Who's Afraid of Maxwell's Demon—and WhichOne?

Aug 24, 2026

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Explores the long-standing philosophical controversies surrounding Maxwell’s demon, tracing key complaints and explaining why they persist. Builds on Popper’s starting point and analyzes the attempts to “exorcise” such demons, while also posing foundational questions about other putative demons. Emphasizes the systematic distinction between different second laws and different demons, presenting the talk as a map of the logical terrain linking thermodynamics with these challenges.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages9

Reading Time23 min

AccessFree to download

Introduction

I. The First Demon

# Who's Afraid of Maxwell's Demon—and WhichOne? Craig Callender Department of Philosophy,UCSD,LaJolla,CA 92130,USA Abstract.Beginning with Popper,philosophers have found the literature surrounding Maxwell'sdemon deeply problematic.This paper explains why,summarizing various philosophicalcomplaints and adding to them.The first part of the paper critically evaluates attempts toexorcise Maxwell's demon;the second part raises foundational questions about some of theputative demons to be summoned at this conference. ## INTRODUCTION In 1866 J.C.Maxwell thought he had discovered a Maxwellian demon—though notunder that description,of course [1].He thought that the temperature of a gas undergravity would vary inversely with the height of the column.From this he saw that itwould then be possible to obtain energy for work from a cooling gas,a clear violationof Thompson's statement of the second law of thermodynamics.This upsettingconclusion made him worry that "there remains as far as I can see a collision betweenDynamics and thermodynamics.” Later,he derived the Maxwell-Boltzmanndistribution law that made the temperature the same throughout the column.However,he continued to think about the relationship between dynamics and thermodynamics,and in 1867,he sent Tait a note with a puzzle for him to ponder.The puzzle was hisfamous “neat-fingered being”who could make a hot system hotter and a cold systemcolder without any work being done.Thompson in 1874 christened this being a“demon”;Maxwell unsuccessfully tried to rename it"valve."However named,thedemon's point was to “show that the second law of thermodynamics has only astatistical validity.”Since that time a large physics literature has arisen that asks aquestion similar to that asked in theology,namely,does the devil exist? Beginning with Popper,philosophers examining the literature on Maxwell's demonare typically surprised—even horrified [2,3,4,5,6,7].As a philosopher speaking at aphysics conference exactly 100 yrs after Popper's birth,I want to explain why this isso.The organizers of this conference instructed me to offend everyone,believers andnon-believers in demons.Thus my talk,apart from an agnostic middle section,contains a section offending those who believe they have exorcized the demon and asection offending those who summon demons.Throughout the central idea will be toclearly distinguish the various second laws and the various demons.Since to everydemon there is a relevant second law,and vice versa,my talk will therefore be a kindof map of the logical geography of the underworld of physics. ## I.THE FIRST DEMON In the work of Clausius,thermodynamics was considered to be completely universaland applicable without limit.Just as Newton's law of gravitation applied to all matterin motion—atoms,apples,and planets—so too did thermodynamics apply toeverything.Uffink [8]describes many difficulties in the arguments of Clausius andothers in proving the universal applicability of the second law even before aconsideration of mechanical theories,but still the idea that nonequilibrium states go toequilibrium states seems to have been applied to everything. With the fall of caloric theory and the realization that heat was due to matter inmotion,a natural question emerged about the relationship between classical mechanicsand thermodynamics.Does thermodynamics apply to everything,even microscopicsystems that weren't yet seen? The German physicists Clausius and(early)Boltzmann took the most natural path:thermodynamics still did apply to everything atevery scale.With the theory so successful,induction suggests that there is no reasonto doubt that it applies at the microscopic level too.The two Germans thus providedarguments for believing that the laws of thermodynamics were corollaries ofHamiltonian mechanics.The H-Theorem was of course the most famous of these.But as people thought about the relationship between the two more,problems arose.(See [9]for an extensive philosophical

FAQs

What problem does Maxwell’s demon raise for the second law of thermodynamics?

Maxwell’s demon is presented as challenging the idea that the phenomenological second law always follows from underlying Hamiltonian mechanics, suggesting violations could occur in principle under certain assumptions.

Why do philosophers become surprised or horrified when examining the literature on Maxwell’s demon?

The text explains that, starting with Popper, philosophers reacting to the literature often experience shock, and the speaker intends to clarify why this reaction happens in a physics-conference context.

How does the paper frame the relationship between demons and second laws?

It states that each demon corresponds to a relevant second law and vice versa, so the overall discussion distinguishes multiple second laws and multiple demons to map their logical connections.

Aug 24, 2026

Upload by Miles