Re: Reversible Computation and Experience

Date: Sat May 05 2001 - 19:59:38 MDT

Lee writes:
> In any case, I think that a Turing machine would be just
> as conscious of an experience as it un-calculated as it
> was when it calculated. What persuaded me is that the
> un-calculation is exactly what would transpire if the
> arrow of time were reversed; but the direction of the
> arrow of time is symmetrical (i.e., there's no place to
> stand and claim that it's going now one way, and now the
> other). So un-experiencing something would be just as
> fun as experiencing it... not "un-fun" at all!

It's a good question, and I think this is an excellent thought experiment
for shedding light on the matter. This is an example of what I was
getting at earlier when I suggested that it doesn't always make sense
to ask where and when consciousness is occuring.

A reversible computer is as likely to take a step backwards as forwards.
So even if it manages to complete a calculation from A to B, the process
will be a random walk, moving forward and backwards many times over each
portion of the path from A to B.

Now if we interpret each step of the computer as instantiating a little
bubble of conscious experience, then the consciousness will move forward
and backwards in time just like the computer. You'll think thought 1,
then thought 2, then you'll think thought 2 backwards, then forwards
again, then thought 3, then thought 3 backwards, 2 backwards, 1 backwards,
1 forwards, and so on.

However of course the conscious mind is not aware of this. The structure
of the computer is such that it would have no memory of having gone back
and forth like this. In the end when it thinks back over what happened,
it will remember thought 1, then 2, then 3, and so on, straight on to
the end.

So there is a real disconnect in this case between what we hypothesize
happening from a functional perspective, and what we have evidence for
happening based on the reports of the conscious mind. All we truly
have evidence for is that the computation did exist, did complete.
We don't know whether the mind really experienced going backwards and
forwards like that.

And we especially don't know what form the consciousness took during
the reverse steps. What does it feel like to have memories erased
rather than stored? There is no way to ask a person and find out
(that I can see). Certainly if you ask them after a step is reversed,
the answer will obviously be the same as if the step never occured.

I posted an article several years ago [1] about an actual machine which
raised a somewhat similar question about what happens to consciousness
if the machine can't remember certain kinds of transformations. But I
think Lee's example is even more pointed.



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