Thursday, December 24, 2015

Testability of String and Multiverse theories

Illustration by Vasava from Ellis and Silk 2014
V interesting report in Nature of a conference between physicists and philosophers to explore some of the philosophical issues behind String Theory and Multiverse theories.

This was provoked to a significant extent by George Ellis' and Joe Silk's Comment article in Nature a year ago called "Scientific method: Defend the integrity of physics" in which they remarked that "debates in physics circles took a worrying turn. Faced with difficulties in applying fundamental theories to the observed Universe, some researchers called for a change in how theoretical physics is done. They began to argue - explicitly - that if a theory is sufficiently elegant and explanatory, it need not be tested experimentally, breaking with centuries of philosophical tradition of defining scientific knowledge as empirical. We disagree. As the philosopher of science Karl Popper argued: a theory must be falsifiable to be scientific."

They singled out String/M- theory and Multiverse theory as two prime examples. They were very explicit that:
The multiverse is motivated by a puzzle: why fundamental constants of nature, such as the fine-structure constant that characterizes the strength of electromagnetic interactions between particles and the cosmological constant associated with the acceleration of the expansion of the Universe, have values that lie in the small range that allows life to exist. Multiverse theory claims that there are billions of unobservable sister universes out there in which all possible values of these constants can occur. So somewhere there will be a bio-friendly universe like ours, however improbable that is. Some physicists consider that the multiverse has no challenger as an explanation of many otherwise bizarre coincidences. The low value of the cosmological constant — known to be 120 factors of 10 smaller than the value predicted by quantum field theory — is difficult to explain, for instance.
Now of course as readers of this Blog will know there is at least one testable-in-principle alternative  to the Multiverse that would "explain" the fine tuning: the MaxHELP hypothesis. I don't know, of course, whether this hypothesis is correct but it is certainly an alternative.

Ellis and Silk called for a conference on this and it was duly held.

David Gross drew a distinction between the two theories. He classified string theory as testable “in principle” and thus perfectly scientific, because the strings are potentially detectable. Much more troubling, he said, are concepts such as the multiverse because the other universes that it postulates probably cannot be observed from our own, even in principle.

Richard Dawid proposed an extension of the concept of theory confirmation (to be called “non-empirical confirmation”) that allows for confirmation by observations that are not predicted by the theory in question.

Carlo Rovelli rightly stressed the need for a clear distinction between scientific theories that are well established by experiments and those that are speculative. “It’s very bad when people stop you in the street and say, ‘Did you know that the world is made of strings and that there are parallel worlds?’.”

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