The history of the detectability radius of civilizations
At what distance from Earth can we see hypothetical brothers in mind? There are many good reviews on this subject (for example, [ 1 , 2 , 10 ]). It follows from them, in particular, that with the help of the most powerful radio telescope in Arecibo today, we can send a signal to another civilization with the same telescope, up to several thousand light-years away. Of course, for this communication to take place, you need to know exactly where, when and at what frequency to transmit and listen. If you simply scan the entire sky, “listening” to each point for a long time to catch a signal, then the coverage of our interstellar communication decreases to a radius of 5-10 light years [ 10table 4]. It is at such a distance that we can today notice cosmic neighbors at a level of development comparable to us.
Question: How has this distance changed in the past? Is it possible to understand something by looking at the values of this parameter in a global historical perspective? It turns out yes. And that’s what we’ll do. We will see how the radius of self-detection R s , that is, the distance at which terrestrial civilization could detect another civilization at the same level of development, has changed over time.
Why "on the same"? Because it’s simpler and more definite. No need to speculate, probably mistaken whether the ancient Egyptians could notice someone else's satellite (and whether we can recognize someone else's super-technology, having it before our eyes).
Even in such a simplification, quantitative estimates are extremely difficult and may contain significant inaccuracies. Therefore, amendments, of course, are accepted, but only if I made a mistake, at least an order of magnitude.
So let's get started.
Australia began to populate about 40 thousand years ago, and Polynesia - 3-5. Between these milestones, people did not know either horses or any serious sailing.
Take a look at the map of the native languages of Australia [ 15 , Image Credit: Wikipedia]: The
colors on it indicate not just languages, but their families . That is, groups of languages that differ much more than Russian from Polish. A very similar picture emerges on the map of the languages of the natives of North America [ 16 ].
According to these cards, the cultural isolation between colorful shreds was extremely high. Otherwise, languages, for millennia, would probably have mixed up. So, the characteristic “radius of self-discovery” in those days was the size of the territory where they spoke the same family of languages. Just because if people knew their immediate neighbors somehow, what was happening behind their lands remained almost unknown.
Based on these considerations, we take R s ≈ 10 6 meters in the interval between the emergence of modern man (50 thousand years ago) and the first states (about 6 thousand years ago).
On the world map of Ptolemy’s work of the second century AD, China is already marked with the word “Sinae” ([ 20 , Image Credit: Wikipedia]):
According to the same source, the first exchange of ambassadors between the Roman and Chinese empires took place in the year 166. Thus, the two largest civilizations of the planet, at a similar level of development, find each other at a distance of R s = 8 * 10 6 m.
In 1522, Magellan’s expedition completed a trip around the world ([ 30 , Image Credit: Wikipedia]):
If there was another European civilization on Earth, there would have been practically no chance of it being unnoticed by this time. That is, in 1522, R s = 2 * 10 7 m.
In 1609, Galileo built a telescope with an almost 30-fold increase, which made it possible, with some luck, to distinguish details on the moon with a size of 8-10 kilometers [ 40 ]. Unfortunately, London (one of the largest terrestrial cities at that time) then reached only about one mile in size [ 50 ].
By 1657, with a 100-fold Huygens telescope on the moon, it was possible to observe details of 2-3 kilometers [ 40 ]. London reached this mark around 1677 [ 52 ]. That is, if he had been there, they would have already seen him.
Technically, this is already the date of the inclusion of the lunar orbit in the circle of our self-discovery. However, due to observational difficulties, separate “discoveries” of the traces of civilization on the Moon took place until the 19th century [ 60 ], while the spectrographic works of Fraunhofer in 1823 [ 70 ] completely closed this question.
Subject to this amendment, we assume that R s reached a value of 4 * 10 8 meters by the year 1700.
From the 19th to the middle of the 20th century, R s grew mainly due to the increasing influence of our civilization on nature. Our buildings, cities, canals became larger and more noticeable.
Here, for example, the Karakum Canal:
An analysis of the parameters of telescopes [ 80 , 90 ] shows that it, in the form of a dark strip in this picture, could be seen on Mars by 1897. Which roughly coincides with the opening of the famous "Martian channels" (1877, [ 100]). And although those channels turned out to be an optical illusion, it is important to understand: they, in principle, could be real. Observational astronomy even then made it possible to notice a fairly large real channel due to seasonal darkening of adjacent vegetation. True, the Karakum Canal began to be built only in 1954, but channels of comparable size on Earth were laid earlier.
Another sign is the glow of the electric lights of night cities. Using the calculator [ 110 ] (or recounting the conclusions from [ 1 15 ]), we can estimate that if Venus had its own New York, then its night lights on the night side could be seen somewhere between 1900 and 1960 th years.
Based on the totality of these data, we take 6 * 10 10meters as the radius of self-discovery of terrestrial civilization by the 1930th year.
And then radio astronomy entered the scene.
The United States and the USSR, fearing a sudden nuclear attack by a neighbor, created Ballistic Missile Early Warning Systems, [ 120 , 122 ]. These systems have become one of the most powerful continuously operating radio sources on Earth.
They are based on an ordinary radar that runs around the horizon every few seconds and listens if a reflected signal comes from a flying warhead (Image Credit: Wikipedia):
What happens to a radar beam if it does not come across a warhead? Right. The beam flies on. Into space, past the Moon, planets of the Solar System, stars, and so on to infinity.
The beam strength is such that if you know in advance which star to “listen to”, you can catch it with the Arecibo telescope in 19 light years, and with good probability in 0.7 light years, inspecting the entire sky [according to 10 , table 4].
Thus, already around 1965, the parameter R s reaches a value of 0.7 light years, or 7 * 10 15 meters.
To say exactly what this radius is today is rather difficult. Many new tools, methods, but few systematic descriptions. Estimates often have to be done on the run by indirect evidence.
So, Allen Telescope Array [ 145], being brought to mind, it should confidently detect a radar equivalent to Arecibo at a distance of up to 300 parsecs (and 105 parsecs in its current form). This is assuming directional transmission. It is clear that at such a distance the probability of transmission is small for us. However, if we take the next 5 parsecs, then the stars in this sphere are already only fifty, of which the “decent” spectral class is only 30 in general. This amount can easily be grasped by directional transmissions. Accordingly, if “they” are not idiots, want to communicate, and live within a radius of several parsecs, then our chances of hearing “them” turn out to be very high.
What else? In the work [ 150] people searched for spectral tritium signatures in the radio range in a radius of about 20 light-years from the Sun. Tritium, as you know, is almost never found in nature, so any discovery of it would almost certainly indicate the activity of a cosmic civilization that actively uses thermonuclear energy. For example, for interplanetary flights on OrionHius rockets . The level is slightly higher than ours, but still comparable and understandable.
In [ 153] it is alleged that modern or near-expected radio telescope construction has reached a level sufficient to record the noise of alien television signals at distances of 10-500 parsecs. Of course, in the mode of careful listening to "interesting" stars, and not the whole sky in a row, which in practice means rather the lower limit of this range.
In addition to radio, other interesting channels have recently become significant.
Thus, laser transmissions (also directional) with a power of only 90 watts are detected by modern methods from distances up to 100 light years, according to [ 155 ]. That, taking into account the argument already cited about focusing on the nearest neighbors, again comes down to a highly probable detection on units of parsecs.
The consequences of a global nuclear war (in the form of a luminescence of ionized air) are at the limit of detectability by modern methods for nearby stars [ 156 ]. It is also reported that the James Webb Space Telescope, scheduled for launch in 2018, will be able to "feel" the pollution of planetary atmospheres with technogenic freons at interstellar distances.
Finally, if “they” guess to dump their radioactive waste into the local sun (we have such projects), then along the spectral lines of the rarest “fission fragments” (Tc, Pr, Nd, Pu, Ba, Zr), similar activity, with sufficiently serious its scale, it is possible to detect in general almost thousands of light years [ 160 , 156 ].
Summing up all this diversity, we conclude that if we had neighbors within a radius of a dozen light years from the Sun, we would probably have already noticed them.
For starters, it would be nice to plot R s versus time. It turns out to be far from easy! Our tools and eyesight pass before an addiction that extends over 20 thousand years and 11 orders of magnitude, but gaining six of them in some half a century. Even on a logarithmic scale, it just looks like a "wall."
In order to convey this dependence, one has to invent very unnatural coordinate systems. For example, the logarithm of the radius of self-discovery as a function of the logarithm of the number of years in the past from the (artificially chosen) year 2016:
Drawn. Growth, as you see, is fast and accelerating all the time. Interestingly, on a global historical scale, it looks like an integral process that began long before the SETI program or the invention of the telescope. It seems that the search for other civilizations is a natural part of the growth of our civilization.
The second question: what is the analytical form of this dependence? And can it be extrapolated to the future?
In a good way, you can’t do this. Through seven points, you can draw almost anything, and getting burned by extrapolation is easier than easy. It does not at all follow from the child’s growth rate in the first 10 years that by the age of 300 he will begin to step over tower cranes.
Therefore it is impossible. But if veryI want it, then a little bit is possible. At least as an exercise in working with such sharp addictions?
From the very first experiments, it becomes clear that this dependence is stronger than exponential. An attempt to fit an exponent into it ends in failure (R 2 = 0.227). Consequently, power functions are also excluded. They grow slower than exhibitors. We need something that grows, on the contrary, faster.
What do we know about our addiction? That, by its logic, it is always positive, everywhere is growing monotonously, and has no features in the past. Therefore, neither the Gamma function (there are features) nor the (shifted) exponent of a degree of the form R = exp ((YY 0 ) n) - because it has features and / or non-monotony for any n, except for odd integers, the adoption of which would be shamanism and fit for an answer.
The next in the speed class is the exponent of the exponent. It is finally possible to enter into the dependence (R 2 ≈ 0.92). In the chosen crazy coordinate system, it looks like this:
In the more natural coordinates for it “the logarithm of the radius, the exponent of a century”, everything turns out almost even elegantly: The
analytical expression takes the form:
Ln ( R s ( t )) = (4.2878708257 * 10 -8 ) * e ( Y / 100 ) + 16.0063874034
Where Y is the current year, and R s is expressed in meters.
In a more convenient form for perception, this dependence can be rewritten as follows:
Ln ( R s / 8940 km) = e С -16.9648904452
Where C is the current moment, expressed in centuries (i.e., for example, for 2015, C = 20.15). Amusing "coincidences" attract attention:
Alas, it remains unclear whether these figures correspond to any objective reality, or whether they are artefacts of approximation at too few points.
By extrapolating this dependence into the future, it can be found that the radius of self-detection covers our Galaxy (180 thousand light-years) by the year 2046, and the visible Universe (14 billion light-years) by 2075. Can these numbers be believed? Of course not. But it can be reliably argued that the radius of self-detection is growing very quickly. And if we, as a civilization, do not ruin ourselves with a nuclear war or some shameful collapse of education, then we will have every chance to learn a lot of new and interesting things even during the lifetime of the current generation.
[ 1 ]. An interesting fact about the modern possibilities of interstellar communication .
[ 2 ]. Estimation of the probability of detecting a random radio signal from an extraterrestrial civilization .
[ 10 ]. Detectability of Extraterrestrial Technological Activities by Guillermo A. Lemarchand .
[ 15 ]. List of Australian Aboriginal languages (Wikipedia) .
[ 16 ]. Native American Languages .
[ 20 ]. Sino-Roman relations (Wikipedia) .
[ 30 ]. Ferdinand Magellan (Wikipedia).
[ 40 ]. The Developmental History of the Telescope .
[ 50 ]. History of London (Wikipedia) .
[ 52 ]. Map of London 1677 (City of London Ogilby and Morgan's Map of 1677.jpg) (Wikipedia) .
[ 60 ]. Communication with extraterrestrial intelligence, #History (Wikipedia) .
[ 70 ]. MOON INHABITATION, p 277 .
[ 80 ]. Timeline of telescope technology (Wikipedia) .
[ 90 ]. Yerkes Observatory (Wikipedia) .
[ 100 ]. Martian canal (Wikipedia) .
[ 110 ]. Telescope Limiting Magnitude Calculator .
[ 150 ]. Detection Technique for Artificially Illuminated Objects in the Outer Solar System and Beyond .
[ 120 ]. Ballistic Missile Early Warning System (Wikipedia) .
[ 122 ]. Radars for the Detection and Tracking of Ballistic Missiles, Satellites, and Planets .
[ 145 ]. Allen Telescope Array, #Key science goals (Wikipedia) .
[ 150 ].A Search for the Tritium Hyperfine Line from Nearby Stars .
[ 153 ]. Eavesdropping on Radio Broadcasts from Galactic Civilizations with Upcoming Observatories for Redshifted 21cm Radiation .
[ 155 ]. A Search for Optical Laser Emission Using Keck HIRES5 .
[ 156 ]. Observational Signatures of Self-Destructive Civilizations .
[ 160 ]. Nuclear waste spectrum as evidence of technological extraterrestrial civilizations .
Evgeny Bobukh, 10/18/2015.
Question: How has this distance changed in the past? Is it possible to understand something by looking at the values of this parameter in a global historical perspective? It turns out yes. And that’s what we’ll do. We will see how the radius of self-detection R s , that is, the distance at which terrestrial civilization could detect another civilization at the same level of development, has changed over time.
Why "on the same"? Because it’s simpler and more definite. No need to speculate, probably mistaken whether the ancient Egyptians could notice someone else's satellite (and whether we can recognize someone else's super-technology, having it before our eyes).
Even in such a simplification, quantitative estimates are extremely difficult and may contain significant inaccuracies. Therefore, amendments, of course, are accepted, but only if I made a mistake, at least an order of magnitude.
So let's get started.
Primitive times: R s ≈ 1000 km
Australia began to populate about 40 thousand years ago, and Polynesia - 3-5. Between these milestones, people did not know either horses or any serious sailing.
Take a look at the map of the native languages of Australia [ 15 , Image Credit: Wikipedia]: The
colors on it indicate not just languages, but their families . That is, groups of languages that differ much more than Russian from Polish. A very similar picture emerges on the map of the languages of the natives of North America [ 16 ].
According to these cards, the cultural isolation between colorful shreds was extremely high. Otherwise, languages, for millennia, would probably have mixed up. So, the characteristic “radius of self-discovery” in those days was the size of the territory where they spoke the same family of languages. Just because if people knew their immediate neighbors somehow, what was happening behind their lands remained almost unknown.
Based on these considerations, we take R s ≈ 10 6 meters in the interval between the emergence of modern man (50 thousand years ago) and the first states (about 6 thousand years ago).
Roman Empire: R s = 8000 km
On the world map of Ptolemy’s work of the second century AD, China is already marked with the word “Sinae” ([ 20 , Image Credit: Wikipedia]):
According to the same source, the first exchange of ambassadors between the Roman and Chinese empires took place in the year 166. Thus, the two largest civilizations of the planet, at a similar level of development, find each other at a distance of R s = 8 * 10 6 m.
Magellan's voyage: R s = 20,000 km
In 1522, Magellan’s expedition completed a trip around the world ([ 30 , Image Credit: Wikipedia]):
If there was another European civilization on Earth, there would have been practically no chance of it being unnoticed by this time. That is, in 1522, R s = 2 * 10 7 m.
Moon Observations, 1610-1820s. R s = 384 thousand km
In 1609, Galileo built a telescope with an almost 30-fold increase, which made it possible, with some luck, to distinguish details on the moon with a size of 8-10 kilometers [ 40 ]. Unfortunately, London (one of the largest terrestrial cities at that time) then reached only about one mile in size [ 50 ].
By 1657, with a 100-fold Huygens telescope on the moon, it was possible to observe details of 2-3 kilometers [ 40 ]. London reached this mark around 1677 [ 52 ]. That is, if he had been there, they would have already seen him.
Technically, this is already the date of the inclusion of the lunar orbit in the circle of our self-discovery. However, due to observational difficulties, separate “discoveries” of the traces of civilization on the Moon took place until the 19th century [ 60 ], while the spectrographic works of Fraunhofer in 1823 [ 70 ] completely closed this question.
Subject to this amendment, we assume that R s reached a value of 4 * 10 8 meters by the year 1700.
Mars and Venus, R s ≈ 60 million km
From the 19th to the middle of the 20th century, R s grew mainly due to the increasing influence of our civilization on nature. Our buildings, cities, canals became larger and more noticeable.
Here, for example, the Karakum Canal:
An analysis of the parameters of telescopes [ 80 , 90 ] shows that it, in the form of a dark strip in this picture, could be seen on Mars by 1897. Which roughly coincides with the opening of the famous "Martian channels" (1877, [ 100]). And although those channels turned out to be an optical illusion, it is important to understand: they, in principle, could be real. Observational astronomy even then made it possible to notice a fairly large real channel due to seasonal darkening of adjacent vegetation. True, the Karakum Canal began to be built only in 1954, but channels of comparable size on Earth were laid earlier.
Another sign is the glow of the electric lights of night cities. Using the calculator [ 110 ] (or recounting the conclusions from [ 1 15 ]), we can estimate that if Venus had its own New York, then its night lights on the night side could be seen somewhere between 1900 and 1960 th years.
Based on the totality of these data, we take 6 * 10 10meters as the radius of self-discovery of terrestrial civilization by the 1930th year.
And then radio astronomy entered the scene.
1960s Radar Leaks, R s = 0.7 light years
The United States and the USSR, fearing a sudden nuclear attack by a neighbor, created Ballistic Missile Early Warning Systems, [ 120 , 122 ]. These systems have become one of the most powerful continuously operating radio sources on Earth.
They are based on an ordinary radar that runs around the horizon every few seconds and listens if a reflected signal comes from a flying warhead (Image Credit: Wikipedia):
What happens to a radar beam if it does not come across a warhead? Right. The beam flies on. Into space, past the Moon, planets of the Solar System, stars, and so on to infinity.
The beam strength is such that if you know in advance which star to “listen to”, you can catch it with the Arecibo telescope in 19 light years, and with good probability in 0.7 light years, inspecting the entire sky [according to 10 , table 4].
Thus, already around 1965, the parameter R s reaches a value of 0.7 light years, or 7 * 10 15 meters.
2015. Approximately 10 light-years
To say exactly what this radius is today is rather difficult. Many new tools, methods, but few systematic descriptions. Estimates often have to be done on the run by indirect evidence.
So, Allen Telescope Array [ 145], being brought to mind, it should confidently detect a radar equivalent to Arecibo at a distance of up to 300 parsecs (and 105 parsecs in its current form). This is assuming directional transmission. It is clear that at such a distance the probability of transmission is small for us. However, if we take the next 5 parsecs, then the stars in this sphere are already only fifty, of which the “decent” spectral class is only 30 in general. This amount can easily be grasped by directional transmissions. Accordingly, if “they” are not idiots, want to communicate, and live within a radius of several parsecs, then our chances of hearing “them” turn out to be very high.
What else? In the work [ 150] people searched for spectral tritium signatures in the radio range in a radius of about 20 light-years from the Sun. Tritium, as you know, is almost never found in nature, so any discovery of it would almost certainly indicate the activity of a cosmic civilization that actively uses thermonuclear energy. For example, for interplanetary flights on Orion
In [ 153] it is alleged that modern or near-expected radio telescope construction has reached a level sufficient to record the noise of alien television signals at distances of 10-500 parsecs. Of course, in the mode of careful listening to "interesting" stars, and not the whole sky in a row, which in practice means rather the lower limit of this range.
In addition to radio, other interesting channels have recently become significant.
Thus, laser transmissions (also directional) with a power of only 90 watts are detected by modern methods from distances up to 100 light years, according to [ 155 ]. That, taking into account the argument already cited about focusing on the nearest neighbors, again comes down to a highly probable detection on units of parsecs.
The consequences of a global nuclear war (in the form of a luminescence of ionized air) are at the limit of detectability by modern methods for nearby stars [ 156 ]. It is also reported that the James Webb Space Telescope, scheduled for launch in 2018, will be able to "feel" the pollution of planetary atmospheres with technogenic freons at interstellar distances.
Finally, if “they” guess to dump their radioactive waste into the local sun (we have such projects), then along the spectral lines of the rarest “fission fragments” (Tc, Pr, Nd, Pu, Ba, Zr), similar activity, with sufficiently serious its scale, it is possible to detect in general almost thousands of light years [ 160 , 156 ].
Summing up all this diversity, we conclude that if we had neighbors within a radius of a dozen light years from the Sun, we would probably have already noticed them.
Observations and Conclusions
For starters, it would be nice to plot R s versus time. It turns out to be far from easy! Our tools and eyesight pass before an addiction that extends over 20 thousand years and 11 orders of magnitude, but gaining six of them in some half a century. Even on a logarithmic scale, it just looks like a "wall."
In order to convey this dependence, one has to invent very unnatural coordinate systems. For example, the logarithm of the radius of self-discovery as a function of the logarithm of the number of years in the past from the (artificially chosen) year 2016:
Drawn. Growth, as you see, is fast and accelerating all the time. Interestingly, on a global historical scale, it looks like an integral process that began long before the SETI program or the invention of the telescope. It seems that the search for other civilizations is a natural part of the growth of our civilization.
The second question: what is the analytical form of this dependence? And can it be extrapolated to the future?
In a good way, you can’t do this. Through seven points, you can draw almost anything, and getting burned by extrapolation is easier than easy. It does not at all follow from the child’s growth rate in the first 10 years that by the age of 300 he will begin to step over tower cranes.
Therefore it is impossible. But if veryI want it, then a little bit is possible. At least as an exercise in working with such sharp addictions?
From the very first experiments, it becomes clear that this dependence is stronger than exponential. An attempt to fit an exponent into it ends in failure (R 2 = 0.227). Consequently, power functions are also excluded. They grow slower than exhibitors. We need something that grows, on the contrary, faster.
What do we know about our addiction? That, by its logic, it is always positive, everywhere is growing monotonously, and has no features in the past. Therefore, neither the Gamma function (there are features) nor the (shifted) exponent of a degree of the form R = exp ((YY 0 ) n) - because it has features and / or non-monotony for any n, except for odd integers, the adoption of which would be shamanism and fit for an answer.
The next in the speed class is the exponent of the exponent. It is finally possible to enter into the dependence (R 2 ≈ 0.92). In the chosen crazy coordinate system, it looks like this:
In the more natural coordinates for it “the logarithm of the radius, the exponent of a century”, everything turns out almost even elegantly: The
analytical expression takes the form:
Ln ( R s ( t )) = (4.2878708257 * 10 -8 ) * e ( Y / 100 ) + 16.0063874034
Where Y is the current year, and R s is expressed in meters.
In a more convenient form for perception, this dependence can be rewritten as follows:
Ln ( R s / 8940 km) = e С -16.9648904452
Where C is the current moment, expressed in centuries (i.e., for example, for 2015, C = 20.15). Amusing "coincidences" attract attention:
- The radius of self-discovery is “naturally” expressed in units comparable to the radius of the planet.
- 1696 is a turning point. Before it, the growth of R s (t) could also be described as exponential. After - as fundamentally faster.
- The characteristic scale for updating the growth rate of our review is 100 years.
Alas, it remains unclear whether these figures correspond to any objective reality, or whether they are artefacts of approximation at too few points.
By extrapolating this dependence into the future, it can be found that the radius of self-detection covers our Galaxy (180 thousand light-years) by the year 2046, and the visible Universe (14 billion light-years) by 2075. Can these numbers be believed? Of course not. But it can be reliably argued that the radius of self-detection is growing very quickly. And if we, as a civilization, do not ruin ourselves with a nuclear war or some shameful collapse of education, then we will have every chance to learn a lot of new and interesting things even during the lifetime of the current generation.
References and Sources
[ 1 ]. An interesting fact about the modern possibilities of interstellar communication .
[ 2 ]. Estimation of the probability of detecting a random radio signal from an extraterrestrial civilization .
[ 10 ]. Detectability of Extraterrestrial Technological Activities by Guillermo A. Lemarchand .
[ 15 ]. List of Australian Aboriginal languages (Wikipedia) .
[ 16 ]. Native American Languages .
[ 20 ]. Sino-Roman relations (Wikipedia) .
[ 30 ]. Ferdinand Magellan (Wikipedia).
[ 40 ]. The Developmental History of the Telescope .
[ 50 ]. History of London (Wikipedia) .
[ 52 ]. Map of London 1677 (City of London Ogilby and Morgan's Map of 1677.jpg) (Wikipedia) .
[ 60 ]. Communication with extraterrestrial intelligence, #History (Wikipedia) .
[ 70 ]. MOON INHABITATION, p 277 .
[ 80 ]. Timeline of telescope technology (Wikipedia) .
[ 90 ]. Yerkes Observatory (Wikipedia) .
[ 100 ]. Martian canal (Wikipedia) .
[ 110 ]. Telescope Limiting Magnitude Calculator .
[ 150 ]. Detection Technique for Artificially Illuminated Objects in the Outer Solar System and Beyond .
[ 120 ]. Ballistic Missile Early Warning System (Wikipedia) .
[ 122 ]. Radars for the Detection and Tracking of Ballistic Missiles, Satellites, and Planets .
[ 145 ]. Allen Telescope Array, #Key science goals (Wikipedia) .
[ 150 ].A Search for the Tritium Hyperfine Line from Nearby Stars .
[ 153 ]. Eavesdropping on Radio Broadcasts from Galactic Civilizations with Upcoming Observatories for Redshifted 21cm Radiation .
[ 155 ]. A Search for Optical Laser Emission Using Keck HIRES5 .
[ 156 ]. Observational Signatures of Self-Destructive Civilizations .
[ 160 ]. Nuclear waste spectrum as evidence of technological extraterrestrial civilizations .
Evgeny Bobukh, 10/18/2015.