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Comment by cwillu | Hacker News Reader

The irreversible component of a computation is what actually generates heat, more or less.

IMO it would be more accurate to say: the delete operation is the one that we know thermodynamics and information theory must charge us for, on a physics level.

In current computers, we’re nowhere near those limits anyway. Reversible computing is interesting research for the future.

Quantum computers, albeit useless, are real life example of reversible computers. Those can be achieved.

Quantum dot cellular automata looked like a good candidate for reversible computers in some post-cmos future, last time I looked at that stuff (years ago—so, it is probably time for a check-in). Notably, they do classical computing, they just exploit quantum effects for the logic gates.

I'm wondering... What happens when I do a reversible computation where no information is lost, does cutting the power on the unit create heat?

I guess it would depend on the physical design of the compute elements (reversible computing is generally associated with post-CMOS tech). But, cutting power doesn’t necessarily reset a machine in general… think of a mechanical computer, cutting power removes the ability to change state.

Idk how accurate it is by my mental model of this would be that electricity is like a liquid and when you flip the switch, it drains out of the components creating heat through friction.

“Cutting power” is a thing because current computers require a periodic (and rather frequent) refresh to maintain the state of the system (primarily, but not exclusively, RAM). And indeed, one useful tactic to maintain that state when you need to cut the power for some reason is to supercool the ram so that it doesn't dissipate its charge as fast, basically making the system approach that ideal world.

A fully reversible computers would consume no power whatsoever, so wouldn't require power to function. Instead, you would need power to (re)initialize its state, or to copy the results of its computations to e.g. your monitor. An unplugged reversible computer would be free to compute (and uncompute) its bits perpetually, unconcerned about the rest of the world. The functional programmer's dream, in a sense.

Thanks for that and the preceding comments. I was thinking about the possible paradox of getting a reset for free. But, the cost invariably comes at some point, e.g. when you restart and need to reset your state.

Only at the theoretic limits of efficiency. In real life, not true.

i don't think that's categorically true. if your footprint from expanding out a circuit to make it reversible gets sufficiently large other things like resistance can dominate.

Other things like resistance already dominate (and always have). Reversible computing is the result of exploring the thermodynamic/information theory limits of what computation must cost.

In current chips we just charge and dump a bunch of parasitic capacitances every clock cycle.

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