Path: utzoo!news-server.csri.toronto.edu!rutgers!aramis.rutgers.edu!athos.rutgers.edu!nanotech From: cphoenix@csli.stanford.edu (Chris Phoenix) Newsgroups: sci.nanotech Subject: Re: Is this stuff for real? Keywords: reality nanotech questions Message-ID: Date: 14 Mar 91 00:09:23 GMT Sender: nanotech@athos.rutgers.edu Organization: Center for the Study of Language and Information, Stanford U. Lines: 94 Approved: nanotech@aramis.rutgers.edu Seems to me that some people have a basic confusion here, perhaps caused in part by the nanomachine/cell parallels that are drawn to show nanotech is possible. People see cells replicating, and cells mutating. People are told that nanomachines will replicate, and so they wonder if they also will mutate. The replication process will be totally different, and this is the key to preventing mutations. When I started to write this I thought I could prove that it was possible to prevent mutations, but now I realize I don't know enough to prove it. I've even managed to unconvince myself. But at any rate, I hope this article will remove some of the red-herring of cellular type mutation. In a cell, chemicals float around in water, bump into each other randomly, and cause changes. For example, producing other chemicals. For example, copying DNA. The process is essentially highly parallel, with no controls except feedback caused by chemicals changing some parameter in the cell. For example, a chemical may "turn on" a section of DNA which produces another chemical which catalyzes a reaction which ... and the desired end product has the ability to deactivate the first chemical, so it's a self-limiting process. Since the cell has so many feedback loops and so many things happening in parallel, if something is changed it has a good chance of leaving the cell alive. If a mutation to the DNA does not kill the cell outright, there are error correction processes; but the error-correction, like everything else, is dependent on chemicals bumping into the DNA at the right time. So it's possible for a change in the DNA to occur, the cell to remain viable, and the change to go uncorrected. This is mutation. (If this is wrong, please correct me; if it's oversimplified, please don't bother.) Picture the following nanomachine, designed to prevent mutation: *everything* will be under the control of one or more computers. If these computers don't like what they see, they can shut down the machine permanently. If they choose not to copy the machine, they won't. And the copying process will also be under their direct control. The nanomachine can't turn itself on--that has to be done from outside. When a nanomachine is copied, it sends the contents of all its computer programs back to the original. If the original verifies that the program is correct, it can turn the copy on. Otherwise, it won't. A machine under total computer control is probably the easiest kind to build, anyway. Now let's consider two kinds of mutation: A change in computer memory, and any other "hardware" change. There is one parallel between cells and nanomachines that I'm willing to leave in: computer memory corresponds to DNA. It contains all the instructions for running and replicating the machine. However, computer memory has one feature that the DNA doesn't: it has a computer. The computer can manipulate the memory far more easily and reliably than the cell can manipulate the DNA. It can store many copies of it, can do calculations on it, and can compare large chunks of it. As far as I know, cells can't do any of these. I know DNA is "redundant", but there are still only two copies of any given chromosome, and the copies are different, and there is no way to compare the chromosomes anyway. A computer can store enough information about its memory, and do enough checking of its memory, that virtually any error in the memory can be detected. I don't know enough theory to do the calculations, but I think it should be relatively easy to ensure that the probability of any undetected memory error in any nanomachine that will ever be created is less than the probability of . Can any information-theory people confirm this? If it's true, then mutation of the kind that cells do is impossible. Now, let's consider non-memory changes. This is where I started to wonder if nanomachines could mutate after all. A hardware change should *not* be duplicated when the machine copies itself. It will change the behavior of the original, but will not be transmitted to the copy. This is the theory, anyway. But this is where I started to wonder. It may be possible to change the copying hardware in a way that causes mistakes in the copy, but the change itself is undetectable to the original. In this case, the mistakes might be missed. I think this is unlikely, because the "copying" hardware will probably be a large part of the machine and will probably be used for many other things as well. But consider: a change in the precision of a manipulator arm might cause very few errors, and it's possible that the only error made in copying would be to reduce the precision of the copy's manipulator arm... Although computers can "see" all their internal state, they are dependent on sensors to "see" the outside world. How does a computer know if its dioxin-detector is detecting the right molecule? Well, it has to look at the molecule. With what? With a dioxin-detector... Now suppose the dioxin-detector uses the same arm that is used in duplicating the machine. One mistake in the hardware could be both self-perpetuating and dangerous, and the computer wouldn't have anything to do with it, and couldn't detect it. Now we're getting into nanomachine engineering. The problem, it seems, has come down to this: Is it possible to build a copying mechanism which will have detectable errors whenever it is broken enough to make even slightly imperfect copies? When I put it that way, I get worried...