Understanding Entropy
Consider the following article in Nature
Conflicting arrows of time
 Entropy is an important indicator in physics of the passage of time.
 Briefly, time increase is usually matched by entropy increase.
 It is possible that the Universe contains regions in which time runs in a direction opposite to ours?
 This exciting, but unlikely, possibility has been investigated by L.S. Schulman, who has studied the entropy of two interacting gasses of particles with opposing arrows of time.
 If such regions exist, can each see the radiation emitted by the other?
 The laws of electrodynamics have also been proposed as indicators of the passage of time, because radiation, once emitted, must be subsequently absorbed elswhere.
 But electrodynamics, unlike entropy, can be shown to be time symmetric.
 Indeed, according to the simple cosmological equations for a model universe, such a universe may contract just as easily as it may expand.
 We know which solution applies to our Universe by comparison with observation.
 This problem can be solved by using WheelerFeynman absorber theory.
 Suppose that electromagnetic radiation is emitted by a system, S, in both directions of time and as equal parcels of energy. The normal radiation, R, is later absorbed by matter at S' and the remainder, r, goes backwards in time. The particles in the absorber S' are stimulated to emit radiation themselves which also proceeds with equal energy: half (R') into the future and half (r') into the past. In due course r' reaches S, where it cancels the original wave r emitted by S into the past, leaving only the wave going forward in time, in agreement with observation.
 Schulman extends this theory to his two gases with opposing arrows of time, assuming a purely theoretical interaction, He then finds that the entropy histories of the two systems are affected by their interaction, but not as strongly as might be expected; and the results are "still consistent with electrodynamics".
 On this basis, systems with opposing arrows of time may be able to coexist and see each other.
 Bizarre as it seems, he proposes that such objects could exist in our Galaxy, and might have properties expected of dark matter.
Detailed Comments
 This sentence can be rewritten as follows:
 Entropy services as a clock how long ago something happened.
The problem with that statement is:
 If you can not measure entropy accurate then your prediction about time is not very accurate.
 If your definition of entropy (complexity, order) is not clear then the any conclusion becomes even more ambiguous.
 This sentence can be rewritten as follows:
 Entropy increase services as a clock how long ago something happens.
There is one major problem with this sentence:
What happens if in some experiment entropy decreases ?
 This sentence can be rewritten as:
 It is possible that the Universe contains both regions where entropy increases and regions where entropy decreases ?
This question can only be answered if you have a clear understanding what entropy means.
 I assume that L.S Schulman, has studied the interacting of two different gasses of particles. What where the differences between those particles ? was it charge ? was it spin ? was it polarization ?
 The second part of this sentence can be rewritten as:
 can each gas be influenced by the radiation emitted (particles transmitted) by the other gas ?

 I expect the authors mean here the radio active decay of certain isotopes of radio active elements. The current relation between those isotopes services as a clock how long ago that element was created.
However that does not mean that all electrodynamic interactions (electrons, neutrons, photons, quarks) can serve as an accurate clock.
 There exist no reaction in nature which is truelly time symmetric. Always one extra parameter is involved which controls if the rest of the reaction goes from A to B or from B to A. This is so for electrodynamics as for all other reactions (with or without entropy)
 Our universe may contract and our Universe may expand, but that is not caused by certain equations. It is possible that this contraction and expansion can be described by certain equations, but that does not prove that those equations are correct. First we have to observe that our Universe actually does expand and contract.
 Maybe we know by observation that our universe expand but that does not prove
 That our universe can contract
 That the afore mentioned comological equations are correct
 Which problem can be solved by using WheelerFeynman theory ? Electrodynamic absorbation ? The following example ? How ?
 Electromagnetic radiation A can first split in C and D. B can combine with C to form E. A different approach with the same result is also possible: B splits first in C' and E. A combines with C' form D.
Carefull measurement, including the intermediate results (C or C'), should distinquish which reaction took place. Final observation of D and E is not enough.
The reverse reaction (going from D and E to A and B) is also possible. Whatever the reaction the concept "backwards in time" is not required
 It seems to me that Schulman finally did not perform any experiment ?
 On that basis you can not make any conclusion.
 On that basis for sure you can not explain dark matter.
General Comments
IMO the whole article is written in a rather difficult style. IMO it is possible to rewrite the whole article without the following concepts: Entropy, arrow of time, direction of time, backward in time, time symmetrie, regions and Universe.
It would be much simpler if the authors

First started with a more detailed description how L.S. Schulman performed his experiment with the two gasses. How he started the experiment, what he did, what he measured and what he observed i.e. what the final outcome was.
 Secondly explained this experiment using current understanding. This should not be enough to explain everything.
 Finally introduced the new concepts required, in order to explain all the results better.
IMO the result will be that there will not be any relation between this experiment and the state (or evolution) of the Universe and or of our Galaxy.
Created: 5 June 2000
Modified 28 October 2000
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