The concept of universes that would be “parallel” to ours, or of multiverses (our universe would therefore be only one among an infinity of others), was indeed born among scientists decades ago. The theory was developed by American physicist Hugh Everett in the 1950s. But the concept has been popularized in recent years in cinema. We can think in particular of the multiverse of the Marvel cinematographic universe, where superheroes travel from one world to another in order to save their own reality.
The interest is not limited to the big screen. In 2024, Google claimed that its new quantum chip could exploit parallel universes to obtain its results.
A quantum explanation?
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Some physicists believe that the existence of parallel universes is only the logical conclusion of the laws of quantum physics. In a text published in 2020, British astrophysicist and author John Gribbin said that in the 1950s, Hugh Everett wanted to find an explanation for an important problem in quantum mechanics, that of measurement.
Indeed, in classical physics, we can predict the location of an object if we know its starting point, its speed and all the conditions that can influence its movement. However, in quantum physics, it is impossible to make this prediction for a particle before having observed it. Scientists can only estimate the likelihood of it being found in different locations. However, once the measurement is made, the particle is only located where it is observed and the probabilities that it is elsewhere immediately drop to zero.
So what happens at the moment of measurement, that is to say when the particle goes from several possible positions to just one in a specific location? According to the most common interpretation of quantum mechanics, the mathematical equation representing probabilities simply “collapses,” summarized British science journalist Philip Ball in 2018.
However, Everett was not satisfied with this interpretation. And he wasn’t the only one: it doesn’t really explain what happens in reality, we can read in the article on quantum physics in Encyclopedia Britannica. Rather, this interpretation describes what is happening from a mathematical point of view.
Everett therefore offered his own interpretation. According to this, at the time of measurement, the universe would be divided into several versions: one for each possibility predicted by the mathematical formulas. This is how, in addition to the Universe that we know, there would exist an infinite number of others.
Isolated and independent worlds
According to the multiple worlds interpretation, however, no observer is aware of the existence of worlds, apart from his own, emphasized Philip Ball. We only see one result of the measurement, but all the other results would exist in other worlds. There would, therefore, be several other versions of ourselves who would have witnessed these alternative outcomes.
Thus, everything that is physically possible in our universe could be achieved in a parallel universe. All universes that respect the laws of physics therefore exist somewhere in the multiverse, John Gribbin further emphasized. For example, the world of Misérables exists, but not that of Harry Potter with its rules of magic that contradict the laws of physics.
These universes are somehow superimposed in the same physical space, but mutually isolated: they evolve independently of each other. The different universes cannot communicate with each other.
Interpretation criticized
The interpretation of multiple worlds is far from unanimous among physicists. While some believe this to be the only plausible interpretation, others describe it as absurd.
The main problem with this theory is that it is impossible to prove the existence or non-existence of multiple worlds, since they cannot communicate with each other: this is what a South African mathematician and an American physicist deplored in an article published by Nature in 2014. However, this constitutes a problem which goes back to the very foundations of the scientific method: as summarized by the philosopher Karl Popper, a theory must be refutable to be considered scientific.
Some theoretical physics researchers and proponents of this interpretation, however, claim that if a theory is sufficiently elegant and explicit, it would not need to be tested experimentally. But, as the authors of the article pointed out Naturethere are many examples in the history of science where a theory, no matter how elegant, has been proven wrong. This is the case, for example, of Ptolemy’s theory of the cosmos, which placed the Earth at the center.
One interpretation among others
According to a survey carried out in 2013 among 33 scientists (physicists, mathematicians and philosophers), only 6 preferred the multiple worlds interpretation.
Other interpretations also have some popularity. In this same survey, researchers named 11. Wikipedia counts 13.
From a mathematical point of view, John Gribbin concluded, no interpretation is better than another… even if the proponents of each tend to believe the opposite.
Verdict
The many-worlds interpretation seeks to reconcile a fundamental problem in quantum physics, that of measurement. But it remains very controversial among physicists. And we cannot use this theory to justify superheroes or futuristic technologies that would be able to travel from one universe to another.
