Ebook Physics in the multiverse

Is our entire universe a tiny island within an infinitely vast and infi- nitely diversified meta-world? This could be either one of the most important revolutions in the history of cosmogonies or merely a
misleading statement that reflects our lack of understanding of the most fundamental laws of physics.

The idea in itself is far from new: from Anaximander to David Lewis, philosophers have exhaustively considered this eventuality. What is especially interesting today is that it emerges, almost natu- rally, from some of our best – but often most speculative – physical theories. The multiverse is no longer a model; it is a consequence of our models. It offers an obvious understanding of the strangeness
of the physical state of our universe. The proposal is attractive and credible, but it requires a profound rethinking of current physics.

At first glance, the multiverse seems to lie outside of science because it cannot be observed. How, following the prescription of
Karl Popper, can a theory be falsifiable if we cannot observe its predictions? This way of thinking is not really correct for the mul-tiverse for several reasons. First, predictions can be made in the multiverse: it leads only to statistical results, but this is also true for any physical theory within our universe, owing both to funda-mental quantum fluctuations and to measurement uncertainties. secondly, it has never been necessary to check all of the predictions of a theory to consider it as legitimate science. General relativity, for example, has been extensively tested in the visible world and this allows us to use it within black holes even though it is not possible to go there to check. Finally, the critical rationalism of Popper is not the final word in the philosophy of science.
Sociologists, aestheticians and pistemologists have shown that there are other demarcation criteria to consider. History reminds us that the definition of science can only come from within and from the praxis: no active area of intellectual creation can be strictly delimited from outside. If scientists need to change the
borders of their own field of research, it would be hard to justify a philosophical prescription preventing them from doing so. It is the same with art: nearly all artistic innovations of the 20th century have transgressed the definition of art as would have been given by a 19th-century asthetician. Just as with science and scien-
tists, art is internally defined by artists.

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