Full text of DOI:10.1089/153110702762027880

ASTROBIOLOGY
Volume 2, Number 3, 2002
©
Mary Ann Liebert, Inc.
A Search for Optical Beacons:
Implications of Null Results
DAVID G. BLAIR and MARJAN G. ZADNIK2
1
ABSTRACT
Over the past few years a series of searches for interstellar radio beacons have taken place
using the Parkes radio telescope. Here we report hitherto unpublished results from a search
for optical beacons from 60 solar-type stars using the Perth–Lowell telescope. We discuss the
significance of the null results from these searches, all of which were based on the interstel-
lar contact channel hypothesis. While the null results of all searches to date can be explained
simply by the nonexistence of electromagnetically communicating life elsewhere in the Milky
Way, four other possible explanations that do not preclude its existence are proposed: (1) Ex-
traterrestrial civilizations desiring to make contact through the use of electromagnetic bea-
cons have a very low density in the Milky Way. (2) The interstellar contact channel hypoth-
esis is incorrect, and beacons exist at frequencies that have not yet been searched. (3) The
search has been incomplete in terms of sensitivity and/or target directions: Beacons exist, but
more sensitive equipment and/or more searching is needed to achieve success. (4) The search
has occurred before beacon signals can be expected to have arrived at the Earth, and beacon
signals may be expected in the future. Based on consideration of the technology required for
extraterrestrial civilizations to identify target planets, we argue that the fourth possibility is
likely to be valid and that powerful, easily detectable beacons could be received in coming
centuries. Key Words: Optical search for extraterrestrial intelligence—Search for intelligent
life. Astrobiology 2, 305–312.
INTRODUCTION
tend their exploration of the universe by initiat-
ing the exchange of information. In the case of
beacons one can expect a civilization to go to
HE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE
T
(SETI) has focused on the detection of elec- some lengths to ensure a signal that is reasonably
tromagnetic radiation either deliberately beamed easily detectable, whereas leakage radiation can
as a beacon for the purposes of establishing com- have arbitrary properties. This paper focuses on
munication, or by the detection of leakage radia- the search for beacons.
tion, that is artificial sources of electromagnetic
In previous work it has been argued that there
radiation escaping as a result of activities such as exist a small number of interstellar contact chan-
communication and radar (Cocconi and Morri- nel (ICC) frequencies that can be expected to be
son, 1959; Schwartz and Townes, 1961). Beacons universally chosen on the basis of physical and
provide the means by which civilizations can ex- mathematical laws (Blair, 1986; Blair and Zadnik,
1
Department of Physics, University of Western Australia, Perth, Western Australia, Australia.
Department of Applied Physics, Curtin University of Technology, Perth, Western Australia, Australia.
2
3
05

3
06
BLAIR AND ZADNIK
1
993). Since there are a vast number of possible known to compensate for planetary motion. De-
choices, one might expect that an advanced civi- tection of photon bunches is simple using photo-
lization, desiring to make contact, would utilize multiplier technology. The photon pulses need
a suite of several such ICC frequencies to increase not be periodic for detection. Detection can be
the chance of communication with a less ad- achieved simply by searching for coincident pho-
vanced civilization. In its simplest form an ICC is tons arriving at the same detector, or between
defined by multiplying or dividing a base fre- several nearby detectors. The coincident pulse de-
quency determined by atomic hydrogen by a civ- tection may be performed in a narrow band about
ilization signature constant, a small irrational the ICC wavelength, or at the cost of extra back-
number characteristic of a technological and ground light, across a much broader band. This
mathematical civilization, usually chosen to be p has the danger of increasing the probability of ac-
or 2p (Blair, 1986; Blair and Zadnik, 1993). Inter- cidental coincident photons unless the pulse
estingly, p times the hydrogen hyperfine transi- width is made smaller. Interestingly, the de-
tion frequency sets a frequency close to the min- tectability becomes easier if the apparent lumi-
imum noise temperature for interstellar radio nosity is lower. In an optical SETI, the narrow
communication, at 4.46 GHz, while transition band requirements are far less stringent than in
from ionized hydrogen to the ground state mul- radio SETI, especially if multiple coincidence de-
tiplied by 2p sets a wavelength of 572 nm, near tection is used between several closely spaced
the middle of the Sun’s visible spectrum. Both the telescopes.
optical and the radio channels are ideally suited
Transmitter and detector requirements for op-
to the creation and detection of beacons at inter- tical SETI are described by the following formula:
stellar distances well within the scope of modern
technology as discussed below.
2
L
EL 5 Nohcl 3 }2
(1)
2
d D
Here E is the laser energy per pulse, N is the
L
o
METHODS
required mean number of simultaneous photons
within the coincidence window) entering the re-
(
It is important to demonstrate the feasibility of ceiving telescope to register positive detection, L
creation of possible beacons to allow rational is the distance over which the beacon is trans-
evaluation of results. Any scenario that involves mitted, D is the transmitting telescope aperture,
enormous extrapolation of known technology is and d is the receiving telescope aperture. Factors
not falsifiable and beyond the realm of scientific that relate to beam geometry and optical effi-
disputation. We must place ourselves in the con- ciency are of order unity and have been ne-
text of an advanced civilization trying to make glected. Photomultiplier noise is not an issue be-
contact with the newly emerged technological cause single photon counting is possible using
civilization on Earth. In the case of optical bea- cooled photomultipliers. Using the above for-
cons it is necessary to ask whether such a beacon mula we may estimate plausible parameters for
is capable of outshining the host star, which with optical SETI. In calibration experiments (see be-
our astronomy is irresolvable, from its supposed low) we have shown that N ,2 is easily resolved.
o
Earth-like planet. The answer is that a pulsed Results are given in Table 1. For distances up to
high-power laser transmitting kilojoule pulses 100 pc the technological requirements are very
through a 10-m optical telescope is easily capable modest, at worst requiring a laser 2.5 times larger
of outshining a host star in a moderately narrow than the Nova inertial fusion laser at Lawrence
wavelength band. The main limitation on the Livermore Laboratory. Results are independent
laser is not that it should outshine the star, but of optical pulse width as long as the pulse width
that several photons per square meter should ar- is smaller than the mean period between back-
rive at the target planet in each short pulse. The ground photon arrival times. A 0.1% bandwidth
signature of the beacon is the presence of bursts filter (centered on the ICC wavelength of 572.3
2
6
29
of photons arriving in pulses 10 –10 s in du- nm) suppresses the background light due to a so-
ration. The transmitting telescope is capable of lar type host star, as seen by a 1-m telescope, to
6
4
concentrating a large fraction of its pulse energy ,10 photons/s at 10 pc and 10 photons/s at 100
onto the planet of the target system alone as long pc. Thus nanosecond pulses, as commonly pro-
as the distance of the system is sufficiently well duced by pulsed lasers, satisfy the pulse width

IMPLICATIONS OF NULL RESULTS FOR OPTICAL SETI
307
TABLE 1. PARAMETERS FOR OPTICAL BEACONS COMPARED WITH THE NOVA LASER
Pulse
energy
Transmitter
aperture
Distance
Nova laser
-m telescope detection
40 kJ
200 Jk
20 kJ
300 kJ
300 kJ
1
10 pc
10 m
10 m
30 m
3 km
1
00 pc
Human eye detection
10 pc
00 pc
1
The numbers indicate a 1-m telescope, and even the human eye could detect nearby beacons using laser power of
10 times the Nova Laser.
,
requirement for all stars, while for distant stars obviously need to be located in space to eliminate
the filter is unnecessary.
atmospheric scintillation.
It is interesting to point out that the transmit-
The large number of radio SETI experiments
ting capabilities do not have to be dramatically over recent years have emphasized the techno-
increased for optical pulses to be brought to the logical ease of interstellar radio communication.
amplitude necessary for unaided eyes to observe For example, Fig. 1 shows the radio power re-
a flashing optical source. For example, 3 kJ opti- quired using a 1-km transmitter telescope to cre-
cal pulses at a repetition rate of 0.1 Hz, broadcast ate a detectable narrow band beacon for nearby
1
5
2
across an area of 10 m (substantially larger than solar-type stars searched (Blair et al., 1991, 1992;
the cross-sectional area of the Earth), would be Zadnik et al., 1997).
visible to the unaided human eye (,20 photons
Power requirements are again relatively mod-
per 3-mm-diameter aperture per pulse). To est. Indeed, at Alpha Centauri a mobile phone
achieve the narrow beam required at a distance transmitter would have sufficient power to be de-
of 10 pc, a transmitting telescope aperture of 30 tected on Earth if coupled to an Arecibo dish. We
m would be required. This increases to 3 km at note that a 1-km telescope is similar to the pro-
1
00 pc distance, assuming a laser power within posed International Square Kilometer Array Tele-
present technological capability. Such large-aper- scope and thus reasonable for an advanced trans-
ture transmitters could be achieved using an in- mitting civilization.
terferometric telescope with multiple elements
The ability to send beacon signals of the type
spaced across the desired aperture. They would discussed here arises because of the enormous an-
FIG. 1. Minimum transmitted power histogram for a 1-km transmitter for 176 targets.

3
08
BLAIR AND ZADNIK
tenna gain achievable. The antenna gain, which multaneous photons could have been detected,
is the ratio of power density achievable with a fo- and ,50 such pulses could easily be detected in
cusing device compared with that of an isotropic a typical 600-s observation.
2
2
radiator, is of order 4p 3 D /l . For a 10-m op-
1
5
tical telescope the gain is ,4 3 10 , whereas for
9
a 1-km radio telescope the gain is ,4 3 10 . The
RESULTS
lower gain of the radio system is compensated by
the low photon energy, low background noise,
and narrow-band search capability. Note that the
absurdly large power required by an isotropic
beacon—larger by the telescope gain factor—ap-
pears to rule out the possibility that single
isotropic beacons might exist to alert all emerg-
ing civilizations to make contact.
The 57 target stars observed are listed in Table
. Results were interpreted in terms of the mini-
2
mum laser power required to create a detectable
beacon (with our setup) using a 10-m telescope,
for a planet located at each of the target stars. The
results show that in most cases beacons of ,1 kJ
per pulse at a 1-Hz repetition rate could have
been detected. Figure 3 is a histogram of the re-
sults.
SUMMARY OF OPTICAL AND
RADIO SEARCHES
Teams of researchers in Australia have made
several efforts to search for beacons using the
Parkes radio telescope (Blair et al., 1991, 1992;
Zadnik et al., 1997). In the first two cases narrow-
DISCUSSION: IMPLICATION OF
NULL RESULTS
SETI searches are clearly still not complete.
band searches were undertaken around the fre- There are many more target stars that could be
quency p 3 HI, or 4.46 GHz. The search bands searched, at greater sensitivity. However, it is
covered Doppler shifts and the possibility that the clear that only a small fraction of nearby target
beacon was broadcast in the cosmic microwave stars can be emitting beacons directed at the
background reference frame. When the Project Earth. What implications can be drawn from this
Phoenix equipment was available, the search was result?
repeated on 49 stars ,11.5 pc from the Solar Sys-
It is worth commenting that the absence of pos-
tem and included the frequencies of p 3 HI, 2p 3 itive results does not imply that “we are alone,”
HJ (8.925 GHz), e 3 OH (4.532 GHz), e 3 (H 1 or that extraterrestrial life does not exist, or that
3
OH) (9.393 GHz), and He (8.666 GHz). A total of technological civilizations do not exist amongst
1
76 target stars were searched.
the stars that have been searched. If technologi-
In 1995 and 1996 an optical search was under- cal civilizations were characterized by the trans-
taken at Perth Observatory, using the 61-cm mission of narrow-band radiation such as televi-
Perth-Lowell telescope. In this case a wavelength sion carrier frequencies or mobile phones (total
of 2p 3 H —a value of 572 nm—was chosen, narrow-band power across a range of frequencies
o
7
where H is the hydrogen ground state wave- ,10 W), we would expect a planet at 10 pc to
o
length. A cooled photomultiplier was used to de- have a radio signature with a flux of ,1 milli-
tect light after transmission through a 1-nm band- Jansky. It is difficult to imagine that a highly ad-
width bandpass filter. A multichannel analyzer vanced telecommunicating civilization would
was used in real time to conduct a pulse height squander so much narrow-band power in free-to-
analysis of the incoming light. Calibration of the air broadcasting. Even if they do, it is most un-
photomultiplier with a pulsed LED was used to likely that any of the narrow-band radio searches
determine the threshold for two or three photon to date could have detected the Earth at 10 pc.
events. The analysis consisted of a search for ex- However, a radio telescope 100 times the diame-
cess two or more photon events. In Fig. 2a we ter of Arecibo (say 30 km) should be able to de-
show some calibration data created using artifi- tect television-type transmissions at 100 pc. It is
cial pulses. A large hump demonstrates the pres- plausible that an advanced civilization should
ence of optical pulses. This is in contrast to Fig. use a dish of this scale to identify planets when
2
b, in which multiphoton events are very rare. In they reach a telecommunicating phase, with the
almost all cases, pulses containing only two si- view to establishing communication. The uncer-

IMPLICATIONS OF NULL RESULTS FOR OPTICAL SETI
309
FIG. 2. a: Artificial pulses created with an LED (recorded for 740 s). As well as a thermal noise background below 6
eV, a distinct hump of multiphoton pulses is clearly visible. The vertical axis shows the count rate N/(observation
time). b: Data from the multichannel analyzer for a 600-s observation of the star Delta Pavonis. The data show a ther-
mal noise background below 2 eV and characteristic nonthermal tail due to electrical interference. No target star
showed a resolvable excess of high-energy events characteristic of optical pulses. The vertical axis shows the count
rate N/(observation time).
tainties in these estimates and the relatively small
As already discussed, sufficiently powerful
margin by which existing telescopes fail to reach beacon transmitters are reasonably easily achiev-
sufficient sensitivity to detect leakage from ,10 able. In this case, why does the alien civilization
pc provide strong arguments for continuation not direct beacons at all possible life-bearing
and improvement of radio leakage searches.
planets, in the hope of establishing communica-
The above argument explains why leakage ra- tion? If this were the case, the absence of beacons
diation has not been detected in SETI searches: would surely indicate that we are alone.
Our telescopes are too small. However, an ad-
The reason that this is incorrect can be deter-
vanced extraterrestrial civilization could have de- mined by considering the physical limits to the
tected the Earth’s radio leakage (which has been capabilities of advanced astronomy. The presence
substantial since ,1950) as long as it is closer than of water, oxygen, and climatic variation should
5
0 light years away. A civilization at 100 pc still be discernible by a civilization with the capability
has 275 years to wait before the Earth’s radio of constructing large-scale interferometric tele-
sphere arrives. If it replies with a beacon, the re- scopes. Thus the civilization can determine that
ply will arrive about the year 2600.
the planet is life-bearing. However, assuming that

TABLE 2. SUMMARY OF OPTICAL SETI OBSERVATIONS
Right ascension
Declination
Degree Minutes
Mean
power
(W)
Date observed
1996
Spectral
type
Distance
(pc)
Star
Hours
Min
VMag
Alpha Ophiculus
Alpha Serendis
Altair
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 17th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 18th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
May 19th
Sept 13th
Sept 13th
Sept 13th
Sept 13th
Sept 13th
17
15
19
19
15
19
18
15
5
34.90
44.27
50.78
55.31
17.05
25.49
59.62
56.45
44.46
46.77
18.18
32.27
41.05
50.70
3.66
18.40
6.42
3.36
57.67
24.97
8.72
21.90
5.51
37.15
10.24
45.38
18.63
31.36
36.54
53.49
34.49
38.67
39.49
46.52
51.69
28.00
29.15
31.36
54.49
25.72
39.49
54.97
12.05
14.18
37.20
0.00
12
6
34
A5
K2
A7
G8
B8
F0
K2
F6
F6
G5
K3
G0
G8
F8
F4
G6
F8
19
26
5.1
11
37
16
65
12
8.1
30
46
19
9.1
10
23
8.4
12
3.4
36
18
5.7
42
13
15
8.7
25
23
21
23
20
9.5
23
33
10
26
30
15
21
2.08
2.65
0.77
3.71
2.61
3.36
4.02
3.85
3.6
190
534
25
25.5
52.1
24.4
23
8
Beta Aquili
Beta Librae
Delta Aquili
Epsilon Aquili
Gamma Serpendis
HR 1983
6
74
29
842
202
2,598
114
23
870
5,205
285
80
61
325
43
76
6
455
342
23
775
178
119
60
220
93
155
325
211
71
4
5
4.1
15
15
39.7
26.9
25.2
31.7
15.8
12.1
45.9
26
18.7
3.8
47.2
22.5
56.7
10.9
9
28.4
28.2
45.2
47.6
6.6
42.9
13.8
8.2
49.9
12.1
23.9
30
50.1
26
222
249
31
HR 4216
10
11
11
11
11
12
13
19
22
16
19
20
16
5
2.64
3
5.8
HR 4375
HR 4443
229
34
HR 4496
5.33
3.61
5.15
4.74
4.21
4.69
3.2
5.16
3.56
3.75
4.72
5.65
5.09
5.92
5.81
4.89
5.7
5.24
5.98
5.48
5.17
4.91
5.85
2.81
4.63
4.89
5.93
5.17
5.17
5.89
4.85
5.6
5.63
5.8
5.41
5.46
5.64
5
5.67
5.47
5.5
5.4
5.38
3.4
3.9
3.2
4.21
6.2
HR 4540
1
HR 4600
242
218
237
256
9
HR 5019
HR 7226
HR 8387
K5
K2
G8
G5
A9
F8
Kappa Ophiclus
3
1 Aquili
11
266
19
Delta Pavonis
Gamma Hercules
HR 1674
HR 2022
HR 2261
HR 2500
HR 4061
HR 4134
HR 4157
HR 4251
HR 4458
HR 4488
HR 4492
HR 4523
HR 4548
Lambda Sagittarius
HR 3759
HR 4134
HR 4266
HR 4413
HR 4492
HR 4903
HR 4979
HR 4995
HR 5113
HR 5257
HR 5459
HR 5544
HR 5567
HR 5698
HR 5700
HR 5854
HR 6060
HR 6094
HR 3750
HR 6595
HR 6761
HR 6998
HR 7226
HR 7631
257
280
274
231
256
252
212
220
232
213
265
240
230
225
22
253
261
263
265
244
237
219
261
225
246
19
221
247
260
216
28
239
26
221
262
221
237
233
5
G3
G5
F6
6
6
10
10
10
10
11
11
11
11
11
18
9
F8
F6
F8
F6
K0
F5
232
861
61
G8
G5
F5
K2
F6
F6
238
395
435
891
738
1,361
1,052
1,395
193
712
457
224
9,567
5,949
44
46.1
42.9
49.6
58.4
23.9
9.1
48.2
55.9
41.5
0.6
10
10
11
11
12
13
13
13
14
14
14
14
15
15
16
16
16
9
K5
F3
22
33
38
13
30
G8
G2
G5
G5
F5
F3
14
110
92
5.6
20
27
37.33
51.38
57.48
22.14
23.17
0.32
8
6.1
K0
K4
K4
F6
24.9
55.7
39.4
32
22.2
11.6
4.3
351
704
25,389
330
178
791
492
511
914
76
F5
F8
G1
G5
G2
F5
170
17
15.62
24.02
27.78
43.43
10.44
38.89
6.42
15
15
17
18
18
19
20
0.41
20
0.1
3.1
3.8
G3
G5
F8
23
34
12
0.34
42.2
K0
140
12,053
The final column gives the threshold for detection for this search as a function of the laser power, in short pulses
from a 10-m transmitting telescope.

IMPLICATIONS OF NULL RESULTS FOR OPTICAL SETI
311
FIG. 3. Mean laser power required to transmit a detectable beacon from target stars using a 10-m diameter tele-
scope.
the Earth is typical, evolution occurs over several 2. The beacons could be optical or radio, they
billion years, with many macroscopic parameters
of the planet remaining remarkably constant.
While the Earth’s atmosphere has previously un-
might use an ICC frequency, but they could
be bright enough to be seen by the naked eye,
making the ICC concept redundant.
dergone profound chemical changes, the temper- 3. Improved searches have a possibility of de-
ature and atmospheric composition have not var-
ied greatly the past 100 million years. It therefore
appears to be impossible that an alien civilization
could obtain any data with which to predict the
emergence of communication technology. It is in-
conceivable that an alien civilization would main-
tecting leakage radiation if extraterrestrial
civilizations emit substantially more narrow-
band radiation than does the Earth. However,
to detect leakage radiation at 100 pc could re-
quire telescopes with 10 times the collecting
area of Arecibo.
4
tain an array of, say, a million beacons (one for 4. The search for beacons should be continued
each target planet) for a few hundred million
years waiting for each to reach the telecommuni-
cating phase. Unless a method of predicting the
emergence of telecommunications can be identi-
fied, the alien civilization has no alternative than
to wait for the emergence of the radio sphere.
recognizing that they can, at present, only be
expected from stars within the Earth’s radio
sphere horizon, which, at present, is less than
,10 pc from the Solar System. The probabil-
ity of receiving a beacon should increase as
time cubed as the volume of space and the
number of stars intersected by the Earth’s ra-
dio sphere increase.
CONCLUSIONS
5. The case for maintaining an intensive search
today would be greatly strengthened if mech-
anisms could be identified whereby advanced
astronomy could predict the emergence of
technological civilization during the preced-
ing centuries or millennia, so that beacons
could be implemented for a reasonable length
The above arguments and null results lead to
the following conclusions:
1
. The absence of signals need not imply the
nonexistence or low density of civilizations de-
siring to make contact: They will reply with
beacons when they detect the Earth’s radio
sphere.
4
8
of time (say, 10 years instead of 10 years).
This would provide a rationale for expecting
beacons today. Examples of human activity

3
12
that one might hope to observe might be the
BLAIR AND ZADNIK
REFERENCES
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ACKNOWLEDGMENTS
We gratefully acknowledge the work done by
Mark Lockett as part of his Physics Honours pro-
ject and the Perth Observatory where the observa-
tions were carried out, in collaboration with the
Perth Astronomy Research Group. We thank the
reviewers for insightful and constructive criticism.
Address reprint requests to:
Assoc. Prof. M.G. Zadnik
Department of Applied Physics
Curtin University of Technology
G.P.O. Box U 1987
ABBREVIATIONS
Perth, 6845, WA, Australia
ICC, interstellar contact channel; SETI, search
for extraterrestrial intelligence.
E-mail: m.zadnik@curtin.edu.au