1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is increase in entropy synonymous with the flow of time?

  1. Oct 1, 2012 #1
    Hello, I was just reading how the second law of thermodynamic is the only principle of physics that is irreversible, and that fact appears to have a correlation with how we view time and how time can only move in one direction, being the same as the flow of entropy. So, are entropy and time really synonymous?
    I was doing a thought experiment to try to settle the issue and envisioned our universe with entropy sped up at twice the rate. Clearly, if entropy was to increase at twice the rate, things would age twice as fast, plants would grow at twice the rate and live for only half as long. For most anything, it would appear, as though time itself had been sped up. But, then I thought about the spin of the Earth. For time to run at twice the speed, then naturally the Earth would have to rotate at twice the speed, but I don't see how an increase in the rate of entropy could have any effect on the speed of the rotation of the Earth, or its period of revolution around the sun. So, now I'm thinking entropy and time are not the same after all. My question is simply: Would a doubling in the rate of the universe's entropy cause the rotation and revolution of the Earth to become twice as fast?
  2. jcsd
  3. Oct 1, 2012 #2
    Be aware that thermodynamic irreversibility refers to spontaneous processes.

    The Second law dows not refer to anything that cannot be reversed. It just describes the 'cost' of doing so.
  4. Oct 2, 2012 #3
    But, I would think not at the universal scale. The question remains, would a doubling of the entropy rate result in the earth spinning twice as fast? If that is not the case, I believe time and entropy cannot be considered synonymous.
  5. Oct 2, 2012 #4
    They are certainly not synonymous.
  6. Oct 2, 2012 #5
    Ok, but you can't just claim something. The argument that since the second law of thermodynamics is irreversible, thus it may not just require an arrow of time, but is responsible for the arrow of time makes a lot of sense to many people. Entropy and time appear to be closely connected. I'm still trying to sort it out by trying to get an answer to my question. Is there any mechanism by which an increase in the rate of universal entropy would cause the earth to spin twice as fast?
  7. Oct 2, 2012 #6
    You are the one making the proposition (claim) and as such you must define your terms.

    The term 'entropy rate' has no formal meaning in thermodynamics so you need to define what you mean before you can make claims about it.
  8. Oct 2, 2012 #7
    You can't just double the entropy and "see what happens". That's the wrong way around. Entropy is a property of the state of a system. If you change the state, then you may get a change in entropy. To suggest that you're able to change entropy and then see what happens to the state is meaningless.

    Can you describe this thought experiment in a way that it could be hypothetically viable?
  9. Oct 2, 2012 #8
    Entropy can be measured as Joules per Kelvin I believe, so it would appear it can be gauged. The system I’m speaking of here is the universe itself and the rate of entropy in it. There must be some rate that entropy transpires in the universe. The universe is expanding and running down, and the time at which it will reach heath death can be predicted. If the present rate of entropy were 1X Joules/Kelvin and tomorrow we woke up to a universe where it is 2X Joules/Kelvin, then would that result in 12.5 hour days / or require an acceleration of all motion? If the system must change first and that change requires everything move at twice its current speed, such as the spin of the earth, for entropy to double in rate, then what’s the difference? That would just prove that entropy and time are inexorably linked and really just two words for the same thing.
  10. Oct 2, 2012 #9
    The above is not a definition of what you mean by the 'rate of entropy'

    Since you will not define what you mean how can the discussion proceed further?
  11. Oct 2, 2012 #10
    How can Joules per Kelvin not be considered a rate, just like miles per gallon? Let's call the rate the time it will take the universe to reach its half-life. If the half-life of the universe were to be shortened by half because the dissociation of order to disorder became twice as fast, would that mean all motion in the universe had to have doubled, such as the spin of the earth?
  12. Oct 2, 2012 #11
    I don't know how many different ways I can answer no.

    I did not say entropy cannot be gauged, I said it cannot be changed by some magical lever.
    Although how would you propose gauging it for a universe?
    Just because we define a quantity with known units, it doesn't mean it can be measured.
    How can entropy have a half-life? What makes you think it decreases exponentially with time?
    By what possible mechanism can the Earth's rotation instantaneously double?
    Last edited: Oct 2, 2012
  13. Oct 2, 2012 #12
    Joules / Kelvin is a rate, miles / per gallon is a rate. I’m not measuring anything. I’m speaking of the rate of change in something. That’s very straight forward. The universe has to have some rate of entropy, and how can it not have a half-life if its energy is running down? Whether one has a magical lever or not, theoretically if you did, and could push that lever forward to twice the rate of its present rate of entropy, would everything in the universe have to speed up? It’s a simple question and a simple concept.
    Saying no without an explanation is no answer. “By what possible mechanism can the Earth's rotation instantaneously double?” How is that an answer? If time were to double then the rotation would have to double; yes, of course. If entropy were to double would the result be the same?
    You cannot claim that entropy and time are not the same just by saying No. That’s tautology. If you know anything about physics then you should consider the question and try answering it intelligently as to why or why not motion would double -- according to laws of physics.
  14. Oct 2, 2012 #13
    OK that's fine now we understand what you mean by a rate we can find your source of difficulty.

    By 'rate of entropy' you actually mean the quantity itself, since entropy is defined as energy per degree.

    So to translate this sentence you mean

    'The universe has to have some particular value of entropy, and how can it not have a half-life if its energy is running down?'

    As far as I know there is no evidence whatsoever to suggest that the energy of the universe is running down.
    Thermodynamically the universe is either an infinite or a isolated system, either way it is not suffering diminution of its energy.
    However much you remove from infinity still leaves you with infinity.
    If, on the other hand the universe is finite it is an isolated system since it incorporates all there is. The First Law of Thermodynamics tells us that the energy of an isolated sytem is constant.
    Last edited: Oct 2, 2012
  15. Oct 2, 2012 #14
    You haven't any evidence that universe's energy is decreasing, nor that this would imply the universe's entropy must decay exponentially, which allows for a half-life to be defined. You can't talk about "theoretically pushing the lever forward" because such a statement doesn't make any sense in that form. A thought experiment still has to be explainable.

    You are describing something that doesn't make sense, and when I ask you to elaborate, you tell me it's not a counterargument. There is no counterargument if you cannot present your argument clearly. I think I'll let someone else continue this struggle, but for the record, as soon as you start insulting someone's intelligence, their motivation to help you drops quite suddenly.
  16. Oct 2, 2012 #15
    Beginning with the Big Bang, there has been a progression from order to disorder (net disorder), which is called entropy. If that was X Joules/Kelvin, what would have happed if it had been 2X Joules/Kelvin? Would everything now be moving twice as fast? If so, then entropy must be considered no different than time itself. So, would all motion be moving at twice the current velocity?

    Though total energy in the universe cannot decline; entropy is how much of that energy is available for doing work. The amount of available energy is what is declining. So once the universe is homogenous, there well be no more energy available to do any work, unless the universe can somehow reverse entropy (sometimes called sentropy). Entropy would be at maximum and the universe, without some means of reversing entropy, will be dead. Since the actual state of complete maximum entropy is probably a limit approaching ?? (whatever maximum is defined as), it would probably be more appropriate to speak of the longevity of the universe in terms of half-life, but that is not important.
  17. Oct 2, 2012 #16

    Many implications...you can envision some from the above article.

    As most entropy in our universe is hidden within black holes, your hypothesis would seem
    to imply twice the proportion of black holes in our universe....in addtion it would seem our sun might be twice as hot and we would not even be here.
  18. Oct 2, 2012 #17
    Yes, but twice as hot in a universe that uses energy twice as fast, so what's the difference? If time was sped up twice as much, you would expect twice the number of black holes. If the sun burnt at twice the rate, it would live half as long, just as you’d expect if time were to speed up twice as fast. So what is one difference between the rate of entropy speeding up and time speeding up? Nothing it appears. The spin of the earth had to come from some energy input to push it at some time in the past, so if the rate of entropy had been twice as much during the Big Bang, it and everything would be moving twice as fast. Yes? The point is, is that entropy only goes in one direction (universally speaking) and so does time. Is that just coincidence? Thus, it appears that time is emergent from entropy.
  19. Oct 2, 2012 #18

    What does that mean?

    Time doesn't have a direction anymore than length does.

    Unless you retort that length "goes forward" too. :tongue2:

    That is saying spacetime is isotropic. There is no flow to it.

    This is the "stage" that entropy plays out on.

    Consider what it is you mean with the term time, specifically the assumption that time is what "flows.
    Last edited: Oct 2, 2012
  20. Oct 3, 2012 #19
    Can you remember tomorrow or change yesterday? Obviously time has direction.

    "This is the "stage" that entropy plays out on." You can't just claim things; you must support your claim with some clear reasoning. How can you know that?

    If anything is not isotropic it would be entropy, not time. And, that fact may be the only thing that would distinguish entropy from time.
  21. Oct 3, 2012 #20


    User Avatar
    Science Advisor

    I can burn a log on a fire.
    I can let a log rot.

    I can look at my watch and see that one process takes longer than the other.

    It seems clear that the rate of increase of entropy is not constant with respect to what we call time.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook