X-Message-Number: 26054 Date: Mon, 18 Apr 2005 20:45:52 -0400 From: Daniel Crevier <> Subject: To Tom Donaldson on brain and weather simulation References: <> To Tom Donaldson, on brain & weather simulation You can't blame the inaccuracy of weather forecasts on the fact that they are performed on digital computers. The real reason is the well known 'butterfly effect': the extreme sensitivity of air mass dynamics to small disturbances. These disturbances can be small inaccuracies in initial conditions, or small perturbations like, yes, butterflies (although airliners and power plant exhausts are probably more to blame). Increasing the accuracy of the computations themselves would make no difference: it is our own knowledge of the details of the system that is at fault. This means that you could replace present weather computers, which substitute, as you say, real simultaneity with 'make-believe simultaneity', by very many analog computers, one per cell in the simulation. These analog computers would interact with perfect simultaneity, exactly like the neurons in a brain. Yet, if you use the same intial conditions and don't include every living butterfly or airliner, the results would be just the same as those obtained with a digital computer. Weather simulation is thus a legitimate example of digital computers simulating a very complex distributed system faster than real time, and just as accurately as would an analog and perfectly parallel simulation. Therefore it is not clear that modelling a brain with hard-wired neurons would be any more accurate or economical than with digital computers. Daniel Crevier Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=26054