
Wave-Based Turing Machine: Time Reversal and Information Erasing [pdf] - seycombi
https://blog.espci.fr/efort/files/2016/07/Turing.pdf
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matthewmcg
I (semi-seriously) expected to see the fictional "L. P. Waterhouse" listed as
an author of this paper.

Edit: relevant passage from _Cryptonomicon_ :

The sand at the surf line has been washed flat. A small child’s footprints
wander across it, splaying like gardenia blossoms on thin shafts. The sand
looks like a geometric plane until a sheet of ocean grazes it. Then small
imperfections are betrayed by swirls in the water. Those swirls in turn carve
the sand. The ocean is a Turing machine, the sand is its tape; the water reads
the marks in the sand and sometimes erases them and some times carves new ones
with tiny currents that are themselves a response to the marks. Plodding
through the surf, Waterhouse strikes deep craters in the wet sand that are
read by the ocean. Eventually the ocean erases them, but in the process its
state has been changed, the pattern of its swirls has been altered. Waterhouse
imagines that the disturbance might somehow propagate across the Pacific and
into some super-secret Nipponese surveillance device made of bamboo tubes and
chrysanthemum leaves; Nip listeners would know that Waterhouse had walked that
way. In turn, the water swirling around Waterhouse’s feet carries information
about Nip propeller design and the deployment of their fleets—if only he had
the wit to read it. The chaos of the waves, gravid with encrypted data, mocks
him.

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anta40
I had a hard time figuring what is the paper about until I read this comment.
Kinda helpful. Thanks.

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KirinDave
This is remarkable. This paper has a high probability of being called "the
watershed mark" in 30 years...

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Kinnard
Why?

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vilhelm_s
There is an accompanying video here:
[https://www.youtube.com/watch?v=4lqiZqmS-
bY](https://www.youtube.com/watch?v=4lqiZqmS-bY)

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agumonkey
Read his blog, there's an article about shifing the propagation medium to
reverse a 2d Wave. Splendid.

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djyaz1200
Can someone please explain this to me in layman's terms?

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vilhelm_s
Here's a description taken from Alyssa M. Adams' blog
([http://www.curiosityspaces.com/computation/](http://www.curiosityspaces.com/computation/)):

> Droplets bouncing around on a surface is apparently a great way to think
> about quantum mechanics, or at least some system with a wave encoding
> information about a discrete object. Here's a nice video showing how a
> bouncing particle on a liquid surface has a chaotic trajectory. The
> probability distribution of that particle's position ends up being something
> like the most unstable mode of the boundaries for the particle. But this
> paper shows that the droplet's trajectory can be totally reversed, even
> though the trajectory is chaotic. This is because the droplet's trajectory
> is completely encoded in the resonating wave it's riding. In other words,
> the system totally remembers what the droplet is doing and that memory can
> be accessed and then totally reversed. A great physical (well, MORE physical
> than usual) example of what memory is.

