Where is quantum theory headed

Public talk at the EmQM13 conference opening event on ”The Future of Quantum Mechanics” The organizers have asked me to state my views on the direction of the future development of quantum mechanics. Will it evolve within the standard framework, without the addition of new foundational physics? Or will the foundations require modification in an, at least in principle, experimentally detectable way? First, let’s discuss the current status of quantum theory. Quantum mechanics is our most successful physical theory. It underlies our detailed understanding of atomic physics, chemistry, and nuclear physics, and the many technologies based on this knowledge. Additionally, relativistic quantum mechanics is the basis for the very successful standard model of elementary particles. However, from its beginnings there have been conceptual problems associated with the nature of measurement in quantum mechanics. These can be simply illustrated with the famous Stern-Gerlach experiment (Fig. 1). Stern-Gerlach Experiment Figure 1: Schematic representation of the Stern-Gerlach experiment Silver atoms boiled off from a furnace are sent through a non-uniform magnetic field, and impinge on a photographic plate. Instead of a continuous distribution of spots, one sees two spots, corresponding to spin up and spin down relative to the magnetic field axis. Each atom goes up OR down, but one cannot predict which in any given run the results of the experiment are probabilistic. There is a 50% chance of an atom going up, and a 50% chance that it will go down. From the point of view of the Schrodinger equation of quantum theory, this result has no explanation. In quantum theory, the state of the particle is described by its wave function, and the Schrodinger equation says that at a post-measurement final time Tf , the wave function is related to that at a pre-measurement initial time Ti, by a deterministic relation Ψ(Tf) = U(Tf , Ti)Ψ(Ti) U(Tf , Ti) = efi with the transition operator U completely specified by the Hamiltonian H. To explain what is observed, the Schrodinger equation must be supplemented by the reduction postulate and the Born rule. These state that the wave function only gives a description of probabilities when a measurement is made, with the probabilities for an “up” outcome and a “down” outcome given by the squares of the coefficients of the corresponding components in the initial wave function Ψ(Ti), Born Rule for Probabilities Ψ(Ti) = CupΨup +CdownΨdown probup = ∣Cup∣ probdown = ∣Cdown∣ ∣Cup∣ + ∣Cdown∣ = 1 with the sum of the up and down probabilities equal to one. The reduction postulate and Born rule are an add-on to the Schrodinger equation. According to the Copenhagen interpretation of quantum mechanics, the Schrodinger equation applies when a microscopic system, the silver atom, is time-evolving in isolation. But when the atom interacts with a macroscopic measuring apparatus, as in the Stern-Gerlach setup, you have to use the reduction postulate and Born rule. This situation leads to puzzles that have been debated for over eighty years. If quantum mechanics describes the whole universe, then why can’t one use the Schrodinger equation to describe the system consisting of the silver atom