Tuesday, March 12, 2013

Causal completeness of physics

Massimo Pigliucci writes about reductionism:
I had an interesting exchange with physicist Steven Weinberg, who played what he thought was a trump card in favor of reductionism “all the way down”: he mentioned the causal completeness of the laws of physics. I asked him to elaborate on the point, and he said that the laws of Newtonian mechanics, for instance, are causally complete in the sense that there is no room within the equations for any unaccounted parameters. It follows, according to Weinberg, that those equations are a complete description of the causality of the system, leaving no room for emergent properties. ...

To set the frame for the discussion I cannot do better than to quote Vicente’s abstract verbatim: “According to an increasing number of authors, the best, if not the only, argument in favor of physicalism is the so-called ‘overdetermination argument’. This argument, if sound, establishes that all the entities that enter into causal interactions with the physical world are physical. One key premise in the overdetermination argument is the principle of the causal closure of the physical world, said to be supported by contemporary physics.”
Are the laws of physics really causally complete? Is there a consensus with that opinion? News to me.

I thought that physicists were almost entirely of the opinion that the laws of physics were stochastic, and not deterministic. Therefore I don't see how Weinberg could say that the laws are causally complete. If the laws are stochastic, then a complete set of causes (or initial conditions) cannot predict the future. Events will happen by chance, and will not be completely explainable by causes.

There are a few physicists, like Einstein, who have a semi-religious belief in determinism. For them, I guess I can see how they can believe in causal completeness. But everyone else says that determinism has been disproved by quantum mechanics theory and experiment.

Am I wrong here? Please let me know if I am misunderstanding someone.

Meanwhile Lumo writes:
Much of the irrational disgust by string theory is caused by people's widespread mathphobia. People think that if the number of possibilities or solutions to certain conditions is large, the topic ceases to be a science and it can't be analyzed. But as this paper and others show, it may often be analyzed and the possibilities may be, in fact, fully listed and classified. In principle, one may also find the right vacuum that describes the Universe around us if it exists in a given set.
I would bet my last dollar that the universe vacuum state is not one of the 921,497 complete intersection Calabi-Yau four-folds. Maybe the irrational disgust of string theory is caused by mathphobia, but the rational disgust is based on the failure to find any relation between the real world and these mathematical models.

1 comment:

  1. Interesting post. Maybe physicists are confused by this issue. I don't know, but I get the impression that collectively they are.