Why is the entropy of the universe not decreasing?

The second law of thermodynamics

Contradiction to the formation of order and evolution of life?

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1 The Second Law and its Misinterpretations

The macroscopic1 Thermodynamics knows the following main principles:

  • 0) Existence of the variable "temperature"
  • 1) The energy of a closed system is retained
  • 2) The entropy of a closed system remains constant or increases.
  • 3) The entropy disappears at absolute zero
Sentences 0 and 3 are not always mentioned, number 1 is historical and almost trivial.

The second law is therefore the most interesting. A popular interpretation says that one day the whole universe will assume a state of equilibrium of maximum entropy in which life is no longer possible. But this is not to be mentioned here.

The casual misinterpretation is just as popular
P) Entropy is a measure of disorder and is always increasing.
Two main "conclusions" are drawn from this:
A) There are many cases where order increases or entropy decreases, or both; therefore the second law must be wrong.
B) The second law is correct and therefore the independent emergence of ordered structures, especially of life and social structures, is not possible. This argument is mostly used in religious contexts.

The mistake of the premise P is that the second law is based on closed systems relates. Assertions A and B are already invalid. However, on a fairly abstract level.

Therefore, some specific examples are given in the remainder of this article. They demonstrate how thermodynamics promotes the independent creation of order and the reduction of entropy. The common denominator is that entropy is in one partsystem decreases without violating the Second Law.

2 heat emission

+ ---------------- + | + ------------ + | | | T1 | | dS1 = - | dQ | / T1 | + ------------ + | | | | | | dQ | | \ | / | | + -----'------ + | | | T2 | | dS2 = + | dQ | / T2 | + ------------ + | + ---------------- +

The graphic shows a closed system, built up from two sub-systems with initially different temperatures T.1> T2. The thermal energy dQ is transferred from subsystem no. 1 to no. 2. The entropy of the overall system increases, namely by dS = dS1+ dS2> 0. In the end, both will be at the same temperature and in equilibrium.

But if we consider subsystem 1 alone, then its entropy is constantly decreasing. The other term, "order," is a subjective definition, but it usually increases. One example is the popular lead pouring. The following graphic looks different at first glance, but the principle is the same: heat passes from 1 to 2.

+ -------------- + | + ------------ + | || T2 || || + - + || || | T1 | -> dQ || | + - + - + ------ + | + -------------- +

First is the lead (T.1) liquid. So pretty messy. After it has solidified, you can see under the electron microscope that it is made up of crystals.

Here, in accordance with the Second Law, a closed system has reduced its entropy in a subsystem and created order. Without any human intervention.

In principle, this applies to most cooling processes, if not so obviously.

3 heat flow

It gets even more interesting when a subsystem between two heat baths2 is working.

3.1 heat engine

+ ------------------ + | + ------------ + | | | T1 | | dS1 = - | dQ | / T1 | + ------------ + | | | dQ | | \ | / | | + -'-------- + | | | ERM -> dA | | dA = w dQ | + ---------- + | | | (1-w) dQ | | \ | / | | + -----'------ + | | | T2 | | dS2 = + (1-w) | dQ | / T2 | + ------------ + | + ------------------ +

The heat engine (WKM) branches off a little mechanical energy dA. An example of an ERM is the steam engine. "w" denotes the efficiency.

The diverted usable energy dA is already something ordered. They can be used to separate white and black balls within the WKM. Or oxygen and nitrogen molecules.

In this way, order arises independently again in a subsystem.

3.2 Benard effect

This effect is a prime example of synergetics3.

In a thin layer of liquid heated from below, various regular convection patterns occur depending on the temperature difference. For example, cells arranged next to each other like a honeycomb, inside which the water circulates like in a saucepan.

3.3 laser

The laser behaves analogously to the Benard effect: it is excited by disordered, i.e. thermal, energy from a pump lamp. on the other hand, it is connected to the general electromagnetic field, which obviously greedily absorbs the energy from the laser, i.e. can be regarded as cold.

The laser is the other prime example of synergetics3.

3.4 Weather, life, etc.

The subsystem made up of the oceans and the atmosphere (sometimes called the biosphere) is also traversed by disordered energy. It flows from the sun (hot) through the biosphere into space (cold). This is how weather (movement = directed energy = ordered), ice (crystals), life and its evolution arise.

4 Spontaneous segregation

If you put smaller macromolecules in a solution of large macromolecules, the former will spontaneously assemble into ordered structures.

On closer inspection it is found that the total entropy of all macromolecules has increased. The smaller molecules now have a little more freedom of movement, which automatically increases entropy.

The effect is the subject of current research. It is believed that living cells make extensive use of it. For more information, search the web for the keyword "entropic forces".


1 In the quantum (field) theory the entropy is as -kB.* Trace (density operator * log (density operator)) defined and in a closed system always constant. The macroscopic entropy that can increase is only created by averaging it. More on this in another FAQ article.

2 Heating baths are simply further subsystems, one of which is only interested in the absorption or release of thermal energy dQ.

3 Book: Hermann Haken, "Synergetik", 1976 (?). Laser, Benard effect and a few other examples are carefully calculated. Is in all university, physics and similar libraries. Search engines can also find information on synergies and hooks.


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