The authors of Rennie et al. (2017) paper think microbes are not so different from robots designed artificially with the capability of responding to the surrounding environment.
The authors first argue that the slime mold has a sense of intelligence by advancing towards in the direction with more nutrients by pumping cytoplasm in that direction. Additionally, slime mold has the ability of leaving chemical trails which reminds it places not worth visiting.
Further, the authors discuss the intelligence of bacteria, especially biofilms. First, they argue that biofilms have sophisticated structures which allow them to absorb nutrients nearby as much as possible. Moreover, the cells in biofilms have different functions depending on the location of the cells in the biofilms. Similar to slime mold, biofilms have the capability of moving towards locations with more nutrients by secreting materials which help them move more energetically and more rapidly.
Additionally, biofilms have different spiral migration pattern depending on their genes. Biofilms also are intolerant of other strains so they develop boundaries between each other when forming.
This paper discusses the possibility of detecting artificial illuminated objects in the outskirt of solar system by measuring the flux variation with respect to distance.
The authors begin by arguing that there are two basic illumination classes we use, one is thermal (light bulb) and the other one is quantum (LED). The spectra of those light sources should be very different from natural sources. Therefore, it is possible to detect those sources.
Further, the authors discuss whether we could detect those sources with our current technology and their conclusion is that we could be able to detect illumination level on the equivalent scale of large cities on Earth out to the outskirt of Solar system.
Additionally, the authors quantitatively calculate the flux versus distance slope difference between artificial objects (-2) and natural sources (-4). There are other factors that could affect this slope, including changing phase angle. Those factors influence the flux variation on the scale of 0.1 magnitude and should be able to be averaged out by long period of observation.
Finally, the authors argue that the chances of detecting such objects will be higher when the “dark side of the planet is more in view” or the host star of planet has went into a white dwarf so that the light contrast will be higher.
The paper discusses the possibility of searching indirect ETI evidence using the images taken from the Lunar Reconnaissance Orbiter (LRO).
The authors begin by arguing the advantages of searching for ETI on the Moon. First, the Moon is very close to the Earth, so we could observe the features on the Moon in detail. Second, the surface of the Moon is rarely changed due to the low impact rate from meteorites. Therefore, any large artifacts left by ETI on the Moon should preserve for a quite long time for us to search.
Then the authors classify the kind of artifacts into four categories: 1) messages 2) scientific instruments 3) trash 4) geo-engineering structures. For messages, the authors argue that the chance of finding such messages on the surface might be low. Further recovery of such message might depend on radar technology or excavation. For scientific instruments, the authors argue that it might be worthwhile to look for solar panels at the poles of the Moon but LRO did not find anything. As for trash and geo-engineering structures, the authors suggest future excavation missions might find interesting features.
Finally, the authors discuss how they would carry out the search using the existing data base. First, they suggest either hiring people to examine the images or do a computer automated scan. The problem with computer automation is that it could only find specific features. But we could expect any kind of strange features from ETI.
This paper discusses the possibility of detecting Bracewell probes in the Earth and Moon vicinity.
The author begins by classifying the probes into three categories: class one, objects intended to be found, class two, objects intended not to be found and class three, objects for which detection is not important or relevant. Since we have not detected any probes yet, it is likely we will not detect any class one or class two objects. Therefore, the author argues that the only observable objects will be class 3 objects.
Further, the author argues the sizes of those probes to be 1 to 10 m, taking into account 1) the long-duration of exploration 2) capability of withstanding meteoroid 3) radiation pressure 4) capability of returning information.
Next, the author discusses the search space in geocentric orbits, selenocentric orbits, Earth-Moon libration orbits, and Earth-Moon halo orbits. In each of the orbits, the author discusses the size of the search space and also the search speed.
Finally, the author argues that it is possible to detect such probes in a SETI program which extends for 2 years both from the space or ground. Additionally, the author also points out the possibility of detecting those probes in radio and infrared band.
The reason this paper is important is because this is the first time people have tried quantitatively to define where to search for Bracewell probes near the Earth and Moon.
The author in this paper discusses the possibility of detecting artificial non-spherical objects with Kepler and COROT, including single objects and multiple objects. The three single object cases discussed in this paper are triangle, two-screen and louver-like six screen shaped objects. The transit depths generated by their simulations are on the order of 100ppm which is detectable with Kepler. Then the author moves on to discuss detecting multiple-object transit signals, specifically, multiple transits, grouped by prime numbers.
Further, the author discusses the efficiency of using the above mentioned artificial transits as a communication tool. The author compares this way of communication with laser beacons and finds similar communicating efficiencies. The author additionally shows that with our current technology, to communicate with each target star using laser pulses, the time required is on the order of days. The communication efficiency will be much improved now since the launch of GAIA satellite.
Finally, the author argues that transit signals will be used for attention-getting and laser pulses will be used for data transfer since it is more directional.
There are several limitations to this paper, first, in the communication efficiency part, the author does not take into account the factor of distance. Second, in the communication efficiency part, the author assumes ETI has knowledge of proper motions and distances to the target stars which is not always the case.
The main concern I have with this paper is that the author does not at all discuss what could mimic the artificial signals during the limb-darkening period for natural sources such as stellar activities.
This paper presents an empirical way to find type 3 Kardashev civilizations, which is to identify outliers in the Tully-Fisher relation for spiral galaxies and in the fundamental plane for elliptical galaxies.
The author begins by arguing that it is possible to observe galaxy scale interruption of starlight. Then the author derives the scaling relation for galaxies argues that the scatter seen in empirical data around the scaling relation is as low as 10 percent.
This tight relation could be used to identify outlier galaxies which have very low surface brightness, probably with a infrared excess if the type 3 Kardashev civilizations use technologies such as Dyson sphere.
Then the author describes the empirical effort to find such outliers in 31 spiral galaxies and 106 elliptical galaxies. However, no significant outlier has been detected in those galaxies. The author argues that this could be due to setting a too strict cut on the surface brightness deviation. Newer samples could reduce the cut down to 50 percent. However, the author leaves it for another time. Further, the author discusses the limitation of photographic plates not being able to detect low surface brightness galaxies which could give us biases towards regular galaxies than galaxies harnessed by type 3 Kardeshev civilizations.
Finally, the author discusses how long it would take for a type 3 Kardashev civilization to rise and the upper limit is 304 billion years which is much longer than the age of the universe. There are several reasons which could lower this upper limit. First, the sample is incomplete and they do not have any very low surface brightness galaxies. Second, not all type 3 Kardashev civilizations have to be star-fed. They could harness other types of energy such as dark matter and dark energy. Third, the time for those civilizations to develop may be much shorter than 10 billion years. One thing I do not understand is that why it is an upper limit than a lower limit because if those civilizations do not exist, the upper limit should be infinite.
Compared to the Cocconi and Morrison’s proposal to search for radio signals in the nearby stars. Dyson (1960) pointed out it was also useful to look at infrared signals at 10 microns radiated by his hypothesized Dyson sphere. He argued much more advanced civilizations would have built thin shells around their host stars to more efficiently harvest the energy of the host star.
He argued the Malthusian pressure would drive some of the advanced civilizations to build such a sphere to catch up with the exponential population growth. Additionally, he also argued that the such a sphere would have similar orbit size as the Earth and same temperature as well so that we could possibly detect it at the 10 microns which was also transparent to the Earth atmosphere.
I do not really understand how he concluded that the temperature of the sphere would be at the same temperature as the Earth. Would not the energy harvesting efficiency be much higher if the sphere was radiating at a much lower effective temperature. It seems to me that his conclusion about the temperature of the sphere came from our understanding of the Earth and it was 10 microns which was also transparent to the Earth atmosphere made him draw the conclusion that we should observe at 10 microns.
In the later reply letter, he clarified the idea of Dyson sphere to not being a solid sphere which was physically impossible to build but a swarm of objects traveling on independent orbits around their host star. He also admitted whether the alien civilizations obeyed the Malthusian principle or not was a question of taste. Finally, he argued that even if we did not find any aliens, the search for intense infrared signals could help us identify interesting astronomical objects.
The Oliver (1979) paper addresses the question which frequency range is optimal for SETI search.
The author first argues that advanced civilizations will choose the least expensive means of communication to establish contact with other civilizations. And clearly detecting electromagnetic waves is much more cost effective than sending space probes and spaceships. Then the author begins arguing why the electromagnetic waves, especially low-energy photons suits best for SETI search because they are massless, chargeless.
Further, the author makes the case for the optimal spectral region for SETI by analyzing where the galactic and cosmic radiation background are the smallest. This constrains the frequency range to be 1 to 60 GHz. Further complications such as the water absorption lines in the atmosphere further constrains the terrestrial microwave window to be between 1 to 10 GHz. The author also argues that the noise level will be much higher if we go to optical frequency range. There are additional merits using the 1 to 10 GHz frequency range such as great collecting area, higher beacon powers and narrower bandwidths.
Next, the author considers the doppler shift caused by the rotation of the Earth on the microwave signal. Then the minimum of total receiver noise could be achieved around 1.5GHz. Finally, the author makes his comment on the frequency range chosen for the Cyclops report (1420 MHz to 1662 MHz). He is pretty optimistic because we are now at least know where to look for SETI signal and waterhole is best for searching for life since all life we know so far depends on water.
The reason the paper is assigned is because unlike the Cocconi and Morrison (1959) paper, the paper theoretically derives the best frequency range to observe. One possibility the paper does not consider as important as radio SETI is the possibility of optical SETI, which is a more recent SETI research direction.
One limitation of this paper is that it assumes alien civilizations possess the same technology as we do so they will also look in the waterhole frequency range which is not always the case.
The problem the paper is trying to address is that whether Fermi’s paradox is truly Fermi’s original idea and whether it is a paradox or not. This is important because the misinterpretation of Fermi’s original idea has lead to two time cancellations of NASA funding for SETI.
Fermi’s original question was “where is everybody?” which did not question the existence of extraterrestrial intelligence. He was questioning the feasibility of conducting interstellar travel. However, Hart (1975) and Tipler (1980) mistakenly treated this question as a conclusion which is the aliens do not exist because we do not see them. The difference between the two is that one is a question and one is a conclusion.
Further, the author argues that the Fermi paradox which really should be called the Hart-Tipler argument is not a paradox. Instead it is a reductio ad absurdum argument. They are disproving the existence of extraterrestrial intelligence by showing the absurd results they will get if they assume the extraterrestrial intelligence exists. However, there also exist many preconditions Hart and Tipler assumed for their argument which includes the feasibility of interstellar travel, the lifetime of civilization. They made assumptions which might not be true. Therefore, their conclusion depends on the assumptions they make. So it is not a true paradox since we can tweak the assumptions and make the counter argument.
The author also rebuts a similar argument which is there is no other extraterrestrial intelligence because we have not detected anything. The author argues that the incompleteness in the searching for SETI signal could be the real cause of not detecting anything.
My takeaway for this paper is how easily people will misinterpret other people’s ideas. Especially I think for Fermi’s case, the reasons this happened is because Fermi has never published his idea and he was already dead when the Hart and Tipler papers came out. If he was alive, he would clarify his argument clearly and it will not be called Fermi’s paradox today.
The Hart (1975) is a complete rejection of SETI studies. The problem the paper is trying to address is that the whether we are the only intelligent life in our galaxy. The author supports this point by disproving three alternative hypotheses that could lead to the fact that we have not detected or encountered any other alien intelligent life. The first one is the technology argument that it could be very energy and time consuming to do interstellar travel. However, the author argues that the lifespan and form of technology might be completely different from human so there could be alien civilizations that can perform long distance interstellar travel. The second alternative argument is that the alien civilizations have cultures that prevent them from visiting Earth or they might have destroyed themselves quickly through nuclear wars. The author argues that these hypotheses are simply inadequate since the culture for an alien civilization could change over time and we are assuming they could destroy themselves based on our own current global situation. The third alternative argument is the alien civilizations have not reached Earth yet. The author counters this argument by saying it would only take 2 million years to explore the whole galaxy so the chances that alien missed us is very small. The fourth and final argument is that alien might have visited Earth before. The authors counters this argument by questioning why the alien civilizations have stopped visiting us and we could not provide sufficient answers to this question.
By countering all the alternative explanations that might explain why we have not encountered any aliens so far, the author concludes that we are the only intelligent civilization in the galaxy. However, I think the author missed one important point that even alien does not exist now. It does not mean it did not exist before. I could reference the video called “HALO” that there existed a life form called the swarm that depended on other form of life in the universe. Maybe the aliens in our galaxy were almost all killed by the swarm and we are one of the few civilizations that survived. The other possibility is that instead of Earth as a zoo, the whole galaxy, even the whole universe is a zoo, specifically created for human. Finally, maybe the advanced civilizations in our galaxy have cloaking devices that hide them from the rest of the galaxy for safety purposes and that is the reason we have not detected any of them. It could be very dangerous to send our location information out into the universe.