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

A question of entropy

  1. Nov 7, 2003 #1
    the entropy level (disorder) increase with the time flow (as it the entropy is bigger in the future), so if we would go against the flow of time, time travel to the past then the entropy will decrease my question is if we will go back to the past till the big bang the moment of creation does that mean the entropy is at it's lowest there is relative order to the future now how can that be?
    if im not mistaken the creation of big bang is a thing of disorder isnt it?
  2. jcsd
  3. Nov 7, 2003 #2
    Are you speaking of the evolution of life or matter???

  4. Nov 7, 2003 #3


    User Avatar

    Staff: Mentor

    The direction of increasing entropy is called the "thermodynamic arrow of time" and it is one of the main reasons you can't go back in time.

    Now, your question itself is a fairly incoherent run-on sentence, so I'm not exactly sure what you are getting at. But since time and the universe start at the big bang (or rather, at a point in time just after the big bang), there is no "before" with which to measure a change in entropy before and after the big bang.

    Overall, I guess you are asking if the big bang theory contradicts the second law of thermodynamics. No, it doesn't.
  5. Nov 7, 2003 #4
    The entropy of the universe is thought to have been very low near the beginning of the universe. Nobody can calculate the entropy of the Big Bang itself, since it's a singularity, but there is no reason to assume that it must have been a high-entropy state.
  6. Nov 8, 2003 #5
    so it means order has been in the big bang.
    to me it's contradictory because in the beginning there wasnt structure to the universe and now there is and as i understood it entropy is another word for disorder now how can it be that in the big bang there was more order than now?
  7. Nov 8, 2003 #6


    User Avatar
    Science Advisor
    Gold Member

    'Order' is such a vague term, so when applying it to something quantifiable like entropy you have to be precise. Entropy is usually thought of as dS = dq/T, where dS is the change in entropy, dq is the energy absorbed by the system and T is the thermodynamic temperature, but in terms of the Boltzmann entropy theory it can be thought of as the following

    S - S0 = k ln(W/W0) ----------------(1)

    Where W is the number of microscopically distinct states that give rise to the same macroscopic state of the system (in terms of quantum mechanics this would be the number of solutions to the Schroedinger wave equation giving the same energy distribution), k is the Boltzmann constant, S0 is the entropy of a standard conditon and W0 is the probabilty of a standard condtion.

    The third law of thermodynamics state that a perfect crystal at absolute zero has an entropy of zero using this fact and (1) you can then show that any non-perfect molecular configuration has a certain amount of what is known as configurational entropy that is intrinstic to it and this is what can be thought of as disorder. The early stages of the universe by this defintion would be very ordered, infact incredibly so, IIRC the probabilty of the current universe sponateously revrting back to this amount of order would be 10-123.
  8. Nov 8, 2003 #7
    As jcsd, "order" is a vague term. What one intuitively thinks of as "order" doesn't necessarily correspond to "low entropy".

    For instance, take a cloud of gas, which gravitationally collapses to form a solar system, with a star, planets, etc. Would you say that the universe is more "ordered" after the formation of the solar system, than it was before? Maybe ... but does it have less entropy? No: it has more entropy. See:


    (These articles are very similar to each other, but they have slightly different references and details, so I cited all three.)
  9. Nov 8, 2003 #8
    that's what puzzled me too.
    the universe has structure and although it is ordered it's entropy is high.
    what is the term of "order" in physics? (i thought it was low entropy).
  10. Nov 8, 2003 #9
    As far as I know, there is no mathematical definition of "order" that corresponds in all cases to our intuition.
  11. Nov 8, 2003 #10


    User Avatar
    Science Advisor
    Gold Member

    Read through the links that Ambitwistor posted, as they explain how in large systems where gravity pre-dominates, as entropy increases so does inhomogenity. The early universe was incredibly homogenous.
  12. Nov 8, 2003 #11
    also now the universe is homogenous.
    if it wasnt then how could you say that the laws of physics apply everywhere in the universe?
  13. Nov 8, 2003 #12
    i think it's more a philosophical question then a mathematical one.
  14. Nov 8, 2003 #13


    User Avatar
    Science Advisor
    Gold Member

    You are right the universe still is pretty homogenpus, but nowhere near as homogenous (and by homogenopus I mean homogenous in the distribution of it's matter) as it was in it's early stages.
  15. Nov 8, 2003 #14
    becuase the space where the universe just begun was.... how should i put it, "small" and now as it's evolving so has matter evolved with space.
  16. Nov 8, 2003 #15


    User Avatar
    Science Advisor
    Gold Member

    And what has this evolution amounted to in terms of entropy? An increase.
  17. Nov 8, 2003 #16


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Which brings us to the 4th law --- "It is impossible to do any useful thermodynamic work with a thesaurus." Entropy is a thermodynamic state function defined in the first half of the 19th century. This is prior to the statistical mechanical arguments. Statistical mechanics gives us partition functions for systems; if you must use the stat. mech. approach to analyzing questions involving entropy, WRITE THE PARTITION FUNCTION FOR THE SYSTEM! Don't appeal to the lame, intuitive (and incorrect) analogy that entropy is equivalent to a measure of disorder. And, NEVER take that lame analogy and run with it to the thesaurus --- order=structure=sequence=a thousand other untenable arguments (running with analogies is every bit as dangerous as running with scissors --- you WILL cut yourself).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: A question of entropy
  1. Entropy question (Replies: 6)

  2. Question about entropy (Replies: 3)

  3. Question about entropy (Replies: 1)