Does information weigh anything?
Erasing one bit releases a minimum quantity of heat. It was predicted in 1961 and measured in 2012.
The question
In the twentieth century, physicists as eminent as von Neumann, Gabor and Brillouin believed that every elementary step of information processing had an unavoidable cost in heat.
Measuring and copying turned out to be free in principle. One operation is not, and it is the humblest: throwing something away.
What has been shown
Known. Strongly supported by evidence and broadly established.
Strongly supported by evidence and broadly established.
The principle
In 1961 Rolf Landauer argued that logically irreversible operations, those that map two different inputs to the same output, must dissipate energy. Erasing a bit reduces the number of states the system can be in, and that reduction has to be paid for in heat released to the environment.
The minimum is small: at room temperature, about three zeptojoules per bit. Today’s computers dissipate far more, so the bound is not what currently limits them.
5 sources for this passage. Show them.
- Irreversibility and Heat Generation in the Computing ProcessLandauer, IBM Journal of Research and Development · 1961
- Information and thermodynamics: experimental verification of Landauer’s erasure principleBerut, Petrosyan and Ciliberto, Journal of Statistical Mechanics · June 2015
- Experimental test of Landauer’s principle in single-bit operations on nanomagnetic memory bitsHong, Lambson, Dhuey and Bokor, Science Advances · March 2016
- Notes on Landauer’s principle, reversible computation, and Maxwell’s DemonBennett, Studies in History and Philosophy of Modern Physics · September 2003
- Information Processing and Thermodynamic EntropyMaroney, Stanford Encyclopedia of Philosophy · September 2009
Where the argument came from
The 1961 paper is about computing hardware rather than about philosophy. It appeared in IBM’s research journal and opens on how fast and how compact computing circuits could get, and what sets the floor.
His answer was that the floor is not in the electronics. It is in the logic: an operation that loses information about its input cannot be run backwards, and irreversibility in the logic forces dissipation in the physics.
It was measured
In 2012 an experiment trapped a single microscopic bead in a double-well potential, used the two wells as the two states of a bit, and measured the heat released while the bit was erased.
The slower the erasure, the closer the dissipated heat came to the predicted minimum, where it levelled off. A fifty-year-old theoretical claim about the relationship between information and thermodynamics became an experimental result.
Experiments since, with a particle in a feedback trap and with nanoscale magnetic bits of the kind used for data storage, have found the same floor.
Why erasing is the expensive operation
- Observed
- Copying a bit onto a blank one costs nothing in principle.
- Inferred
- Nothing is lost. The operation can be run backwards.
- Observed
- Erasing a bit costs at least kT ln 2 of heat.
- Inferred
- Two possible states become one. The lost distinction has to go somewhere, and it goes into the surroundings.
- Observed
- The measured heat approaches that floor as erasure slows.
- Inferred
- The bound is physical rather than a bookkeeping convention.
4 sources for this passage. Show them.
- Experimental verification of Landauer’s principle linking information and thermodynamicsBerut and others, Nature · March 2012
- Information and thermodynamics: experimental verification of Landauer’s erasure principleBerut, Petrosyan and Ciliberto, Journal of Statistical Mechanics · June 2015
- High-Precision Test of Landauer’s Principle in a Feedback TrapJun, Gavrilov and Bechhoefer, Physical Review Letters · November 2014
- Experimental test of Landauer’s principle in single-bit operations on nanomagnetic memory bitsHong, Lambson, Dhuey and Bokor, Science Advances · March 2016
The usual answer to a famous demon
Maxwell imagined a being that sorts fast molecules from slow ones, creating a temperature difference without work and apparently breaking the second law.
The standard resolution, reached by O. Penrose in 1970 and independently by Charles Bennett in 1982, is that the demon must record which molecules are which, and its memory is finite. Erasing those records costs at least as much as the sorting gained. On this account the second law survives because information processing is part of thermodynamics rather than outside it.
Earman and Norton, writing in 1998 and 1999, do not accept that this is what does the work. If the demon already obeys the second law, no claim about information is needed to save the law, and if it does not, no claim about information will. Bennett grants the point, and defends the principle for what it explains.
3 sources for this passage. Show them.
- Information and thermodynamics: experimental verification of Landauer’s erasure principleBerut, Petrosyan and Ciliberto, Journal of Statistical Mechanics · June 2015
- Notes on Landauer’s principle, reversible computation, and Maxwell’s DemonBennett, Studies in History and Philosophy of Modern Physics · September 2003
- Information Processing and Thermodynamic EntropyMaroney, Stanford Encyclopedia of Philosophy · September 2009
The demon fails not because it cannot sort, but because it has to remember.
What is claimed beyond it
Uncertain. Meaningful scientific, historical or philosophical disagreement remains.
Meaningful scientific, historical or philosophical disagreement remains.
How far the identification goes
That erasure has a thermodynamic cost is widely accepted and has been measured. That the universe is in some deeper sense made of information is a much stronger claim, and it is a research programme rather than a result.
Ideas connecting entanglement to the structure of spacetime are among the more concrete versions, and they are mathematical relationships in particular model spacetimes rather than statements about this one.
4 sources for this passage. Show them.
- Experimental verification of Landauer’s principle linking information and thermodynamicsBerut and others, Nature · March 2012
- Experimental test of Landauer’s principle in single-bit operations on nanomagnetic memory bitsHong, Lambson, Dhuey and Bokor, Science Advances · March 2016
- Information Processing and Thermodynamic EntropyMaroney, Stanford Encyclopedia of Philosophy · September 2009
- Building up spacetime with quantum entanglementVan Raamsdonk, General Relativity and Gravitation · 2010
The evidence
Primary research, scientific institutions and archives. Links open at the publisher.
- Experimental verification of Landauer’s principle linking information and thermodynamicsBerut and others, Nature · March 2012
- Irreversibility and Heat Generation in the Computing ProcessLandauer, IBM Journal of Research and Development · 1961
- Information and thermodynamics: experimental verification of Landauer’s erasure principleBerut, Petrosyan and Ciliberto, Journal of Statistical Mechanics · June 2015
- Notes on Landauer’s principle, reversible computation, and Maxwell’s DemonBennett, Studies in History and Philosophy of Modern Physics · September 2003
- High-Precision Test of Landauer’s Principle in a Feedback TrapJun, Gavrilov and Bechhoefer, Physical Review Letters · November 2014
- Experimental test of Landauer’s principle in single-bit operations on nanomagnetic memory bitsHong, Lambson, Dhuey and Bokor, Science Advances · March 2016
- Information Processing and Thermodynamic EntropyMaroney, Stanford Encyclopedia of Philosophy · September 2009
- Building up spacetime with quantum entanglementVan Raamsdonk, General Relativity and Gravitation · 2010
The bottom of things
Empty space and information, then the smallest length, black holes and time, where quantum theory and gravity meet and no tested theory joins them, and last, why there is anything at all.
02 of 06This question is stop 2 of 6 on The bottom of things, a route through the Ocean of Reality.Where this question leads
- Ocean of RealityWhy does time only move one way?Shared ground: thermodynamics.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Read
- Ocean of SpaceWhat happens inside a black hole?Shared ground: information.Abyssdepth, meaning: Fundamental questions where our understanding is extremely limited.Read
- Ocean of RealityCan everything be computed?These two name each other.Surfacedepth, meaning: Things we understand reasonably well.Read
- Ocean of RealityAre the constants of nature constant?Deepdepth, meaning: Major unanswered questions.Read