Creationists sometimes claim that (i) the Big Bang was a “disordered” explosion, that (ii) galaxies and solar systems are ordered, and that (iii) the second law of thermodynamics forbids order from coming from disorder. Hence, they claim, galaxies and solar systems could not have formed naturally.
The short response
- The Big Bang was not a literal explosion but rather the rapid expansion of spacetime itself.
- The formation of galaxies and solar systems involves only a local decrease in entropy at the expense of an increase in entropy elsewhere. This is consistent with the second law of thermodynamics.
The longer response
I. Entropy vs. disorder
First, read the section on entropy vs. disorder in Did the Big Bang violate the second law of thermodynamics? That section makes it clear that the creationist argument must be recast as the argument about entropy rather than about order and disorder. The recast claim would be that the Big Bang was an explosion, that states resulting from explosions have high entropy, but that galaxies and solar systems have low entropy, and that therefore they could not have arisen naturally after the Big Bang.
There are at least two serious problems with this recast argument.
II. The Big Bang was not an explosion in the familiar sense of the word
The Big Bang was not an explosion in the familiar sense of the word, but instead the smooth and rapid expansion of spacetime containing an almost perfectly uniform distribution of matter. In fact, the distribution of matter in the early universe was so uniform that until the results of the Cosmic Background Explorer (COBE) came in, cosmologists were hard pressed to explain how this matter could eventually clump up into galaxies (Gribbin 1993). Likening the Big Bang to an explosion therefore is misleading on several counts, and there is no reason to expect its products to be anything like products of an explosion. (In case you are wondering, COBE revealed tiny irregularities in the early distribution of matter that became the seeds around which galaxies eventually formed).
III. The second law of thermodynamics permits local decrease in entropy
The second law of thermodynamics permits local decreases in entropy as long as they are offset by increases in entropy elsewhere. Galaxies and solar systems form by dissipating gravitational energy (as in models that employ gravitational attraction in galaxy formation), or by dissipating kinetic energy (as in models that employ shockwaves in galaxy formation), thereby increasing the entropy of the universe as a whole. Since their local reduction of entropy is purchased at the cost of an increase in entropy elsewhere, their formation does not violate the second law of thermodynamics.
Some creationists have asserted that the entropy of every system increases over time in the absence of a sophisticated conversion mechanism; however, neither is this true (as the example of crystallization is sufficient to show), nor does the second law of thermodynamics say anything like this, so creationists who make that claim are making up a false law rather than appealing to an established scientific one.
Gribbin J. 1993. In the Beginning: After COBE and Before the Big Bang. Boston: Little, Brown, and Company.