The Astronomer Arthur Eddington once said, "If you try to go
against the second law of thermodynamics
I can give you no hope. There's nothing but for your
theory to collapse in deepest
humiliation." And that is exactly what we're
try to do today. The second law of thermodynamics
comes in many forms, but the most
well-known is that the entropy or
disorder of the universe is always
increasing. But this could also be
rephrased as the energy in the
universe available to do useful work is always decreasing. Let me explain. When
things are in a very low entropy or ordered
state, they can do what scientist call
useful work. For example imagine a box
with hot air on one side and cold
air on the other. This is a very ordered or
low entropy state, and the temperature
difference can be used to create
electricity tor to power engines. As the gas reaches
thermal equilibrium the energy available
to do useful work becomes less and less
until eventually there's none left at all. This messy
high entropy state is pretty useless and will
never spontaneously reverse into hot and
cold air again. Unless outside work is put in. If
we use something like a heat pump to
separate the hot molecules from the cold, this
box can do a lot of useful work again.
But it would never have gone back to that state
unless outside work like the heat pump
had been put in.
What's more is that the work used by
the heat pump to create this temperature
difference is always going to be more than
the work the temperature difference itself
can create. So we always need to put in
more than what get out. In other words, you
can't get something for nothing. As the
entropy of the air decreases the
entropy of the system is still increasing
because of all the wasted heat used to power the heat pump.
But James Clerk Maxwell of the famous
Maxwell's equations didn't like this
conclusion one bit. He came up with a
surprisingly simple thought experiment in hope of
finding another way.
What if we introduced a tiny demon to our box
of gas? Stick with me. A demon that could
see which air molecules were hot and which were cold.
Maxwell's Demon has one job. To let only hot air
molecules through to the left side and reallyonly
cold air molecules through the right, so
that eventually, the box would have
hot air on one side and cold air on the
other. The entropy of the air has decreased
and is available to do useful work again.
We know that this doesn't really sit well
with the second law of thermodynamics, so let's try and find the source of outside work.
Surely the demon opening and closing the
door creates heat, so that must be the extra
work put in. Well imagine the scenario where
the demon is a bit confused and is
opening and closing the door at random.
There will still be the same amount of
work put in but with no extra work to put out.
So that's not it. Then where is the
increase in entropy? It seems like with
information alone Maxwell's Demon has
defied this almighty universal law. This
baffled scientists for over a century. Confused as
they were, there will also be extremely
excited!
This idea of getting something for nothing
is enough to excite anybody, but for so long scientists
everywhere were trying to create self-powered
or perpetual motion machines.
A perpetual motion machine could run without any
outside work put in. In other words,
a machine that could get something for nothing. There were so many
attempts, the over balanced wheel,
the capillary bowl, the Oxford electric bell,
but unfortunately just as many fails.
Perpetual motion machines don't work. So what
was it about Maxwell's Demon that was able
to break this unbreakable law? Is there
something special about life? Is this law only for
machines and not for tiny omniscient
demons? It wasn't until a hundred years
later when physicists Leo Szilard came along with
the answer. The flaw in our system was
coming from the most unexpected place.
The demons brain. To do his job, Maxwell's
demon needs to be constantly measuring
molecules, obtaining information and
making memories. He uses this information
to lower the entropy of the box. But inn doing
this, he is using energy to organize all
this information in his brain. Imagine the
demon's brain as a clean slate. As he
starts to make measurements and form
memories, the entropy in his brain is increasing
as he tries to sort through all this random
information. The demon's brain is also a finite size,
so when it gets full, he needs to delete
some information to make room for more.
This erasing of information also costs energy. Heat
is created in the process, entropy of the
system increases and the second law of thermodynamics prevails. So thanks,
Leo Szliard, turns out no, you can't get
something for nothing. Sorry if that was a bit
anticlimactic. Thanks so much for watching I
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