Silas comments on True Sources of Disagreement - Less Wrong

8 Post author: Eliezer_Yudkowsky 08 December 2008 03:51PM

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Comment author: Silas 08 December 2008 06:05:18PM 1 point [-]

I'm going to nitpick (mainly because of how much reading I've been doing about thermodynamics and information theory since your engines of cognition post):

Human neurons ... dissipate around a million times the heat per synaptic operation as the thermodynamic minimum for a one-bit operation at room temperature. ... it ought to be possible to run a brain at a million times the speed without ... invoking reversible computing or quantum computing.

I think you mean neurons dissipate a million times the thermodynamic minimum for an irreversible one-bit operation at room temperature, though perhaps it was clear you were talking about irreversible operations from the next sentence. A reversible operation can be made arbitrarily close to dissipating zero heat.

Even then, a million might be a low estimate. By Landauer's Principle a one-bit irreversible operation requires only k*T*ln2 = 2.9e-21 J at 25 degrees C. Does the brain use more than 2.9e-15 J per synaptic operation?

Also, how can a truly one-bit digital operation be irreversible? The only such operations that both input and output one bit are the identity and inversion gates, both of which are reversible.

I know, I know, tangential to your point...