Multiverse: is another world possible? By Etienne Klein

Multiverse is another world possible By Etienne Klein

Everything Everywhere All at Once by Daniel Kwan and Daniel Scheinert scooped up seven statuettes at the 2023 Oscars, including Best Picture. It tells the story – or rather THE stories – of a heroine overwhelmed by her management of a laundromat and who jumps more or less happily from one universe to another, each time incarnating herself in a different version of herself: movie star, chef cook, woman with sausages instead of fingers, etc.

If this breathtaking scenario is difficult to follow, it is because it dissolves the very notion of personal destiny. It must be said that it is based directly on the concept of “multiverse” as it was set up by theoretical physicists. In general, we speak of the Universe in the singular, supposing by this that it can only be unique, “alone in the world” if one can say so. But this sort of evidence has been called into question by the very structure of certain physical theories that are in their infancy today.

Let’s pick up the thread.

Our observable Universe has two enigma-like properties: it is homogeneous on a large scale and its global curvature, which could have been negative or positive, happens to be perfectly zero. In 1981, two cosmologists, Alan Guth and Alexei Starobinsky, advanced a hypothesis likely to explain these two properties: the primo-primordial universe would have undergone a gigantic “inflation”, that is to say a phase of furious expansion accelerated when its density was extremely high: distances would have been multiplied by a factor of 10 to the power of 50 in a duration of 10 to the power of 32 seconds.

This hypothesis may well be very speculative (it is still awaiting confirmation), but it has made its way into a few cosmological models, where it comes in multiple variants. Some make you dizzy. For example, when the hypothesis of inflation is inserted into the “superstring theory”, which aims to describe all the forces of nature in a unified way, a multiverse of fascinating diversity emerges. Physicists have realized that there are an immense number of different versions of this theory, each describing a particular universe having its own set of physical laws and fundamental constants with different values ​​than they happen to have in our universe. If all these theoretical possibilities had an echo in reality, then we would have to imagine that our universe is part of a vast nested universe composed of a very large number of “bubble universes”. In short, the theory of superstrings would describe not only our world, but also all the other worlds possibly generated by the process of inflation.

“In another life, I could have been you”

Is this a new fable? Of a delirium of theoreticians? Of a provisional auxiliary? No one knows. Some consider the hypothesis of the multiverse as inescapable, others consider it epistemologically dangerous on the grounds that it would be exaggeratedly metaphysical. The fact remains that the universes that it allows us to think about offer us the possibility of carrying out experiments… of thought! For example, to better question the foundations of our cosmological models by asking the question of the determining role played by the universal constants of physics: what would have happened if their values ​​were different from those they have? Would the appearance and maintenance of life remain possible?

In his ChroniclesBob Dylan reported the following dialogue:

– Her: In another life, I could have been you.

– Him: Yeah, but in another life, I would have been someone else.

– Her: Ah, you’re right. We’ll have to figure this out.

“Unraveling that” is precisely what is not easy when one has fun asking counter-factual questions in this way. Who has not wondered one day or another: “And if I had been born elsewhere, in another time, what would my life have been like?” In this matter, all daydreams are allowed, including those carried by the most unbridled imagination. But still, isn’t there a limit? Milan Kundera (in The curtain) saw at least one of them: “We are all nailed to the date and place of our birth. Our “I” is inconceivable outside the concrete situation of our life. It is only comprehensible in and through this situation.”

“I am another”, so they say. No doubt, but never quite different. Otherwise, it is the very idea of I which is drowning in its multiple avatars.