This was another very enjoyable chapter for me and would be for any condensed matter physicist. Here Ellis digs down deep into the lower levels where physics dominates. Reductionists see everything being driven by physics at these levels. “At the bottom level, what happens is based on physics: it enables the emergence of higher level entities, which then in turn act down on the lower level components. Hence top-down causation takes place also in the context of physics.”
At the bottom level we find quantum dynamics. In sharp contrast to classical dynamics, we find inherent unpredictability. The state of a system evolves dynamically and collapses into an eigenstate only when a measurement is made or there is an interaction with another physical entity.
“Because the electronic band structures and the resultant lower level entities such as phonons are based on the higher level crystal structure rather than simply being based solely on properties of the lower level constituent, they are both emergent phenomena. They simply would not exist if the macro-structure did not exist.” Ellis goes on to use my favorite examples of emergence from my own fields of expertise, superconductors and semiconductors. Both materials exhibit unique particles such as Cooper pairs and holes and excitons that could exist only through the structural nature of a large ensemble of lower level particles, namely the ions and electrons that comprise the material. The top-down causation is the structural crystal lattice and the way in which the ions vibrate and respond to the electron cloud. None of these effects could be predicted on the basis of the independent particles alone. It is their interaction that enables the new particles and new behavior.
“…the higher levels of the hierarchy of complexity and causation provide the context within which the lower level actions take place. By setting the context in terms of initial conditions, boundary conditions, and structural relations, the higher levels determine the way the lower level actions occur.”
“The lower levels do the work, but the higher levels decide what is to be done.”
Ellis includes an interesting discussion of Olber’s Paradox which ponders why the night sky is dark. Integrating over an infinite universe would imply that the night sky should be dark. The fundamental reason for a dark night sky is that the universe is not infinite and the visual horizon is such that it is dominated by the cosmic background radiation. In other words, our universe is essentially a thermal bath at a temperature of 2.73K. The thermal gradient from the sun’s radiation to this bath temperature enables the energy driving force for evolution on the earth. Hence, he says “…the reason we observe the night sky to be dark is that if that were not true, we would not be here to see it.”
He also addresses the fascinating question of the arrow of time. Since all of the basic equations of physics are time invariant, being the same forward in time as backward, why should there be an arrow of time? Ellis says “The direction of time must be derived by a top-down process from cosmological to local scales.” In other words, the initial conditions and special boundary conditions at the big bang set up the entropy of the universe to be very small initially so that entropy always increases as the universe evolves.