Universe, Multiverse and Simulation
(A few remarks regarding the Multiverse)






Jean-François COLONNA
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CMAP (Centre de Mathématiques APpliquées) UMR CNRS 7641, École polytechnique, Institut Polytechnique de Paris, CNRS, France

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[en français/in french]


Keywords: Multiverse, Multivers.



Long ago, our ancestors believed that the Earth was stationary and at the center of the Universe. This view was not at all ridiculous, as it was what our senses revealed to us (and still reveal today...), in the absence of appropriate means of observation. This was geocentrism.


Even though before him, Aristarchus of Samos (~310-230 BC) and Hypatia of Alexandria (4th century AD) had intuited heliocentrism, it wasn't until 1543 that Nicolaus Copernicus (in De revolutionibus orbium caelestium libri VI) "placed" the Sun at the center of the Universe and made Earth a planet like the others.


As observational tools advanced, it was then the turn of our Sun to be "downgraded", becoming just another star among its siblings at the heart of a single galaxy, the Milky Way.


New advancements later revealed, during the twentieth century, that our galaxy was just one among hundreds of billions of others in a possibly infinite Universe...


What about today? Are we facing another demotion?


For a long time, there was hope to justify Earth's privileged position based on the Mathematical laws of Physics: its optimal distance from the Sun (allowing water to be liquid), the presence of a protective magnetic field (against solar or cosmic threats...), the existence of a stabilizing satellite (the Moon), etc. Kepler attempted this by associating each of the five known planets with one of the Platonic solids. Newton's laws, although allowing for precise calculations of the trajectories of the planets in the solar system, required "initial conditions" (the positions of the bodies in space at a given moment) to be provided, and these could only be measured, not deduced from the laws themselves. Today, it is considered that Earth's fortunate position is the result of the conditions under which the Sun and its planetary system formed: it is most likely a stroke of luck!

Since the discovery of the first exoplanet (51 Pegasi b) in 1995 by Michel Mayor (Nobel Prize in Physics 2019), thousands of others have been identified, and it appears that most stars have such a planetary system. However, it was also discovered simultaneously that the solar system might not be representative. It seems that almost anything is possible: terrestrial planets (like Earth) close to or far from their star, gas giants (like Jupiter) close to or far from their star, etc. It is worth noting that the detection methods currently used (such as the so-called transit method, where a planet passes in front of its star, or observing the oscillations of a star caused by its potential planetary system) tend to favor giant planets with short orbital periods. Thus, with the Universe appearing vast, even infinite, there must be a number of favorable configurations, with the solar system and Earth being just one of them: and we are here to make this observation!

Today, a similar question pertains to the Universe: it is described by about thirty parameters that are "finely tuned", allowing for the existence of atoms, stars, planets, and life. So, why and how is such fine-tuning possible? Could the answer to this question be of the same nature as that given above to explain Earth's privileged position? Might there be a (quasi-)infinite number of Universes where nearly all combinations of fundamental parameter values are represented, including ours? This collection of Universes would form a (the?) Multiverse...

It should be noted that this hypothesis is, on the one hand, contrary to Occam's Razor on the other hand, logically implies the existence of a Multi-Multiverse, a Multi-Multi-Multiverse, and so on ad infinitum...

Nevertheless, this metaphysical hypothesis emerges from a number of theories (which are not mutually exclusive!):

According to Karl Popper, a scientific theory must be falsifiable. In this context, for example, the Big Bang model would not be considered scientific because it is not falsifiable (it seems difficult to recreate the experiment!). The current practice is to accept a model as long as some of its predictions are verified and others are not falsified (this was the case with General Relativity concerning black holes and gravitational waves). As it stands, Multiverse models have not made any predictions, and we should await their further development, particularly within the framework of Grand Unification (Quantum Mechanics and General Relativity)...





Copyright © Jean-François COLONNA, 2024-2024.
Copyright © CMAP (Centre de Mathématiques APpliquées) UMR CNRS 7641 / École polytechnique, Institut Polytechnique de Paris, 2024-2024.