Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Entropy vs Life

  1. Jul 10, 2012 #1
    I have gotten to the thermodynamics portion of my physics class, and right now I'm reading about entropy. It is taking me forever to read the chapter though because I keep pondering so many questions.

    - How can life be possible with the concept of entropy? Cells must organize themselves in very distinct ways in order to function correctly, but this seems to defy entropy (though maybe I'm thinking to broadly). Also, the formation of planets is an example of organization of matter in nature.

    - This paragraph really caught my eye

    "No decrease in energy occurs when hot and cold water are mixed. What has been lost is not energy, but opportunity, the opportunity to convert part of the heat from the hot water into mechanical work. While removing heat from the hot water and giving it to the cold water, we could have obtained some mechanical work, but once the hot and cold water have been mixed and have come to a uniform temperature, the opportunity to convert heat to mechanical work is lost irretrievably." ​

    I can't help but draw connections to this from everyday life. There have been "opportunities" that I have let pass me by (although macroscopic), that may never present themselves again. And yes I know this is very corny, but it made me think!

    - Another sentence

    "When entropy increases, energy becomes less available, and the universe becomes more random or 'run down'".​

    This absolutely blows my mind! Neglecting the big crunch and assuming that the universe will continue on forever, is it possible that eventually there will be no structure in the universe at all? Would this include the structure of time and space and the laws of physics and mathematical axioms all together? How can something that seemingly has structure which is only an abstract idea such as mathematical axioms deteriorate?

    Anyone care to comment on these questions?
  2. jcsd
  3. Jul 10, 2012 #2


    Staff: Mentor

    You have seriously misunderstood the second law of thermodynamics. It doesn't say that there can never be any decrease in entropy for any system, but rather that in order for one system to decrease entropy another system must increase entropy by a greater or equal amount. In the case of the earth the sun dumps a large amount of low entropy (high temperature) radiation on earth, earth radiates that energy with a much higher entropy (low temperature). The entropy decrease corresponding to life is a miniscule fraction of that overall entropy increase.
  4. Jul 10, 2012 #3
    When ice is formed in might be in a more ordered state that it was as a gas. But in order for ice to form heat energy must have been transferred somewhere else thus the entropy elsewhere must have increased. Same idea for life and planets.

    I personally like to think of this theme for your next question (and the word opportunity) since I am a biochemical guy that likes physics because its tidy on the classical level: In so many processes in which human beings use energy to do work we look for differences. Differences in temp., differences in concentration of some ion, differences in the height of diff objects above the earth. So we are constantly looking for ways to take advantage of differences because they have the potential to let us use their energy for some other sort of work. Cells are very good at this. Look at the amazing hydrogen ion gradient that helps produce ATP. A conc. gradient turned to a chemical bond through an amazing mechanism that is super complex. Or a rubber band stretched to a very unstable energy state that allows us to fly a little balsa wood air plane.

    Hope some of this helps.
  5. Jul 10, 2012 #4

    If we deteriorate math axioms deteriorate.

    There are others that think differently. Apparently there are some ideas that claim true math exists outside of human thought. I personally dont understand this, but I read it somewhere...

    pi in the sky I think it was...
  6. Jul 10, 2012 #5
    This was very helpful in clarifying things. So if heat is transferred elsewhere when ice freezes, then does it really increase the amount of entropy in the universe? Entropy is not conservative correct?

    I guess what I'm confused about is that if a system loses entropy then the entropy of another system has to be increased by a greater or equal amount (never less), then eventually will structure be lost? Also, is it possible to calculate the actual entropy of a system or just the change in entropy?
  7. Jul 10, 2012 #6

    And you gotta be real careful about clarifying your system. Some religious types that dont like evolution have used the 2nd law to claim life is a miracle since it defies the 2nd law but it does not. Miracles and magical poofs are not science.
  8. Jul 10, 2012 #7

    That's very interesting. It's amazing how quickly people will come to a conclusion about something without trying to fully understand it. For instance, I just came up with these questions after reading about thermodynamics for 30 minutes and clearly I have no idea what I'm talking about. Apparently other people are trying to argue their religion with it...
  9. Jul 10, 2012 #8
    You are obviously inquisitive and want to attempt to understand.

    Evolution gets a standard attack at a fairly consistent level in the US. The latest is intelligent design. The Early Universe and of course the age of the earth and universe also take their fair share of disingenuous hits. And then of course the science people hit back with "and the earth is still flat, eh, ya hoser?" *cue a Canadian accent*

    And then a food fight ensues.

    "Do not attempt to reason a man out of something he did not reason himself into". I try to follow the Big John Swift saying. Im getting silly and tired. Good night.
  10. Jul 11, 2012 #9


    User Avatar
    Staff Emeritus
    Science Advisor

    I believe you have hit the nail right on the head with this statement. This is why it is always important to make sure you have tried your best to understand something before commenting on it.
  11. Jul 11, 2012 #10


    Staff: Mentor

    Yes. This is called heat death of the universe.

    The change in entropy is generally more important, however by definition a perfect crystal at 0 K has 0 entropy. So in principle it is possible to calculate the actual entropy.
  12. Jul 11, 2012 #11
    Work must be done on a system to decrease its entropy. You put outside energy into a closed system, and it can lower entropy. Much how our bodies constantly need food and sleep to keep our body and brains functioning. Without outside energy transferred to us, our bodies would fall apart.
  13. Jul 11, 2012 #12
    Hmmm... How can you define a system as having a certain absolute entropy? I take what you are saying is that the crystal at 0K has 0 entropy because the particles are not moving (in theory), but it has 0 entropy with respect to what standard? For instance, if you can define a system as having 0 entropy then you should be able to define a system as having an entropy of 1, 2, 3, etc but I thought this was impossible, but I understand you can find the change in entropy.
    Someone please help clarify
  14. Jul 11, 2012 #13


    Staff: Mentor

    You just adopt a convention and say "this thing is 0 entropy, by definition".

    A perfect crystal at 0 K IS the standard.


  15. Jul 11, 2012 #14
    Well from what my book says, entropy is similar to heat in that it does not make sense to define the entropy of a system, only the change in entropy. You cannot say that a system contains a certain amount of heat, only that a certain amount of heat is transferred into or out of the system. I thought entropy was similar.

    That's what is confusing me. The book says that entropy is always increasing in the universe, but why does that even matter if we are the ones choosing the reference point. For example, when we find gravitational potential energy, the reference point of where we choose height to be zero doesn't matter, only the change in height between the two points matters. We could choose h=0 at 10,000 meters but an object at h=0 is still 10,000 meters off the ground. When the object hits the ground it is now at -10,000 meters but that doesn't mean anything... only the change in it's height matters. (along with gravity, mass, etc)

    Wouldn't entropy be similar in that only the change in entropy is all that matters. If the average entropy of the entire universe is increasing with time, then there wouldn't be an average CHANGE in entropy would there? I guess this would only apply if on average the entropy is uniformally increasing in the universe but I'm guessing this isn't true... can anyone verify?
  16. Jul 11, 2012 #15


    User Avatar
    Science Advisor

    No, just like we can choose "0 point" for potential energy because how it changes is all that is important, so although we can choose a "reference point" for etropy, its change has meaning. And we don't need to talk about "average" entropy because what is important is the change in total entropy in a system. It still can be true that entropy is decreasing in one part of a system but increasing in another, so you could talk about "average" entropy but what is important is that the total in the system is increasing.
  17. Jul 11, 2012 #16


    Staff: Mentor

    That is a good example. Entropy is similar. The change in entropy is what is physically meaningful, i.e. the second law of thermo refers to changes in entropy.

    So, just like in gravitational and electrical potentials, we are free to arbitrarily choose some reference as 0. It doesn't really matter what reference we choose, it is just an arbitrary convention. We have arbitrarily chosen to define the entropy of a perfect crystal at 0 K as the reference 0 entropy point. There is no physical significance, it is just a convention.
  18. Jul 11, 2012 #17
    Can it also be possible that Entropy is cyclic?
    In such case, there was a time when total entropy of the universe was decreasing.
    After it reached the lowest point, it reversed direction and
    we are now living in the increasing cycle of entropy.

    It also appears to me, entropy can be decreased by performing external work on the system.
  19. Jul 11, 2012 #18


    Staff: Mentor

    There is no experimental evidence to suggest that.
  20. Jul 11, 2012 #19
    Yes, but that would get rid of the sort of time-translational symmetry we associate with Conservation of Energy (at least, I think,) and as has already been pointed out, we have no reason to think so.

    Completely true. However, if you claim this is in violation of the Second Law, it isn't, as the Second Law only says that entropy increases in a closed system. Performing this external work makes sure the system isn't closed.
  21. Jul 12, 2012 #20
    If entropy is increasing, life will eventually come to an end, right?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook