Post by Andrei Tchentchik on Jun 17, 2019 10:29:35 GMT 2
(.#192).- Why humanity has found no trace of civilization E.T.
Why humanity has found no trace of civilization E.T.
Two theories make it possible to model the colonization of the Galaxy by an extraterrestrial civilization. One of them implies that certain areas will not be visited. Has our corner of the Milky Way been forgotten?
Roland Lehoucq is an astrophysicist at the Atomic Energy Commission.
Why has humanity so far found no trace of extraterrestrial civilizations? To answer this question he raised in 1950, the physicist Enrico Fermi examines the case of a civilization motivated by planetary colonization and endowed with reasonable technical means giving him control of interstellar travel. He believes that the time that will be required to fully colonize the Galaxy is ultimately quite small.
This, later, will clarify the axioms of the paradox that bears his name: "If technologically advanced extraterrestrial civilizations preceded us and if at least one of them embarked on planetary colonization, then the front colonization progresses at a rate sufficient to fill our galaxy in a very short time before its age. But we do not see traces of such a civilization. There has never been a civilization advanced in the Galaxy. "Sad conclusion for a reasoning that seems impeccable. But is it really possible to estimate the time needed to colonize a galaxy?
Bubonic plague
To start the conquest of the Galaxy, a strategy is needed from the outset: to go into space from his maternal planet to a habitable planet spotted in advance. Then colonize it and develop a complete society before using it as a starting point for a new leap into space. This colonization process can be modeled both by the population dynamics equations that, for example, account for variations in the population of silver foxes in the Far North, and by the equations describing the diffusion of matter, that it is used to describe how a drop of ink is dispersed in a glass of water.
This modeling has already been successfully applied to the description of the 1347 bubonic plague epidemic, which ravaged much of Europe or the spread of modern epidemics. In this context, the pace of galactic colonization depends on three parameters: the time taken to reach two neighboring stars, the speed at which the interstellar ships travel and the time between the arrival of the first settlers from the departure of a new vessel to space. Let's try to estimate these three quantities.
The time required to go from one star to another is obviously a function of the distance that separates them, but also the speed of the interstellar vessel used. In our galaxy, the average distance between two stars depends on the region where they are located. In the Milky Way bulb, this distance is less than 0.4 light-years whereas it is rather of the order of 4 in the solar neighborhood. But the most relevant distance is rather that which separates two stars having habitable planets. It is more difficult to estimate because we are still conjecturing the number of these planets. The nearest potentially habitable planet was discovered in 2011 by French astronomers. It revolves around the star Gliese 581, about 20 light years away. Let's take 10 light-years as a plausible value.
An interstellar journey at one tenth of the speed of light is technologically imaginable, even if the practical realization is not for tomorrow
To reach this planet in a reasonable time, it is necessary of course that the speed of the ship is consequent, much superior to those of our current probes. Voyager 1, currently located more than 21 billion kilometers from the Sun, spins into space with a speed of 17 km / s, barely 0.006% of the speed of light. At this speed, it would reach Proxima Centauri, the closest star to the Sun, in 70,000 years. To visit Gliese 581 would therefore take more than 350,000 years.
To improve these performances, no new physics is necessary. An interstellar journey at one tenth of the speed of light is technologically imaginable, even if the practical realization is not for tomorrow, as the project is ambitious compared to current standards. With such a speed, travel times are then of the order of the century. Once there, it is hoped that the settlers will settle in and that a new wave will leave later to reach another world. To estimate the time needed for this step, note that the world population has increased seven-fold during the last two centuries, from about one billion in 1800 to seven billion in 2011. At this rate, an initial colony of 1000 people reaches a population of one billion people in just 1400 years. This arbitrary choice of population allows us to realize that the "incubation" time, necessary for the preparation of a new departure to the stars, is of the order of one to two millennia.
With the values chosen, we can calculate that the effective rate of advance of the colonization front is of the order of 0.5% of the speed of light. At this pace, 20 million years are enough to cross the distance that separates us from the opposite edge of the Galaxy! By playing on the parameters - while remaining within a reasonable range -, the characteristic duration of this galactic colonization varies from a few tens to a hundred million years. The surprise comes from the fact that this duration is very small in front of the age of our Galaxy, estimated at 10 billion years, but also before the duration that it took to the life to evolve until the current human civilization, that is 3.8 billion years. Let us finally note that it took only 10,000 years for humanity to move from the Stone Age to the space age. As a result, any civilization that appeared before us long enough ago should have had time to fully colonize the Galaxy.
Big and small empty
One way to explain why we have not already encountered an extraterrestrial civilization is to realize that the pattern of diffuse colonization may not be complete. Another galactic colonization scenario is based on the theory of percolation, which describes how fires spread or how water circulates in a porous environment.
We will suppose that a colony can not establish itself on an already occupied world: it is already difficult enough to travel in space to be able to avoid any possibility of interstellar invasion
These models are based on three fundamental assumptions. First, since fire needs a fuel not too far away to propagate, interstellar travel is possible, but difficult, so that there is a maximum distance beyond which it is impossible to establish a colony. Thus, for any planetary system of departure, there is only a small number of colonizable planets situated at a reasonable distance. Secondly, given the distances that separate the stars, the mother civilization can not exercise any control over its colonies. This situation has already occurred in historic land colonies where, because of the slowness of communications, the local administration had a large autonomy vis-à-vis the metropolis.
In our analogy with fire, this means that a new home evolves independently of the one that gave birth to it. The colonies will therefore develop their own culture over the centuries, and possibly different visions on interstellar travel. Thirdly, since interstellar travel is rather difficult and expensive, it may be thought that not all will choose to found new colonies, either because they will not want it, or because of lack of resources, or because lack of suitable planets within their range of action. Each colony therefore has a certain probability P of founding others. In the case of a fire, it can be seen that the fire is likely to occur, depending on factors such as wind speed or fuel properties. Finally, as the fire can not take on a land already burned, we will suppose that a colony can not establish itself on an already occupied world: it is already difficult enough to travel in the space to be able to avoid any possibility of invasion interstellar.
Once these assumptions are fixed, the theory of percolation states that given the number of dimensions of the propagation space (here, it is obviously three) and the average number of colonizable planets, there is a critical probability below from which all waves of colonization will end with a finite number of colonies that will be grouped into "clouds" whose borders will be made up of non-colonizing civilizations. On the other hand, if P is greater than the critical value, these clouds can grow indefinitely and eventually fill the entire Galaxy. There will, however, be many small gaps. For values of P very close to the critical value, the clouds of colonized worlds are grouped together in fractal structures of irregular shape containing large occupied areas, but also large voids.
Critical threshold
The study of this new model of galactic colonization does not call into question the durations estimated by Fermi, but it specifies how the colonies will fill the galaxy. It also makes it possible to advance two explanations to the absence of an extraterrestrial visit while allowing the possibility of advanced civilizations: the probability that one colony establishes another is too weak and the colonization stops quickly; this probability is greater than the critical probability, but the Earth is located in one of the non-colonized regions. As nothing is simple ... The reasoning that leads to the Fermi paradox is essentially the calculation of the time needed to explore the Galaxy, itself based on a scenario of the expansion of the colonizing wave.
It has been shown that the existence of advanced extraterrestrial civilizations is compatible with the fact that we have never crossed paths with them. But a good way to know if it really exists could be to launch ourselves into the colonization of the Galaxy! After all, this long-term adventure is not totally out of reach since in principle the current technology is sufficient, even if the means to implement still far exceed us. If we are persevering and lucky, we may encounter galactic neighbors. Unless the distressing spectacle of our civilization already allows us to settle the question by saying as Calvin, the little boy of the BD Calvin and Hobbes, contemplating a wild dump in the forest: "Sometimes I wonder if the best proof that There are intelligent species somewhere in the Universe, none of them have yet tried to contact us. "
F I N .
Why humanity has found no trace of civilization E.T.
Two theories make it possible to model the colonization of the Galaxy by an extraterrestrial civilization. One of them implies that certain areas will not be visited. Has our corner of the Milky Way been forgotten?
Roland Lehoucq is an astrophysicist at the Atomic Energy Commission.
Why has humanity so far found no trace of extraterrestrial civilizations? To answer this question he raised in 1950, the physicist Enrico Fermi examines the case of a civilization motivated by planetary colonization and endowed with reasonable technical means giving him control of interstellar travel. He believes that the time that will be required to fully colonize the Galaxy is ultimately quite small.
This, later, will clarify the axioms of the paradox that bears his name: "If technologically advanced extraterrestrial civilizations preceded us and if at least one of them embarked on planetary colonization, then the front colonization progresses at a rate sufficient to fill our galaxy in a very short time before its age. But we do not see traces of such a civilization. There has never been a civilization advanced in the Galaxy. "Sad conclusion for a reasoning that seems impeccable. But is it really possible to estimate the time needed to colonize a galaxy?
Bubonic plague
To start the conquest of the Galaxy, a strategy is needed from the outset: to go into space from his maternal planet to a habitable planet spotted in advance. Then colonize it and develop a complete society before using it as a starting point for a new leap into space. This colonization process can be modeled both by the population dynamics equations that, for example, account for variations in the population of silver foxes in the Far North, and by the equations describing the diffusion of matter, that it is used to describe how a drop of ink is dispersed in a glass of water.
This modeling has already been successfully applied to the description of the 1347 bubonic plague epidemic, which ravaged much of Europe or the spread of modern epidemics. In this context, the pace of galactic colonization depends on three parameters: the time taken to reach two neighboring stars, the speed at which the interstellar ships travel and the time between the arrival of the first settlers from the departure of a new vessel to space. Let's try to estimate these three quantities.
The time required to go from one star to another is obviously a function of the distance that separates them, but also the speed of the interstellar vessel used. In our galaxy, the average distance between two stars depends on the region where they are located. In the Milky Way bulb, this distance is less than 0.4 light-years whereas it is rather of the order of 4 in the solar neighborhood. But the most relevant distance is rather that which separates two stars having habitable planets. It is more difficult to estimate because we are still conjecturing the number of these planets. The nearest potentially habitable planet was discovered in 2011 by French astronomers. It revolves around the star Gliese 581, about 20 light years away. Let's take 10 light-years as a plausible value.
An interstellar journey at one tenth of the speed of light is technologically imaginable, even if the practical realization is not for tomorrow
To reach this planet in a reasonable time, it is necessary of course that the speed of the ship is consequent, much superior to those of our current probes. Voyager 1, currently located more than 21 billion kilometers from the Sun, spins into space with a speed of 17 km / s, barely 0.006% of the speed of light. At this speed, it would reach Proxima Centauri, the closest star to the Sun, in 70,000 years. To visit Gliese 581 would therefore take more than 350,000 years.
To improve these performances, no new physics is necessary. An interstellar journey at one tenth of the speed of light is technologically imaginable, even if the practical realization is not for tomorrow, as the project is ambitious compared to current standards. With such a speed, travel times are then of the order of the century. Once there, it is hoped that the settlers will settle in and that a new wave will leave later to reach another world. To estimate the time needed for this step, note that the world population has increased seven-fold during the last two centuries, from about one billion in 1800 to seven billion in 2011. At this rate, an initial colony of 1000 people reaches a population of one billion people in just 1400 years. This arbitrary choice of population allows us to realize that the "incubation" time, necessary for the preparation of a new departure to the stars, is of the order of one to two millennia.
With the values chosen, we can calculate that the effective rate of advance of the colonization front is of the order of 0.5% of the speed of light. At this pace, 20 million years are enough to cross the distance that separates us from the opposite edge of the Galaxy! By playing on the parameters - while remaining within a reasonable range -, the characteristic duration of this galactic colonization varies from a few tens to a hundred million years. The surprise comes from the fact that this duration is very small in front of the age of our Galaxy, estimated at 10 billion years, but also before the duration that it took to the life to evolve until the current human civilization, that is 3.8 billion years. Let us finally note that it took only 10,000 years for humanity to move from the Stone Age to the space age. As a result, any civilization that appeared before us long enough ago should have had time to fully colonize the Galaxy.
Big and small empty
One way to explain why we have not already encountered an extraterrestrial civilization is to realize that the pattern of diffuse colonization may not be complete. Another galactic colonization scenario is based on the theory of percolation, which describes how fires spread or how water circulates in a porous environment.
We will suppose that a colony can not establish itself on an already occupied world: it is already difficult enough to travel in space to be able to avoid any possibility of interstellar invasion
These models are based on three fundamental assumptions. First, since fire needs a fuel not too far away to propagate, interstellar travel is possible, but difficult, so that there is a maximum distance beyond which it is impossible to establish a colony. Thus, for any planetary system of departure, there is only a small number of colonizable planets situated at a reasonable distance. Secondly, given the distances that separate the stars, the mother civilization can not exercise any control over its colonies. This situation has already occurred in historic land colonies where, because of the slowness of communications, the local administration had a large autonomy vis-à-vis the metropolis.
In our analogy with fire, this means that a new home evolves independently of the one that gave birth to it. The colonies will therefore develop their own culture over the centuries, and possibly different visions on interstellar travel. Thirdly, since interstellar travel is rather difficult and expensive, it may be thought that not all will choose to found new colonies, either because they will not want it, or because of lack of resources, or because lack of suitable planets within their range of action. Each colony therefore has a certain probability P of founding others. In the case of a fire, it can be seen that the fire is likely to occur, depending on factors such as wind speed or fuel properties. Finally, as the fire can not take on a land already burned, we will suppose that a colony can not establish itself on an already occupied world: it is already difficult enough to travel in the space to be able to avoid any possibility of invasion interstellar.
Once these assumptions are fixed, the theory of percolation states that given the number of dimensions of the propagation space (here, it is obviously three) and the average number of colonizable planets, there is a critical probability below from which all waves of colonization will end with a finite number of colonies that will be grouped into "clouds" whose borders will be made up of non-colonizing civilizations. On the other hand, if P is greater than the critical value, these clouds can grow indefinitely and eventually fill the entire Galaxy. There will, however, be many small gaps. For values of P very close to the critical value, the clouds of colonized worlds are grouped together in fractal structures of irregular shape containing large occupied areas, but also large voids.
Critical threshold
The study of this new model of galactic colonization does not call into question the durations estimated by Fermi, but it specifies how the colonies will fill the galaxy. It also makes it possible to advance two explanations to the absence of an extraterrestrial visit while allowing the possibility of advanced civilizations: the probability that one colony establishes another is too weak and the colonization stops quickly; this probability is greater than the critical probability, but the Earth is located in one of the non-colonized regions. As nothing is simple ... The reasoning that leads to the Fermi paradox is essentially the calculation of the time needed to explore the Galaxy, itself based on a scenario of the expansion of the colonizing wave.
It has been shown that the existence of advanced extraterrestrial civilizations is compatible with the fact that we have never crossed paths with them. But a good way to know if it really exists could be to launch ourselves into the colonization of the Galaxy! After all, this long-term adventure is not totally out of reach since in principle the current technology is sufficient, even if the means to implement still far exceed us. If we are persevering and lucky, we may encounter galactic neighbors. Unless the distressing spectacle of our civilization already allows us to settle the question by saying as Calvin, the little boy of the BD Calvin and Hobbes, contemplating a wild dump in the forest: "Sometimes I wonder if the best proof that There are intelligent species somewhere in the Universe, none of them have yet tried to contact us. "
F I N .
