From: hal@finney.org
Date: Sun Feb 06 2000 - 13:36:59 MST
Robert J. Bradbury, <bradbury@aeiveos.wa.com>, writes:
> What I want to know is are there people who have read Ralph's
> technical essay:
> The Molecular Repair of the Brain
> http://www.merkle.com/cryo/techFeas.html
> and who *still* believe cryonics is infeasible?
>
> By infeasible, I mean do you have concrete reasons to believe
> that something Ralph says is in gross error or missing in the
> details of how one would molecularly reassemble the damage caused by
> freezing? Another way of asking the same question is -- How
> can you believe that trillions of nanobots with collective
> ultra-human intelligence could *not* reassemble the 3D jig-saw
> puzzle left by your frozen brain?
I am signed up for cryonics, but I have never found this essay
completely convincing. It seems to gloss over the hard part, which
is the possibility of near-universal damage to the synaptic structures
which we believe encode memories. Cryonicist Mike Darwin has written
many times about the damage he sees to brain tissue which has been
cryopreserved using current protocols. He sees devastating damage,
ice crystals cutting everywhere, breaking and tearing tissues apart as
they grow, making things look like a bulldozer has come through.
Ralph writes of evidence that brain tissue can withstand conversion of
up to 60% of water to ice and maintain some level of functionality,
and says that current protocols have kept the ice level below 40%.
But these experiments are not able to rule out the possibility of damage
sufficient to eradicate memory. They are able to get some brain waves
and other evidence that neurons are functioning to some degree, but
there is obviously significantly damage.
Ralph also suggests that because synaptic structures are large, composed
of tens of thousands of atoms, that this will facilitate the task of
reconstructing them. However this size is meaningless if the damage is
occuring at this scale. If a large fraction of these 10000 atoms are
badly disrupted it may not be possible to figure out exactly how they
were arranged before freezing.
Most of the latter part of the essay discusses the feasibility of
measuring atoms and moving them around, and I am willing to stipulate
to that. It is the middle step here, where we use our computer models to
figure out where things were before the damage occured, that troubles me.
The best analogy I can suggest is that the brain may be like a city.
When it is frozen, it undergoes a terrible bombardment, and buildings are
damaged or destroyed, rubble lying in the streets everywhere. In the
aftermath, people can sweep up and get things going after a fashion,
piles of rubble here and there. It can function to a limited extent,
but it is not at all the city it was before. This is like the cat brain
spontaneously recovering brain waves.
To rebuild the brain is like a team of archeologists reconstructing
the city, figuring out where each piece of rubble goes, getting the
architecture back where it was. That's probably not going to be possible.
We can put something together that may look nice, but it is an educated
guess at best. Real cities have idiosyncracies and quirks, and when
those are lost in the rubble they will never be restored. What we end
up with is a ficticious city, one which has similarities to what was
there before, but the original city is lost forever.
I don't know that the brain necessarily works like this; maybe there
will be enough information for a full restoration. But it seems to me
that it is possible that this is the situation, and that this model is
consistent with the limited studies which have been done. If so then
Ralph's reasoning is incorrect, and from the available data we cannot
conclude that cryonics is especially likely to work.
Hal
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