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.
According to string theory, there may be a large number of universes. All of these universes are believed to come into existence through a process called eternal inflation, in which at least one universe continually expands at an incredible rate, while others form and grow within it like bubbles. This pool of universes has been dubbed the multiverse.
Many of these other universes could be chock-full of conscious creatures early in their histories when, like the universe we see today, the past is distinct from the future. That could help make our point of view the standard one. But if these universes eventually become featureless and continue to linger, they will all accrue Boltzmann brains, tipping the balance away from us again.
Zukowski and Bousso’s latest work suggests this won’t happen. Universes are constantly budding off a parent universe in the multiverse, so the parental characteristics can determine what kinds of “baby” universes form within it – and whether those universes will stick around long enough to be filled with Boltzmann brains or decay first.
Bousso and Zukowski performed a mathematical analysis of multiverses that start out in one of two different initial states: an older model first suggested by Stephen Hawking and his colleague James Hartle, and a newer model that has come out of mathematical treatments of the string multiverse. While the Hartle-Hawking model ended up overrun with Boltzmann brains, ordinary human-like consciousnesses prevailed in the newer model. That makes our view of the universe reassuringly normal in such a multiverse (Physical Review D, doi.org/mkj).
The very idea of string theory and the multiverse is still controversial. It is often attacked for being overly complicated and difficult to prove. If Bousso and Zukowski are correct, though, and it can help resolve the problem of Boltzmann brains, the theory may just win a few more backers.
“This is potentially an added experimental success for string theory and eternal inflation,” says Daniel Harlow, a physicist at Princeton University. “We need to understand it better – [but] the fact that it potentially explains something is motivation to understand it better.”