The Future of Computing Depends on Making It Reversible

["Dave Farber" <farber () gmail com>]

Begin forwarded message:

> From: Dewayne Hendricks <dewayne () warpspeed com>
> Date: September 10, 2017 at 10:25:20 AM EDT
> To: Multiple recipients of Dewayne-Net <dewayne-net () warpspeed com>
> Subject: [Dewayne-Net] The Future of Computing Depends on Making It Reversible
> Reply-To: dewayne-net () warpspeed com
>
> [Note:  This item comes from friend Judi Clark.  DLH]
>
> The Future of Computing Depends on Making It Reversible
> It’s time to embrace reversible computing, which could offer dramatic improvements in energy efficiency
> By MICHAEL P. FRANK
> Aug 25 2017
> <https://spectrum.ieee.org/computing/hardware/the-future-of-computing-depends-on-making-it-reversible>
>
> For more than 50 years, computers have made steady and dramatic improvements, all thanks to Moore’s Law—the 
> exponential increase over time in the number of transistors that can be fabricated on an integrated circuit of a 
> given size. Moore’s Law owed its success to the fact that as transistors were made smaller, they became 
> simultaneously cheaper, faster, and more energy efficient. The ­payoff from this win-win-win scenario enabled 
> reinvestment in semi­conductor fabrication technology that could make even smaller, more densely packed transistors. 
> And so this virtuous ­circle continued, decade after decade.
>
> Now though, experts in industry, academia, and government laboratories anticipate that semiconductor miniaturization 
> won’t continue much longer—maybe 5 or 10 years. Making transistors smaller no longer yields the improvements it used 
> to. The physical characteristics of small transistors caused clock speeds to stagnate more than a decade ago, which 
> drove the industry to start building chips with multiple cores. But even multicore architectures must contend with 
> increasing amounts of “dark silicon,” areas of the chip that must be powered off to avoid overheating.
>
> Heroic efforts are being made within the semiconductor industry to try to keep miniaturization going. But no amount 
> of investment can change the laws of physics. At some point—now not very far away—a new computer that simply has 
> smaller transistors will no longer be any cheaper, faster, or more energy efficient than its predecessors. At that 
> point, the progress of conventional semiconductor technology will stop.
>
> What about unconventional semiconductor technology, such as carbon-nanotube transistors, tunneling transistors, or 
> spintronic devices? Unfortunately, many of the same fundamental physical barriers that prevent today’s complementary 
> metal-oxide-semiconductor (CMOS) technology from advancing very much further will still apply, in a modified form, to 
> those devices. We might be able to eke out a few more years of progress, but if we want to keep moving forward 
> decades down the line, new devices are not enough: We’ll also have to rethink our most fundamental notions of 
> computation.
>
> Let me explain. For the entire history of computing, our calculating machines have operated in a way that causes the 
> intentional loss of some information (it’s destructively overwritten) in the process of performing computations. But 
> for several decades now, we have known that it’s possible in principle to carry out any desired computation without 
> losing information—that is, in such a way that the computation could always be reversed to recover its earlier state. 
> This idea of reversible computing goes to the very heart of thermo­dynamics and information theory, and indeed it is 
> the only possible way within the laws of physics that we might be able to keep improving the cost and energy 
> efficiency of general-purpose computing far into the future.
>
> In the past, reversible computing never received much attention. That’s because it’s very hard to implement, and 
> there was little reason to pursue this great challenge so long as conventional technology kept advancing. But with 
> the end now in sight, it’s time for the world’s best physics and engineering minds to commence an all-out effort to 
> bring reversible computing to practical fruition.
>
> The history of reversible computing begins with physicist Rolf Landauer of IBM, who published a paper in 1961 titled 
> “Irreversibility and Heat Generation in the Computing Process.” In it, Landauer argued that the logically 
> irreversible character of conventional computational operations has direct implications for the thermodynamic 
> behavior of a device that is carrying out those operations.
>
> Landauer’s reasoning can be understood by observing that the most fundamental laws of physics are reversible, meaning 
> that if you had complete knowledge of the state of a closed system at some time, you could always—at least in 
> principle—run the laws of physics in reverse and determine the system’s exact state at any previous time.
>
> To better see that, consider a game of billiards—an ideal one with no friction. If you were to make a movie of the 
> balls bouncing off one another and the bumpers, the movie would look normal whether you ran it backward or forward: 
> The collision physics would be the same, and you could work out the future configuration of the balls from their past 
> configuration or vice versa equally easily.
>
> [snip]
>
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