
Irreversibility and Heat Generation in the Computing Process (1961) [pdf] - tardygrade
https://www.pitt.edu/~jdnorton/lectures/Rotman_Summer_School_2013/thermo_computing_docs/Landauer_1961.pdf
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9214
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
Shouldn't that be the other way around? Plenty of things leak heat (like
cooking dinner) that don't do any computation?

~~~
goldenkey
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)

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messe
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
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.

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ouid
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
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)

~~~
ouid
These are interesting reads, and it seems that reversible computers suffer the
same problem, physically. If I want to actually make a reversible computer, I
still have to make the blank tape that it uses.

~~~
scottlocklin
Precisely: and that issue (among many others) is the one ding dongs in
"quantum information" sweep under the carpets and hope you don't notice. It's
HUGELY OBVIOUS if you try to do it for an HP42 calculator tier computer.
Somehow the mystification of adding entanglement to the mess pushes it off
into "Hilbert space" and nobody notices.

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karmakaze
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.

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

