In the
previous four posts in this series, I have introduced rudiments of the Big Bang
theory, and some elementary ideas from quantum mechanics. Another crucial notion
for understanding the evolution of our universe, and of life on Earth, is that
of ORDER vs. DISORDER.
Can you
scramble an egg? Yes. Can you unscramble a scrambled egg? No way. The physics
behind this common-sense fact has far-reaching consequences. It is so important
that it is stated in the form of a law in science. It is called the second
law of thermodynamics.
The law states
that, in any system not interacting with the surroundings, things cannot become
more ordered than they were to start with, but they can become more
disordered. (Scrambling an egg amounts to creating a state of more disorder. A
scrambled egg getting unscrambled would amount to the emergence of order out of
disorder.)
If there is a second
law, there must also be a first law of thermodynamics. Indeed there is. It
states that, although energy can be transformed from one form to another, it
cannot be created or destroyed. It is thus the law of conservation of energy.
Historically,
the science of thermodynamics emerged in the 19th century when
efforts were intensified for using heat energy for doing mechanical work. The
steam engine was an embodiment of this effort. People tried to maximize the
amount of mechanical work (locomotion) they could get from a given amount of
fuel, or from a given amount of heat energy. They soon concluded that, no
matter how efficient the design of an engine, there is a limit to the
percentage of heat energy that can be converted to mechanical work. Why should
there be a limit?
Let us denote
heat energy by Q, mechanical work by W, and something called 'internal
energy' by U. Imagine a gas in a container at some temperature T1.
Suppose you add some heat Q to this system. Naturally, its temperature
would go to some higher value T2. This is because the atoms
or molecules of the gas are moving around randomly, with an average speed of
motion, and adding heat raises this average speed, and therefore the temperature.
The motion and
internal vibration of the molecules of the gas implies the existence of energy;
and internal energy U is a measure of that. The temperature increases
from T1 to T2 because the internal energy
has increased from, say, U1 to U2 when heat
Q was supplied. What the first law of thermodynamics says is that, when
heat Q is expended, some part of it goes into doing mechanical work (in
this case thermal expansion and the consequent lifting of the piston in the
figure below), and the rest goes into increasing the internal energy, or
temperature. Thus
Q = W
+ (U2 – U1).
But why can we
not have W = Q? That is, why can we not convert all heat to work?
That would amount to having U2 = U1 in the
energy-conservation equation above, which amounts to expecting that temperature
would not rise (from T1 to T2) when heat Q
is supplied to the system. This is impossible. What is needed for that to
happen is that all the chaotically moving molecules in the container in the above
diagram should move in a concerted or special way to move the piston by such a
distance that W = Q. This is clearly impossible because there is no
reason for the randomly moving molecules to move in that special way, even on
an average. Therefore W < Q always.
When heat Q
flows into the system, its temperature T1 must rise to a
higher value T2. Similarly, if there are two systems (or
two parts of the same system), one at temperature T1 and the
other at a higher temperature T2, then heat must flow from
the hotter part to the cooler part. This will happen spontaneously, and will go
on till T1 = T2; i.e. till equilibrium
has been reached.
What has happened
here is that there was a temperature GRADIENT (T1 ≠ T2),
and Nature destroyed the gradient. In fact, a valid way of stating the second
law of thermodynamics is to say that NATURE ABHORS GRADIENTS. We see this
happening everywhere. Those who work with vacuum technology know how difficult
it is to maintain vacuum in any system. Vacuum in a container means a pressure
gradient w.r.t. its surroundings. The vacuum deteriorates with time, in keeping
with the law that gradients must decrease spontaneously.
Similarly,
concentration gradients tend to be annulled with time. The sugar poured into your cup of
tea gets dissolved with time, even when you are not stirring it. The figure
below illustrates this for the case of two gases (coloured red and blue for
fun). If you remove the partition, the gases mix, rather like the scrambling of
an egg.
But the gases will never unmix on their own, just as the egg will not unscramble on its own. Any isolated system always progresses towards a state of maximum disorder. That is what the second law says.
But we also
see so much order around us:
- We are able to grow highly ordered objects called crystals out of a highly disordered precursor, namely a solution or a melt.
- At the moment of the Big Bang, there was no order or structure at all. Then how so much order has emerged in the universe?
- Most important of all, how has life, which signifies a very high degree of order, emerged out of nonlife?
- Why is anything alive at all? Why not a state of total equilibrium, namely death and complete decay? What stops this from happening?
Watch this
space for more.
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