I thought that an interesting part of this chapter to discuss was the energy intake discussion for plants and humans. So we know that the energy producing pathways in plants and humans are somewhat spontaneous processes overall, once all the various components of those pathways have been accounted for (i.e. breakdown of food for energy, to then create more ATP helping to create an ordered system, and then a final release of energy through various function), but what is interesting to note is what the systems take from and give back to the surrounding system. That is, as the chapter states, is that they consume order, and not energy, which I find to be a bit of a knockback to my understanding of a systems functioning. By my understanding of this quote, I believe that through the conservation of energy through the systems, what happens is that equal energy is taken in and released, but that the energy is of a different quality (that is a different level of order), and since we're consuming order, that means we release disorded energy back to our surroundings.
What is most interesting about all this is how the organisms take in a high-quality form of energy (sunlight for plants, food for animals) and can use this energy to build up a low-quality form of matter (water, CO2 etc) into high ordered material. Again, this obviously involves an input of energy such as ATP to allow for this reaction to occur, but I still find it amazing how these organisms have their way of making non-spontaneous reactions happen.
The other important point that should be made is in relation to the understanding that the entropy of the universe is always increasing, which can be illustrated through the plant and animal systems. Though both systems take in both high- and low-quality energy, they also both are left at the end of the cycle no more high or low in their quality of matter, and all they will have released back to the surrounding system is low-quality energy (disordered energy) which backs up the theory of ever-increasing disorder (note to mention that when the systems die they only degrade, releasing disorded energy).
I would like to see what others say on this topic, and to see if they may have some more insight to provide.
Last year Prof. Charlie Lineweaver of ANU gave the Friday Physics Colloquium*. The topic of his talk was a VERY wide view of how life happens. The point was that the structure of life happens so that free energy gradients can be discharged. Increasing structure leads to faster discharge etc.
ReplyDeleteIn a closed isolated system, the entropy does not change. In classical mechanics, this is Liouville's theorem (I think) which says that the volume of phase space is conserved by Hamilton's equations. In quantum mechanics, the statement that the propagation operator is unitary implies the (von Neumann) entropy is conserved.
So where does the entropy come from? If the universe is isolated, then the entropy of the universe CANNOT ACTUALLY BE CHANGING AT ALL. Lineweaver (and others) say that the entropy is coming from the gravitational field. The idea (as I understand it) was that the distribution of matter in the early universe was uniform, and the state of the gravitational field was one of high entropy. I suppose that means that, more locally, the entropy that we produce is ultimately entropy that was in the gravitational field (presumably we are harvesting it by eating plants that were fed on the suns rays).
The third law reads the way it does presumably because the entropy of the gravitational field is left out.
Why is it left out? This has something to do with a point that I didn't understand until I watched some very nice lectures on Stat. Mech. by Leonard Susskind. He points out that gravitationally bound systems have a negative specific heat, and therefore can never be in equilibrium with their surroundings. The more energy the gravitationally bound system gives off, the higher it's temperature gets, the more it has to give off, etc. It seems then that you can't REALLY do equilibrium stat. mech. with them. This is probably why the entropy is left out of the third law. (It also begs the question of how one would define the thermodynamic entropy of the graviational field, by the way - I have definitely been playing loose with this idea here).
This point will have almost nothing to do with anything we will deal with in this course, but it is interesting, in a cosmic sense.
*http://www.physics.uq.edu.au/colloquium/?p=31
(Re Seth's comment): So are you saying that the third law of thermodynamics omits gravitational field effects? I suspect I am misunderstanding this comment because I don't understand the concept of the entropy of a gravitational field. The next paragraph illustrates what I currently suspect the entropy of a gravitational field to be.
ReplyDeleteSay we had a cube of space slightly more than the size of earth, which was randomly distributed with particles such that the total mass of the particles was equal to the mass of the earth. Furthermore let the temperature of this system be 0K (so there is no heat energy). This initial state has only gravitational potential energy, and maximum entropy, since the particles are randomly distributed. The gravitational potential energy causes the particles to be attracted to the centre of mass of the system. The motion of the particles causes the temperature to increase, the energy of this comes from the decrease in gravitational potential energy of the system. The entropy decreases, since the particles now occupy a smaller volume. Is the entropy lost in this system equivalent to the initial entropy of the gravataional field?
(Re Matt's post): I really liked the idea that living things consume order, rather than energy. I never thought of the bioshpere like that before. This concept is quite obvious when thinking about how animals breakdown complex sugars and proteins into water and CO2 to from energy. But when you apply this to processes like photosynthesis, one can begin to understand how much more 'quality' light energy is than heat energy. All the order and structure on earth comes from the loss in 'quality' when the light energy from the sun is degraded into heat energy (obviously omitting the small environments that are sustained by thermal vents in the ocean).
ReplyDeleteTo be honest, I am out of my depth here, because I am not up to date with cosmology. I didn't really understand all of what Lineweaver was saying here, for the same reasons you state. It would seem that if the matter is evenly distributed, then this is a state of maximum entropy constrained by the total amount of matter.
ReplyDeleteIt is true that the entropy of a closed isolated system does not change in classical (Liouville's thm.) or quantum (unitarity) mechanics. So, if the "universe" is a closed isolated system, then one would expect the entropy doesn't change.
Applying these ideas to the universe might be premature though, since there are well-known inconsistencies between theories describing molecular and mesoscale systems to cosmological systems.
I do not know how one writes the entropy of the gravitational field, and I think this is an issue of current research in cosmology. So it is probably off-base of me to have referred to the entropy of the field as glibly as above.
Also, if gravitational systems cannot be at thermodynamic equilibrium, then defining the entropy of an equilibrium state is obviously problematic. This would carry over to ideas of reversible processes as normally defined in thermodynamics.
So, I wouldn't try to pick apart my statements too much. They are not meant to be rigorous. If you are interested, there is a paper by Lineweaver with the same title as the talk, and this might be a good place to look for further information.
I really like that bit of the chapter. To me it seems to go hand in hand with the typical thermodynamic laws. The human body cannot be described as a closed system and to maintain biological order we have to consume ordered energy. You can't get something for nothing.
ReplyDeleteIt would be interesting to see whether plants or animals are more efficient at exchanging the ordered input for disordered output though?
That is a really interesting point about the entropy of gravitational fields being ommitted, especially due to its fluxing state and never being able to be in equilibrium with its surroundings. So I guess essentially what the third law of thermodynamics would be breaking down to is that we always know that there is more entropy being released into the universe, but whatever is being released is then counter-acted by a gravitational flux (and by flux, I mean the inability to find that equilibrium with its surroundings).
ReplyDeleteIn response to Heather's comment, that is indeed a good question to ask, especially once you've really looked at the processes that occur. I personally have the belief that plants would likely be better with their exchange processes, in that essentially their materials made, including waste product goes into building up the plant, and strengthening it. Then, at some point, an animal will come along to eat it, then using the materials of the plant as both low and high quality energy sources. I suppose in that regard, animals are also useful, but there are obviously distinct moments of high and low quality output from animals, though of course some of those outputs still are useful to both plants and animals.