Why the laws of physics will never fully explain the universe
It is better to think of the cosmos as a living organism, and not as a machine.
Originally Posted by Professor Andrew Pontzen, Author of Universe in a Box: A New Space Story [The Universe in a Box: A New Cosmic History]
It’s hard to come to terms with the sheer scale of the cosmos: there are hundreds of billions of stars in our galaxy and at least trillions of galaxies in the universe. But for a cosmologist, something even more intriguing than the mind-boggling numbers themselves is the question of how all these stars and galaxies came into existence in 13.8 billion years. This is the most real prehistoric adventure. Life cannot develop without a planet, planets cannot form without stars, stars must be contained within galaxies, and galaxies could not exist without the rich structure of the universe to support them. The history of our origins is written in the sky, and we are only learning to read it.
It once seemed that, for all its vastness, the cosmos could be understood with the help of a small number of strict physical laws. Newton brought this idea to life by showing that apples falling from trees and the orbits of planets around our Sun are due to the action of the same force – gravity. A similar radical unification of terrestrial and celestial phenomena has survived in modern teaching: it is assumed that all countless molecules, atoms and subatomic particles in the universe obey the same set of laws. Most of the evidence suggests that this assumption is correct, and it follows that the improvement of our understanding of these laws will allow us to resolve all remaining questions about cosmic history.
However, this is a logical fallacy. Even if we imagine that humanity will eventually discover “the theory of everything”, covering all individual particles and forces, the explanatory value of this theory for the Universe as a whole is likely to be negligible. During the 20th century, even as particle physics unraveled the secrets of atoms, it became clear that the behavior of nature at the macro level could not be understood through the study of individual objects.
An example is social insects on Earth. For example, army ants gather in swarms to find colonies of smaller prey, which they then devour. While swarming, they perform extraordinary feats, using their bodies to level terrain and even build bridges over uneven ground.
From the human point of view, the collective behavior of the ants might suggest that there is a leader in the nest who develops a strategy to reach the prey efficiently, but there is no such leader. There are only solitary ants that follow simple and unchanging rules, such as joining an ant bridge if there are many individuals pushing behind them, and leaving it if no one is crawling over. The apparent complexity arises from the huge number of individuals that follow these rules. As physicist Philip W. Anderson said, “More obeys different rules“.
Seemingly the epitome of clockwork predictability, the solar system therefore has an uncertain long-term future. Separately, one planet around one star could rotate indefinitely, but in reality there are several planets, and each of them, albeit very subtly, pushes the other. Over time, a series of tiny shocks can lead to serious consequences that require a huge amount of calculations to predict.
To a certain extent, computers can solve this problem by modeling the overall result by adding individual influences using fast and reliable arithmetic. The problem is that the simulations don’t agree with each other. Some predict that the solar system is stable despite constant pushes, others suggest that in a few billion years, Mercury under the influence of these forces may enter a collision course with Venus or even fly out of the system.
Models of the solar system diverge in predictions because no calculation can fully account for all influences, and even the slightest disagreement about individual points leads to a completely different result in the end. This is an example of such a phenomenon as chaos, and this both interests and worries scientists. Interested because it becomes clear that planetary systems can exhibit much richer behavior than the cold and lifeless law of gravity would suggest. Worrisome, because even if the solar system is chaotic and unpredictable, then there are fears that an attempt to understand and describe the entire universe is doomed to failure.
Take galaxies, which are, on average, tens of millions of times larger than the solar system and feature a rich variety of shapes, colors, and sizes. In order to understand how galaxies turned out to be so diverse, it is necessary, at a minimum, to know how and where stars formed in them. However, star formation is a chaotic process in which diffuse clouds of hydrogen and helium slowly condense under the influence of gravity, and no computer is able to keep track of all the necessary atoms (only in our Sun there are about 10 of them57). Even if the calculations were feasible, the chaos would exponentially increase the smallest uncertainties, preventing us from getting a definitive answer. If we strictly adhere to the traditional laws of physics as an explanation for galaxies, then this is the end of the road.
To “fit” in a computer, a simulation of galaxy formation must combine a huge number of molecules, describe how they all move, push each other, transfer energy, react to light and radiation, etc., all without explicit reference to an infinite number of individual objects inside. This requires us to be creative, to find ways to describe the essence of many different processes, allowing for various options for the development of events, without getting hung up on details that are unknowable anyway. Our models inevitably rely on extrapolations, compromises, and mere speculations developed by experts. Uncertain details cover not only stars, but also black holes, magnetic fields, cosmic rays, as well as not yet understood “dark matter” and “dark energy”, which, apparently, determine the overall structure of the universe.
But this will never lead to the creation of a literal digital copy of the universe in which we live. Such a reconstruction is as impossible as an accurate prediction of the future of the solar system. But simulations, even based on loose descriptions and assumptions, can serve as a guide, suggesting how galaxies may have evolved over time, allowing us to interpret results from increasingly sophisticated telescopes, and telling us how to learn more.
Ultimately, galaxies are less like machines and more like animals: obscure, interesting to study, but only partly predictable. Accepting this fact requires a change of perspective, but it enriches our view of the universe.
