# Trying to understand the entropy of different situations...

I just read a book by nuclear physicist Carlo Rovelli on the subject of "Time" and he says that 'entropy' is the only non-reversible process in the basic equations of physics, and he believes time and entropy are related (if I understand him correctly). So this started me thinking on entropy:
1. Suppose you have a finite universe that is filled uniformly with Hydrogen atoms. Does this universe have less entropy than a universe that is half empty and the other half filled with the same quantity of Hydrogen atoms?
2. If gravity pulls those atoms together into a star so that they fuse into many different types of atoms (such as Oxygen, Uranium, Silicon etc), then is there more entropy now, or less? Normally, I would think that having a variety of atoms (from which you can build planets, and even life itself, would be a state of less entropy, but that's just intuitive, and must be wrong.
4. Thermal energy is supposed to be the most entropic type of energy. The amount of energy in the universe is conserved. Mass can be converted into energy. Is Mass a low-entropy form of energy? Is kinetic energy a form of energy that has more entropy than 'potential energy'?
5. Carlo Rovelli also says that often the universe contains objects that are stable for a long time. I know this is true of chemical reactions, because sometimes you need a catalyst to make chemicals react, but I think he is talking of objects with entropy and energy that also don't change for a long time - he uses the phrase 'locked in'. Does the universe progress over time to have more such 'locked-in' objects? For example, a universe that started with just Hydrogen atoms would, over time, end up with many other types of atoms that could not easily be broken back down into Hydrogen atoms. Is so, does this increasing 'locked in' direction correlate with time and entropy?
6. An atom bomb can convert mass into energy, but how is energy converted into mass?
7. At the end of the lifespan of the universe, will atoms such as Oxygen and Carbon exist?

That is enough for now. Thanks

stockzahn
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1. Suppose you have a finite universe that is filled uniformly with Hydrogen atoms. Does this universe have less entropy than a universe that is half empty and the other half filled with the same quantity of Hydrogen atoms?
2. If gravity pulls those atoms together into a star so that they fuse into many different types of atoms (such as Oxygen, Uranium, Silicon etc), then is there more entropy now, or less? Normally, I would think that having a variety of atoms (from which you can build planets, and even life itself, would be a state of less entropy, but that's just intuitive, and must be wrong.
6. An atom bomb can convert mass into energy, but how is energy converted into mass?

I'm not able to answer all your questions, but I would like to contribute at least to some of them - I'm sure there will be more posts clarifying all points by more qualified people:

1) Yes, the entropy of the half-filled universe should be lower than of the uniformly filled one.
2) When the atoms are accelerated and heated up, part of the energy will be transformed into thermal radiation. Also, when fusioning, part of the mass will be transformed into kinetic energy and thermal energy and the entropy overall increases.
6) In particle colliders the particles are accelerated (increased in energy). When they collide, part of the contained energy is transofrmed into mass, forming new particles which then can be analysed.

• Mark Springer