Let’s start with a warning.
While some colleagues conjecture the existence of more than one universe, today we will limit ourselves and our imagination to our own information bubble: the sphere with a radius equal to the distance traveled by light from the beginning of time about 13.8 billion years ago. Taking into account the expansion of the Universe, our information bubble has a radius of about 46 billion light-years. Other theoretical universes, with other laws of physics, reside outside our cosmic bubble and are therefore beyond our power of examination.
Here, we might as well add another clarification to base our discussion on the plausible: by life, I mean any self-sustaining network of chemical reactions capable of metabolizing the energy of the environment and reproducing itself, following the rules of Darwinian natural selection. So no spiritual machine is more advanced than us; no weird, stellar smart clouds; and no swarms of wormhole-dwelling nanobots with some sort of collective self-awareness. Flying Spaghetti Monsters are fine. (See below.)
What the whole Universe shares
With that out of the way, now we can really get started.
Perhaps the most striking result of modern science is our understanding that the same laws of physics and chemistry apply across the vastness of space and time. We are now able to look at stars and baby galaxies billions of light years away from us and billions of years away. When we look at them and analyze their properties, we find that they have the same chemical elements (although in different ratios) and that they evolve according to the same dynamic laws that our own sun follows. The physical and chemical laws are the same everywhere and at all times. This allows us, earth creatures, to extend our research to the whole universe.
We also know – and this is another striking discovery in modern astronomy – that most stars come with a court of planets, and that planets tend to have their own set of moons. Each of them is its own world, with unique physical properties and chemical compositions. There are large and small planets; rocky and gaseous ones; planets with many moons, few or none. The planets spin like tops, with a large or a small tilt. (Earth’s tilt is 23.5° from vertical; Uranus’s is 97.7°.) Planets can have thicker or thinner atmospheres containing different gases. The list continues.
The Flying Spaghetti Monster
In round numbers, our Milky Way galaxy alone should contain around a trillion worlds, each of which is a unique entity with its own history.
If we add the hundreds of billions of other galaxies in our cosmic bubble, we have about a trillion trillion worlds in our universe, plus or minus a factor of one hundred. (A geeky comment: it’s funny that this is so close to Avogadro’s number, the number of atoms in a gram of hydrogen.)
At this point, you could reasonably suggest that in this staggering diversity of worlds, almost anything is possible. It may seem so at first glance. But this apparent freedom of very large numbers is not as free as it seems. The unity of the laws of physics and chemistry acts as a very powerful constraint on what can and cannot exist in nature.
In science, we can’t really rule out what may exist, as long as it satisfies the laws of physics as we know them. But we can use the laws of physics and chemistry to deduce what could to exist. Example: The Flying Spaghetti Monster is entirely plausible. We can imagine a cousin of the octopus that ventured out of the water a few billion years ago on the planet MumbaXX. After millions of years, our creature sprouted feathers from its tentacles and took flight. Or, if not feathers, a balloon mechanism using hot air from its digestive tract or thermal vents where it feeds.
Rules set in carbon
So what can we expect to find by browsing the vast collection of worlds and searching for living creatures? Although no one can answer this question, we can establish some ground rules.
Rule number one: life will be based on carbon. Why? Because carbon is the easygoing atom, with a chemical versatility no other element can match. Carbon has four unpaired outer electrons. It can form tight chemical bonds by sharing these electrons with other chemical elements. A potential alternative is silicon, but its biochemistry would be severely limited in comparison, with bonds about half as strong as those of carbon. Life needs versatility to thrive.
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Rule number two: Life needs liquid water. Yes, you can find frozen bacteria in permafrost, but they are not alive. Since life is, in essence, a web of complex biochemical reactions that move compounds one way or another, it needs a solvent – a medium where reactions can take place. Made up of oxygen and hydrogen, two of the most abundant chemical elements anywhere in the universe, water has a distinct advantage. In addition, it has the very unique property that ice floats – water in a solid state is less dense than in a liquid state.
Ammonia is sometimes offered as a possibility. But it is a gas at room temperature and only becomes liquid below -28°F at normal pressure. A cold planet with a heavy atmosphere might have liquid ammonia, but that takes a lot out of life. Indeed, any form of life under these conditions would have a very slow metabolism. Water is that magic substance that is transparent, has no smell or taste, and expands when it freezes (a key property for aquatic life in colder climates, as there is liquid water under the ice). It is also the main ingredient of our own body.
No other human in the Universe
Given these two constraints, the essence of life should be simple. It will include carbon, water and other elements (at a minimum, nitrogen).
The details, however, are not straightforward. Each planet that can contain life will have its own history. As a result, life there will also have its own history – a history contingent on the history of the host planet. The properties of a planet shape life on it. In turn, everything that lives on a planet will shape the properties of the planet. In each world, natural selection acts as a historically contingent pressure for survival. As conditions on the planet change, often due to the very presence of life on the planet, life will adapt in unique ways. It will never be the same on different worlds.
As a result, and despite the common carbon-water essence of life, there will not be identical life forms on different planets. The more complex the life form, the lower the chances that it will be reproduced elsewhere, even approximately.
If the Flying Spaghetti Monster exists, it will only exist on one world. In the same way, we only exist on one world. We are the only humans in this universe. And if we consider what we’ve learned from the history of life on Earth, chances are intelligent life is extremely rare. While intelligence is clearly an asset in the fight for species survival, it is not a goal. devolution; evolution has no goal.
Until it becomes intelligent, life is happy to simply reproduce. With intelligence, it will be unhappy to be content to reply. This, in a nutshell, is the essence of the human condition.
Putting it all together, we propose that we are indeed chemically connected to the rest of the cosmos and that we share the same basis of life as any other hypothetical living thing. At the same time, we are unique, just like all other living creatures. Life is an incredible force. From a carbon-based code and a common genetic ancestor, he can create a staggering variety of wonders – in this world, and perhaps in others.