Science News - August 1, 2009

Lead or poverty’s later toll Multiversal logic

Most toxic materials have the most del- Dealing with the question of multi-
eterious effects at the earliest exposure verses requires coping with logic as
ages, so I was puzzled by the study out- well as with infinity (“Success in cop-
come in “School-age lead exposures may ing with infinity could strengthen
do more harm than earlier exposures” case for multiple universes,” SN:
(SN: 6/6/09, p. 13). Did the study control 6/6/09, p. 26). Alex Vilenkin’s argu-
for social and financial background? It ment “is based on the belief that people
would make sense for effects of back- aren’t special” and thereby is circular.
ground to be greater at age 6 than age 1. “Humans would most likely live in an
Tom DuBois, Glens Falls, N. Y. average bubble” only if A) the distribu-
tion of bubbles under consideration

Richard Hornung of Cincinnati Chil- resembles a normal distribution (such dren’s Hospital Medical Center says the that near-average examples are more study looked at socioeconomic status likely than far from average ones) and indirectly through maternal IQ and a B) the sample of studied universes is standard measure of the home environ- randomly selected and large enough ment. He also notes that each of the study to be statistically representative. B is populations was relatively homogeneous, certainly untrue; A might not be true. made up almost exclusively of inner city Similarly, an infinite universe alone is children, further reducing the chance that not sufficient to produce Boltzmann the findings were a result of differences in brains — there must be an infinite sup- social and financial factors rather than ply of appropriate matter in it (all lead exposures. — Janet Raloff neutrons wouldn’t do, for example)

with a sufficiently random distribution
to cover all possible configurations.
As the laws of physics and chemistry
constrain the distribution of matter,
matter distribution may not be random
enough to include Boltzmann brains.
Conversely, the existence of humans in
no way disproves the existence of Boltz-
mann brains. Just because the highly
nonrandom sample of the brains that
we know of are all in animals doesn’t
tell us what the average randomly sam-
pled brain in the universe is like. These
difficulties go both ways — without a
statistically significant random sample
of universes, we cannot tell scientifi-
cally what is normal.
David Campbell, Tuscaloosa, Ala.