X-Message-Number: 25438
Date: Wed, 29 Dec 2004 06:13:58 -0800
Subject: Brain Food: The Identity Machine
From: <>

I have previously argued that blueprint-following does not result 
in the original. In the case of the World Trade Center, I argued 
that following its blueprint would lead to a duplicate of the World 
Trade Center, but would not result in the one that was destroyed.

Clearly, if we can use the blueprint to build one World Trade 
Center, we can use it to build N identical World Trade Centers, 
simultaneously.  Which one of these would be the one that was 
destroyed? If they are *all* the one that was destroyed, i.e. if 
they are numerically identical to the one that was destroyed, then 
they share all properties, and would therefore appear to us to be 
one thing. However, presumably if we performed the experiment, we 
would see N World Trade Centers, and not one, establishing that 
they do not share all properties, and hence, that they cannot be 
the same thing. In particular, then, they could not all be the one 
that was destroyed.

You can say 'the one on the left' is the one that was destroyed, or 
'the one on the middle', or 'the one constructed in Hong Kong,' but 
such assertions are arbitrary and unscientific. Clearly, *none* of 
them are the one that was destroyed. Blueprint-following does not 
lead to the 'recreation' of the original. 'Recreation,' strictly 
speaking, is not possible, only duplication to some upper limit of 
precision, fixed (at the least) by quantum mechanics.

That said, suppose some day in the far future, we are able to 
construct a machine that takes a blueprint-created duplicate, and 
makes it numerically identical to the original (or perhaps just 
constructs something that is numerically identical to the 
original).

I call such a machine an 'Identity Machine'. What would the 
Identity Machine look like, if it could be built?

Well, let's take our Identity Machine and use it on a duplicate of 
something that already exists, such as an apple (since this is a 
simpler case than using it on a copy of something that doesn't 
exist). The duplicate now becomes the original, in the sense of 
being numerically identical to the original apple. What happens to 
the apple that used to be the original? There are two 
possibilities:

1) The old original ceases to exist;
2) The old original ceases to be the original, and becomes a 
duplicate.

The first possibility would allow faster-than-light communication, 
if we could perceive the old original going out of existence. 
Therefore, we cannot allow it, at least not with perception, and no 
perception of the event would imply an instantaneous revision of 
spacetime.

The second possibility is more appealing to intuition: the 
duplicate becomes the original, and the original becomes a 
duplicate. However, this has implications beyond what you might 
expect.

The original apple was grown in an apple orchard, while the 
duplicate was created by advanced duplication technology. Assuming 
the second possibility, once we use the Identity Machine on the 
duplicate, it becomes the apple that was grown in an apple orchard, 
while the old original becomes a duplication of that apple. 
However, if the 'Identified' apple was grown in an apple orchard, 
then all our records of its history, including our own personal 
memories, will reflect that fact. This means, for one, we will 
believe the 'Identified' apple to have been grown in an apple 
orchard, because in fact it was grown in an apple orchard; 
similarly, we will believe the old original to be a duplicate, 
because it is a duplicate.

What this means is that even for the second possibility, using the 
Identity Machine results in a revision of spacetime. This brings 
with it the logical contradictions of time travel. 

Therefore, I conclude that an Identity Machine is not possible. 

Best Regards,

Richard B. R.

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