On 8/13/2012 7:45 AM, Bruno Marchal wrote:

The infinite tape is only a rather misleading pedagogical folklore. Example of universal "number" are brain, computer, programming language interpreters, etc. Universal pattern in the game of life are finite pattern. The infinite tape here is the infinite plan. The infinite tape of the human has been provided by the wall of the cavern, the pebble, knots, the papers, the books and diaries, the magnetic tapes, the physical reality itself, etc. The infinite tape plays the role of a potential infinite neighborhood in which the memory of the machine can extend itself. It is not part of any machine, as the notion of machine requires finiteness. And in that sense bacteria have infinite tapes: the soil, or liquid, or gel in which they multiply.

That's why I carfully wrote "bacterium" instead of "bacteria". Bacteria can reproduce and evolve and so can be universal computers. But when Evgenii asked to compare rocks and bacteria, he meant a rock as compared to a bacterium.

This seems to come back to my previous question about a robot. Yes, I can see that a bacterium *could be* a universal Turing machine, that is with the right code in it's DNA/RNA. But I don't see that it follows that if I pick a bacterium a random it will be a UTM or even that there is any bacterium on Earth that is a UTM.


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