Researchers Re-Examine Second Law of Thermodynamics

Many readers have written to tell us that researchers are examining the possibility of using Brownian ratchets to help combat the problem of heat dissipation in miniaturized electronics. "Currently, devices are engineered to operate near thermal equilibrium, in accordance with the Second Law of Thermodynamics which states that heat tends to transfer from a hotter unit to a cooler one. However, using the concept of Brownian ratchets, which are systems that convert non-equilibrium energy to do useful work, the researchers hope to allow computers to operate at low power levels, and harness power dissipated by other functions. 'The main quest we have is to see if by departing from near-equilibrium operation, we can perform computation more efficiently,' Ghosh told iTnews. 'We aren't breaking the Second Law — that's not what we are claiming,' he said. 'We are simply re-examining its implications, as much of the established understanding of power dissipation is based on near-equilibrium operation.'"

Comments

Hmmmm, help me out here.

[SatanicPuppy]

I may just be too stupid to follow this, so feel free to slap me down.

The article sucks, obviously, but they repeat the phrase "Brownian Ratchet" incessantly, and I know what those are: a theoretical molecular machine able to extract energy from a heat source that is in thermal equilibrium. Obviously this would be interesting because normally we use heat transfer to generate energy and if there is no excess to transfer one would suppose (based on the second law) that there is no extra energy to be converted to whatever work needs to be done.

But the article and the summary both use the phrase "non-equilibrium" which suggests the existence of heat energy in excess of what is naturally dissipated, which is, gosh, the source of almost all the power that we use, in one form or another.

So either I'm unclear on the concept of a non-equilibrium thermodynamic state, or they don't know what the fuck a Brownian Ratchet is, and are trying to grab a sensationalist headline by making a wild claim that has nothing to do with what they're actually doing (e.g. running the system fans off steam power or something).

Re:Hmmmm, help me out here.

[m50d]

The point as I understand it is to use the thermal gradient you have to do useful work. You have a hot CPU core and cold air around it - and currently all we do is try and move heat from one to the other as quickly as possible. But in theory this is a power source that could be used - kind of like regenerative braking on hybrid cars. The idea is that you design the chip to run with some parts - probably the middle - at higher temperatures than others (the edge), and use these gradients for powering; ultimately you would perhaps only need to directly power the most intensive part of the CPU (at a guess, the ALU) and things like instruction decode could be powered entirely off the waste heat from this.

Re:Hmmmm, help me out here.

[SatanicPuppy]

I think Feynman's objection there was that, without a gradient, the system would lock up because an equal force would be exerting in both directions...Effectively a system that can be moved in one direction by equilibrium heat can necessarily be moved in the other direction as well, and therefore the net effect would end up being zero.

Re:Hmmmm, help me out here.

[gardyloo]

Exactly. Feynman showed that unless the ratchet itself is at a lower temperature/higher enthalpy/lower entropy than the surroundings, there's no way to extract the energy which sits in the heat reservoir. Once you've stuck that ratchet in there, in full thermodynamic contact with those surroundings, it's going to quickly heat up and its "ratcheting" action quickly become just as random as everything else.

Re:Hmmmm, help me out here.

[JustinOpinion]

Yeah the article is unclear. Here's my best shot at clarification:

A "Brownian ratchet" is a thought-device about extracting energy from the random Brownian motion of a hot gas. Similar to Maxwell's Demon, it can't work in a system at equilibrium. Without a temperature gradient, there is no way to extract useful work. The ratchet will be undergoing random motion equal in magnitude to the energy we hope to extract, so we can't actually extract anything.

However, if we're not at equilibrium, the rules are different. These researchers are talking about "non-equilibrium Brownian ratchets", which you could also call a "Brownian motor". In an non-equilibrium situation, you will have a gradient of heat or chemical potential that could, in principle, be converted into useful work.

So my guess is the researchers are trying to do something like:
1. Build devices that exploit the temperature gradient that exists in the device. So a bunch of nano-sized ratchets that convert the heat gradient on the outside of the chip (relative to the cool air) to recharge a capacitor or something.
2. Build switching elements (e.g. transistors) that directly store the excess switching energy in some way. That is, build switching elements that both do computational switching, but immediately utilize the resulting temperature gradient of the dissipated heat.

In either case, all they are suggesting is to take advantage of the heat gradients that inherently occur when you have imperfect switching elements dissipating heat. It's not really that novel, conceptually... although if they actually have a specific way to do this in mind, then that could be quite interesting.

Re:Hmmmm, help me out here.

[blueg3]

The article is terrible. They're actually looking at non-equilibrium Brownian ratchets, which is very different from a Brownian ratchet. Much like how they're not reexamining the second law of thermodynamics, they're reexamining its implications.

As I read it, the general idea seems to be that instead of simply burning electricity and disposing of the waste heat, they're considering reclaiming some of the waste heat to help power the device (which could help reduce its heat output). Of course, since they're consuming energy to perform calculations (which are entropy-reducing), they're required to emit a certain amount of uncapturable heat.

Re:Hmmmm, help me out here.

[JustinOpinion]

Ok, I've got some further details. The researchers involved are Avik Ghosh and Mircea Stan, at the School of Engineering and Applied Science, University of Virginia.

On his webpage, Ghosh has this to say:

FYI -- there is some sensational press out there that makes it sound like we're planning to break/have already broken the 2nd law of thermodynamics. This is, of course, absurd -- but I think it's imperative we set the record straight before everyone starts jumping all over us.

The context.... a colleague and I received funding to study non-equilibrium switching invoking a concept called 'Brownian Ratchets' that has been well studied in nonequilibrium statistical physics over the years. The potential benefactor of this study is the chip industry, in a very broad way, as it is worried about rapidly increasing thermal budgets (chips are becoming very hot). We're simply trying to examine the physics of Brownian ratchets in a device context. A popular model for heat dissipation in binary switching (proposed by Victor Zhirnov and co-workers) looks at a two well one barrier geometry, with a gate controlling the barrier and a drain controlling the overall directionality. Each such raising and lowering of a barrier at the end dissipates energy irreversibly (during the reset step where one erases information), leading to a kTln2 dissipation per operation (kT is the thermal energy). And this analysis is usually done by assuming that you wait after you raise or lower a barrier and then let the electrons move and reach equilibrium with the surroundings. The analysis is thus based on equilibrium Boltzmann statistics -- since the electron was at equilibrium before a computation and reaches equilibrium after. What is not clear is what happens during the non-equilibrium transition phase, or if you switch before the equilibrium is reached. The aim of the study is not to attempt to deviate from cherished physical principles, but on the contrary to see what these cherished principles posit for such a situation. A ratchet is known to be able to rectify non-equilibrium noise to produce directed motion by transducing spatial asymmetries in the system (this is well recognized in nonequilibrium statistical mechanics and has been mulled over for years). The physics is well studied, but the context is perhaps new... we are interested in seeing if rectifying such non-equilibrium noise (as a ratchet does) can perhaps shave off some of the power dissipation limit associated with a drain bias in the regular example.

This is, of course, still at a toy model -- we need to worry about how to deal with compatibility of input and output, for example. Simply put, we don't know if this will bear fruit for the big picture of low-power device operation, but it's worth investigating.

That's about it... but then, cooling laptops as hot as the sun through the power of thinking or by breaking the 2nd law sounds fancier ... doesn't it?

(Emphasis added.)

So this seems like still very early work (just an idea, really)... and it appears that the intention is to build new kinds of switches (e.g. transistors) that exploit the fact that switching is inherently non-equilibrium, and extract some of the energy that is dissipated during these switching events.

Re:Hmmmm, help me out here.

[gardyloo]

What if you could create a nano ratchet that can only go in one direction?

It's an interesting question. That's what people thought ratchets were in the first place. Then Smoluchowski (1912), Callen and Welton (1951), and later Feynman (as popularizer, mainly) showed that once the ratchets come to the same temperature as the "working fluid" in which they're placed, the ratchets can no longer be one-way devices. In fact, for any ratchet above absolute zero, it will occasionally "miss" and slip backwards. In other words, people who want ratchets to consistently extract energy from a fluid have to keep the ratchets at absolute zero, which means they're not in equilibrium with the fluid.

Linke's lab at UO.

[gardyloo]

One of my friends got her degree in Linke's lab: http://www.uoregon.edu/~linke/res_ratchet.html . She was good at explaining the ratchets, and one of the things always stressed was that they don't work in thermal equilibrium---by definition!. In any case, Linke's website has good explanations.

Not so sure this helps

[sjames]

To get the questions out of the way, the Brownian ratchet at equilibrium has been shown not to work, exactly as we might expect from the laws of thermodynamics.

But that's not what they're talking about. They are hoping to use a Brownian ratchet at a temperature differential, which is a clever way to extract work from a temperature differential to be sure, but is fully in line with thermodynamics as we understand it today.

The difficulty I have with this is that the problem in electronics is dissipating the heat fast enough to avoid a meltdown. Extracting work from the differential actually slows the heat transfer down (acts as an insulator) and so would make the device run hotter. It is NOT a cooling solution.

Where it could be useful is in low power devices that typically run well under their heat tolerance with a passive heatsink. In that case, the device could be run hotter in exchange for 'recycling' some of the energy they consume to make them even lower power.

Re:Obligatory Wikipedia reference

[Spy der Mann]

You're right, but when a part of the chip is at a scorching 70C or more, I wouldn't really say that's really equilibrium.

The article (which *IS* a summary, btw) as I understand it, says: Let's use the excess heat in some parts of the chip and use that as a secondary power source.

In other words, it's not about breaking the 2nd law, but identifying the points of excess heat dissipation (read-as: Low efficiency) to minimize energy waste. I find that feasible, I read an article in physorg about using the excess heat in car exhausts to power up the electronics, for example.

Re:Obligatory Simpsons:

[bloobloo]

Maybe there should be a department for that or something.

Obligatory Flanders & Swann quote

[canthusus]

"Heat won't pass from a cooler to a hotter
You can try it if you like, but you'd far better notter"