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The usual extropian retort to this limit has been "reversible computing," but as far as I can tell there has been little work on reversible computing hardware -

The usual extropian retort to this limit has been "reversible computing," but as far as I can tell there has been little work on reversible computing hardware -- far less than quantum computing. It looks like the University of Florida made some hardware in the early 2000s. Can anyone offer insights about why hardware research is so scarce here? Are there reasons to think it's a dead end prior to fabricating anything? https://en.wikipedia.org/wiki/Reversible_computing https://en.wikipedia.org/wiki/Reversible_computing EDIT: searching Google Scholar, it looks like only ~50 papers/books mention reversible computing in the past year, compared to 1000+ for quantum computing.

bo1024 9y ago
Just my guess, but the practical motivation is probably not there -- we are so far from the levels of efficiency at which the Landauer principle is noticeable.
Strilanc 9y ago
Reversible computing still requires energy (or rather neg-entropy). It's used to perform error correction. Otherwise the computer won't last very long.
- kobeya 9y ago
  It’s also used to offload bits. Real computers don’t have infinite tape.
tgb 9y ago
Reversible computation is inherently slow computation and the main advantage is it lets you get below this limit but current computers are still many (dozens?) of orders of magnitude off from reaching that limit. Until then, you'd be better off just improving efficiency of non-reversible computers.
- abecedarius 9y ago
  That wiki page says "Although the Landauer limit was millions of times below the energy consumption of computers in the 2000s and thousands of times less in the 2010s,[3] proponents of reversible computing argue that this can be attributed largely to architectural overheads which effectively magnify the impact of Landauer's limit in practical circuit designs, so that it may prove difficult for practical technology to progress very far beyond current levels of energy efficiency if reversible computing principles are not used.[4]" On a skim it looks like [4]'s argument is that people have been criticizing the overhead of fully reversible circuits, but only 'conditionally reversible' circuits are needed for the power benefits.
  - tgb 9y ago
    Wow have we improved that much in efficiency? I learned about this from the Feynman lectures on computing which are perhaps a little out of date.
    - tgb 9y ago
      Adding: I looked it up and Feynman quotes 10^8 times as much energy dissipated per step as the Van Neumann-Landauer limit when using the transistors of his era (1984-86 is when he taught the course).
  - GTP 9y ago
    Based on this statement there is also another possible way: remove the architectural overheads that magnify the impact of Launder's limit. So probably he is right saying that there are other improvements that are easier to accomplish that we can do before caring about Launder's limit.
deleted 9y ago
[deleted]
kkylin 9y ago
I remember talking to someone who had worked on reversible computing about 13 or 14 years ago, and he had said that (i) it does work (presumably that it is indeed more energetically efficient), but (ii) the state of computing was such that it may be a couple decades before it can make a real impact (). Then he added, "someone else can have that fun." Around that time he switched to working on other things. () Unfortunately I can no longer recall exactly what he meant.
tgb 9y ago
Oh it's worth noting that quantum computing research is reversible computing research. Quantum operations are unitary operators which necessarily have inverses. Famous quantum computing results like the "no cloning" theorem are basically just consequences of the reversibility of QC.
- akvadrako 9y ago
  I suspect any non-trivial reversible computer will be a quantum computer. Quantum gates can perform almost any reversible operation and losing information (by not being reversible) might be physically equivalent to decoherence.
hadsed 9y ago
To make a circuit reversible generally requires more gates, because you need a place to store the information that you normally might have gotten rid of (say in an AND gate you simply store one of the variables). Practically this would make chips much slower if you assume everything else in fabrication stays the same (e.g., die size). Not a whole lot of benefits it seems, except for energy consumption, but that's the consumer's problem.

bo1024 9y ago

Just my guess, but the practical motivation is probably not there -- we are so far from the levels of efficiency at which the Landauer principle is noticeable.

Strilanc 9y ago

Reversible computing still requires energy (or rather neg-entropy). It's used to perform error correction. Otherwise the computer won't last very long.

kobeya 9y ago
It’s also used to offload bits. Real computers don’t have infinite tape.

tgb 9y ago

Reversible computation is inherently slow computation and the main advantage is it lets you get below this limit but current computers are still many (dozens?) of orders of magnitude off from reaching that limit. Until then, you'd be better off just improving efficiency of non-reversible computers.

abecedarius 9y ago
That wiki page says "Although the Landauer limit was millions of times below the energy consumption of computers in the 2000s and thousands of times less in the 2010s,[3] proponents of reversible computing argue that this can be attributed largely to architectural overheads which effectively magnify the impact of Landauer's limit in practical circuit designs, so that it may prove difficult for practical technology to progress very far beyond current levels of energy efficiency if reversible computing principles are not used.[4]" On a skim it looks like [4]'s argument is that people have been criticizing the overhead of fully reversible circuits, but only 'conditionally reversible' circuits are needed for the power benefits.
- tgb 9y ago
  Wow have we improved that much in efficiency? I learned about this from the Feynman lectures on computing which are perhaps a little out of date.
  - tgb 9y ago
    Adding: I looked it up and Feynman quotes 10^8 times as much energy dissipated per step as the Van Neumann-Landauer limit when using the transistors of his era (1984-86 is when he taught the course).
- GTP 9y ago
  Based on this statement there is also another possible way: remove the architectural overheads that magnify the impact of Launder's limit. So probably he is right saying that there are other improvements that are easier to accomplish that we can do before caring about Launder's limit.

deleted 9y ago

[deleted]

kkylin 9y ago

I remember talking to someone who had worked on reversible computing about 13 or 14 years ago, and he had said that (i) it does work (presumably that it is indeed more energetically efficient), but (ii) the state of computing was such that it may be a couple decades before it can make a real impact (). Then he added, "someone else can have that fun." Around that time he switched to working on other things. () Unfortunately I can no longer recall exactly what he meant.

tgb 9y ago

Oh it's worth noting that quantum computing research is reversible computing research. Quantum operations are unitary operators which necessarily have inverses. Famous quantum computing results like the "no cloning" theorem are basically just consequences of the reversibility of QC.

akvadrako 9y ago
I suspect any non-trivial reversible computer will be a quantum computer. Quantum gates can perform almost any reversible operation and losing information (by not being reversible) might be physically equivalent to decoherence.

hadsed 9y ago

To make a circuit reversible generally requires more gates, because you need a place to store the information that you normally might have gotten rid of (say in an AND gate you simply store one of the variables). Practically this would make chips much slower if you assume everything else in fabrication stays the same (e.g., die size). Not a whole lot of benefits it seems, except for energy consumption, but that's the consumer's problem.