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V_V comments on Steelmaning AI risk critiques - Less Wrong Discussion

26 Post author: Stuart_Armstrong 23 July 2015 10:01AM

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Comment author: V_V 26 July 2015 09:08:12AM *  1 point [-]

As the efficiency of a logically irreversible computer approaches the Landauer limit, its speed must approach zero, for the same reason why as the efficiency of a heat engine approaches the Carnot limit its speed must approach zero.

I don't have an equation at hand, but I wouldn't be surprised if it turned out that biological neurons operate close to the physical limit for their speed.

EDIT:

I found this Physics Stack Exchange answer about the thermodynamic efficiency of human muscles.

Comment author: Wei_Dai 26 July 2015 11:15:14AM *  4 points [-]

Hmm... after more searching, I found this page, which says:

The faster the processor runs, the larger the energy required to maintain the bit in the predefined 1 or 0 state. You can spend a lot of time arguing about a sensible value but something like the following is not too unreasonable: The Landauer switching limit at finite (GHz) clock speed:

Energy to switch 1 bit > 100 k_B T ln(2)

So biological neurons still don't seem to be near the physical limit since they fire at only around 100 hz and according to my previous link dissipates millions to billions times more than k_B T ln(2).

Comment author: jacob_cannell 27 July 2015 12:07:21AM 2 points [-]

A 100kT signal Is only reliable for a distance of a few nanometers. The energy cost is all in pushing signals through wires. So the synapse signal is a million times larger than 100kT to cross a distance of around 1 mm or so, which works out to 10^-13 J per synaptic event. Thus 10 watts for 10^14 synapses and a 1 hz rate. For a 100 hz rate, the average dist would need to be less.

Comment author: V_V 27 July 2015 03:54:56PM -1 points [-]

Energy to switch 1 bit > 100 k_B T ln(2)

Not my field of expertise, but I don't understand where this bound comes form. In this paper for short erasure cycles they find an exponential law, although they don't give the constants (I suppose they are system-dependent).

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