## bookmark_borderThe Option for Life

We start right at the beginning of Cixin Liu’s novel Death’s End when the theoretical physicist Yang Dong finds herself in the control room of a supercomputer facility. In former times it was used to model particle collisions. But since the sophon block was put into action by the extraterrestrial Trisolarans, progress in fundamental physics has ceased. Instead, the facility is now used to simulate the evolution of planet earth.

Yang Dong is about to leave, as suddenly, a thought strikes her mind. She turns toward the facility operator and, out of the blue, asks him: “Do you believe in God?”

The facility operator looks up from a multitude of monitors, facing her through a pair of green glasses. “I don’t.”, says Green Glasses, a brave scientist, and turns back to the monitors.

“But if the physical parameters of the Big Bang would have been just slightly different, there would be, for example, no heavy elements, and thus no life. How can you believe that they weren’t fine-tuned in some way?” Yang Dong persists.

The operator faces her again, shaking his head.

“I can’t tell for the Big Bang …” he turns back to his monitor, “… but let’s see how the earth would have evolved without life.”

The supercomputer quickly runs a coarse-grained simulation, revealing an orange-colored planet featuring endless deserts and lonesome mountain ranges sprinkled by impact craters. Where did all the oceans, all the rivers, and all the groundwater go?

The simulation demonstrates that the earth would be massively different without life: Mountains not covered by vegetation are more likely to erode. Plains without plants are subject to desertification. And without life, our atmosphere’s composition would be vastly different, which might result in less protection against meteorite impacts or massive global warming like on Venus, eventually evaporating entire oceans.

“Earth is shaped by life. It is a home constructed by life for itself.”, the facility operator concludes.

### Selection Bias and Hyperbolic Fixed Points

When you start asking why there is life on earth instead of just a plain desert, a vantage point might be the anthropic principle, which you might come across when reading through the books of Stephen Hawking.

It tells us, in short, that as we observe life on earth, in the first place, our universe must be able to support life. If we are given a multiverse, only a universe with physical parameters amenable to the existence of life will finally end up hosting forms of life. So this is a sort of selection bias: If these parameters weren’t fine-tuned the way they are, there just wouldn’t be any living beings to rumor about their very existence (Weak Anthropic Principle).

Taking the perspective of theoretical physics and dynamical systems theory, to me, this seems pretty much as if the laws and parameters governing our universe were fine-tuned towards a hyperbolic fixed point, like a pendulum being stuck at its top position. If they were slightly different, thus, if the pendulum in our analogy would move even just slightly from the top, the resulting state would be entirely different: Moving away from the narrow fixed point that accommodates life and floating towards a multitude of dead universes.

A dead universe would be like the ground state, the globally stable solution, but also a trivial solution. Along these lines, a universe featuring life would be in an excited, non-equilibrium state, giving rise to a non-trivial fixed point solution. And the probability of hitting such a lively, hyperbolic fixed point straight would be almost zero. Thus, following the (weak) anthropic principle, there must be a multitude of dead universes out there (somewhere outside our universe, no one knows where exactly), and just by chance, our universe hit a fixed point of life.

### Life might be Attractive

Odds are getting better if one takes into account the impact of life itself. As the simulation of earth’s evolution in Cixin Liu’s novel demonstrated, the earth would be entirely different and way less hospitable without life. But on the contrary, life supports itself by forming sustainable, circular cybernetic systems, which dramatically increases the chance of finding forms of life once conditions get sufficiently close to a life-supporting state.

The odds of reaching such a life-supporting state remain still pretty low. But rather than hitting such a state straightaway, now one would have to get “just” sufficiently close to a state that is supporting at least primitive forms of life. And from there, life builds itself a home.

This should significantly increase our chances of ending up in a lively universe. While complex forms of life, such as human beings, require rather specific environmental conditions for survival (unless they are technologically developed), primitive life forms might develop even under extreme conditions such as on Mars or the Jupiter moon Europa.

Getting back to dynamical systems, this sounds a lot more like an attractive fixed point: It might be that tuning the parameters of our universe towards a lively solution might – at least locally – look like a stable focus rather than a hyperbolic saddle point. Think of an inverted double-well potential, which may describe the energy of a dynamical system / represent in our analogy a very simple universe with just one parameter $$\zeta$$.

An example is shown below: there is a stable fixed point of life, marked by a green “O,” surrounded by repulsive hyperbolic fixed points, marked by a red “X.” If we tune the parameter $$\zeta$$ to the left of the left “X” or the right of the right “X,” we end up in a dead universe for this simple universe. But if we get in between the two “X,” the energetically most favorable state is at the green “O,” and thus our universe might converge to the lively solution at “O.”

And as hitting a range of possible parameters, like the range between the two “X,” is easier than hitting the point “O” straightaway, when tuning the parameters of our universe, the existence of life might not be as improbable as we might think.

Indeed, the often-cited experiments by Miller and Urey have shown in the 1950s that once you expose a soup of inorganic, basic compounds to radiation, after some time, essential amino acids build-up, which are the basis of carbon-based forms of life as we know. For sure, organic molecules won’t make up for a living organism, but it’s a starting point, and so there might be a fair chance to end up with (more complex) forms of life.

This means that even if conditions were not exactly fine-tuned on earth, or, on a more intergalactic level, if the parameters and laws of our universe were not perfectly fine-tuned, there would be some margin of states which would still yield a lively solution. One would “just” have to get sufficiently close to a state similar to Miller and Urey’s experiment where a planet allows for the formation of essential amino acids, which allows for the formation of primitive forms of life, that would alter the environmental conditions of that planet and which eventually would support more complex forms of life.

Life might be like an attractive fixed point, and different states of a planet might be converging to a lively solution, as long as their initial conditions do not deviate too much from these fixed points representing life. Yet, these lively solutions would not be unique, just like dinosaurs and humans represent two different lively fixed point solutions based on carbon compounds, and there might be even forms of life that are not based on carbon.

On the one hand, this should make us aware of our responsibility to shape our planet earth for the future to keep it hospitable. On the other hand, it might not be as improbable as one might think to encounter life on other planets as well. Unlike the characters in Cixin Liu’s novels, we just haven’t encountered alien life yet (at least to my knowledge).

The conversation concludes, and Yang Dong departs from the supercomputer facility, left with the final, terrifying question: “If life has shaped planet Earth that much, how would the universe have evolved, if there were no life?” or vice versa: “Which impact will life have on the universe as a whole?”

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