In a new study, SFI professor David Wolpert, SFI fractal faculty member Carlo Rovelli, and physicist Jordan Scharnhorst take a new look at a famous and disturbing idea in physics and cosmology known as the “Boltzmann brain” hypothesis. This concept suggests that our memories, perceptions, and observations may not reflect the actual past at all. Instead, they may have formed randomly through fluctuations in entropy, giving the appearance of a consistent history that never actually happened.
This puzzle arises from deep tensions in statistical physics. An important foundation for understanding why time appears to move in one direction is Boltzmann’s H theorem, a central principle of statistical mechanics coupled with the second law of thermodynamics. This law explains why entropy tends to increase over time, giving us a sense of past and future.
however, H The theorem itself is time symmetric. That is, one direction of time is not preferred over another. This creates surprising meaning. From a strictly formal point of view, the patterns that make up our memories and observations are more likely to result from random fluctuations in entropy than from an actual sequence of past events. Simply put, physics seems to accept that our memories may not be reliable records, but detailed illusions created by chance. This anxiety-inducing thought is what defines the Boltzmann brain hypothesis.
How assumptions about time shape the discussion
To better understand this issue, the researchers built a formal framework to explore how different assumptions affect their conclusions about entropy and memory. Their work combines the Boltzmann brain hypothesis, the second law of thermodynamics, with the related “past hypothesis,” which posits that the universe began in a state of low entropy.
An important question is which points in time should be treated as fixed when analyzing how entropy evolves. Some approaches take the current state of the universe as a given and work outward from there. Some hypothesize that the Big Bang is the starting point of low entropy. Importantly, the laws of physics do not specify which of these perspectives is correct, leaving room for interpretation.
Circular reasoning in the entropy and memory debate
This study introduces what the authors call the “entropy conjecture” to highlight an important issue in many existing debates. They show that discussions about entropy, time, and memory often rely on subtle circular reasoning. In these cases, assumptions about the past are used to support conclusions such as the reliability of memory or the direction in which entropy increases. These same conclusions are used to justify the original assumptions.
Rather than settling the debate, the researchers are focused on uncovering these hidden structures. By separating the role of physical laws from the assumptions used to interpret them, this research provides a more transparent way to think about long-standing questions surrounding the nature of time, entropy, and memory.
