Physics Links and Notes

Here are three interesting things I read recently.

1. Lee Smolin has an article titled “The unique universe” in physicsworld (hat tip: Not Even Wrong). It covers some of the same ground as the video I had earlier posted here. In it he argues against some ideas which have been recently popular among physicists when considering the shape of the next fundamental theory of cosmology. First, many now argue that our universe is just one of a vast or infinite number of others: the multiverse. Also, it is argued that the fundamental theory will be timeless, since they see our experience of the flow of time as an emergent local phenomenon. This leaves us with a vision of a timeless and static multiverse.

Smolin says advocates of this vision are led by mistaken reasoning. One problem arises when physicists take the essentially Newtonian schema we use to evaluate systems within the universe (deterministic laws + initial conditions) and try to apply it to the entire cosmos. This leads them to try to describe a process for selecting our universe from a landscape of many universes (anthropically or otherwise). Smolin argues that we would do better to explore theories which take time to be fundamental, and where laws can vary in a process of cosmic evolution.

2. Many people are optimistic that an information-theoretic perspective will lead to new insights in exploring the foundations of quantum mechanics, and this multi-authored paper, called “A new physical principle: Information Causality”, is an interesting effort in this regard. Information causality is, according to the authors, a principle which helps pick out QM from a space of possible theories which, like QM, feature entangled correlations but allow no faster than light signaling. While the principle seems simple when stated (“communication of m classical bits causes information gain of at most m bits”), the fact that other (hypothetical) theories which feature strong correlations don’t meet it is notable. Hat tip goes to this post by the Quantum Pontiff which has some helpful discussion.

3. I had come across the essay “Free will, undecidability, and the problem of time in quantum gravity” by Rodolfo Gambini, which was submitted to the FQXi contest (see here), but I didn’t immediately catch on to his arguments. But now having reviewed two papers on Arxiv by Gambini and colleagues (here and here) I have a better idea what his program is. The key starting point is this: the mathematics of quantum mechanics treat time as an external infinitely divisible classical variable; Gambini et.al. think that fundamental limitations on the practical measurement of time within the physical world have implications for how we should interpret the problem of quantum measurement. For instance, if we look at decoherence theory, we see that a quantum superposition involving a system, a measuring device and the environment can evolve such that the degrees of freedom responsible for interference are dispersed. But decoherence itself says nothing about a measurement taking place -- the system is still evolving unitarily. Gambini et.al. argue that a point comes where no possible mechanism is available to tell whether or not a measurement outcome (or event) has or has not taken place. They think this undecidability threshold can be seen as the marker for when an event has occurred. (Then, in the essay, Gambini waxes philosophical and speculates that this undecidability between evolution and collapse might create space for free will.) A thread about this in physicsforums is here.

I liked reading Gambini’s papers, but I think the calculations regarding the undecidability point are controversial, given that a full explanation would require a theory of quantum gravity. And if my preferred approach to QG is right -- where time is a fundamental aspect of a pre-gravity microscopic quantum theory, and the particles and space-time geometry of current theories are emergent regularities -- then I suspect that the constraints on describing a physical clock would not arise in the same way as it does here.