Entropy, Disorder and Evolution
August 2000
The Challenge
A common formulation of the Second Law of Thermodynamics is that the entropy of an isolated system
never decreases. The challenge for anybody who wants to disprove evolution with this form of the Second Law is thus:
- Define an isolated system where the law will be violated. This means no energy or matter comes in or
out. No heat, light, molecules... nothing!
- Show that after some period of time evolution would cause the system to achieve a lower entropy (as measured
in J/K) than what it had at the beginning. (Yes the introductory level physics "trick" of dimensional analysis can be
used here: if you cannot express it in J/K then it isn't entropy.)
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Many years ago somebody came up with the idea that since the Second Law of Thermodynamics
states that the disorder of a system is always increasing, the theory or evolution is contrary
to basic physics. Physicists have been denying this ever since, but our objections have not
been pursuasive enough to end the argument. My own contribution
Entropy, God and Evolution
(location updated in 2008)
is perhaps typical. While the comments I've received (mainly from other physicists) in the last 4 years
have convinced me that I should make a few changes when I get the time... I think the main
reason the page doesn't have a very big impact is that it attempts to teach too much physics.
There's too much to swallow in one bite for anybody who is new to the issues.
In this feature my approach will be different. A few obvious examples should be readily understood
by anybody.
Does the Second Law say that Disorder Always Increases?
NO!
We observe lots of cases in this world where the natural course is increasing order. Here
are just a few examples:
- Crystals: the natural state of most solids is crystal - the atoms line up in orderly
fashions. For most materials it takes much deliberate effort to make it non-crystalline.
- Marbles: try this... take a bag of marbles and randomly drop them into a small bowl.
They come to rest in orderly layers and form hexagonal patterns. Very natural order arising out
of disorder.
- Rain: When it rains we have water molecules distributed over a huge area of sky become
more ordered by forming into tiny droplets, then become more ordered still by falling to the
ground and gathering into relatively small volumes.
BUT: If I throw a stack of papers into the air it starts ordered and ends up disordered
This is true, and it may even be cited in a statistical mechanics class in order to introduce entropy. The reason
is that this is an example of how you can use Laplace's principle of insufficient reason
to analyze likely outcomes in dynamic situations, and we are then going to proceed to do something similar
for entropy. In other words, this is an analogy to help us understand something microscopic and it
is not to be taken literally. When we say "entropy is disorder" after making this analogy, we have
defined a very specific type of disorder. To apply this definition of disorder to the world around us will
be like measuring the "power of an argument" in Watts. Most cases of what we perceive as "disorder" has little
if anything to do with entropy, and hence little to do with the Second Law of Thermodynamics.
Summary
Some counter examples show that the Second Law of Thermodynamics does not forbid systems becoming more ordered. Any attempt to disprove
the theory of evolution using thermodynamics will require proper formalisms.
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