TRUST your senses. Any theory that lets bizarre brains randomly pop into existence can’t be a valid description of the universe.
That might seem obvious, but such conscious observers, called Boltzmann brains, are inevitable in certain versions of cosmology. New work that claims to banish such theories not only suggests your brain isn’t such an oddity, but tells us which frameworks for the cosmos are the most sound.
The notion of a Boltzmann brain is built on 19th-century physicist Ludwig Boltzmann’s idea that the entropy of a closed system – a measure of its disorder – always increases. There are far more ways to be disorderly than orderly, so it’s much more likely that the system will move towards disorder. But there is always an infinitesimal probability that a system will suddenly fluctuate from disorder to order.
What’s more, we know that the expansion of our universe is accelerating, and the standard view is that mysterious dark energy is responsible. If dark energy remains constant throughout time, the universe will expand forever.
“If you have literally forever to wait, you’ll get essentially every single possible thing fluctuating into existence,” says Sean Carroll at the California Institute of Technology in Pasadena. That includes Boltzmann brains.
The idea is that given infinite time, more brains will fluctuate into existence than evolve, so most conscious observers would be the result of fluctuations. In such an old universe, then, the odds are that we are such brains, too.
Carroll isn’t a fan of Boltzmann brains, and now he thinks he can show they are a bridge too far.
If our brains spontaneously fluctuated into existence, he reasons, then we must be living in the very far future, since the universe needs a near-infinite time for such fluctuations to become a reality. But our measurements suggest that the universe began a mere 14 billion years ago.
That discrepancy means that if we truly are Boltzmann brains in an old universe, then our perceptions are befuddled, too. “We’d have no reason to believe that our memories of the past are accurate,” says Carroll.
Hhhhmmm. To read more, click here.