On Sun, 13 Jun 1999 19:16:20 +1000 "Timothy Bates"
<tbates@karri.bhs.mq.edu.au> writes:
>any chance of passing on the algorithm you sued to calculate this ;)
>
>> I calculated that a human-equivalent-intelligence (HEI) would fit
>into
>> several thousand cubic microns,
I have a canned response I sent to another correspondent- it got me a signed book from Halperin!
Mr. Halperin, I'm a professional computer engineer, and an amateur but trained philosopher and biologist, and I don't think that even advanced nanotechnology can pack a human-equivalent brain into the volume of a blood cell. (TFI p208 is wrong)
Let me show you the numbers: The human brain has 10 billion neurons. Each neuron has between 30 and 10,000 synapses, with one associated dendrite for each. If the axon is considered just another connection, its organizational importance is merely to make distant dendrites. Taking the geometric mean of the number of dendrites, say that each cell has 300 dendrites. Then there are at roughly 3x10^12 synapses in the proposed human-equivalent brain.
Now, I thought about ways to reduce this by editing the system, but they won't work. Like most real computing systems, the majority of the logic (>95%) by weight or volume is I/O. (The cerebrum, cerebellum, gyrii, and most of the encephalon) Neural networks are great for I/O: they're robust and compact compared to the digital systems they replace. You would not want to use anything else to construct the phenomenal systems of a robot.
Therefore, the computer needs at least one byte per synapse, 3x10^12 bytes of storage.
So, to get at least human speed, we need roughly 1/1,000,000 the number of processors, about 3x10^6. I assume that each one of these is servicing a million simulated synapses. I'm going to throw in the CPUs for free (I know pretty good CPUs that have as few as 7,000 gates; see the web site for computer cowboys).
The computers' program memories are therefore the major system expense. Can we get rid of them? Now let's say that the engineer goes for broke, and designs a system with no computers. It's totally analog, maybe with frequency-modulated hysteresis devices acting as neurons, and carbyne pushrods acting as dendrites. In this case, the system volume should grow substantially, because the dendrites have to physically exist, each with a few thousand carbon atoms, rather than just being simulated from 8 bits on <50 atoms of tape.
Possibly one could substitute a custom logic machine that _only_ processes neural nets? The problem with these is that they tend to be larger and more complex than the computers they replace. Random logic is bulkier and more power-hungry than the random-access memories that store software. Faster, maybe, but then we might stall waiting for the tape, right?
The computers therefore take about 9,800 cubic microns. The tape storing the synapses takes about 1,500 cubic microns. Now remember, this is a _low_ estimate. I actually think that the storage for a synapse would have to store an address of a neuron as well, thus having 4 bytes of address in addition to the byteof weight. This quintuples the tape system to 7,500 cubic microns. Also, the tape drive and computers might double in size. Drexler doubled them.
<snip stuff about Halperin's book>