From: karr@cs.cornell.edu (David Karr)
Subject: Re: Virtual reality puzzle
Organization: Cornell Univ. CS Dept, Ithaca NY 14853
Date: Wed, 19 Aug 1992 14:56:36 GMT

In article <9882@uqcspe.cs.uq.oz.au> warwick@cs.uq.oz.au writes:
>karr@cs.cornell.edu (David Karr) writes:
>
>>In article <9881@uqcspe.cs.uq.oz.au> warwick@cs.uq.oz.au writes:
>>>Trivial:  Open up the VR machine in your Real/VR homes, and examine EVERY
>>>  component.  The VR machine cannot possibly store the details of its own
>>>  construction (where would it store them?).
>
>>Nonsense....
>
>Nonsense it's nonsense...

All right, maybe "nonsense" was a little strong, but the argument is still
fundamentally and fatally flawed.

>>machine, including an Acme 500 gigabyte 3 ms hard disk which stores
>>needed data including the details of the components of the VR machine
>
>To what level of detail?  "1 x 150ohm resistor connecting points 142 and 421"?

YES!  Why not?

>Is there a file on the disk that describes the contents of the disk?

Yes, but ...

>If so, I can just keep that file, and use the rest of the disk for
>my GIFs...

No, you can't.  The "file" that describes the contents of the disk is
the very contents of the disk.  Ever seen a computer do a core dump?
Do you believe it is technically possible for a computer with a single
hard disk to dump the entire contents of this disk to a printer?
Including the sectors that contain the program that dumps the disk?
:-)

>>The human body stores the details of its own construction,
>>in fact it has many millions of copies of the plan.
>
>Again - to what level of detail?  Certainly not the level of detail
>present in the REAL THING (tm).  My DNA does NOT say how tall I am.

The level of detail is greater than you seem to think.  Ever had
trouble telling one twin from another?  In any case, my point is that
the human body is a complex machine, yet it carries millions of copies
of its genetic code, any one of which carries sufficient information
"simulate" another human being in full detail.

>As I said originally, this all relies on you ability to tell the difference
>between a mock-up VR machine and the real thing... maybe that's the hard part.

Yes, well, this is a good point.  I would expect that the simulated
dust motes that settled inside the simulated VR machine are not in the
same locations as in the real one.  But without being able to go back
and look at the real machine to compare, how are you ever going to be
able to tell?  Even if you'd had the foresight to take detailed notes
and photographs before starting the simulation, if you're still in the
simulation these will be fake anyway and can be altered to suit.  Even
if you think you see a discrepancy between what you see and what you
remember, how much can you trust your memory to that level of
detail--in real life?

------

In fact, there's a theorem of computability that says there is a
computer that can describe itself.  Specifically, there is a Turing
machine M that, given any input, prints out a complete description of
M (itself) on its tape.  You can easily extend the proof of this
result to have the machine print out a text that describes exactly
what a Turing machine is in general, and how you should interpret the
description of M.

This result is considered non-intuitive, so you may be excused for
having skepticism, but it's true nevertheless.

------

This VR thread reminds me of an SF story I read once--unfortunately I
forget the author and title--about a man who is leaving his house on a
rainy day, suddenly remembers something he forgot back in the house,
and when he goes in he looks out a window and sees sunny skies behind
the house.  Meanwhile, back at the control center for the simulation
the man is in, there is hell to pay for whoever was so careless as to
allow this discrepancy to show up.

Anyone remember the author and title?

-- David Karr (karr@cs.cornell.edu)

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