LW - Digital brains beat biological ones because diffusion is too slow by GeneSmith
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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Digital brains beat biological ones because diffusion is too slow, published by GeneSmith on August 26, 2023 on LessWrong.I've spent quite a bit of time thinking about the possibility of genetically enhancing humans to be smarter, healthier, more likely to care about others, and just generally better in ways that most people would recognize as such.As part of this research, I've often wondered whether biological systems could be competitive with digital systems in the long run.My framework for thinking about this involved making a list of differences between digital systems and biological ones and trying to weigh the benefits of each. But the more I've thought about this question, the more I've realized most of the advantages of digital systems over biological ones stem from one key weakness of the latter: they are bottlenecked by the speed of diffusion.I'll give a couple of examples to illustrate the point:To get oxygen into the bloodstream, the body passes air over a huge surface area in the lungs. Oxygen passively diffuses into the bloodstream through this surface where it binds to hemoglobin. The rate at which the body can absorb new oxygen and expel carbon dioxide waste is limited by the surface area of the lungs and the concentration gradient of both molecules.Communication between neurons relies on the diffusion of neurotransmitters across the synaptic cleft. This process takes approximately 0.5-1ms. This imposes a fundamental limit on the speed at which the brain can operate.A signal propogates down the axon of a neuron at about 100 meters per second. You might wonder why this is so much slower than a wire; after all, both are transmitting a signal using electric potential, right?It turns out the manner in which the electrical potential is transmitted is much different in a neuron. Signals are propagated down an axon via passive diffusion of Na+ ions into the axon via an Na+ channel. The signal speed is fundamentally limited by the speed at which sodium ions can diffuse into the cell. As a result, electrical signals travel through a wire about 2.7 million times faster than they travel through an axon.Delivery of energy (mainly ATP) to different parts of the cell occurs via diffusion. The fastest rate of diffusion I found of any molecule within a cell was that of positively charged hydrogen ions, which diffuse at a blistering speed of 0.007 meters/second. ATP diffuses much slower. So energy can be transferred through a wire at more than 38 billion times the speed that ATP can diffuse through a cell.Why hasn't evolution stumbled across a better method of doing things than passive diffusion?Here I am going to speculate. I think that evolution is basically stuck at a local maxima. Once diffusion provided a solution for "get information or energy from point A to point B", evolving a fundamentally different system requires a large number of changes, each of which individually makes the organism less well adapted to its environment.We can see examples of the difficulty of evolving fundamentally new abilities in Professor Richard Lenski's long-running evolution experiment using E. coli. which has been running since 1988. Lenski began growing E. coli in flasks full of a nutrient solution containing glucose, potassium phosphate, citrate, and a few other things.The only carbon source for these bacteria is glucose, which is limited. Once per day, a small portion of the bacteria in each flask is transferred to another flask, at which point they grow and multiply again.Each flask will contain a number of different strains of E. coli, all of which originate from a common ancestor.To measure the rate of evolution, Lenski and his colleagues measure the proportion of each strain. The ratio of one strain compared to the others gives a clear idea of its "fitness ad...
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