From: hal@finney.org
Date: Fri Aug 27 1999 - 17:50:06 MDT
I was re-reading Isaac Asimov's 1966 novelization of the movie Fantastic
Voyage, in preparation for Robert Freitas' upcoming opus, Nanomedicine,
which is supposed to be out next month. In the novel, a crew and
submarine are miniaturized and injected into the blood stream of an
ailing mathematician, so that they can operate and remove a blood clot
from the inside.
Asimov does his best to paper over the many scientific and technical
inaccuracies in the movie. He wrote an article about it some time
later which I read many years ago, but I can't recall in which of his
essay collections it was published. He described some of the same
frustrations that other technical advisers have had when trying to keep
science fiction movies on the scientific straight and narrow.
The story takes on new meaning for the modern reader if you imagine it,
not as the experiences of a band of human explorers, but as a model for
what a nanomedical device might face as it performs some task within
the body.
The submarine, Proteus, starts out "some 50 feet in length" (p. 58) and
will be shrunk to a size that Asimov quaintly describes as "3 micra"
(p. 48). This is a factor of approximately 5 million, which is roughly
the size ratio for the nanotech devices Drexler envisions.
The book has the first reference I remember to what is now a commonplace
nanotech vision. From page 106:
Owens said, "Arterial wall to the right."
The Proteus had made a long, sweeping curve and the wall seemed about
a hundred feet away, now. The somewaht corrugated amber stretch of
endothelial layer that made up the inner lining of the artery was
clearly visible in all its detail.
"Hah," said Duval, "what a way to check on atherosclerosis. You can
count the plaques."
"You could peel them off, too, couldn't you?" asked Grant.
"Of course. Consider the future. A ship can be sent through a
clogged arterial system, loosening and detaching the sclerotic regions,
breaking them up, boring and reaming out the tubes. Pretty expensive
treatment, however."
"Maybe it could be automated eventually," said Grant. "Perhaps little
housekeeping robots can be sent in to clean up the mess. Or perhaps
every human being in early manhood can be injected with a permanent
supply of such vessel-cleansers..."
This passage does illustrate one of the technical flaws in the book,
which is that the people see by light rays. In fact, from what I have
read, it is not practical to focus light with a microscopic eye, and
in fact organisms of that size do not use vision. Presumably nanotech
devices will follow the same rules, and rely on touch and "taste"
(chemical sensors) to navigate and sense their environment.
The other main flaw I found was the assumption that the shrunken people
would work on the same time scale as unshrunken humans. Generally, it
is necessary and appropriate for microscopic entities to work at scaled
up speeds proportional to their degree of shrinkage. Characteristic
times for vibrations and other mechanical phenomena shrink along with
the size of the objects.
As a simple example, how long does it take a dropped object to fall
one micron? About 400 microseconds. If a member of the Proteus crew
dropped a tool, it would hit the ground in less than a millisecond.
Asimov does not seem aware of this discrepency.
He does attempt to address the problem that at the shrunken scale of the
ship, ordinary microscopic phenomena happen very, very fast. Blood in
the artery travels at a bit less than 1 mph. This is 5 million miles
per hour at the scale of the ship! Asimov does not really discuss the
difficulties of navigating among cells jostling about at this speed,
but he does realize that there is a paradox here. Red cells and other
residents of the blood bump into the walls and each other without damage,
but we know that bumping things at five million mph will cause great
damage.
The actual resolution is to understand the the appropriate time scale
also shrinks, so that, holding the strength of the ship constant, it
should be thought of as travelling 1 mph on its accelerated time scale.
Drexler discusses this in the first chapter of Nanosystems. However,
Asimov is unaware of this, so his resolution is to say that the Proteus,
being made of shrunken atoms, is more delicate and less robust than the
cellular components around her. The red cells can bump walls at this
speed without damage, but for Proteus, it would be terribly destructive.
IMO this doesn't make much sense, but after all the idea of shrinking
is a fantasy and we need to grant Asimov some license. And it does
add drama to the voyage, as the crew is easily menaced by ordinary and
undamaging phenomena like passing through the heart, and being exposed
to the sound vibrations of the inner ear.
But from the point of view of a hypothetical nanomedical device, this
artistic device must be discarded. The nanomachine would not be any
less robust than the cells surrounding it, and indeed if made of rock
as proposed, it should be much stronger. Further, the appropriate time
scale for its actions will be on the order of tenths of microseconds.
Hal
This archive was generated by hypermail 2.1.5 : Fri Nov 01 2002 - 15:04:55 MST