Physics

The Milky Way Galaxy contains an unknown number, N , of civilizations that emit electromagnetic radiation (of unknown wavelengths) over a finite lifetime, L . Here we are assuming that the radiation is not produced indefinitely, but within L as a result of some unknown limiting event. When a civilization stops emitting, the radiation continues traveling outward at the speed of light, c , but is confined within a shell wall having constant thickness, cL . We develop a simple model of the Galaxy that includes both the birthrate and detectable lifetime of civilizations to compute the possibility of a SETI detection at the Earth. Two cases emerge for radiation shells that are (1) thinner than or (2) thicker than the size of the Galaxy, corresponding to detectable lifetimes, L , less than or greater than the light-travel time, ∼100,000 years, across the Milky Way, respectively. For case (1), each shell wall has a thickness smaller than the size of the Galaxy and intersects the galactic plane in a donut shape (annulus) that fills only a fraction of the Galaxy's volume, inhibiting SETI detection. But the ensemble of such shell walls may still fill our Galaxy, and indeed may overlap locally, given a sufficiently high birthrate of detectable civilizations. In the second case, each radiation shell is thicker than the size of our Galaxy. Yet, the ensemble of walls may or may not yield a SETI detection depending on the civilization birthrate. We compare the number of different electromagnetic transmissions arriving at Earth to Drake's N , the number of currently emitting civilizations, showing that they are equal to each other for both cases (1) and (2). However, for L<100,000 years, the transmissions arriving at Earth may come from distant civilizations long extinct, while civilizations still alive are sending signals yet to arrive.

A recurring topic in interstellar exploration and the search for extraterrestrial intelligence (SETI) is the role of artificial intelligence. More precisely, these are programs or devices that are capable of performing cognitive tasks that have been previously associated with humans such as image recognition, reasoning, decision-making etc. Such systems are likely to play an important role in future deep space missions, notably interstellar exploration, where the spacecraft needs to act autonomously. This article explores the drivers for an interstellar mission with a computation-heavy payload and provides an outline of a spacecraft and mission architecture that supports such a payload. Based on existing technologies and extrapolations of current trends, it is shown that AI spacecraft development and operation will be constrained and driven by three aspects: power requirements for the payload, power generation capabilities, and heat rejection capabilities. A likely mission architecture for such a probe is to get into an orbit close to the star in order to generate maximum power for computational activities, and then to prepare for further exploration activities. Given current levels of increase in computational power, such a payload with a similar computational power as the human brain would have a mass of hundreds to dozens of tons in a 2050 - 2060 timeframe.

The large distances involved in interstellar travel require a high degree of spacecraft autonomy, realized by artificial intelligence. The breadth of tasks artificial intelligence could perform on such spacecraft involves maintenance, data collection, designing and constructing an infrastructure using in-situ resources. Despite its importance, existing publications on artificial intelligence and interstellar travel are limited to cursory descriptions where little detail is given about the nature of the artificial intelligence. This article explores the role of artificial intelligence for interstellar travel by compiling use cases, exploring capabilities, and proposing typologies, system and mission architectures. Estimations for the required intelligence level for specific types of interstellar probes are given, along with potential system and mission architectures, covering those proposed in the literature but also presenting novel ones. Finally, a generic design for interstellar probes with an AI payload is proposed. Given current levels of increase in computational power, a spacecraft with a similar computational power as the human brain would have a mass from dozens to hundreds of tons in a 2050-2060 timeframe. Given that the advent of the first interstellar missions and artificial general intelligence are estimated to be by the mid-21st century, a more in-depth exploration of the relationship between the two should be attempted, focusing on neglected areas such as protecting the artificial intelligence payload from radiation in interstellar space and the role of artificial intelligence in self-replication.

The Drake equation has been used many times to estimate the number of observable civilizations in the Galaxy. However, the uncertainty of the outcome is so great that any individual result is of limited use, as predictions can range from a handful of observable civilizations in the observable universe to tens of millions per Milky Way-sized galaxy. A statistical investigation shows that the Drake equation, despite its uncertainties, delivers robust predictions of the likelihood that the prevalent form of intelligence in the universe is artificial rather than biological. The likelihood of artificial intelligence far exceeds the likelihood of biological intelligence in all cases investigated. This conclusion is contingent upon a limited number of plausible assumptions. The significance of this outcome in explaining the Fermi paradox is discussed.

The last two decades have seen the number of known exoplanets increase from a small handful to nearly 2000 known exoplanets, thousands more planet candidates, and several upcoming missions that are expected to further increase the population of known exoplanets. Beyond the strictly scientific questions that this has led to regarding planet formation and frequency, this has also led to broader questions such as the philosophical implications of life elsewhere in the universe and the future of human civilization and space exploration. One additional realm that hasn't been adequately considered, however, is that this large increase in exoplanets would also impact claims regarding astrology. In this paper we look at the distribution of planets across the sky and along the Ecliptic, as well as the current and future implications of this planet distribution.

A hard difficulty in Astrobiology is the precise definition of what life is. All living beings have a cellular structure, so it is not possible to have a broader concept of life hence the search for extraterrestrial life is restricted to extraterrestrial cells. Earth is an astronomical rarity because it is difficult for a planet to present liquid water on the surface. Two antagonistic bioethical principles arise: planetary protection and terraforming. Planetary protection is based on the fear of interplanetary cross-infection and possible ecological damages caused by alien living beings. Terraforming is the intention of modifying the environmental conditions of the neighbouring planets in such a way that human colonisation would be possible. The synthesis of this antagonism is ecopoiesis, a concept related to the creation of new ecosystems in other planets. Since all the multicellular biodiversity requires oxygen to survive, only extremophile microorganisms could survive in other planets. So, it could be carried out a simulation of a meteorite by taking to other planets portions of the terrestrial permafrost, or ocean or soil, so that if a single species could grow, a new ecosystem would start, as well as a new Natural History. As a conclusion, ecopoiesis should be the bioethical principle to guide practices and research in Astrobiology.

One of the most unusual military aircraft programs V / STOL was the Avro VZ-9 "Avrocar". Designed to be a real flying saucer, the Avrocar was one of the few V / STOL to be developed in complete secrecy. Despite significant changes in the design, during flight tests, the Avrocar was unable to achieve its objectives, and the program was eventually canceled after an expenditure of 10 million US dollars between 1954 and 1961. But the concept of a lift fan, driven by a turbojet engine is not dead, and lives today as a key component of Lockheed X-35 Joint Strike Fighter contender. Was held in a data research and information related to Avrocar project carried out during the Second World War, which was directly linked to advances in aircraft that were built after it, and correlate them with the turbo fan engines used today.

The motion of the floating bridge of the banjo, in conjunction with the break angle of the strings over that bridge, produces string tension modulation that is first order in the amplitude of the string motion. This note refines a previous suggestion regarding the impact on the frequencies of the strings' and bridge's motion. For a given mode frequency pair of string and bridge, the resulting tension modulation produces a new, additional motion characterized by the sum and difference of the original ones. Strictly speaking, this corresponds to canonical "frequency modulation" only in the limit of modulation slow compared to the string frequency. The more general result is precisely an example of what is known as "parametric oscillation," first analyzed by Rayleigh. The qualitative impact of tension modulation on banjo timbre remains as suggested previously. It is only the precise math and physics that warrants this correction.

The interaction of a drum's head with its enclosed air is presented in the simplest possible form appropriate to the questions and issues that arise in understanding the timbre of the banjo. The inherent air-head impedance mismatch allows treating the head as driver of the air and the air's effect, in turn, as back reaction. Any particular question can then be addressed with a calculation in simple wave mechanics. The analysis confirms and quantifies the notion that internal air resonances enhance the response of the head at its and their frequencies. However, the details of just how are fairly complicated.

11" D mylar heads over a normal range of tensions (DrumDial 85 to 91) and "open-back" backed pots of depths 2", 2 3/4", and 5 5/8" are studied over the range 100 to 2000 Hz. Normal modes and resonant frequencies of the heads and of the pot air separately are easily identified and agree with simple expectations. The present focus is the head - pot air interaction. There is no "gold-plated" example of a pair of head-air interacting modes that are distant in frequency from all others. (Had there been such a pair, their interaction could have been isolated and studied in detail.) Nevertheless, there are a few cases where there are hints of the kind of interactions expected from a simple theory. The investigations also offer several examples of banjo physics, including aspects of bridge position and rim flexibility, and some dramatic examples of the perils of sound recording, including floor bounce and room sound.