In a recent interview with Curt Jaimungal @TOEwithCurt Prof. Leonard Susskind admits that string theory does not describe the real world. And that string theorists have put the project of unification on the back-burner, and that new ideas beyond string theory are needed.
It has for long been said by several researchers that if we want to quantise gravity, and unify it with the standard model of particle physics, we need to first fix foundational problems of quantum theory. These problems include the following:
(i) The problem of time in quantum theory: Quantum theory assumes classical time to describe evolution. However, spacetime and its geometry can be assumed to be classical if and only if the universe is dominated by macroscopic classical bodies. Such classical objects are a limiting case of quantum systems. It therefore follows that the current formulation of quantum theory depends on its own limit. This can only be an approximate description, and an exact formulation will not depend on classical time. Such an exact formulation is sought at all energy scales, and not just at the Planck scale, for in principle even a low energy universe can be entirely devoid of classical objects. Such a formulation turns out to be a gateway to quantum gravity, and from thereon to unification. The key idea is to replace classical spacetime (labeled by real numbers) by a non-commuting spacetime labeled by quaternions / octonions. The use of octonions unifies space-time symmetries [GR] with internal (gauge) symmetries [standard model].
(ii) Why do macroscopic entangled systems not obey the quantum principle of linear superposition in spatial position? Even though such macroscopic systems are composed of microscopic ones which obey such superposition.
(iii) Why does the wave-function collapse during a quantum measurement? Why are the outcomes of the collapse random and why do they obey the Born probability rule?
(iv) How do correlated quantum systems manage to influence each other outside the light-cone? [i.e. the quantum non-locality puzzle, the EPR paradox].
It is true that the current formulation of quantum theory is extremely successful and is not contradicted by any experiment. However, from here it does not follow that the foundational problems can be ignored. They were ignorable in the standard model [SM] of particle physics described by quantum field theory - but even there only partly so. Because one does not know why the dimensionless coupling constants take the values they do. Foundations cannot be ignored if one is trying to quantise gravity and/or unify it with the SM.
String Theory has paid a heavy price for neglecting these foundational problems of quantum theory.
Many physicists are attempting to approach the unification problem by making the above foundational questions as the starting point of their investigation. One example of such an approach is described in the attached brief review. Here, the pre-quantum theory of Trace Dynamics [due to Stephen Adler and collaborators] is generalised to a pre-spacetime, pre-quantum theory. From here, gravitation and quantum theory both are emergent phenomena. The octonionic theory, as it has come to be known, addresses and answers the above foundational questions, and in the process arrives at a theory of quantum gravity and of unification.
This is an ongoing research program.
https://www.preprints.org/manuscript/202411.0351/v1