<s>
A	O
billiard-ball	B-Application
computer	I-Application
,	O
a	O
type	O
of	O
conservative	B-Application
logic	I-Application
circuit	O
,	O
is	O
an	O
idealized	O
model	O
of	O
a	O
reversible	B-Application
mechanical	B-Device
computer	I-Device
based	O
on	O
Newtonian	O
dynamics	O
,	O
proposed	O
in	O
1982	O
by	O
Edward	O
Fredkin	O
and	O
Tommaso	O
Toffoli	O
.	O
</s>
<s>
Instead	O
of	O
using	O
electronic	O
signals	O
like	O
a	O
conventional	O
computer	B-Architecture
,	O
it	O
relies	O
on	O
the	O
motion	O
of	O
spherical	O
billiard	O
balls	O
in	O
a	O
friction-free	O
environment	O
made	O
of	O
buffers	O
against	O
which	O
the	O
balls	O
bounce	O
perfectly	O
.	O
</s>
<s>
It	O
was	O
devised	O
to	O
investigate	O
the	O
relation	O
between	O
computation	O
and	O
reversible	B-Application
processes	O
in	O
physics	O
.	O
</s>
<s>
In	O
particular	O
,	O
it	O
is	O
possible	O
to	O
set	O
up	O
the	O
paths	O
of	O
the	O
balls	O
and	O
the	O
buffers	O
around	O
them	O
to	O
form	O
a	O
reversible	B-Application
Toffoli	O
gate	O
,	O
from	O
which	O
any	O
other	O
Boolean	O
logic	O
gate	O
may	O
be	O
simulated	O
.	O
</s>
<s>
Therefore	O
,	O
suitably	O
configured	O
billiard-ball	B-Application
computers	I-Application
may	O
be	O
used	O
to	O
perform	O
any	O
computational	O
task	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
simulate	O
billiard-ball	B-Application
computers	I-Application
on	O
several	O
types	O
of	O
reversible	B-Application
cellular	I-Application
automaton	I-Application
,	O
including	O
block	O
cellular	O
automata	O
and	O
second-order	O
cellular	O
automata	O
.	O
</s>
<s>
Logic	O
gates	O
based	O
on	O
billiard-ball	B-Application
computer	I-Application
designs	O
have	O
also	O
been	O
made	O
to	O
operate	O
using	O
live	O
soldier	O
crabs	O
of	O
the	O
species	O
Mictyris	O
guinotae	O
in	O
place	O
of	O
the	O
billiard	O
balls	O
.	O
</s>
