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> A conventional computer is, essentially, an expensive electric heater that happens to perform a small amount of computation as a side effect. Is this really

> A conventional computer is, essentially, an expensive electric heater that happens to perform a small amount of computation as a side effect. Is this really a fair analogy? Isn't it precisely the opposite? The computer performs computation and generates heat as a side effect?

kian 8y ago
With reversible computing, they're talking about being able to perform computation for nearly-zero energy. By comparison, a modern computer is essentially an expensive electric heater that performs a very tiny amount of computation with a vanishingly small fraction of said energy.
Sharlin 8y ago
Given how far we are from the Landauer limit, the comparison is apt. The amount of negentropy gained by computation even in the best case is utterly dwarfed by the increase of entropy in the form of waste heat.
- markbnj 8y ago
  Ah ok I see.... thanks for the explanation!
- peteradio 8y ago
  It's sand lightning what do you expect from a bunch of half haired monkeys?
- jazzyjackson 8y ago
  I was having this conversation recently, trying to think of what about computation produces waste heat - how many watts go to computation vs how many watts are waste due to inefficiency. But we couldn't put our finger on, where does this inefficiency come from? Is is the silicon heating up due to resistance ? Or is there some analogy for friction in switching the state of a flip flop , such that there is heat generated by the state change ?
  - deleted 8y ago
    [deleted]
  - Sharlin 8y ago
    In our current hardware it is almost entirely electrical resistance. But even in a theoretical 100% superconducting computer there’s a fundamental lower bound to heat dissipated by every bit-erasing operation; this is the Landauer limit. Entropy increases when we forget information: we can’t reverse the computation, giving time a direction (the so-called thermodynamical arrow of time). You can think of it as conservation of energy: if you have two electric signals and put them through a gate with only one output, some of the total energy must be dumped to balance the books. If a computation is completely reversible—if we can always get our input back given the output, Landauer limit does not apply, but of course other hardware inefficiencies may still be present.
    - posterboy 8y ago
      Actually, cmos input stages are high impedance, i.e. high resistance, and therefore have close to no current flowing between state changes. The output stage should be very low impedance if driving loads, or very high impedance if turned of. Indeed, switching between states, saturating the gates with enough electrons is the problem that generates heat, when there is voltage and resistance visible for the following node. At that scale, the small resistance of the connections plays a role, too, sure.
- SilasX 8y ago
  Right, it's like the joke the General Motors is a pension and health care bureaucracy with a car-making division. That yes, GM's core business is (rightly regarded as) car-making, but the financial flows related to that other stuff are so big as to dwarf the "car thing".
  - jlawson 8y ago
    I like these jokes. The government is a health and unemployment insurer that also has an army. Your body is a child factory that has a brain.
    - arkades 8y ago
      > child factory that has a brain From the perspective of biology, that’s not even a joke, just an accurate statement.
voldacar 8y ago
An incandescent lightbulb turns 90% of the energy consumed into heat and only 10% or so into light. It's a heating element that produces light as a side effect. Computers are a little like that, given their thermal inefficiency.

kian 8y ago

With reversible computing, they're talking about being able to perform computation for nearly-zero energy. By comparison, a modern computer is essentially an expensive electric heater that performs a very tiny amount of computation with a vanishingly small fraction of said energy.

Sharlin 8y ago

Given how far we are from the Landauer limit, the comparison is apt. The amount of negentropy gained by computation even in the best case is utterly dwarfed by the increase of entropy in the form of waste heat.

markbnj 8y ago
Ah ok I see.... thanks for the explanation!
peteradio 8y ago
It's sand lightning what do you expect from a bunch of half haired monkeys?
jazzyjackson 8y ago
I was having this conversation recently, trying to think of what about computation produces waste heat - how many watts go to computation vs how many watts are waste due to inefficiency. But we couldn't put our finger on, where does this inefficiency come from? Is is the silicon heating up due to resistance ? Or is there some analogy for friction in switching the state of a flip flop , such that there is heat generated by the state change ?
- deleted 8y ago
  [deleted]
- Sharlin 8y ago
  In our current hardware it is almost entirely electrical resistance. But even in a theoretical 100% superconducting computer there’s a fundamental lower bound to heat dissipated by every bit-erasing operation; this is the Landauer limit. Entropy increases when we forget information: we can’t reverse the computation, giving time a direction (the so-called thermodynamical arrow of time). You can think of it as conservation of energy: if you have two electric signals and put them through a gate with only one output, some of the total energy must be dumped to balance the books. If a computation is completely reversible—if we can always get our input back given the output, Landauer limit does not apply, but of course other hardware inefficiencies may still be present.
  - posterboy 8y ago
    Actually, cmos input stages are high impedance, i.e. high resistance, and therefore have close to no current flowing between state changes. The output stage should be very low impedance if driving loads, or very high impedance if turned of. Indeed, switching between states, saturating the gates with enough electrons is the problem that generates heat, when there is voltage and resistance visible for the following node. At that scale, the small resistance of the connections plays a role, too, sure.
SilasX 8y ago
Right, it's like the joke the General Motors is a pension and health care bureaucracy with a car-making division. That yes, GM's core business is (rightly regarded as) car-making, but the financial flows related to that other stuff are so big as to dwarf the "car thing".
- jlawson 8y ago
  I like these jokes. The government is a health and unemployment insurer that also has an army. Your body is a child factory that has a brain.
  - arkades 8y ago
    > child factory that has a brain From the perspective of biology, that’s not even a joke, just an accurate statement.

voldacar 8y ago

An incandescent lightbulb turns 90% of the energy consumed into heat and only 10% or so into light. It's a heating element that produces light as a side effect. Computers are a little like that, given their thermal inefficiency.