Are We Random Boltzmann Brains?

Are We Random Boltzmann Brains?

Characters in sci-fi movies and novels routinely encounter alien life somewhere in the universe. The crew of the USS Enterprise deals with Klingons, Q, and the Borg while Luke Skywalker works with Chewbacca the Wookiee and Ewoks to defeat the evil Galactic Empire. Sometimes the aliens come to Earth, as in Transformers, where Optimus Prime and his cohorts join elite military personnel on Earth to defend humanity against an attack by other villainous robots. Lois Lane encounters Superman, the Krypton native who gains great powers from the solar system’s yellow Sun. Yet despite the diversity of sci-fi aliens, no filmmaker or writer seems to be aware of the one alien that troubles cosmologists as they wrestle to understand the nature of the cosmos: the Boltzmann brain.

Most “normal” aliens have a history analogous to ours, a tangible past. Boltzmann brains do not because they arise from spontaneous fluctuations in the otherwise sterile vacuum of outer space. Although incredibly improbable (scientists use numbers like 1 in 1010^16 or 1 in 1010^120), thermodynamic fluctuations can transform the incredibly high entropy measured in space into a low entropy region occupied, for the briefest moment, by a sentient being, namely a Boltzmann brain. In a single, finite universe with a beginning 14 billion years ago, such small probabilities have allowed scientists to ignore the possibility of Boltzmann brains—but the current cosmological models, which include an ever-expanding universe and multiverse proposals, require a more careful treatment.

For example, in the distant future our universe will grow to enormous proportions. The vast size and incredible time available make Boltzmann brains a real possibility. Add to this the possible existence of a multiverse and Boltzmann brain formation becomes almost certain. In such a scenario, two different “naturalistic” mechanisms exist for explaining life in the universe—the more familiar evolutionary molecules-to-man explanation (where we are ordinary observers or OO) and the quantum/thermal fluctuations that lead to Boltzmann brains (BB).

Given these two alternatives, the key question becomes which pathway produces the most typical form of life. It turns out to be the fluctuation pathway leading to BBs. However, one must exercise caution because the cosmological models where these comparisons happen often involve universes that grow without bound, and that means scientists must find ways of counting and comparing infinite sets (of potential OO or BB)—a notoriously difficult thing to accomplish. In fact, the simplest concepts of how to regulate the infinities (which go by names like “proper time cutoff,” “stationary time,” and “pocket-based”1) lead to predictions at odds with humanity appearing nearly 14 billion years after the creation of the universe.

While all this discussion about life arising by thermal fluctuations can cause one’s brain to hurt (pun intended), it also highlights a few apologetic points. In my assessment, the solutions naturalistic scientists adopt to solve the BB problem buttress fine-tuning arguments for God’s existence.

The first solution argues that a universe that decays on a timescale similar to its current age would remove any consideration of BBs. Research shows that the mass of the Higgs and top quark must be finely tuned so that this decay happens late enough for the universe to support humanity but not so late that the Boltzmann brain problem arises.

The second solution for avoiding the BB problem imposes constraints on how to regulate the infinities involved in counting comparisons. Two scientists evaluated a way to regulate infinities that requires that the universe exhibit an arrow of time so that conditions for significant BB formation never occur. The team also showed how this way of regulating infinities allows for different kinds of vacuum conditions to maintain an arrow of time.2 This work mirrors other research into the early moments of the universe by recognizing that an arrow of time (or causality) represents a fundamental requirement for a habitable universe.

Maybe the reason sci-fi shows don’t encounter BB aliens flows from the recognition that a BB-dominated universe is cognitively unstable. In such a universe (again, from a naturalistic perspective), the best explanation for human existence is that we’re Boltzmann brains. However, if we are BBs, then there is no reason to trust our thoughts about the universe because they simply reflect the fluctuation that produced us.

Of course, none of this matters if humans are not simply an arrangement of atoms but instead the union of a body and a soul as described in the Bible (Genesis 1:27, 2:7). By exhibiting that “arrow of time” or causality, science demonstrates that a transcendent creation event, rather than a BB scenario, better explains human existence.

  1. Andrea De Simone et al., “Boltzmann Brains and the Scale-Factor Cutoff Measure of the Multiverse,” Physical Review D 82 (September 14, 2010): 063520.
  2. Raphael Bousso and Claire Zukowski, “Multivacuum Initial Conditions and the Arrow of Time,” Physical Review D 87 (May 10, 2013): 103504.