The idea that memories might not correspond to real events but could actually be illusions created by chance from cosmic static has been discussed in physics for more than a century. Recently, three physicists examined the logic behind this idea and found that arguments on both sides may be fundamentally circular.
A recent study published in the journal Entropy by Santa Fe Institute Professor David Wolpert, physicist Carlo Rovelli, and Jordan Scharnhorst revisits the Boltzmann brain hypothesis. This thought experiment, based on statistical mechanics, suggests that random fluctuations in entropy could, in theory, create a fully formed brain with false memories and a sense of a coherent past.
Rather than trying to prove or disprove the Boltzmann brain hypothesis, the researchers focused on identifying a structural flaw in the way scientists have debated the issue.
Where the Logic Breaks Down
The Boltzmann brain paradox comes from the H theorem, developed by Austrian mathematician and physicist Ludwig Boltzmann. This idea is key to statistical mechanics and supports the second law of thermodynamics, which explains why disorder (or entropy) increases over time and why we perceive time as moving forward. However, the H theorem itself treats the past and the future identically in its equations.
This symmetry creates a problem. If entropy can decrease in the future just as easily as it increased in the past, then the patterns that form our memories could just as likely come from random fluctuations as from real events. In other words, our memories might not necessarily correspond to actual past events.
The usual response is that this scenario is extremely unlikely. The chance of a functioning brain forming from random thermal noise is so small that it would take much longer than the current age of the universe for it to happen. However, the new study shows that this argument depends on assumptions that may not even be justified.
A Never-Ending Circle
To clarify the debate, the researchers created a mathematical framework that models the universe’s entropy as a time-symmetric Markov process, which they call the “entropy conjecture.” In this framework, they identified a key issue: physics alone cannot determine which moment in time to use as a reference point. That choice must be assumed.
This assumption leads to circular reasoning. Arguments against the Boltzmann brain hypothesis, including those that appeal to the second law of thermodynamics, usually assume that our memories accurately record real events. Yet the main reason to trust our memories is that the second law suggests they should be reliable. In other words, the conclusion relies on the premise, and the premise relies on the conclusion.
Arguments in favor of the hypothesis show the same circularity. The study finds that the Boltzmann brain hypothesis and the standard “past hypothesis,” which assumes the universe began in a low-entropy state at the Big Bang, have the same structure. Each approach analyzes the problem from a different moment in time, changing only which moment it treats as fixed.
Reframing the Question
The researchers stress that their findings are meant to diagnose the problem, not to give a final answer. Their study does not decide whether the Boltzmann brain hypothesis is true or whether our memories are real, but it does show that current arguments do not properly answer the question.
The team formalized the entropy conjecture as a mathematical process and revealed a problem earlier studies overlooked: every argument in this debate depends on assumptions about which facts to treat as fixed, and physics alone cannot resolve the issue.
Fundamentally, any real resolution has to come from outside the math—whether from prior beliefs, or from Bayesian reasoning. That, the authors suggest, underscores why the debate has continued to go in circles for so long.
The recent study, “Disentangling Boltzmann Brains, the Time-Asymmetry of Memory, and the Second Law,” appeared in the journal Entropy.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.