**From:** Tim Freeman (*tim@fungible.com*)

**Date:** Sat Jul 19 2008 - 13:29:16 MDT

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--- On Sat, 7/19/08, Tim Freeman <tim@fungible.com> wrote:

*> Entity A could prove to entity B that it has source code S by
*

*> consenting to be replaced by a new entity A' that was constructed by a
*

*> manufacturing process jointly monitored by A and B. During this
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*> process, both A and B observe that A' is constructed to run source
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*> code S. After A' is constructed, A shuts down and gives all of its
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*> resources to A'.
*

From: Matt Mahoney <matmahoney@yahoo.com>

*>But A cannot know if S is its own source code. (I assume that S
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*>includes current state information needed to make a copy of
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*>itself). If it could know, then A could simulate itself (with infinite
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*>recursion, which is impossible).
*

The above proof technique also leads to the conclusion that backing up

my computer is impossible. My computer cannot possibly know all of

the bits it uses, since if it could, it could simulate itself, with

infinite recursion, which is impossible.

The gap there is that knowing the bits does not imply having the

capacity to run them. Simulating itself requires knowing what to

simulate, and also having the computational capacity to simulate.

Suppose my computer is configured to have 1GB of virtual memory. It

can't simulate a full version of itself, but it can simulate itself

with 750MB of virtual memory. The simulation overhead of 250MB of

memory might not be missed in everyday use, but it does stop the

infinite recursion after a fairly small number of levels, or maybe

after just one level depending on how brittle the definition of "my

computer" is. None of that stops me from backing up my computer.

Talk of "current state information" doesn't make a difference. At one

time there were patches that let Linux hibernate to disk. If I

successfully installed those, I could copy a machine-state to disk and

back that up, as it was before the start of the backup.

-- Tim Freeman http://www.fungible.com tim@fungible.com

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