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Test parallel universes for real?

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Some claim we can.

We are told,

It is important to keep in mind that the multiverse view is not actually a theory, it is rather a consequence of our current understanding of theoretical physics. This distinction is crucial. We have not waved our hands and said: “Let there be a multiverse”. Instead the idea that the Universe is perhaps one of infinitely many is derived from current theories like quantum mechanics and string theory.

Any time people must talk this way, they have a bad theory.

Popular, but bad.

But the critical question is, what if they discovered it wasn’t true? Would they just keep the baffle going?

Well, let’s see …

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Why String Theory doesn't prove physical necessity? https://letterstonature.wordpress.com/2010/04/03/is-that-really-necessary-a-fine-tuned-critique-of-william-lane-craig-part-2/
B. A deeper understanding of physical laws reveals that many/most/all the constants are related. Perhaps there is only one constant of nature, which itself sets the scale of the universe and is thus unobservable. Other physical constants turn out to be mathematical constants. Let me make three quick responses. 1.Suppose string theory achieves this goal. Even if there no alternatives within string theory, there are alternatives to string theory. Paul Davies says it well: “I think [the idea that there is only one possible universe] is demonstrably wrong. There is not a shred of evidence that the universe is logically necessary. Indeed, as a theoretical physicist I find it rather easy to imagine alternative universes that are logically consistent, and therefore equal contenders for reality”. 2.The problem of parameters in the solution, noted above by Vilenkin {The equations of [string theory] have no adjustable constants, but their solutions, describing different vacuum states, are characterised by several hundred parameters-the sizes of compact dimensions, the locations of the branes, and so on. (Alex Vilenkin, “Many Worlds in One”)}. It could turn out that what we think of as parameters of the laws of nature are simply parameters of the solution that describes our universe. They would, then, remain fine-tuned, since other solutions would presumably not be life permitting. 3.It would be the mother of all coincidences that the only universe permitted by the laws of nature would happen to permit intelligent life. In their classic paper on fine-tuning in 1979, Carr and Rees comment that “even if all apparently anthropic coincidences could be explained [by deeper physical law], it would still be remarkable that the relationships dictated by physical theory happened also to be those propitious for life.”
Axel, I'm not expert on the subject, but from past research I've done, I would say yes. And I find the quoted statement somewhat misleading, since it seems to equate the multiverses of Quantum Mechanics with those of String Theory. The multiverse of Quantum Mechanics is derived from the Many Worlds Interpretation. It is a Level III multiverse in which all the laws and constants of physics are the same across all universes. As such, it is no help in addressing the issue of the apparent fine-tuning of the universe for intelligent life (i.e. the fine-tuning of the universe in such a way as to make intelligent life possible). The type of multiverse needed to address fine-tuning is a Level II multiverse in which the laws and constants of physics vary. Models of this kind of multiverse are based on multiple layers of pure speculation and hypothetical physics that is not testable even in principle. It is supposed that perhaps some form of Chaotic Inflation somehow connects to aspects of String Theory in order to somehow create universes, and that it does so in such a way that their physical laws and constants and their initial conditions are somehow determined at random, ultimately actualizing all possible descriptions of reality allowed for by String Theory. Lee Smolin has made some interesting comments on the status of string theory and the multiverse:
"We physicists need to confront the crisis facing us. A scientific theory [the string theory/multiverse] that makes no predictions and therefore is not subject to experiment can never fail, but such a theory can never succeed either, as long as science stands for knowledge gained from rational argument borne out by evidence. There needs to be an honest evaluation of the wisdom of sticking to a research program that has failed after decades to find grounding in either experimental results or precise mathematical formulation. String theorists need to face the possibility that they will turn out to have been wrong and others right."
Really, the primary motivation for clinging to the multiverse and believing it to be real is the belief that it solves the fine-tuning problem. Of course, some people will try to claim that a Level II inflationary multiverse, even if based on overlapping layers of speculation, isn't simply motivated by the fine-tuning problem but is actually just the outcome of extrapolating certain models of inflationary cosmology. Claims of this sort seem somewhat confused, however, because Inflationary theory itself just is an attempt to explain aspects of fine-tuning. And, as I recall, Sean Carroll published a paper showing that the chances of an inflationary process producing a realistic cosmology is around one in 10^660,000,000. So when people try to claim that belief in an infinite multiverse is simply the cool-headed result of accepting necessary implications of well-established science, you should take those claims with a mountain of salt. HeKS
Is not equating QM with string theory an insult to QM? Axel

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