7 December 2019 | New Scientist | 35Boltzmann’s entropy worked surprisingly
well to describe thermal systems such as steam
engines – and it is still hard at work, with
physicists and chemists using it on a daily
basis. But difficult questions were raised as
early as 1867, when James Clerk Maxwell
devised a thought experiment in which a
crafty demon lurked inside a box of molecules
divided in two. The molecules start off evenly
mixed, with no difference in temperature
between the compartments, and thus no
ability to do useful mechanical work. But the
demon uses its knowledge of the molecules’
movements to separate hot ones from slower
cold ones by opening a door between the two.
That posed a problem. The demon seems
to have rendered the system ready to do work:
open the door and the energetic molecules
will be able to push a piston. In other words,
the demon has reduced the system’s entropy,
violating the second law of thermodynamics,
which holds that the entropy of a closed
system will always increase over time.
This is considered the most far-reaching
and robust law of nature. “The second law
cannot be violated, never, in no situation,
under no circumstances,” says Sebastian
Deffner at the University of Maryland,
Baltimore County. “Every time we run into an
apparent violation, we find we have overlooked
a contribution to the entropy production.”
That was certainly the case with Maxwell’s
demon. What had been overlooked turned
out to be a vital component of how we now
understand physical systems: information.
The demon can only perform its trick if it
can store information about the molecules
and their movements. It can’t have an
infinitely large memory, so it will have
to discard some information – and in the
1980s, physicist Charles Bennett calculated
that this has a physical effect involving
an increase in entropy. So the very process
DO that allows the demon to reduce entropy >
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First law of thermodynamics
Broadly speaking, energy can’t be
created or destroyed. Any energy added
to a system goes into raising its internal
energy and allowing it to perform work.Second law of thermodynamics
The total amount of entropy in a
closed system can never decrease.
This is often expressed as the
universe tending towards disorder.UNBREAKABLE LAWS