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Previous Programs

2020 Consciousness in the Physical World

2019 Intelligence in the Physical World

2019 Information as Fuel

2018 Agency in the Physical World
Awardees; RFP download

2016 Physics of the Observer
Awardees; RFP download

2015 The Physics of What Happens
Awardees; RFP download

2013 Physics of Information
Awardees; RFP download

2010 The Nature of Time
Awardees; RFP download

2008 Foundational Questions in Physics and Cosmology
Awardees; RFP download

2006 Foundational Questions in Physics and Cosmology
Awardees; RFP download

John Bechhoefer
Simon Fraser University


David Sivak, Simon Fraser University; Susanne Still, University of Hawaii at Manoa

Project Title

Maxwell's demon in the real world: Experiments on the constraints governing information processing

Project Summary

Why is information processing so costly? 5% of US energy consumption is devoted to computing requirements, and 20% of calories in a human body are devoted to the operation of the brain. These numbers are much greater than the fundamental limits implied by thermodynamics, but why? Some of the energy consumption may be unavoidable, as the most lenient limits require that a device operate very slowly, whereas practical concerns favor faster, costlier operations. But other inefficiencies are potentially cured by a better understanding of the underlying mechanisms. This is our focus.

More than a hundred and fifty years ago, Maxwell first proposed a thought experiment that captivated both physicists and the public alike. In modern language, Maxwell (and later Szilard and others) argued that acquiring information about a system can allow work to be extracted from a single reservoir of heat, something the second law of thermodynamics seems to forbid. The way out of this paradox is to understand that “information is physical”: running the device that acquires and memorizes information about a system also requires work. Indeed, the second law implies that it requires as much or more work than can be extracted using the information. But other factors, such as the speed of operation and details of system design can limit the efficiency of information when used to fuel devices.

We will study these factors using a “feedback trap,” an instrument we have been developing over the last ten years to study and control the motion of microscopic particles. Using light, we can create essentially arbitrary potentials that apply any desired forces. We will then introduce a remarkable experimental realization, where by simply observing a weight on a spring as it bounces up and down, we can lift it without effort. Such a device is a ratchet: it converts thermal fluctuations into stored work, which may later be used as desired to power other machines; in effect, the ratchet is fueled by information. We will investigate its efficiency in converting this information as fuel into stored work, focusing on constraints that prevent maximum efficiency from being realized. One recently identified structural constraint limiting efficiency is a mismatch between what can be seen and what can be done to a system; we will conduct a diverse array of experiments on simple microscopic systems where we can impose and then study these structural constraints in a systematic way. The insights gained in the above experiments will then be leveraged to construct experimental versions of the two elementary logical operations underlying computers; we will test their reversibility under slow operation and study the minimum costs when constrained to operate at a finite speed.

Our work will lead to the creation of real-world, thermodynamically ultra-efficient, information-processing systems. These physical realizations will clarify the way forward to constructing technological systems and to understanding biological information processing and will serve as exemplars that demand engagement in philosophical discussions, thereby clarifying the role information plays in thermodynamics.

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