Is there more than one Boltzmann brain?
In a recent issue of the magazine Krauss used his column to express frustration with what he perceives to be an almost theological obsession among some of his physics colleagues with problems that by their very nature cannot be resolved. Angels dancing on the head of a pin stuff. Krauss has a point of sorts, but the subject he chose to illustrate this is one that I find fascinating. Whether it is worthy of serious attention is another matter.
Physicists of a cosmological persuasion are, says Krauss, becoming worryingly obsessed with the idea of Boltzmann brains. These are one of the more bizarre outcomes of a new type of anthropic argument.
If, as many cosmologists now think, there are many universes, and if human-like life can exist only in universes where the physical parameters that define them are very finely tuned, then the values of the parameters and constants we measure in our own universe should be of no surprise to us. But, as physicists including Krauss have shown, the validity of of the anthropic principle rests on the assumption that humans are typical life forms.
The argument runs something like this: if the expansion of our universe goes on for ever, then it is inevitable that random thermal fluctuations will spontaneously produce complex, conscious entities. These cosmological analogues of the monkeys who end up bashing out the complete works of Shakespeare on typewriters are known as Boltzmann brains, named after the 19th century physicist Ludwig Boltzmann.
Boltzmann brains may be inevitable in an infinitely expanding space, but therein lies a problem. That is, far more of them should exist than intelligences such as our own, which have taken billions of years of evolution to come up with something as truly profound as the Dyson vacuum cleaner. If that is the case then we humans are far from typical, and out goes the anthropic principle.
So what to do? Physicists have attempted to solve this conundrum by forcing their theoretical universes to decay before Boltzmann brains can infect them. Some considerable hacking of hypotheses is required, but this is a trivial exercise when there are no data on which to test the models, and little or no hope of the hypotheses ever becoming proper scientific theories.
And that is what irritates Krauss:
“If debating angels dancing on pins marked the intellectual low point of medieval theology, then we may similarly question the merits of debating problems that require hand-waving arguments involving unknown quantities that differ by billions and billions of orders of magnitude. Let’s focus on other issues, at least until better theories come along.”
Indeed, but on the other hand such speculation can be fun, and if nothing else it is a creative exercise that stretches the imagination. The idea of disembodied intelligences has an obvious appeal, and, theological speculation aside, it makes one think more deeply about the nature of consciousness, and how it might exist outside of a mass of wobbly brain tissue.
“Spooks in space”, Mason Inman, New Scientist, issue 2617, 17 August 2007
“Sinks in the Landscape, Boltzmann Brains, and the Cosmological Constant Problem”, Andrei Linde, arXiv, 2006