6 ms·

Irreversibility and Heat Generation in the Computing Process (1961) [pdf]

36 points by tardygrade 6y ago

messe 6y ago
An interesting paper in somewhat the opposite direction, but still building on Landauer's work is "Ultimate physical limits to computation": https://arxiv.org/abs/quant-ph/9908043 https://arxiv.org/abs/quant-ph/9908043 The end of the abstract: > [...] quantitative bounds are put to the computational power of an `ultimate laptop' with a mass of one kilogram confined to a volume of one liter.
- scottlocklin 6y ago
  That along with "on the computational capacity of the universe" was the paper that stopped me from ever taking Seth Loyd seriously again. Later finding out he was a creepy Epstein croney... well sometimes the universe makes a lot of sense.
  - deleted 6y ago
    [deleted]
9214 6y ago
There's a plethora of air-gap malware studies from Ben-Gurion university [1], abused channels range from thermal, acoustic, optical, to classic TEMPEST. The interesting thing is that there's no escape from such covert attacks, since machines are bound by the laws of physics that demand energy dissipation and generation of noise, heat, etc. If it leaks, then it computes. [1]: https://cyber.bgu.ac.il/air-gap/ https://cyber.bgu.ac.il/air-gap/
- blacksmith_tb 6y ago
  Shouldn't that be the other way around? Plenty of things leak heat (like cooking dinner) that don't do any computation?
  - 9214 6y ago
    Maybe. Or maybe they do but we don't know it ;)
  - goldenkey 6y ago
    The quantum of action, which a layperson can consider as the smallest possible state change, is actually Planck's constant. https://en.wikipedia.org/wiki/Planck_constant https://en.wikipedia.org/wiki/Planck_constant All energy refers to is an amount of state change that is occurring every second that some quantity of energy exists. That's why action has units Energy x Time. You can divide by time and you get Energy = Actions per Second, kinda like APM in Starcraft.. https://en.wikipedia.org/wiki/Action_(physics) https://en.wikipedia.org/wiki/Action_(physics) Principle of least action is really the principle of "least change." In summary, computation is happening -- but most of the computation that energy does while held as mass/matter is cyclic processes (aging.) Not anything interesting, at least to me or you. Unless you like https://en.wikipedia.org/wiki/Radiometric_dating https://en.wikipedia.org/wiki/Radiometric_dating
ouid 6y ago
So, here's a question: If heat dissipation is so fundamental to computation, why aren't we using the amount of required heat dissipation as the fundamental measure of complexity in quantum computing? In particular, say I construct a box with a uniform mixture of all quantum states on n qubits, and all the unitaries that can operate on those n qubits. how much heat do I have to dissipate in order to refine the state in my box to a "particular" quantum state and a "particular" unitary? There are some interesting, and recent, results about how quickly I can dissipate heat. In particular, T(t) is bounded from below by k*T(0)/t^7, where k is some constant. Since the number of quantum computers you could want to build in that box is growing very fast with the size of your input, I suspect that your inability to throw away states in the process of constructing your computer very rapidly becomes the dominant effect in how long it takes to go from "which number do I want to factor" to actually getting the factors of that number.
- scottlocklin 6y ago
  One of the things they don't tell you about quantum computing is it's supposed to be reversible. Which, ceteris paribus, probably means they're not physical. Nobody likes to talk about that as it's terrible for funding. You can build totally reversible computers using ordinary classical physics which ... in principle can be arranged to dissipate no heat (in practice they'll always dissipate heat). The problem is you're basically effectively dissipating the "heat" into a memory system which rapidly becomes practically infinite [1]. Imagine keeping around all the bits that got AND-gated away from .... I dunno, fitting GPT-3. Or even just inverting some big matrix. That's what you got to do for reversible computing: at the individual bit level mind you -many of the fundamental floating point operations are not themselves reversible, so they throw off more "heat" aka fill up memory cells with bits which allow you to reverse them. Landauer, who is an underappreciated genius, wasn't aware of the reversible computing idea, or had too much sense to bother with it. There are others who attempt to defeat his very common sense idea with hand wavey adiabatic relaxation ideas, but I think they're all baloney. All of this is a barrel of monkeys to think about (not QC, which is dumb; the general reversible computing stuff); I recommend the seminal papers listed in the below wiki article if you have an afternoon to burn. Bennett, Toffoli and Vitanyi in particular are real fun to read. [1] https://en.wikipedia.org/wiki/Reversible_computing https://en.wikipedia.org/wiki/Reversible_computing
karmakaze 6y ago
A lighter, non-quantitative starting point https://en.wikipedia.org/wiki/Reversible_computing https://en.wikipedia.org/wiki/Reversible_computing I thought of this when I learned about Rust 'moving' values could--in reversible computing hardware--have a zero thermodynamic lower bound.
dang 6y ago
Sorry for offtopicness: submitter, could you please email hn@ycombinator.com? I would like to send you a repost invite for another submission.

36 points by tardygrade 6y ago

An interesting paper in somewhat the opposite direction, but still building on Landauer's work is "Ultimate physical limits to computation": https://arxiv.org/abs/quant-ph/9908043 https://arxiv.org/abs/quant-ph/9908043 The end of the abstract: > [...] quantitative bounds are put to the computational power of an `ultimate laptop' with a mass of one kilogram confined to a volume of one liter.

scottlocklin 6y ago
That along with "on the computational capacity of the universe" was the paper that stopped me from ever taking Seth Loyd seriously again. Later finding out he was a creepy Epstein croney... well sometimes the universe makes a lot of sense.
- deleted 6y ago
  [deleted]

9214 6y ago

There's a plethora of air-gap malware studies from Ben-Gurion university [1], abused channels range from thermal, acoustic, optical, to classic TEMPEST. The interesting thing is that there's no escape from such covert attacks, since machines are bound by the laws of physics that demand energy dissipation and generation of noise, heat, etc. If it leaks, then it computes. [1]: https://cyber.bgu.ac.il/air-gap/ https://cyber.bgu.ac.il/air-gap/

blacksmith_tb 6y ago
Shouldn't that be the other way around? Plenty of things leak heat (like cooking dinner) that don't do any computation?
- 9214 6y ago
  Maybe. Or maybe they do but we don't know it ;)
- goldenkey 6y ago
  The quantum of action, which a layperson can consider as the smallest possible state change, is actually Planck's constant. https://en.wikipedia.org/wiki/Planck_constant https://en.wikipedia.org/wiki/Planck_constant All energy refers to is an amount of state change that is occurring every second that some quantity of energy exists. That's why action has units Energy x Time. You can divide by time and you get Energy = Actions per Second, kinda like APM in Starcraft.. https://en.wikipedia.org/wiki/Action_(physics) https://en.wikipedia.org/wiki/Action_(physics) Principle of least action is really the principle of "least change." In summary, computation is happening -- but most of the computation that energy does while held as mass/matter is cyclic processes (aging.) Not anything interesting, at least to me or you. Unless you like https://en.wikipedia.org/wiki/Radiometric_dating https://en.wikipedia.org/wiki/Radiometric_dating

ouid 6y ago

So, here's a question: If heat dissipation is so fundamental to computation, why aren't we using the amount of required heat dissipation as the fundamental measure of complexity in quantum computing? In particular, say I construct a box with a uniform mixture of all quantum states on n qubits, and all the unitaries that can operate on those n qubits. how much heat do I have to dissipate in order to refine the state in my box to a "particular" quantum state and a "particular" unitary? There are some interesting, and recent, results about how quickly I can dissipate heat. In particular, T(t) is bounded from below by k*T(0)/t^7, where k is some constant. Since the number of quantum computers you could want to build in that box is growing very fast with the size of your input, I suspect that your inability to throw away states in the process of constructing your computer very rapidly becomes the dominant effect in how long it takes to go from "which number do I want to factor" to actually getting the factors of that number.

scottlocklin 6y ago
One of the things they don't tell you about quantum computing is it's supposed to be reversible. Which, ceteris paribus, probably means they're not physical. Nobody likes to talk about that as it's terrible for funding. You can build totally reversible computers using ordinary classical physics which ... in principle can be arranged to dissipate no heat (in practice they'll always dissipate heat). The problem is you're basically effectively dissipating the "heat" into a memory system which rapidly becomes practically infinite [1]. Imagine keeping around all the bits that got AND-gated away from .... I dunno, fitting GPT-3. Or even just inverting some big matrix. That's what you got to do for reversible computing: at the individual bit level mind you -many of the fundamental floating point operations are not themselves reversible, so they throw off more "heat" aka fill up memory cells with bits which allow you to reverse them. Landauer, who is an underappreciated genius, wasn't aware of the reversible computing idea, or had too much sense to bother with it. There are others who attempt to defeat his very common sense idea with hand wavey adiabatic relaxation ideas, but I think they're all baloney. All of this is a barrel of monkeys to think about (not QC, which is dumb; the general reversible computing stuff); I recommend the seminal papers listed in the below wiki article if you have an afternoon to burn. Bennett, Toffoli and Vitanyi in particular are real fun to read. [1] https://en.wikipedia.org/wiki/Reversible_computing https://en.wikipedia.org/wiki/Reversible_computing

karmakaze 6y ago

A lighter, non-quantitative starting point https://en.wikipedia.org/wiki/Reversible_computing https://en.wikipedia.org/wiki/Reversible_computing I thought of this when I learned about Rust 'moving' values could--in reversible computing hardware--have a zero thermodynamic lower bound.

dang 6y ago

Sorry for offtopicness: submitter, could you please email hn@ycombinator.com? I would like to send you a repost invite for another submission.