Let's start with a qualification. When some researchers speculate that there is more than one universe, today we still limit our imagination to an "information bubble": the universe is like a sphere with a radius equal to the distance traveled by light since the beginning of time 13.8 billion years ago. Considering the expansion of the universe, the radius of this information bubble is about 46 billion light-years.
Other theoretical universes, as well as other different laws of physics, exist outside our cosmic bubbles, beyond our ability to examine.
Here, we might as well add another qualification to our discussion: the life mentioned here refers to any self-sustaining chemical reaction network, which can metabolize energy from the environment and reproduce according to Darwin's natural selection rules. Therefore, there is no more advanced mental machine in the universe than us; There is no strange intelligent nebula that breeds stars, and there is no nano robot that lives in wormholes and is endowed with some kind of collective self-consciousness - "flying face God" (see below).
The foundation shared by the whole universe
Based on these understandings, now we can really start the discussion. Perhaps one of the most striking results of modern science is to make us realize that the same laws of physics and chemistry apply to the vast space and time. We can now observe stars and small galaxies billions of light-years away, and their history has been billions of years. When we observe and analyze their properties, we will find that they have the same chemical elements (although different proportions), and their evolution follows the same dynamic law as the sun. The laws of physics and chemistry are the same whenever and wherever. This enables us living on earth to expand the vision of exploration to the whole universe.
We also know that another amazing discovery of modern astronomy is that most stars have multiple planets, and these planets often have their own moons. Each planet or satellite has its own world, with unique physical properties and chemical composition. Planets are large and small, made of rocks and gaseous; Some have many satellites, some have only one or two, or none. The rotation of the planet is like a top, with a large or small axis inclination (the axis inclination of the earth is about 23.5 degrees, while Uranus is an amazing 97.7 degrees). The atmosphere of planets is thick and thin, and the gas composition in it is different. Such examples are numerous.
Flying face God
By integer estimation, the Milky Way Galaxy alone has about 1 trillion worlds, each of which is a unique entity with its own history.
If we add hundreds of billions of other galaxies in the cosmic bubble, we can calculate that there are about 1 trillion worlds in the universe, with an error of about 100 times (some people commented that this value is too close to the Avogadro constant, which is a dimensionless number, about 6 per mole) × 10 ^ 23, originally defined as the number of atoms contained in 1 gram of atomic hydrogen, and later redefined as the number of atoms contained in 12 grams of carbon-12).
From this perspective, you may take it for granted that almost everything is possible in a universe with such amazing diversity. At first glance, it seems so, but this huge number is not as "free" as it seems. The unity of the laws of physics and chemistry is an extremely powerful constraint that limits what can and cannot exist.
In science, we can't really exclude what may exist, as long as it satisfies the laws of physics we know. But we can infer what may exist through the laws of physics and chemistry. For example, the existence of "flying face God" makes sense in science. We can imagine that billions of years ago, an octopus like creature ventured out of the water on mumbaxx; Millions of years later, the creature grew feathers on its tentacles and began to fly. Alternatively, instead of growing feathers, they may have developed a mechanism for using hot air - from the digestive tract or the hydrothermal vents they feed on - to fly like balloons.
Carbon rules
So what can we expect to find when we search for life in many worlds of the universe? At present, no one can answer this question, but we can make some basic rules.
Rule 1: life will be based on carbon. Why? Because carbon is an easy-going atom with chemical diversity unmatched by other elements. Carbon has four unpaired outer electrons, which can form close chemical bonds by sharing these electrons with other chemical elements. Another potential alternative element is silicon, but compared with carbon, the biochemical composition of silicon will be seriously limited, and the strength of chemical bonds formed by silicon is only about half that of carbon. Life needs pluripotency to thrive.
Rule 2: life needs liquid water. Of course, we can find frozen bacteria in permafrost, but they are not living. In essence, life is a complex biochemical reaction network. It needs a solvent and a medium that can unfold the reaction to move the compound in this direction or that direction at any time. Water is composed of oxygen and hydrogen, which are the two most abundant chemical elements in the universe and have obvious advantages. In addition, water has a very unique characteristic: solid water (ice) is less dense than liquid water and can float.
Ammonia is sometimes considered another possibility, but it is a gas at room temperature and becomes a liquid at atmospheric pressure only below minus 28 degrees. A cold, heavy planet may have liquid ammonia, but it requires too much life. In fact, under these conditions, the metabolism of any life form will be very slow. Water is a magical substance. It is transparent, colorless and tasteless, and will expand when frozen (this is a key attribute for water-based life in cold climate, because there can be liquid water under the ice). Water is also the most important component of our body.
Are there any other humans in the universe
With these two constraints, the essence of life is very simple. It will include carbon, water and other substances (at least nitrogen).
However, if we continue to study deeply, the situation will not be so simple. Every planet that may contain life has its own history. Therefore, life there will also have its own history, and their history depends on the history of the host planet. The characteristics of planets determine the form of life. Conversely, any life on a planet will shape the attributes of the planet. In every world, natural selection is a kind of survival pressure under historical conditions. As planetary conditions change - often due to the existence of life - life adapts in a unique way. In different worlds, life will be presented in different forms.
Therefore, although life has a common carbon water essence, different life forms will be produced on different planets. The more complex a life form is, the less likely it is to be replicated elsewhere, even if it is similar.
If the "flying face God" really exists, it should only exist in one world of the universe. Similarly, we can only exist in the world of earth. We are the only human beings in the universe. If we review the life history of the earth, we will find that intelligent life is likely to be extremely rare. Although intelligence is an obvious advantage in the survival struggle between species, it is not the purpose of evolution; Evolution has no purpose.
Before having intelligence, life is just copying happily; With intelligence, their replication process may not seem so happy. In short, this is the essential state of human beings.
In conclusion, it can be considered that humans do have chemical connections with other parts of the universe - we have the same basis of life as any other hypothetical alien creature. At the same time, humans are unique, as are all other creatures. Life is a magical power. Starting with a carbon based code and a common genetic ancestor, it can create amazing miracles of diversity in this world and possibly in other worlds.