  Molecular machines consist of either a single protein or a macromolecular
complex composed of protein and RNA molecules. Just like their macroscopic
counterparts, each of these nano-machines has an engine that "transduces" input
energy into an output form which is then utilized by its coupling to a
transmission system for appropriate operations. The theory of heat engines,
pioneered by Carnot, rests on the second law of equilibrium thermodynamics.
However, the engines of molecular machines, operate under isothermal conditions
far from thermodynamic equilibrium. Moreover, one of the possible mechanisms of
energy transduction, popularized by Feynman and called Brownian ratchet, does
not even have any macroscopic counterpart. But, {\it molecular machine is not
synonymous with Brownian ratchet}; a large number of molecular machines
actually execute a noisy power stroke, rather than operating as Brownian
ratchet. The man-machine analogy, a topic of intense philosophical debate in
which many leading philosophers like Aristotle and Descartes participated, was
extended to similar analogies at the cellular and subcellular levels after the
invention of optical microscope. The idea of molecular machine, pioneered by
Marcelo Malpighi, has been pursued vigorously in the last fifty years. It has
become a well established topic of current interdisciplinary research as
evident from the publication of a very influential paper by Alberts towards the
end of the twentieth century. Here we give a non-technical overview of the
strategies for (a) stochastic modeling of mechano-chemical kinetic processes,
and (b) model selection based on statistical inference drawn from analysis of
experimental data. It is written for non-experts and from a broad perspective,
showing overlapping concepts from several different branches of physics and
from other areas of science and technology.
