Can a living being be the size of a galaxy?

Why life is limited to the size we meet on Earth

The size of the objects of our Universe varies from a tiny scale of 10 ^-19 m, on which quarks interact, to the cosmic horizon, located 10 ²⁶ meters from us. In these admissible 45 orders of magnitude, the life we ​​know is limited to a relatively small interval, only 9 orders of magnitude, located approximately in the middle of the universe: bacteria and viruses are less than a micron, 10 ^-6 meters, and the height of the largest trees reaches about 100 meters. [Americans call them honey mushrooms - approx. trans.], living at the foot of the Blue Mountains in Oregon, are likely to be a single organism, extending 4 kilometers across. The known intelligent life scale is even smaller, in the region of three orders of magnitude.



Can it be any different?



Progress in computational theory suggests that consciousness and intelligence require quadrillions of primitive “circuit” elements. Since our brain consists of neurons, which in themselves, in essence, are specialized cooperative unicellular organisms, it can be concluded that a biological computer needs to be of a size comparable to the brain in order to demonstrate our capabilities.







We can assume the possibility of creating neurons smaller in size than ours, for artificial intelligence systems. Elements of electrical circuits are now much smaller than neurons. But their behavior is also simpler; in addition, they require a supporting superstructure (energy supply, cooling, communication), which takes up a lot of space. Most likely, the first intelligent systems will be comparable in size to our bodies, despite the fact that they will be based on fundamentally different materials and architectures. This also suggests that there is something special on the scale.



What about the end of the scale with gigantic sizes? William Burroughs in his novel "The Ticket that Burst", presented that under the surface of the planet is "a huge inorganic consciousness near absolute zero thinking in unhurried crystal deposits." Astronomer Fred Hoyle wrote dramatically and convincingly about the intelligent and super-intelligent “Black Cloud”, comparable in size to the distance from Earth to the Sun. His idea preceded the Dyson spheres, massive structures that completely surround the star and take most of its energy. It is supported by the calculations that we carry out with my colleague Fred Adams. It turns out that the most efficient structures for processing information in today's galaxies may be located in smoky winds raised by dying red giants. For tens of thousands of years, red giants surrounded by dust provide the necessary amount of energy, a large enough entropy gradient and enough raw material to potentially exceed the estimated biosphere thickness of a billion terrestrial planets.



How big can such life forms be? Interesting thoughts require not only a complex brain, but also sufficient time for formulation. The speed of information transmission in neurons is 300 km per hour, that is, the signal crosses the human brain in about 1 ms. It turns out that 2 trillion such transitions fit in a person’s life (and each of them is enhanced by a rich and extremely parallel structure). If our brain and our neurons were 10 times larger, and the lifetime and speed of the signals did not change, we would have 10 times less thoughts in our entire life.



If our brain grew to the size of the solar system, and the signals were transmitted in it at the speed of light, then the transfer of a similar number of messages would require the entire current age of the Universe, which would leave no time for evolution. If the brain were the size of our galaxy, the problem would have become even more acute. From the moment of its formation, there would be enough time only for 10,000 messages crossing it from end to end. Therefore, it is rather difficult to imagine life forms with a complexity comparable to that of a human being, occupying scales much larger than the size of a star. If they existed, they would have no time for anything.



Interestingly, the environmental constraints applied to physical bodies also limit life to the size that is necessary for the emergence of intelligence. The height of the tallest sequoias is limited by their inability to lift water higher than 100 meters upwards - this restriction is a combination of gravitational force on Earth (pulling water down) and evaporation, wetting and surface tension in the xylem (pulling it up). If we assume that gravity and atmospheric pressure on other planets will not differ from the Earth one by more than 10 times, then we will get the same restrictions, which differ by no more than a couple of orders of magnitude.



If we also assume that most of life is tied to planets, moons or asteroids, then gravity also sets a natural scale. With the increase of the planet and the increase of its gravity, the force acting on the bones (or their equivalent) of hypothetical animals increases - Christian Huygens wrote about this as early as the 17th century. The animal would need to increase the cross-section of the bones in order to withstand such force, and it increases as the square of the size of the animal. However, these effects quickly fade away as the body mass increases, like a cube of size. On average, the maximum mass of mobile terrestrial organisms decreases approximately linearly with increasing gravity. Accordingly, on a planet with gravity is 10 times less than on Earth, animals could live 10 times more.







But there are minimum sizes for planets - if it is smaller (less than one-tenth the mass of the Earth), it will not be able to hold the atmosphere. Again we are limited to a factor of 10 in relation to the size that we see on Earth.



Life also needs to be cooled. Computer chip designers are constantly faced with the difficulties of removing the heat generated by computing. Living things have the same problem: large animals have a high ratio of volume to surface area, or skin. Since the skin is responsible for cooling the animal, and heat is generated by volume, large animals cool less effectively. As Max Kleiber calculated for the first time in the 1930s, the metabolic rate per kilogram of terrestrial animals decreases in proportion to the weight of the animal in the 0.25 degree. Indeed, if the heating rate did not decrease, large animals would simply boil. If we assume that for the normal functioning of a mammal, the minimum metabolic rate should be one trillion watts per nanogram, we will arrive at a maximum size of organisms of the order of a million kilograms - which is less than 10 times the mass of the blue whale, probably the largest ever Earth organisms.



It is possible, in principle, to imagine animals larger in size. Based on the Landauer principle , which describes the minimum amount of energy required for calculations, and assuming that the energy resources of a supermassive sluggish multicellular organism are spent only on the slow reproduction of its cells, we find that the problems of its mechanical support overtake the problems of heat removal and are the main limiting factor for growth . But on such a scale it becomes incomprehensible what would such a creature do or how it would appear as a result of evolution.



The classic film “Degrees of Ten” was shot four decades ago, but its influence is very deep. It can be linked, for example, with the fact that ordinal scores are firmly established in scientific use, and it served as an inspiration for the creation of cartographic software like Google Earth.







The influence of the film is enhanced by an amazing symmetry in the story, acting between immersion in the microcosm (in which the observer plunges in from the picnic scale on the shore of the Chicago lake to the subnuclear scale) and movement in the macrocosm (in which we fly away from the Earth and its contents into gigantic space scales) .



Are we, intelligent beings, lucky enough to have the opportunity to move in both directions and study the large and small scales of the Universe? Probably not.



Gregory Laughlin is a professor of astronomy and astrophysics at the University of California, Santa Cruz. Co-author of the book "The Five Epochs of the Universe - Inside the Physics of Infinity ", maintains a blog on the site oklo.org.