
Oil Drop Navigates Complex Maze - icey
http://sciencenow.sciencemag.org/cgi/content/full/2010/113/4
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charltones
This bears a striking resemblance to the HN posting on antiobjects:
<http://news.ycombinator.com/item?id=981738>

(representing a pac-man game by having ghosts detect a "smell" of pac-man that
diffuses through the maze)

~~~
icefox
This is of course the Ant Colony Optimization paper from 91
<http://en.wikipedia.org/wiki/Ant_colony_optimization>

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phaedrus
Consider this: replace "acid gradient" in the experiment description with "air
pressure": Imagine constructing a larger version of this maze from cardboard.
A vacuum cleaner is placed at the exit of the maze. Sucking the air out of the
maze creates a pressure gradient such that there is higher air pressure nearer
the entrance and lower air pressure nearer the exit. This pressure gradient
causes the ping pong ball to roll through the maze to exit.

This is a brilliant application of a novel idea BUT the physical process at
work only seems miraculous because we do not deal with acid gradients every
day. Consider: would the same claims that it "performs a computation" be as
credible in the vacuum cleaner and air pressure version?

If any computation is being done, it is a search space being covered in
parallel by billions of independent agents (the molecules of the medium). The
movement of the water droplet (or ping pong ball!) merely serves to display
the answer.

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kaddar
His claims about np-completeness implies that his research needs to be
seriously questioned.

~~~
hendler
The quote was: "Computers and mathematics could also benefit. Maze navigation
can fall into a class of problems known as NP-complete, "which computers have
a surprisingly hard time solving, as the effort to solve them goes up
exponentially with the scale of the problem," says chemist Irv Epstein of
Brandeis University in Waltham, Massachusetts. "The kind of approach shown
here with these mazes might be a very efficient approach to address this
problem."

There are other cases of analog chemical processes beating out digital
computations. But the reference to medicine also seems like a stretch -
navigating a simple maze with a couple of acidic and basic chemicals seems
very far away from a complex chemical hunting cancer in the human body.

~~~
Groxx
Clearly, given varied acidic / basic areas in human bodies, this is not a
"magic bullet" that'll _only_ target cancer cells. They're also not claiming
that this is the case (though the wording is quite vague).

At best, it'll increase the concentration of a drug in the cancer (among other
places) above that of a "plain" drug, which is an improvement, thus should be
investigated.

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bioweek
Wait, so if this oil drop is solving an NP complete problem in N time,
couldn't we map other NP complete problems onto a maze, and have the oil drop
solve them for us?

~~~
jws
Shortest path from one point to another in a graph is polynomial. Shortest
path to all nodes is NP complete.

~~~
ondra
No, all-pairs shortest path problem is in P as well.

~~~
jws
I spoke too loosely. Shortest path traversing all nodes is NP (traveling
salesman). Obviously shortest path between each pair of nodes is going to be
polynomial since the number of pairs is polynomial.

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icefox
Hmm, could this be exploited in some way to find the longest path?

~~~
phaedrus
Actually your question perfectly illustrates the limitations of thinking of
this as a computation device: I doubt it could be used to find the longest
path, for the reason that whatever way the oil droplet responds at one
intersection, it will employ the same type of response at the next.

The oil droplet is able to (usually) find the shortest path by always moving
in the direction of higher acid content. Would you expect it to find the
longest path by always going away from the higher acid content? That's wrong;
that's not the longest path to the exit, it's the SHORTEST path back to the
entrance!

OK, so how about the least acidic path that it hasn't already taken; i.e. not
allow it to double back? Also wrong: that will lead it into the first blind
alley that faces away from the exit, where it will be trapped.

So given that 1. We cannot use the highest acidic value as a criterion and 2.
We cannot use the lowest acidic value as a criterion and 3. We cannot use the
lowest acidic value not already visited, that leaves us with expecting the
droplet to neither always take the most acidic exit from an intersection nor
the least acidic. However, surely SOMETIMES the least acidic exit might be the
longer path, and SOMETIMES the most acidic exit is. Then we are forced to
conclude that the droplet must act differently at different times with the
same stimulus!

That implies it has to maintain some kind of state, or memory. As the droplet
is undifferentiated oil, it seems unlikely that it has a large enough state
space to solve the longest path problem.

