This blog post is part of my series delving into the belief in progress. This particular post will be about the second law of thermodynamics and the claim that this law amounts to a final limit on progress.
The second law (1) states that "All processes result in a reduction in the free energy of the universe.". The ability to maintain islands of order against the trend towards disorder depends on using free energy. Once free energy is all gone we will be unable to maintain civilisations and even life. This decent into chaos is known as the heat death of the universe.
Many people have stated that the second law makes a heat death inevitable. In fact there are two other physical principles that must also hold to imply the collapse of civilisation:
(2) The universe is finite in space or matter.
If this were not the case there could be a constant influx of new hydrogen into our region of the universe allowing the formation of new stars. In such a universe there would be no heat death.
(3) There is a lower bound to how much energy is necessary to perform a given calculation.
Most of the useful things that civilisations do can be described in terms of computations in one medium or another. If the amount of energy required to perform a computation decreases over time (which is possible without ) then civilisations may be able to stay ahead of the game redesigning themselves constantly to use less energy. In such a universe a heat death might occur but it would not necessarily destroy all civilisations.
So in deciding whether civilisations can survive indefinitely or not we need to know how much credibility to give these two principles and how much we can trust the second law of thermodynamics.
Before getting into this we need an estimate of how long away the heat death of the universe is according to our current understanding of physics. According to wikipedia stars will stop shining in roughly 100,000 billion years. However, blacks holes are also sources of energy (via hawking radiation) and civilisations could probably eek out an existence on bodies orbiting super massive black holes. Hawking radiation could be used as a source of useful energy. There is no obvious reason in the known laws of physics that civilisation couldn't survive until approximately 10^40 years. Hence, in order for the principles (1), (2) and (3) to form an absolute limit for progress we must be able to trust them to hold without significant error over a period of ten thousand billion billion billion billion years.
In reverse order let us look at these principles and judge their likelihood in this context:
(3) If there are only finitely many different types of particle then their behaviour may put an absolute limit to the amount of computation that can be performed in any given volume or for any given amount of energy. It is conceivable that there are only a finite number of elementary particles possible, however, the history of physics includes at least two occasions where it was believed that we had discovered the fundamental constituents of matter only to be proved wrong. It is not obvious that there are only a finite number of types of particles. It may be the case that there is an infinite hierarchy of particles each harder than the last to produce.
(2) Current cosmology maintains that the universe is finite in extent. However, there are major problems with the standard cosmological model. We have mysterious entities such as dark matter, dark energy as well significant unexplained phenomena such as inflation and the uniformity of the universe. Several new cosmological theories do not imply a finite universe and we are not yet in a position to form a reasonable estimate of how likely it is that the universe is finite.
(1) The second law of thermodynamics is one of the best justified principles in science. Formulated in 1824 there has been no experimental data contradicting it since despite the best efforts of many. Hoards of cranks over this period have claimed to have produced perpetual motion machines violating the second law but none of these claims has ever been independently verified. It certainly would be a good bet that no one will find a violation of the second law in the 21st century. It might even be reasonable to assert that we won't find a violation in the next millenia. However, in order to justify the second law as an absolute bar to progress it must not only hold for a thousand, a million or a billion years but for 10^40 years.
So in conclusion although the second law of thermodynamics would seem to pose an absolute barrier to progress it does not do so on its own. Two other principles are needed neither of which we can be particularly sure of. Furthermore the time spans under consideration are so vast that even the second law of thermodynamics may well breakdown.
As a cautionary example here I present the principle of parity conservation:
(4) If you take the entire universe and reflect it in a perfect mirror then the reflection will behave in exactly the same way as the original universe.
This was believed to be an absolute conservation law of the same type as the first law of thermodynamics. However, in 1956 the decay of Cobalt-60 was shown to violate partiy conservation. Since then several other conservation laws have been found to fail.