Hoisted from the Archives: Review of Johnson and Broder, The System
links for 2007-10-30

Lawrence Hall

Lawrence Hall is the name of a building (the Lawrence Hall of Science) and a professor (Lawrence Hall of Physics). The second came to the Berkeley Monday Faculty Lunch Forum to argue that there is empirical content to the Anthropic Cosmological Principle.

What is this principle? Put it this way. Suppose somebody asks you why the universe is pervaded by an 80-20 nitrogen-oxygen gas mixture, or why it is 300K outside. The answer is that the universe isn't like that but that where you are is like that because if you wet surrounded by chlorine gas or in a place where it is 400K you--and all life like you--would be dead. Our confidence in these "anthropic" explanations is strong because we can point to places we know of that lack the 80-20 atmosphere--the asteroid belt--and places where it is not a shirtsleeve 300K--Cambridge, Massachusetts.

Can this anthropic principle be applied to more fundamental issues? Can we say that the mass of the d quark is what it is because if it were 20% lower than the neutron would be absolutely stable and there would be no stars? We cannot see any places in the multiverse where the mass of the down quark is lower, but our predecessors did not know about the asteroid belt and other places lacking an 80-20 atmosphere, and the anthropic explanation for why there is oxygen around for us to breathe was just as valid then for them as it is for us. Is it doing science to use this anthropic principle--or is it just meaningless and tautological? After all, pretty much everything in the universe has to be the way that it is for there to be a physicist with the same name as a building talking in the Seaborg Room of the Berkeley Faculty Club Monday at lunchtime--start with Lawrence Hall as your premise, and you have "explained" everything, in some sense.

Lawrence Hall thinks that there is empirical content, and his argument goes like this:

  • The section of the multiverse that we are in has picked its version of the laws of physics through some process.

  • We describe the laws of physics by picking theories and parameter values.

  • The way we describe does not match one-to-one to the way this part of the multiverse picks the parameter values.

  • Therefore there is a probability pressure gradient out there as we look at possible free parameter values for our theories--either low values are much more probable for this part of the multiverse, or high values.

  • So, from our perspective, we would expect to find ourselves in a part of the universe that seems "close" to catastrophe--we should expect, over and over again, to do our calculations and then exclaim: "Wow! We are lucky that parameter X isn't 20% lower (or higher) than it is! If it were, then we couldn't exist!"

  • If we find over and over again that our part of the multiverse appears to us, given the way we describe parameter values, to be remarkably close to a knife-edge of fundamental change and disruption is an empirical prediction of the anthropic cosmological principle.

  • Thus it is testable, and we are testing it.