Does Gravity wipe the increase in 'Entropy' slate clean

  • #1

Main Question or Discussion Point

I'm just getting into physics and know that the entropy of the universe tends to a maximum.

So in the long term stars burn out creating alot of high entropy.

However the left overs of dead stars - the dust and bits of left over radiation then can reform under the influence of gravity to form new stars. This process taking an awful long time.

This graviational effect seems to me to be reversing entropy. and creating new 'high' powered energy stars from bits of detritus -

where am I going wrong!? on this
 

Answers and Replies

  • #2
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It doesn't. Gravity can bring dust and bits of together and they might be able to form new stars and this cycle may continue for a while, but from what I understand from atomic theory, this has a limit. Iron has the most binding energy per nucleon and because of this, all matter will eventually form into iron. The lighter elements will undergo some sort of fusion and increase in atomic number to reach iron and the heavier elements will undergo fission or radioactive decay until they reach iron. In addition, energy will go through many different transformations until it all gets converted into heat and then the temperature gradients will even out until the universe is one temperature. This is the "heat death" of the universe. This is maximum entropy.
 
  • #3
I would not be so quick to claim that everything will form iron. Stars can produce heavier elements than iron by combining very light elements, hydrogen and helium, with heavier nuclei. The reactions are still exothermic. And once formed elements are very stable, unless naturally radioactive. If you are going to claim that everything goes to iron, then you might as well claim that everything goes to energy. The proton may be radioactive itself. For some reason a half life of about 10^36 years sticks in my head.

Back to the OP. You have to remember that energy figures into entropy calculations as well. A dust cloud may seem to be of very low entropy, but that mass of dust represents a huge amount of potential gravitational energy. When it collapses it will release this energy as heat and light, well technically infrared radiation is light. The releasing of energy results in a drop of entropy. So at first glance the formation of new stars may seem to violate the laws of thermodynamics, but that is because you are looking at only part of the problem. If something in a well regarded theorem seems to you to break the laws of thermodynamics the odds are that you are wrong and not the countless scientists who have reviewed the work involve. Trust me, scientists know that you cannot break the laws of thermodynamics.
 
  • #4
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The only thing I was saying was that iron is at the maximum of the nuclear binding energy curve. Smaller particles (i.e. Z < 26) can release energy by fusion and the maximum energy from fusion is the hydrogen to helium type. As you combine the hydrogen to helium and that helium into particles with larger Z, you release energy, but it's a diminishing returns effect. For elements heavier than iron, you can't release energy by fusion. A similar effect happens with fission. As the heavier elements that are splitting apart approach iron, the energy released is subject to diminishing returns. With iron, you can't get more energy out than you have to put in to either fusion or fission. With the original "gravitational energy creating stars" concept, you eventually get a "star" comprised of iron.
 
  • #5
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The formation of a star produces a local decrease in entropy, but the entropy of the universe increases.
 
  • #6
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It is also worth noting that even black holes have entropic decay by evaporating through Hawking Radiation. Black body radiation ensures that energy can never be contained forever.
 
  • #7
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Entropy is not just a measure of the disorder of position, it is also a measure of the disorder of momentum. As a cold dust cloud collapses into singularity (assuming Newtonian mechanics), the disorder ("chaos") in momentum increases.

Mathematically, entropy is a measure of area a bunch of particles occupy in phase space- the six dimensional space consisting the three position and three momentum coordinates. It was noted by Henri Poincaré that a classical gravitational system was chaotic- trajectories in phase space tend to diverge. (Because of this, it can not be proven whether or not the solar system is stable).

The divergence of trajectories in phase space means that the observed entropy would increase , despite the fact that, yes, a cold dust cloud will collapse onto itself.
 
  • #8
Andrew Mason
Science Advisor
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I would not be so quick to claim that everything will form iron. Stars can produce heavier elements than iron by combining very light elements, hydrogen and helium, with heavier nuclei. The reactions are still exothermic.
I think you should check this. The binding energy per nucleon decreases after Fe. This means that the addition of any further nucleons results in a lower binding energy per nucleon (less tightly bound - meaning energy has to be added) so the process is endothermic, not exothermic. Uranium cannot be formed from stellar nucleosynthesis. That only goes as far as iron. Higher elements including U must be formed in more complicated events such as super novas.

AM
 

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