What do the interpretational problems of quantum mechanics have to do with the failed unification programme of string theory?

What do the interpretational problems  of quantum mechanics  have to do with the failed unification programme of string theory?

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The context of my present post is this beautiful video chat:

Steven Weinberg and Andrew Strominger in conversation [April, 2021] YouTube video https://youtu.be/PFJ46G8BflQ

Towards the end of this very watchable video, a member of the audience put a question to both of them:

In your opinion, is there some problem in our understanding of quantum mechanics [i.e. interpretational issues, measurement problem]. And if so, could this unsolved problem be holding up progress in theoretical high energy physics etc. ?

Weinberg: Yes there is a problem. Quantum mechanics gives a great importance to observers. It pre-assumes a quantum-classical divide so as to be able to make sense of the theory [classical apparatus, measurement]. A reductionist theory must not have to depend on its own limit for us to understand the theory. Rather, working from bottom up, the theory should be able to explain the classical properties of the measuring apparatus as a consequence of the theory itself, instead of having to assume them a priori without proof. Essentially, Weinberg is dissatisfied with Bohr's Copenhagen interpretation. Then he correctly points out that Everett had the same dissatisfaction and therefore came up with what became known as the Everett interpretation. Wave functions never collapse during measurements, and the universe is forever in a state of quantum superpositions of everything. Weinberg says in the conversation that he does not agree with / like the Everett interpretation either. Hence, according to him there is something missing in our understanding of quantum theory.

Strominger: No. There is no problem. We can calculate marvellously with quantum theory. The Lamb shift has been calculated to an unprecedented accuracy. No experiment has ever disagreed with quantum mechanics. As for the interpretational issues, these are just words. It is not physics. You could side with Bohr, or you could side with Everett. It does not make a difference. You can still do your excellent calculations with quantum field theory and predict the world. In other words, Strominger is saying: Shut up and calculate.

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These contrasting responses by Prof. Weinberg and by Prof. Strominger are noteworthy, and have a deep connection with the current status of string theory.

Fast forward to the present: "Strominger: It does not make a difference'. ?? Interestingly, it does!! And I try to say this as clearly as possible: The reason string theory has failed in its unification programme, and fails to predict the standard model despite being almost there, is because string theory adheres to the Everett interpretation of quantum mechanics. If string theory is modified a little bit so as to allow for [a dynamical implementation of] Bohr's Copenhagen interpretation and wave function collapse, it can predict the standard model, uniquely. So Bohr vs. Everett makes a huge difference. As big as the difference between success and failure. As I now try to explain.

It is well-known that string theory can be consistently formulated as a unified theory in a higher dimensional space-time [not four]. Ten space-time dimensions, to be precise. (Eleven, for M-theory). So far so good. However the universe we live in is four dimensional, not ten. Why do we not see the remaining six spatial dimensions? The answer proposed in string theory is that the extra dimensions are curled up, compactified, too small to be seen, say as small as the Planck length scale. This proposed solution ruins the theory. For it turns out there are a very large number of inequivalent ways of compactifying the extra dimensions, all of which produce different particle physics theories in four dimensions. String theory loses predictive power. Which compactification to use? In fact it is not clear whether the standard model is even there as one of the compactifications. As a consequence, as Strominger notes, the ambitious unification programme of string theory was over in the 1980s itself, in a couple of years after the excitement set in.

There is a different mechanism, other than compactification, for recovering a four dimensional space-time from a ten dimensional space-time. It requires us to preferentially and deliberately pick Bohr over Everett, and modify quantum mechanics a little bit [while still remaining consistent with all lab tests of quantum theory] and allow for a dynamically induced rapid collapse of the wave function in macroscopic systems. This is known as the Ghirardi-Rimini-Weber (GRW) mechanism of spontaneous localisation, and would happen very naturally in string theory too, provided we remove the restriction that at the Planck scale as well, the Hamiltonian of the theory must be self-adjoint. Instead, allow for the possibility that under suitable circumstances, evolution at the Planck scale can be non-unitary. Such a possibility is certainly not ruled out by current experiments.

How does this help with the compactification problem in string theory? When we say that the universe is four dimensional, what we mean is that classical objects in the universe live and evolve in four spacetime dimensions. Nobody can claim that quantum systems live in a four dimensional space-time!! A quantum system can well be thought of as living in ten space-time dimensions [as string theory does] even in today's universe, provided the support of its wave-function is non-vanishing only over microscopic distances. Here, microscopic does not mean Planck length. Microscopic can be as large as a micron, roughly before the classical-to-quantum transition takes place around an Angstrom, into the world of atoms, which are of course quantum.

Consider then a string theory type quantum field theoretic system living in a ten dimensional spacetime. Except that the dynamics is now given by the GRW modified quantum theory. and the Hamiltonian possesses an anti-self-adjoint part. When sufficiently many degrees of freedom living in 10D get entangled, the GRW mechanism of spontaneous localisation sets in, and the entangled system becomes classical. And now is the key point, in becoming classical, the entangled system descends from ten to four spacetime dimensions. The support of its wave-function over the extra six spatial dimensions is vanishingly small, smaller than Planck length - this has actually been proved. While the size of their extent in the 4D spacetime remains large. This way we have achieved effective dynamical compactification, or so to say, compactification without compactification. Quantum systems, including those in today's universe, continue to live in ten dimensions. The forces that curve the extra six spatial dimensions are precisely the internal symmetries of the standard model.

We have developed a unification theory in ten spacetime dimensions, very similar to string theory. Except that the dynamics is modified quantum dynamics. The theory has a very promising potential to unify gravity and the standard model, and to predict the values of the free parameters of the standard model (work in progress).

So dear Prof. Strominger , it matters: Bohr or Everett. These are not mere words; foundational questions of quantum mechanics are important. And now they are important in your own backyard Prof. Weinberg is absolutely right on this count; sadly he is no longer with us to witness the unfolding of this story.

We can hence have a failed string theory and an unmodified quantum mechanics. Or we can have a successful string theory and a modified quantum mechanics. It confounds me that string theorists, extremely smart physicists though they are, do not get this. Why do they not consider that the problem is not with strings, but with quantum theory? I sincerely hope they change their mind.