200 light-years 132
14-1 Aliens choosethe receiversizefor us 136
14-2 Probesaspart of a network searching the galaxy 138
15-1 Pulseinterval modulation using asmall amount of the time
interval 144
15-2 Duty cycles,intervals and bits per pulse for pulse interval
modulation 144
15-3 Converting bits per pulse into apicture using alow-duty-cycle
system 146
15-4 Asimple black and white picture from the receivedbits 147
15-5 Atmospheric absorption in the infrared 148
15-6 Ultraviolet lasers improve antenna gain 149
15-7 A 'photon bucket' in space 150
16-1 Keysystemtiming parameters 155
16-2 A lasertransmitter scanning system 155
16-3 Timing sequences 156
16-4 Timeline of ascanning transmitter 157
16-5 Blockdiagram for beacon and high-data-rate channels 160
Author's preface
In writing thisbook, Isetoutto educate,entertain,and stimulatepublic support
forintelligenteffortsbyhumankindto search foralien signalsfrom space.Asthe
book explains the primary factsand concepts in layman's terms, the reader will
need only elementary mathematics to grasp the major issues. Whilst some
oversimplification has been inevitable, the coverage is scientifically accurate. I
hope the reasoning willresonate with the public. Once readers have grasped the
fundamentals, Ibelieve they willagree with me that the SETI effort to-date has
been extremely limited in scope; in particular, it has not really explored the
electromagnetic spectrum outside the very narrow radio-frequency region. In
fact, not even 1per cent of the spectrum has been explored!
The people involved in SETI are trying to achieve a technically challenging
taskin amannerconsistentwith the lawsofphysics.Ithas absolutely nothingto
do with the "I was kidnapped by aliens" reporting of the tabloid press.
Unfortunately, in the public mind UFOs and alien signals tend to get lumped
together, with the result that SETI workisoftenportrayed asbeing on the fringe
of science. Certainly, in the past, many engineering and scientific academics
riskedSETI tarnishing their careers.The low probability ofsuccessisabarrier to
academics interested in furthering their careers. Some members of the public
may believe a massive effort to detect alien signals is underway, but this is far
from being the case.Infact, no USgovernment funding currentlyexistsforSETI.
The limited effortsthat are in progressrelyon charitable funding.SETI has been
mainlythe workofacademicswellversedin aparticulardiscipline. However,asI
show in the text, the detection ofextraterrestrialsignalsisasystemsengineering
problemwhose solution- constrainedbylimited resources- both minimizes the
chance offailure and maximizes the chance ofsuccess.SETI efforts to-date have
been marked by basic deficiencies which make it unlikely they will succeed.
Section 4 of this book offers some suggestions for how a systems engineering
approach might be developed.
There hasbeen agreat deal ofspeculationin the literature aboutwhy wehave
had no success,with the inference being that this isbecause there isnobody to
signaltous.Thisisknown asthe FermiParadox, but itisfartoo earlyto conclude
thatwearealone. Littlehasbeen done, especiallyat laserwavelengths. Thisbook
explains what has been done, what isbeing done, what can be done, and what
may have to be done before wecan in good conscience acknowledge the Fermi
Paradox.
Arelated effort isthat underway to identify Earth-like planets orbiting other
stars. This was initially done using terrestrial telescopes, but recently satellites
xii The Search for Extraterrestrials
have joined the hunt. After years of frustration, in excess of 300 extrasolar
planets have been detected in the last decade, in some cases with a single star
having several planets. The early methods were best suited to finding gasgiants
even larger than Jupiter, but the latest systems are designed to find Earth-like
planets. The latest addition to this effort is the Kepler spacecraft launched by
NASA in March 2009. When it does find a planet with a mass comparable to
Earth that isorbiting a solar-type star at just the right distance for the surface to
be conducive to life,NASAwill certainlyhail it asamajor discovery. And indeed
it will be, but a dozen or so further requirements must be satisfied before a star
system can producean intelligencecapable ofattemptingto communicateacross
interstellar distances.
This book seeks to inform the public of what has been done, what is being
done, what can be done andwhat needs to be done, much of it at little cost and
potentially great benefit to mankind. The obstacles to success are discussed in
detail. There isalongwayto go.SETIislimitedto, at best, several milliondollars
per year globally. If we are serious, then we should do better than this. With
public support, moneycan be forthcoming or the challenge may be taken up by
a wealthy foundation.
SETI has been of interest to me personally since 1965, soon after the birth of
the laser. In 1961 Robert Schwartz and Charles Townes wrote a scientific paper
which explained the potential for lasers in extraterrestrial communications.
However, little was done while attempts to detect signals concentrated on radio
frequencies. Interest in optical SETI was kept alive by a few people, notably
myself, Stuart Kingsley and Ragbir Bhathal, until the lack of success at radio
frequencies forced reconsideration. Ipointedoutin 1965 thatlasersignals would
be best sent by short pulses, since this would allow a modest transmitter to
readilyovercomethebrightnessofthehoststar.Today, thereisaserious effort at
Harvard devoted to pulsed laser signals, but all the efforts at all wavelengths are
still quite limited in relation to what can be done. Thus, this book is in four
sections. The first section discusses the likelihood of intelligent life, taking into
account the many constraints which could conceivably mean that we are the
only intelligence in the galaxy. The second section discusses the basics of space
communication and the technical approaches and issues. The third section
describes the SETI efforts that have been attempted and are currentlyunderway.
The final section explains how we might proceed further. The bibliography
should assist anyone seeking a guide to the literature on the subject.
In my career, lasershave gone from beinganew subject ofinterest, to finding
use in awide range ofapplications. Iaddressed lasercommunication in my 1965
book LaserReceivers, and spent my career helping to develop laser communica
tion. It is gratifying that the advances in power and efficiency of lasers have
facilitated such varied applications, and in my role astechnical editor of aLaser
ApplicationsseriesbytheAcademic PressIkept up to date withthemyriadusesof
lasers in industry, defense, medicine and entertainment. I participated in these
advances, being the first to patent a semiconductor diode pumped crystal laser
that is ubiquitous today from a green laser pointer for public speakers, to a