Why Boltzmann Brains Are Bad

TL;DR

The paper argues that cosmological theories predicting Boltzmann Brains are unacceptable because they are cognitively unstable, undermining their credibility regardless of observational data.

Contribution

It identifies the core problem with Boltzmann Brain theories as their cognitive instability, challenging their scientific validity beyond empirical considerations.

Findings

Boltzmann Brain theories are cognitively unstable.

Such theories cannot be both true and justifiably believed.

The problem is independent of observational data.

Abstract

Some modern cosmological models predict the appearance of Boltzmann Brains: observers who randomly fluctuate out of a thermal bath rather than naturally evolving from a low-entropy Big Bang. A theory in which most observers are of the Boltzmann Brain type is generally thought to be unacceptable, although opinions differ. I argue that such theories are indeed unacceptable: the real problem is with fluctuations into observers who are locally identical to ordinary observers, and their existence cannot be swept under the rug by a choice of probability distributions over observers. The issue is not that the existence of such observers is ruled out by data, but that the theories that predict them are cognitively unstable: they cannot simultaneously be true and justifiably believed.