Talk:Reversible computing

Too technical?

I am disputing the "too technical" marker that someone put on this page earlier. Some subjects are simply technical in nature. In my opinion, it is far, far better for an article to be technical and correct, rather than being non-technical but too vague, incorrect or misleading.

In my opinion, if a Wikipedia reader feels that a particular article is too technical or difficult to understand, the most constructive thing that they could do is take the time to learn about the subject in depth themselves, and then try their best to find a new way to explain it that is somehow "less technical" while still remaining accurate and of high quality.

For now, I am removing the marker, although I will take it under advisement and will endeavor to find ways to make the article more accessible to readers in the future.

--Mpfrank 6/14/05

It is too technical. Provide an example of a digital circuit that represents reversible computing. E.g. a two bit adder. Mre5765 06:29, 15 November 2005 (UTC)

It is NOT too technical. Landauer's principle (which is the main idea in the article as it is written) is one of those fascinating discoveries that came well before its time, and therefore lay more or less forgotten for many years. It really is a simple application of the second law of thermodynamics to the field of computer science. Only recently, after many decades of steady exponential increase in the amount of computation that can be performed with a given amount of heat generated, are we even getting close to this limit. You cannot get around this limit by refrigerating the processor, because although the processor may produce less heat per clock cycle at a lower temperature, (since it can then be run at a lower voltage :-), the refrigeration cycle(s) must then reject a proportionately greater amount of heat to the environment. That's kind of long-winded, and I'm sure it's actually better explained in the article, but all this theory is important if you want to overclock your processor to 10 GHz. Perhaps it could be somewhat better organized and retitled, but on the whole it's a good interesting article. 130.94.162.64 23:55, 25 November 2005 (UTC)

Explaining all the gory details of specific circuits would make the article too technical for general readers, IMHO. If someone wants more detail, there are plenty of reversible circuits already described in the adiabatic circuits literature. For example, Saed Younis' 1994 MIT Ph.D. thesis (Asymptotically Zero Energy Computing Using Split-Level Charge Recovery Logic) contains a number of examples including adder and multiplier circuits. --Mpfrank 1/12/06

Technical or not so technical this first sentence does not make much sense: "The term reversible computing refers to any computational process that is (at least to some close approximation) reversible, i.e., time-invertible, meaning that a time-reversed version of the process could exist within the same general dynamical framework as the original process."

First of all the term is dynamic, I don't know what dynamic-al means. Secondly to substitute revert with invert for an explanation is not sufficient. Does it mean if run backwards through time it would work. And whats dynamic(al) framework? By itself it means absolutely nothing if not defined within a, well, certain framework. Does it mean that the same general conditions and potentialities exist, as in an experiment?

Points taken. "Invertibility" is a standard concept in mathematics; here it refers to the invertibility of the transition function that takes old states to new ones. Essentially it means that the transition function is one-to-one. In any case, I added a link to the "invertible function" page to help clarify this. As for "dynamical", as is usual with the "-al" suffix in English, it means "of or relating to" dynamics, and by "the same general dynamical framework" I meant the same general type of dynamical system (whether it's physical or mathematical, discrete or continuous), and added a link to the "Dynamical system" page. Hopefully these links will be helpful. I will try to compose a more accessible introductory paragraph that is still correct. --Mpfrank 4/25/06

I didn't go on to read the rest of the article, because if its of the same standards there's not much point, please whoever can edit this to be more readable.

Too many links...

Someone started going through the article linking common words like "time" and "exist" to their Wikipedia entries. This sort of thing can get ridiculous, and is against the Wikipedia guidelines. If the user really doesn't know what "time" means, let him search it himself! IMHO, links should be reserved for specific technical terms that relate to the original article and that the reader might plausibly want to learn more about. So, I removed some of the extra links. --Mpfrank 6/14/05

Removed reference to UF group

I removed the reference to the UF reversible computing group which someone had added, since it violated Wikipedia guidelines not to use the site for promotional purposes. Anyway, I was the leader of the UF group, and (contrary to some misleading reports by the UF press office) we didn't design any new reversible CPUs there. However, I did participate in the design of several reversible processors (Tick, FlatTop, and Pendulum) earlier during my Ph.D. work at MIT; most of this work took place during the period 1996-1999. --Mpfrank

Entropy

Although in practice no physical process can be exactly physically reversible or isentropic

Ought this be "... no macroscopic physical process ..." ?

Macro/micro is not the relevant distinction here. Here, I mean that the process cannot be reversible as far as our knowledge of the state of the system is concerned. Although macroscopic processes are particularly prone to irreversibility, no microscopic process can be perfectly reversible for all practical purposes either, from the perspective of an observer who doesn't precisely know the laws of physics, by which I mean, e.g., knowing the values of all of the coupling constants and the masses of the fundamental particles to infinitely many decimal places. If you don't know the exact laws of physics, than your description of the state of any system (macro or micro) will necessarily become more uncertain over time, unless the system is already in a stationary state such as a thermal equilibrium state or a quantum ground state, in which case it seems over-generous to refer to its evolution as a "process." --Mpfrank 6/14/05

It's all tosh, and anyone with a clue about entropy and physics/electronics will understand that much of what has been written on "Reversible Computing" is about as reliable as BBC Panorama's report on the spaghetti harvest.

Go right back to basics. When you accelerate (or decelerate) a charge, you cause electromagnetic energy to be radiated. But you always need to accelerate (and/or decelerate) a charge to change a logic 0 to a logic 1 or vice versa. The energy radiated cannot be recovered except in certain very special cases (such as a tuned circuit formed by a capacitor and an inductor, or various equivalents, elements not useful in a general computing sense, and even there the recovery ratio is never 100% (go look up "Q" in the context of electronic tuned circuits)). Thus entropy is increased by meaningful operational circuit, never mind a computational circuit, and that is not reversible on any meaningful scale.

Also, what on earth have backup/restore and "undo" got to do with "reversible computing" meaning thermondynamically reversible? It's a bit like claiming that because a vehicle has a reverse gear and can therefore go back to where it was previously, it is therefore potentially reversible in the entropy-related sense, which surely is clearly rubbish?

"Undo"??

I'm reinforcing the point above regarding the Practical logical reversibility section, which at the very least needs an initial sentence explaining that the reversibility of an "undo" command is not the same sort of reversibility in the discipline of thermodynamics. The main thrust of this article is entropy. This section, however, is in no way about the limits and/or reduction of accrued entropy as it applies to computation. Any undo function as discussed in this section increases the statistical disorder of the system by increasing W (the number ways the bits may be arranged to achieve the energy of the system) and heat is wasted to facilitate this additional work. As such entropy is significantly increased. I question this section's relevance to this article.Wolfworks (talk) 06:50, 15 March 2008 (UTC)