LEGIONS of disembodied brains floating in deep space threaten to undermine our understanding of the universe. New mathematical modelling suggests string theory and its multiple universes may just provide our salvation and that could win the controversial theory a few more backers.
Physicists have dreamed up some bizarre ideas over the years, but a decade or so ago they outdid themselves with the concept of Boltzmann brains fully formed, conscious entities that form spontaneously in outer space.
It may seem impossible for a brain to blink into existence, but the laws of physics don't rule it out entirely. All it requires is a vast amount of time. Eventually, a random chunk of matter and energy will happen to come together in the form of a working mind. It's the same logic that says a million monkeys working on a million typewriters will replicate the complete works of Shakespeare, if you leave them long enough.
Most models of the future predict that the universe will expand exponentially forever. That will eventually spawn inconceivable numbers of Boltzmann brains, far outnumbering every human who has ever, or will ever, live.
This means that, over the entire history of the universe, it is the Boltzmann brains' experience of the universe and not ours that is typical. That's a problem, because the starting point for our understanding of the universe and its behaviour is that humans are typical observers. If we are not, our theories begin to look iffy.
"It has to be more likely to be an ordinary observer than a Boltzmann brain," says Claire Zukowski at the University of California, Berkeley.
A particular problem is that most Boltzmann brains will exist in the far future when the universe is no more than an inky void, with a past indistinguishable from the future. This would make our experience of time's arrow highly unusual.
However, if we can demonstrate that the universe has a finite lifespan, that would deny Boltzmann brains the infinite time they need to outnumber us. String theory might be able to help, says Zukowski, who has been studying the problem as part of her PhD research with Raphael Bousso, also at Berkeley.
