13 ms·

Maxwell’s Demon Continues to Startle Scientists

107 points by dnetesn 5y ago

gus_massa 5y ago
> Then in the 21st century, with the thought experiment solved, the real experiments began. “The most important development is we can now realize Maxwell’s demon in laboratories,” said Sagawa. > In 2007 scientists used a light-powered gate to demonstrate the idea of Maxwell’s demon in action; in 2010, another team devised a way to use the energy produced by the demon’s information to coax a bead uphill; and in 2016 scientists applied the idea of Maxwell’s demon to two compartments containing not gas, but light. About the first experiment: https://www.nature.com/news/2010/101114/full/news.2010.606.html https://www.nature.com/news/2010/101114/full/news.2010.606.h... > The experiment does not actually violate the second law of thermodynamics, because in the system as a whole, energy must be consumed by the equipment — and the experimenters — to monitor the bead and switch the voltage as needed. But it does show that information can be used as a medium to transfer energy, says Sano. The bead is driven as a mini-rotor, with a information-to-energy conversion efficiency of 28%. About the second experiment: https://www.quantumlah.org/about/highlight/2016-02-maxwells-demon-light https://www.quantumlah.org/about/highlight/2016-02-maxwells-... > The team also worked out a theoretical description of the setup – accounting not only for the thermodynamic effects on the light, but also for the information gained by the demon – and found a good match with the data. It has long been known that the resolution of the apparent violation of the second law by Maxwell's demon (for it is only apparent) lies with the demon itself. The demon not only does work on the system, but also acquires information about the system that it would take work to erase. When all these factors are budgeted for, there's no net gain. So they didn't make a Maxwell' Demon. They are just very inefficient motor with a overhyped story around it.
- tedsanders 5y ago
  It's a common misconception that the Landauer Limit, which says it takes kT*ln(2) energy to erase a bit, "solves" Maxwell's demon. That argument is actually circular, because the Landauer Limit relies on the Second Law in its derivation. I cannot find the source now, so it's possible I'm wrong, but my recollection is that even Bennett, who advanced the argument in 2003, eventually admitted as much around 2006/2007, if memory holds.[1] I spent years of my PhD reading and thinking about these topics, and made some contributions to these Wikipedia pages years ago. [1] https://en.wikipedia.org/wiki/Landauer%27s_principle#Challenges https://en.wikipedia.org/wiki/Landauer%27s_principle#Challen... (To be clear, I am not saying that Landauer Limit or the Second Law are wrong - just that it's wrong to use the Landauer Limit to "prove" the Second Law.)
  - meroes 5y ago
    Isn't there no "proof" within the laws of physics for the second law? - you must posit a "Past Hypothesis".
    - sharpneli 5y ago
      Depends on what level. Historically it was an empirical observation that was taken in as an axiom to classical thermodynamics. So it couldn't be derived from other laws. If we move to statistical mechanics we can derive the second law from probabilities. In essence it is possible for any system to break it, it's just very very improbable. Just like if you toss a coin billions of times. It's possible for you to get more, or less, than half of the tosses to be heads. It's just very very improbable to get anything more than very tiny deviations.
      - meroes 5y ago
        But when we apply statistical mechanics to "microscopic" arrangments like a few particles in an idealized rigid box or in a vacuum, there is no arrow of time. The laws of physics themsleves are totally symmetric (CPT theorem), and it is not thought the t-asymmetry of the weak interaction is responsible for the global macroscopic arrow of time. So without some statement of initial/prior conditions, we have no reason to see just the "forward" arrow unless we say something about initial/prior conditions. (Loschmidt's Paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox)
    - contravariant 5y ago
      You can prove it in the context of Ergodic theory. Though that's a model so assuming that it correctly approximates the real world is still an assumption.
      - meroes 5y ago
        Are you sure Ergodic theory does the trick? When I hear people like David Albert or Sean Carroll speak, they always make clear you need some kind of initial/prior conditions or else stat mech does not guarantee why we only see the forward arrow of time. If we assume we can reduce all physical process to the behavior of fundamental particles (or fields), and the physical laws that describe the microscopic world are entirely symmetrical (CPT theorem), you get Loschmidt's Paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox. They say the Past Hypothesis (low entropy big bang prior condition) is necessary to resolve it. *And the t-assymetry of the weak interaction isn't the reason according to the wikipedia article on time asymmetry - you still need initial conditions. "Time reversal violation is unrelated to the second law of thermodynamics, because due to the conservation of the CPT symmetry, the effect of time reversal is to rename particles as antiparticles and vice versa. Thus the second law of thermodynamics is thought to originate in the initial conditions in the universe. "
scrooched_moose 5y ago
Possessing only an undergrad level of physics, I've never understood why this is a remotely interesting question. It seems to effectively be equivalent to "Imagine we have a refrigerator with a massless, frictionless motor that also doesn't need to be plugged in. Isn't it crazy that this refrigerator would violate the second law of thermodynamics?"
- bb88 5y ago
  Entropy can't be reversed, even if you have a massless, frictionless, motor that doesn't need to be plugged in. That is effectively what Maxwell's demon is.
  - deleted 5y ago
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  - tsimionescu 5y ago
    Of course entropy can be reversed, I literally do it in my sleep! However, reversing entropy requires an amount of energy commensurate with the amount of entropy you are reducing. Maxwell's demon would appear as a violation of this. A massless, frictionless motor could trivially reverse all the entropy in the universe given enough time, but it is not clear that Maxwell's demon REQUIRES something equivalent.
    - bb88 5y ago
      Maxwell's demon requires erasure which would subsequently increase entropy. https://www.nature.com/news/the-unavoidable-cost-of-computation-revealed-1.10186#:~:text=Landauer's%20theory%20offered%20the%20first,entropy%20lost%20by%20the%20demon https://www.nature.com/news/the-unavoidable-cost-of-computat....
      - tsimionescu 5y ago
        As someone else was noting, this result is based on Landauer's limit, which is itself ultimately based on the second law of thermodynamics. If Maxwell's demon existed, the second law would be wrong, so Landauer's limit would be wrong, so there could be a way to erase information without increasing entropy. I'm also not sure why Maxwell's demon would require erasure.
quelltext 5y ago
I don't really have the necessary background in physics so maybe that's it but this article just doesn't seem to get the point across very well. > Two advances would be crucial to solving Maxwell’s demon. At this point in the article there had been a description of what Maxwell's demon is as: "Imagine that this nonexistent thing does X and has this outcome", however it's utterly unclear what the question is that needs to be solved. Whether such a system can exist? Whether two rooms with more order would violate some law and why? No idea. Can't I have two insulated rooms if some magical creature teleported certain particles magically from one room to the next? I saw other comments about the pointlessness of the thought experiment, but that's not my gripe. But there must be an explanation of why it really matters and what is to be "solved" here when the word "solve" is used. I have the same problem with other thought experiments or rather their dumbed down description, e.g. Schrödinger's cat. They focus too much on the quirky scenario and superficial outcome ("oh, that would mean ...") but not on what specific questions it raised in the end and what the exact follow-ups were. And most importantly they leave out very crucial details. For instance, I think one important piece to Maxwell's demon (from Wikipedia) is that the demon doesn't do "work".
- zackees 5y ago
  Maxwell’s demon lives on today as two sheets of graphene and two diodes. The vibrating sheets (due to virtual particles) causes oscillating electrical currents which are captured by the diodes and stored as charge. https://www.studyfinds.org/graphene-power-circuit-clean-energy/ https://www.studyfinds.org/graphene-power-circuit-clean-ener... The article mentions that Physics is safe. Yet this simple experiment shows that maxwell’s demon disproves the law of increasing entropy.
- aidenn0 5y ago
  The resolution of Maxwell's demon would involve proving that there is no way to implement such a device in an energy-positive manner. People generally suspect this is the case, but nobody can say why it must be the case.
  - Nursie 5y ago
    Thanks for that concise explanation. My immediate thought was "the demon will be doing work to open and close its door, so the overall entropy is likely to be rising, only local order is increased" I guess the problem is around establishing that that has to be true, for any given demon.
raverbashing 5y ago
Thermodynamics are mostly about statistical effects and return to means. Maxwell's demon seems (to me) to turn probabilities into some Deus Ex Machina thing. (Though in the other way, saying that "disorder" is mandatory) Maybe the thermalization hypothesis will give a more adequate answer to it.
- ncallaway 5y ago
  It seems to me that information theory has already provided an adequate answer to Maxwell's demon (as mentioned in the article). To operate, Maxwell's demon must perform operations on information. Those operations can be quantified, and necessary generate more waste heat than it creates by separating the particles.
  - woopwoop 5y ago
    I've heard this explanation alluded to, but I've never understood the details. What does the last sentence mean?
    - dvt 5y ago
      > What does the last sentence mean? "Waste" here means entropy. So the "thinking" that the demon does actually increases the entropy overall. In other words, even though the system's entropy seems to be decreasing (since particles are being neatly sorted), the process used in the sorting must create some kind of random garbage that actually will be a net increase in entropy, in spite of the now-sorted particles.
    - ncallaway 5y ago
      Basically, any temperature difference between two regions of space can be used to do Work* (essentially, there is a maximum amount of Energy that you can extract from that temperature difference to do anything with). If everything in your closed system is the same temperature, then there is no Energy that you can capture in order to do Work. Well, if we have this demon that can open this gate at _just_ the right times so that it can separate hotter particles from colder particles, then it is able to _create_ a temperature difference where there wasn't one. So, this demon would allow us to use our closed system to do an infinite amount of Work. It would act as a sort of perpetual motion machine. - Step 1: Let the demon open the gate at the right times to separate the hotter gasses (more energetic individual particles) from the colder gasses (less energetic individual particles). - Step 2: Replace the barrier/gate with a heat engine, and let the natural flow of energy from hot -> cold, drive our heat engine and push the box. - Step 3: Once the temperature has diffused enough that our heat engine can no longer do any productive Work (that is, the temperature has equalized), then we replace the heat engine with the barrier/gate and let the demon go back to work. That sets us up to discuss the final sentence. With the advent of information theory, we now know that there is a _minimum amount_ of Work that a hypothetical demon would need just to know when to open and close the gate. That Work requires Energy to perform. As it happens, that Work that the demon must do requires more energy than the demon is able to create by separating the particles. Which means the closed system of particles + demon will still increase entropy, because the demon _consumes more energy than it creates_. Basically, we were able to do the math about the theoretical limits of a perfect demon, and if you were able to make one it doesn't break the 2nd law of thermodynamics. The reason this stumped scientists for a long time is we didn't have the ability to calculate the _minimum amount of Energy_ that would be needed by the demon's brain. * Work here being the physics concept of work. For example, we could move a box around by using a heat engine to turn the temperature difference into adding a physical force to the box.
aerovistae 5y ago
I have never understood Maxwell's demon at all and would love if anyone could explain its significance.
- ncallaway 5y ago
  The general idea of Maxwell's demon as a problem is that it conflicts with the second law of thermodynamics, which essentially states that the total entropy of a closed system will increase over time. Which, essentially, implies that if we have a (perfectly sealed closed system) box of gaseous particles well mixed at temperature T, then we will never observe that box eventually separates into two different temperatures of gas with a T(cold) on the left side of the box and a T(hot) on the right side of the box. However, if we already had a box that was separated into two different temperatures of gas, we would expect those gasses to eventually become well-mixed over time. The paradox of Maxwell's Demon was that we had this apparently closed system (gas particles + Demon), that wasn't doing any Work, but apparently had the ability to violate the 2nd law of thermodynamics. The thinking was that this shouldn't be possible, even in principle. With the caveat that the second-law of thermodynamics is a statistical law, so it's really saying "the odds of entropy decreasing by any significant amount is extremely vanishingly improbable"
  - FranOntanaya 5y ago
    What bothers me is, we derive entropy --a blind, involuntary physical law--, from observation of inanimate systems. So, for example, a very big box full of hot hydrogen will eventually cool off. But, there's a tiny chance it may actually coalesce into life bearing stars. Such life may intentionally work towards increasing order, like Maxwell's demon. So we try to extend entropy to life too and strenuously prove that it'll increase even when aware biological organisms work against it, while still excluding the same organisms from the observations entropy is derived from.
    - exporectomy 5y ago
      Are there no natural non-biological processes which decrease entropy locally just as life does? Surely something like a weather system does that, doesn't it?
ffhhj 5y ago
I have worked implementing gambling algorithms for online casinos and also created my own lottery apps, and this article posses the __blind optimism__ that drives poor devils into gambling and make them loss their money for years, and begins like this: > "You can either play all night on the roulette table, or you can stop if you win $100,” The lucky thermodynamic demon that can extract on average more free energy than the work spent over many iterations, will be forbidden from gambling ever again! If he returns to the system, entropy will make sure to balance his earnings in the long run. Losing and winning devils will be balanced as well. For each lottery winner there are lots of losers.
- ramblerman 5y ago
  The analogy was just to help clarify the situation, not to be taken so literally. Where it doesn't match gambling is that there is no risk of ruin, only time waited. In other words, the universe is constantly flipping dice, and when it looks in our favour we stop. But we didn't pay to play.
  - ffhhj 5y ago
    Yeah, but imagine a gambler that wins once and never plays ever again in the same casino. What if he takes his money to another casino? He could lose or win. Let's say he wins again, and goes around the city playing in different casinos. In the long run his earned money will be 0. So, win in the first casino and then stop. The gambler dies, and his son gets the money. Now his son is a new gambler who lives in a different city. If he plays in other casinos he could win or lose, but in the long run his earned money will be 0 too. It means that for a gambler to actually win he must win only once, then he must not play ever again, or any other gambler with his money, since entropy is "connected" to that money, because money makes part of the same system. Transform that money into energy and give it to the gambler demon, what will happen? Now how much money/energy does it cost to create one of these demons?
dumpsterdiver 5y ago
If we believe in the conservation of energy, wouldn’t the universe itself likely be a version of Maxwell’s Demon? Could black holes/white holes be the membrane driving such a perpetual motion machine?
Sosh101 5y ago
I really enjoyed "The demon in the machine" - anyone know of other good books exploring the topic?
- tomxor 5y ago
  Not a book but another article from quanta, and my favourite on the subject, not for it's technical depth but for it's perspective: https://www.wired.com/2017/02/life-death-spring-disorder/ https://www.wired.com/2017/02/life-death-spring-disorder/ I felt like this one brings in the modern understanding combining Maxwell, Shannon and Landauer into a tangible perspective of everyday reality... maxwell demon is not "solvable", it's an extreme example for the purpose of a thought experiment, however practical "versions" of it exist everywhere, once you combine the costs as per landauers contributions, swap an infinite tape for a large tape, you essentially have a cache - when you start to observe biological and complex chemical systems with this perspective it really does feel like life can be thought of as little emergent upstream eddies "anti-entropy" machines in a torrent of entropy, like a dead fish that naturally swims upstream, there is no defying physics here, entropy still increases universally, but these are natural local variations. It's hard to not believe "reducing entropy" is the fundamental "meaning of life" with this point of view, a meaning that is shared among all life: to not dissolve into oblivion, to collect little hacks of physics and chemistry to find better and better ways to evade it.
  - K0balt 5y ago
    This is also my perspective, that living things are fundamentally machines which externalize entropy, and the processing of information is crucial to this task. The task of living is that of using information to harvest order out of entropy, much as maxwells daemon . Of course this increases overall entropy, only conserving order locally....so you could also think of life as a “fire” that burns order out of imperfectly disordered systems.
    - tomxor 5y ago
      I missed this > that living things are fundamentally machines which externalize entropy That's a clearer way to put, externalising or displacing, makes it more palatable for those getting stuck with the laws of thermodynamics.
johbjo 5y ago
> It needed to record and store information about individual particles in order to decide when to open and close the door. This isn't true, all it needs is to store the average speed of particles on either side of the door and it would still converge.
williesleg 5y ago
Just like climate change assholes.
michaelpfrank 5y ago
I made a short Twitter thread relating to this article here: https://twitter.com/mikepfrank/status/1395731216922464263?s=21 https://twitter.com/mikepfrank/status/1395731216922464263?s=... I recommend the books “Maxwell’s Demon” and the second edition “Maxwell’s Demon 2”, both edited by Leff and Rex, which collect a lot of the classic literature on this topic. Bottom line is, of course no real machine can break the Second Law, but the Maxwell’s Demon thought experiment helps illuminate the physical nature of information.
EMM_386 5y ago
I don't understand the concept of Maxwell's Demon. Wouldn't this demon require it's own energy source to perform this sorting of hot/cold particles? It has to move it's arms or whatever it is doing to manipulate the door. It has to use it's brain and eyes to determine which particles to sort. How is this any different then if you had a heat exchanger or other mechanical device sitting there? I must be missing something obvious.
- Covzire 5y ago
  I'm wondering the same, it seems that the common denominator in any system that we can observe where entropy first stops increasing and begins to decrease is the addition of a mind.
- 6gvONxR4sf7o 5y ago
  I believe the idea is that the demon could be arbitrarily efficient since it opens and then closes the door, allowing a thought experiment of perfect efficiency. Like saying infinity when you mean an arbitrarily large number.
tzs 5y ago
It's not really a Maxwell's Demon, but it kind of acts like one: the Ranque-Hilsch vortex tube [1]. It's a simple mechanical device with no moving parts invented in 1931 with one input port and two output ports. Force compressed air into the input port, and hot air (up to 200 ℃) comes out one of the outputs and cold air (down to -50 ℃) out the other. [1] https://en.wikipedia.org/wiki/Vortex_tube https://en.wikipedia.org/wiki/Vortex_tube
steve76 5y ago
It's a shame we are drawn to the ugly. Create a set of equations which determine the speed of light from empty space based on material properties of nothing. Advance the Fundamental Theorem of Calculus to start the unification of space and time for highly accurate models. *crickets* Throw the word "demon" in front of it. Birds start chirping.
coldtea 5y ago
>In the thought experiment, Maxwell imagined splitting a room full of gas into two compartments by erecting a wall with a small door. Like all gases, this one is made of individual particles. The average speed of the particles corresponds to the temperature of the gas—faster is hotter. But at any given time, some particles will be moving more slowly than others. What if, suggested Maxwell, a tiny imaginary creature—a demon, as it was later called—sat at the door. Every time it saw a fast-moving particle approaching from the left-hand side, it opened the door and let it into the right-hand compartment. And every time a slow-moving particle approached from the right, the demon let it into the left-hand compartment. After a while, the left-hand compartment would be full of slow, cold particles, and the right-hand compartment would grow hot. This isolated system would seem to grow more orderly, not less, because two distinguishable compartments have more order than two identical compartments. Maxwell had created a system that appeared to defy the rise of entropy, and thus the laws of the universe. What do I miss? The system only "defies the rise of entropy/laws of the universe" because we've added an arbitrary agent in the middle (the "daemon"). Shouldn't the needs (entropy and otherwise) for that also should be accounted?

107 points by dnetesn 5y ago

> Then in the 21st century, with the thought experiment solved, the real experiments began. “The most important development is we can now realize Maxwell’s demon in laboratories,” said Sagawa. > In 2007 scientists used a light-powered gate to demonstrate the idea of Maxwell’s demon in action; in 2010, another team devised a way to use the energy produced by the demon’s information to coax a bead uphill; and in 2016 scientists applied the idea of Maxwell’s demon to two compartments containing not gas, but light. About the first experiment: https://www.nature.com/news/2010/101114/full/news.2010.606.html https://www.nature.com/news/2010/101114/full/news.2010.606.h... > The experiment does not actually violate the second law of thermodynamics, because in the system as a whole, energy must be consumed by the equipment — and the experimenters — to monitor the bead and switch the voltage as needed. But it does show that information can be used as a medium to transfer energy, says Sano. The bead is driven as a mini-rotor, with a information-to-energy conversion efficiency of 28%. About the second experiment: https://www.quantumlah.org/about/highlight/2016-02-maxwells-demon-light https://www.quantumlah.org/about/highlight/2016-02-maxwells-... > The team also worked out a theoretical description of the setup – accounting not only for the thermodynamic effects on the light, but also for the information gained by the demon – and found a good match with the data. It has long been known that the resolution of the apparent violation of the second law by Maxwell's demon (for it is only apparent) lies with the demon itself. The demon not only does work on the system, but also acquires information about the system that it would take work to erase. When all these factors are budgeted for, there's no net gain. So they didn't make a Maxwell' Demon. They are just very inefficient motor with a overhyped story around it.

tedsanders 5y ago
It's a common misconception that the Landauer Limit, which says it takes kT*ln(2) energy to erase a bit, "solves" Maxwell's demon. That argument is actually circular, because the Landauer Limit relies on the Second Law in its derivation. I cannot find the source now, so it's possible I'm wrong, but my recollection is that even Bennett, who advanced the argument in 2003, eventually admitted as much around 2006/2007, if memory holds.[1] I spent years of my PhD reading and thinking about these topics, and made some contributions to these Wikipedia pages years ago. [1] https://en.wikipedia.org/wiki/Landauer%27s_principle#Challenges https://en.wikipedia.org/wiki/Landauer%27s_principle#Challen... (To be clear, I am not saying that Landauer Limit or the Second Law are wrong - just that it's wrong to use the Landauer Limit to "prove" the Second Law.)
- meroes 5y ago
  Isn't there no "proof" within the laws of physics for the second law? - you must posit a "Past Hypothesis".
  - sharpneli 5y ago
    Depends on what level. Historically it was an empirical observation that was taken in as an axiom to classical thermodynamics. So it couldn't be derived from other laws. If we move to statistical mechanics we can derive the second law from probabilities. In essence it is possible for any system to break it, it's just very very improbable. Just like if you toss a coin billions of times. It's possible for you to get more, or less, than half of the tosses to be heads. It's just very very improbable to get anything more than very tiny deviations.
    - meroes 5y ago
      But when we apply statistical mechanics to "microscopic" arrangments like a few particles in an idealized rigid box or in a vacuum, there is no arrow of time. The laws of physics themsleves are totally symmetric (CPT theorem), and it is not thought the t-asymmetry of the weak interaction is responsible for the global macroscopic arrow of time. So without some statement of initial/prior conditions, we have no reason to see just the "forward" arrow unless we say something about initial/prior conditions. (Loschmidt's Paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox)
  - contravariant 5y ago
    You can prove it in the context of Ergodic theory. Though that's a model so assuming that it correctly approximates the real world is still an assumption.
    - meroes 5y ago
      Are you sure Ergodic theory does the trick? When I hear people like David Albert or Sean Carroll speak, they always make clear you need some kind of initial/prior conditions or else stat mech does not guarantee why we only see the forward arrow of time. If we assume we can reduce all physical process to the behavior of fundamental particles (or fields), and the physical laws that describe the microscopic world are entirely symmetrical (CPT theorem), you get Loschmidt's Paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox https://en.wikipedia.org/wiki/Loschmidt%27s_paradox. They say the Past Hypothesis (low entropy big bang prior condition) is necessary to resolve it. *And the t-assymetry of the weak interaction isn't the reason according to the wikipedia article on time asymmetry - you still need initial conditions. "Time reversal violation is unrelated to the second law of thermodynamics, because due to the conservation of the CPT symmetry, the effect of time reversal is to rename particles as antiparticles and vice versa. Thus the second law of thermodynamics is thought to originate in the initial conditions in the universe. "

scrooched_moose 5y ago

Possessing only an undergrad level of physics, I've never understood why this is a remotely interesting question. It seems to effectively be equivalent to "Imagine we have a refrigerator with a massless, frictionless motor that also doesn't need to be plugged in. Isn't it crazy that this refrigerator would violate the second law of thermodynamics?"

bb88 5y ago
Entropy can't be reversed, even if you have a massless, frictionless, motor that doesn't need to be plugged in. That is effectively what Maxwell's demon is.
- deleted 5y ago
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- tsimionescu 5y ago
  Of course entropy can be reversed, I literally do it in my sleep! However, reversing entropy requires an amount of energy commensurate with the amount of entropy you are reducing. Maxwell's demon would appear as a violation of this. A massless, frictionless motor could trivially reverse all the entropy in the universe given enough time, but it is not clear that Maxwell's demon REQUIRES something equivalent.
  - bb88 5y ago
    Maxwell's demon requires erasure which would subsequently increase entropy. https://www.nature.com/news/the-unavoidable-cost-of-computation-revealed-1.10186#:~:text=Landauer's%20theory%20offered%20the%20first,entropy%20lost%20by%20the%20demon https://www.nature.com/news/the-unavoidable-cost-of-computat....
    - tsimionescu 5y ago
      As someone else was noting, this result is based on Landauer's limit, which is itself ultimately based on the second law of thermodynamics. If Maxwell's demon existed, the second law would be wrong, so Landauer's limit would be wrong, so there could be a way to erase information without increasing entropy. I'm also not sure why Maxwell's demon would require erasure.

quelltext 5y ago

I don't really have the necessary background in physics so maybe that's it but this article just doesn't seem to get the point across very well. > Two advances would be crucial to solving Maxwell’s demon. At this point in the article there had been a description of what Maxwell's demon is as: "Imagine that this nonexistent thing does X and has this outcome", however it's utterly unclear what the question is that needs to be solved. Whether such a system can exist? Whether two rooms with more order would violate some law and why? No idea. Can't I have two insulated rooms if some magical creature teleported certain particles magically from one room to the next? I saw other comments about the pointlessness of the thought experiment, but that's not my gripe. But there must be an explanation of why it really matters and what is to be "solved" here when the word "solve" is used. I have the same problem with other thought experiments or rather their dumbed down description, e.g. Schrödinger's cat. They focus too much on the quirky scenario and superficial outcome ("oh, that would mean ...") but not on what specific questions it raised in the end and what the exact follow-ups were. And most importantly they leave out very crucial details. For instance, I think one important piece to Maxwell's demon (from Wikipedia) is that the demon doesn't do "work".

zackees 5y ago
Maxwell’s demon lives on today as two sheets of graphene and two diodes. The vibrating sheets (due to virtual particles) causes oscillating electrical currents which are captured by the diodes and stored as charge. https://www.studyfinds.org/graphene-power-circuit-clean-energy/ https://www.studyfinds.org/graphene-power-circuit-clean-ener... The article mentions that Physics is safe. Yet this simple experiment shows that maxwell’s demon disproves the law of increasing entropy.
aidenn0 5y ago
The resolution of Maxwell's demon would involve proving that there is no way to implement such a device in an energy-positive manner. People generally suspect this is the case, but nobody can say why it must be the case.
- Nursie 5y ago
  Thanks for that concise explanation. My immediate thought was "the demon will be doing work to open and close its door, so the overall entropy is likely to be rising, only local order is increased" I guess the problem is around establishing that that has to be true, for any given demon.

raverbashing 5y ago

Thermodynamics are mostly about statistical effects and return to means. Maxwell's demon seems (to me) to turn probabilities into some Deus Ex Machina thing. (Though in the other way, saying that "disorder" is mandatory) Maybe the thermalization hypothesis will give a more adequate answer to it.

ncallaway 5y ago
It seems to me that information theory has already provided an adequate answer to Maxwell's demon (as mentioned in the article). To operate, Maxwell's demon must perform operations on information. Those operations can be quantified, and necessary generate more waste heat than it creates by separating the particles.
- woopwoop 5y ago
  I've heard this explanation alluded to, but I've never understood the details. What does the last sentence mean?
  - dvt 5y ago
    > What does the last sentence mean? "Waste" here means entropy. So the "thinking" that the demon does actually increases the entropy overall. In other words, even though the system's entropy seems to be decreasing (since particles are being neatly sorted), the process used in the sorting must create some kind of random garbage that actually will be a net increase in entropy, in spite of the now-sorted particles.
  - ncallaway 5y ago
    Basically, any temperature difference between two regions of space can be used to do Work* (essentially, there is a maximum amount of Energy that you can extract from that temperature difference to do anything with). If everything in your closed system is the same temperature, then there is no Energy that you can capture in order to do Work. Well, if we have this demon that can open this gate at _just_ the right times so that it can separate hotter particles from colder particles, then it is able to _create_ a temperature difference where there wasn't one. So, this demon would allow us to use our closed system to do an infinite amount of Work. It would act as a sort of perpetual motion machine. - Step 1: Let the demon open the gate at the right times to separate the hotter gasses (more energetic individual particles) from the colder gasses (less energetic individual particles). - Step 2: Replace the barrier/gate with a heat engine, and let the natural flow of energy from hot -> cold, drive our heat engine and push the box. - Step 3: Once the temperature has diffused enough that our heat engine can no longer do any productive Work (that is, the temperature has equalized), then we replace the heat engine with the barrier/gate and let the demon go back to work. That sets us up to discuss the final sentence. With the advent of information theory, we now know that there is a _minimum amount_ of Work that a hypothetical demon would need just to know when to open and close the gate. That Work requires Energy to perform. As it happens, that Work that the demon must do requires more energy than the demon is able to create by separating the particles. Which means the closed system of particles + demon will still increase entropy, because the demon _consumes more energy than it creates_. Basically, we were able to do the math about the theoretical limits of a perfect demon, and if you were able to make one it doesn't break the 2nd law of thermodynamics. The reason this stumped scientists for a long time is we didn't have the ability to calculate the _minimum amount of Energy_ that would be needed by the demon's brain. * Work here being the physics concept of work. For example, we could move a box around by using a heat engine to turn the temperature difference into adding a physical force to the box.

aerovistae 5y ago

I have never understood Maxwell's demon at all and would love if anyone could explain its significance.

ncallaway 5y ago
The general idea of Maxwell's demon as a problem is that it conflicts with the second law of thermodynamics, which essentially states that the total entropy of a closed system will increase over time. Which, essentially, implies that if we have a (perfectly sealed closed system) box of gaseous particles well mixed at temperature T, then we will never observe that box eventually separates into two different temperatures of gas with a T(cold) on the left side of the box and a T(hot) on the right side of the box. However, if we already had a box that was separated into two different temperatures of gas, we would expect those gasses to eventually become well-mixed over time. The paradox of Maxwell's Demon was that we had this apparently closed system (gas particles + Demon), that wasn't doing any Work, but apparently had the ability to violate the 2nd law of thermodynamics. The thinking was that this shouldn't be possible, even in principle. With the caveat that the second-law of thermodynamics is a statistical law, so it's really saying "the odds of entropy decreasing by any significant amount is extremely vanishingly improbable"
- FranOntanaya 5y ago
  What bothers me is, we derive entropy --a blind, involuntary physical law--, from observation of inanimate systems. So, for example, a very big box full of hot hydrogen will eventually cool off. But, there's a tiny chance it may actually coalesce into life bearing stars. Such life may intentionally work towards increasing order, like Maxwell's demon. So we try to extend entropy to life too and strenuously prove that it'll increase even when aware biological organisms work against it, while still excluding the same organisms from the observations entropy is derived from.
  - exporectomy 5y ago
    Are there no natural non-biological processes which decrease entropy locally just as life does? Surely something like a weather system does that, doesn't it?

ffhhj 5y ago

I have worked implementing gambling algorithms for online casinos and also created my own lottery apps, and this article posses the __blind optimism__ that drives poor devils into gambling and make them loss their money for years, and begins like this: > "You can either play all night on the roulette table, or you can stop if you win $100,” The lucky thermodynamic demon that can extract on average more free energy than the work spent over many iterations, will be forbidden from gambling ever again! If he returns to the system, entropy will make sure to balance his earnings in the long run. Losing and winning devils will be balanced as well. For each lottery winner there are lots of losers.

ramblerman 5y ago
The analogy was just to help clarify the situation, not to be taken so literally. Where it doesn't match gambling is that there is no risk of ruin, only time waited. In other words, the universe is constantly flipping dice, and when it looks in our favour we stop. But we didn't pay to play.
- ffhhj 5y ago
  Yeah, but imagine a gambler that wins once and never plays ever again in the same casino. What if he takes his money to another casino? He could lose or win. Let's say he wins again, and goes around the city playing in different casinos. In the long run his earned money will be 0. So, win in the first casino and then stop. The gambler dies, and his son gets the money. Now his son is a new gambler who lives in a different city. If he plays in other casinos he could win or lose, but in the long run his earned money will be 0 too. It means that for a gambler to actually win he must win only once, then he must not play ever again, or any other gambler with his money, since entropy is "connected" to that money, because money makes part of the same system. Transform that money into energy and give it to the gambler demon, what will happen? Now how much money/energy does it cost to create one of these demons?

dumpsterdiver 5y ago

If we believe in the conservation of energy, wouldn’t the universe itself likely be a version of Maxwell’s Demon? Could black holes/white holes be the membrane driving such a perpetual motion machine?

Sosh101 5y ago

I really enjoyed "The demon in the machine" - anyone know of other good books exploring the topic?

tomxor 5y ago
Not a book but another article from quanta, and my favourite on the subject, not for it's technical depth but for it's perspective: https://www.wired.com/2017/02/life-death-spring-disorder/ https://www.wired.com/2017/02/life-death-spring-disorder/ I felt like this one brings in the modern understanding combining Maxwell, Shannon and Landauer into a tangible perspective of everyday reality... maxwell demon is not "solvable", it's an extreme example for the purpose of a thought experiment, however practical "versions" of it exist everywhere, once you combine the costs as per landauers contributions, swap an infinite tape for a large tape, you essentially have a cache - when you start to observe biological and complex chemical systems with this perspective it really does feel like life can be thought of as little emergent upstream eddies "anti-entropy" machines in a torrent of entropy, like a dead fish that naturally swims upstream, there is no defying physics here, entropy still increases universally, but these are natural local variations. It's hard to not believe "reducing entropy" is the fundamental "meaning of life" with this point of view, a meaning that is shared among all life: to not dissolve into oblivion, to collect little hacks of physics and chemistry to find better and better ways to evade it.
- K0balt 5y ago
  This is also my perspective, that living things are fundamentally machines which externalize entropy, and the processing of information is crucial to this task. The task of living is that of using information to harvest order out of entropy, much as maxwells daemon . Of course this increases overall entropy, only conserving order locally....so you could also think of life as a “fire” that burns order out of imperfectly disordered systems.
  - tomxor 5y ago
    I missed this > that living things are fundamentally machines which externalize entropy That's a clearer way to put, externalising or displacing, makes it more palatable for those getting stuck with the laws of thermodynamics.

johbjo 5y ago

> It needed to record and store information about individual particles in order to decide when to open and close the door. This isn't true, all it needs is to store the average speed of particles on either side of the door and it would still converge.

williesleg 5y ago

Just like climate change assholes.

michaelpfrank 5y ago

I made a short Twitter thread relating to this article here: https://twitter.com/mikepfrank/status/1395731216922464263?s=21 https://twitter.com/mikepfrank/status/1395731216922464263?s=... I recommend the books “Maxwell’s Demon” and the second edition “Maxwell’s Demon 2”, both edited by Leff and Rex, which collect a lot of the classic literature on this topic. Bottom line is, of course no real machine can break the Second Law, but the Maxwell’s Demon thought experiment helps illuminate the physical nature of information.

EMM_386 5y ago

I don't understand the concept of Maxwell's Demon. Wouldn't this demon require it's own energy source to perform this sorting of hot/cold particles? It has to move it's arms or whatever it is doing to manipulate the door. It has to use it's brain and eyes to determine which particles to sort. How is this any different then if you had a heat exchanger or other mechanical device sitting there? I must be missing something obvious.

Covzire 5y ago
I'm wondering the same, it seems that the common denominator in any system that we can observe where entropy first stops increasing and begins to decrease is the addition of a mind.
6gvONxR4sf7o 5y ago
I believe the idea is that the demon could be arbitrarily efficient since it opens and then closes the door, allowing a thought experiment of perfect efficiency. Like saying infinity when you mean an arbitrarily large number.

tzs 5y ago

It's not really a Maxwell's Demon, but it kind of acts like one: the Ranque-Hilsch vortex tube [1]. It's a simple mechanical device with no moving parts invented in 1931 with one input port and two output ports. Force compressed air into the input port, and hot air (up to 200 ℃) comes out one of the outputs and cold air (down to -50 ℃) out the other. [1] https://en.wikipedia.org/wiki/Vortex_tube https://en.wikipedia.org/wiki/Vortex_tube

steve76 5y ago

It's a shame we are drawn to the ugly. Create a set of equations which determine the speed of light from empty space based on material properties of nothing. Advance the Fundamental Theorem of Calculus to start the unification of space and time for highly accurate models. *crickets* Throw the word "demon" in front of it. Birds start chirping.

coldtea 5y ago

>In the thought experiment, Maxwell imagined splitting a room full of gas into two compartments by erecting a wall with a small door. Like all gases, this one is made of individual particles. The average speed of the particles corresponds to the temperature of the gas—faster is hotter. But at any given time, some particles will be moving more slowly than others. What if, suggested Maxwell, a tiny imaginary creature—a demon, as it was later called—sat at the door. Every time it saw a fast-moving particle approaching from the left-hand side, it opened the door and let it into the right-hand compartment. And every time a slow-moving particle approached from the right, the demon let it into the left-hand compartment. After a while, the left-hand compartment would be full of slow, cold particles, and the right-hand compartment would grow hot. This isolated system would seem to grow more orderly, not less, because two distinguishable compartments have more order than two identical compartments. Maxwell had created a system that appeared to defy the rise of entropy, and thus the laws of the universe. What do I miss? The system only "defies the rise of entropy/laws of the universe" because we've added an arbitrary agent in the middle (the "daemon"). Shouldn't the needs (entropy and otherwise) for that also should be accounted?