Q: Why does the entropy of universe always increase, and what is the heat death of the universe?

Physicist: The increase of entropy is just how a scientist talks about the fact that the universe tends to do the most likely thing.  For example, if you throw a bucket of dice you’ll find that about a sixth of them will be 1, about a sixth will be 2, and so on.  This has the most ways of happening, so it’s the most likely outcome, and for the same reason it’s the outcome with the highest entropy.

High entropy. Arrangements of lots of dice tend, over time, to end up like this.

In contrast, you wouldn’t expect all of the dice to be 4 at the same time, or otherwise assume one particular pattern.  That would be a very unlikely and low entropy outcome.

Audrey Hepburn is one of the lower entropy states you'll find. Or rather, will never find, because it's so unlikely. You have to sit back and squint a little to see it.

“Entropy” is just a mathematical tool for extending the idea down to atomic interactions, where we don’t have a nice idea like “dice” to work with.

One of the things that increasing entropy does is to spread out heat as much as possible.  If you have a hot object next to a cold object, then the heat will spread so that the cooler object heats up, and the hotter object cools down, until the two are at the same temperature.  The idea (the math) behind that is the same as the idea behind mixing fluids or sands together.  There are more ways for things to be mixed than sorted.

The same thing happens on a much larger scale.  The Sun, and every other star, is radiating heat into the universe.  But they can’t do it forever.  Eventually the heat will have spread out so much that there won’t be warmer objects and cooler objects.  Everything will be the same temperature.  The same, very cold, temperature.  The vast majority of the universe is already screaming cold, so the heat death of the universe is just about burning what fuel there is and mixing the heat so created into the ever-expansive, cold, and unyielding cosmos.  Both the burning of fuel (mostly through fusion in stars) and the distribution of heat are processes which increase entropy.

The cold and unyielding cosmos. What's the stupid point of anything?

Once everything is the same temperature, the universe attains a “steady state”.  With no energy differences, there’s no reason for anything to change what it’s doing (all forces can be expressed as an energy imbalance or “potential gradient“).  Heat death is the point at which the universe has finally settled down completely (or almost completely), and nothing interesting ever happens again.

Which is depressing, but it is a fantastically long time away.  There are a hell of a lot of other bad things that’ll probably happen first.

The eminent philosophers Flanders and Swann have a more up beat take on the heat death of the universe:

“Heat is work, and work’s a curse,

and all the heat in the universe,

is gonna cool down.  ‘Cause it can’t increase,

then there’ll be no more work, and there’ll be perfect peace.

That’s entropy, man.”

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22 Responses to Q: Why does the entropy of universe always increase, and what is the heat death of the universe?

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  2. Boz says:

    So, humans are flying around in the space ships, and living on space stations, but the heat death of the universe is approaching. The last stars are slowly dying and no new stars are being born.

    Will it be likely (or even possible) for human civilisation to continue existing when the universe is in a heat death situation?

  3. neen says:

    No, that won’t be possible, since flying around in spaceships (or even living) requires an non homogenous distribution of heat, so the combination of people living and the universe being heat dead is impossible.

  4. The Physicist The Physicist says:

    Kinda by definition, if there are still people around using usable power, then the heat death hasn’t happened yet.
    But keep in mind that the age of the universe (so far) is very small compared to the time it’ll take for the heat death to settle in. If any of our descendents are still around, they definitely won’t be remotely human. We got from single cells to people in a couple billion years, and the heat death is at least many, many trillions of years off.

  5. Cheryl says:

    What about parallel universes and M theory? What do you think of that?

  6. The Physicist The Physicist says:

    Maybe? Haven’t studied it. However, for what very little it’s worth, it’s likely that the laws of thermodynamics hold in other branes.

  7. Seth says:

    The equation: (entropy)=(heat content)/(Temperature) shows that more energy added makes the entropy of that system decrease. What is the mathematical relation of energy as a function of complexity (information)?

    This is one equation i found but it doesn’t explain how probability (P) is related to information… or maybe I am way off.

    S = -k∑[P log(P)]

  8. The Physicist The Physicist says:

    It looks like you may be thinking of one form of the fundamental thermodynamic relation.
    In the case of ideal gases, temperature is proportional to the derivative of the heat content with respect to entropy. This is very similar to (temperature) = (change in heat content)/(change in entropy).
    Non-uniform probabilities don’t tend to show up in thermodynamics. In information theory we routinely talk about state 1 having a particular probability, state 2 having another, etc.
    In thermodynamics we assume that all states have the same probability. This may seem a little strange, but it’s a result of the “asymptotic equipartition property“. When you have a whole lot of identical parts (like you find in most complex physical systems) you find that the chance of randomly picking a state that has a probability different from the average is nearly zero.
    If you’re interested in learning about just the information theory side of entropy, then Shannon’s original paper is a good place to start.

  9. Ron says:

    “What is the mathematical relation of energy as a function of complexity (information)?”

    The short answer: None.

    Claude E. Shannon introduced the term ‘entropy’ into information theory to describe randomness in information, or jumbled-up-ness, or compressibility. In a sense, he borrowed the WORD from physics mostly because the concept ‘appeared’ similar and could be described with similar math. However, that is where the similarities, and more importantly, the connection with thermodynamics end.

    The laws associated with entropy and thermodynamics do not carry over into information theory. They were never meant to. Had Shannon picked a different word(i.e. compressibility), this confusion may have been avoided, but alas, we are stuck with it.

    Unfortunately, that hasn’t stopped some unscrupulous evolution theory deniers from trying to link the two with cheap equivocation logical fallacies. This is where I have personally seen this question come from in most instances.

  10. Mark says:

    I read your answer to the above question, stating that:

    ” The Sun, and every other star, is radiating heat into the universe. But they can’t do it forever. Eventually the heat will have spread out so much that there won’t be warmer objects and cooler objects. Everything will be the same temperature. The same, very cold, temperature. The vast majority of the universe is already screaming cold, so the heat death of the universe is just about burning what fuel there is and mixing the heat so created into the ever-expansive, cold, and unyielding cosmos. Both the burning of fuel (mostly through fusion in stars) and the distribution of heat are processes which increase entropy. Once everything is the same temperature, the universe attains a “steady state”. With no energy differences, there’s no reason for anything to change what it’s doing (all forces can be expressed as an energy imbalance or “potential gradient“). Heat death is the point at which the universe has finally settled down completely (or almost completely), and nothing interesting ever happens again.”

    Okay. But once all heat has left the Universe, and nothing ever changes, then “Entropy” would seem (intuitively) to no longer exist. And even before that, as the Universe cools, shouldn’t Entropy become reduced to a minimum state at 0 (zero) Degrees Kelvin? And if so, wouldn’t that violate the Second Law of Thermodynamics? Or, is there any measure of entropy or disorder/order in the universe outside of thermodynamics?

    Please let me know. Thank you.

  11. James says:

    The universe won’t be at 0 K, but instead at “pretty much” 0 K, and all the energy in the universe will be uniformly distributed. No energy can leave the universe (kind of by definition of a universe) and so it must still exist somewhere, which is everywhere. Think of entropy as a measure of how much energy there is that cannot be used for useful work. All our processes require a gradient of some sort (e.g. a temperature gradient across two things so that one of them can heat up) and the heat death has no gradients, because everything is uniformly distributed, and so the maximum entropy (i.e. max amount of energy that cannot do work) has been reached. So entropy will be unchanging but at the maximum level.

  12. Tasha Lanier says:

    what do you do if things disappear like pictures from the bible other things energy through body at one point hearing voices allthe time I NEED HELP TO BREAK THIS CURSE ITS SCARY

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  15. Kane S. Latranz says:

    WE HAVE MET THE “HEAT DEATH” OF THE UNIVERSE IN PROGRESS, AND IT IS… US?

    I have had an observation for some time now about an aspect of the theoretical heat death which I have since turned into a short book. I queried a science question and answer service regarding a major aspect of the book for a little confirmation, and confirmation I received. “Since stars are very close to being isolated systems, the second law of thermodynamics states that their entropy can only grow with time. Therefore you are right that the production of helium and heavier elements (in their core) out of hydrogen, and the final explosion of massive stars as supernovae can be viewed as particular instances of the thermodynamic arrow of time.” Alexandre Le Tiec MCFP Postdoctoral Fellow in the Maryland Center for Fundamental Physics at University of Maryland, College Park, working in the field of theoretical astrophysics.

    Although it relates to disorder in certain respects, people often become confused about “entropy,” the second law of thermodynamics, as disorder. So, it is being redefined in many introductory textbooks for physics and chemistry as energy dispersal thanks to retired chemistry professor, Frank L. Lambert. “Energetically, the second law of thermodynamics favors the formation of the majority of all known complex and ordered chemical compounds from the simpler elements. Thus, contrary to popular opinion, the second law does not dictate the decrease of ordered structure in its predictions, it only demands a “spreading out” of energy in all processes.”–Frank L. Lambert, from the website: :2ndlaw.oxy.edu.

    This all comes as a confirmation of something I have wrestled with for some time as a non-scientist, although science does run in my family. Doing a lot of reading that sometimes left me scratching my head in regard to the traditional definition of the second law, (Such as work by Stephen Hawking.), I discerned many processes that would be defined as entropy that also happen to be necessary for evolution and life. Chief among these is the theoretical heat death of the universe some trillions of years from now which strikes me as a kind of nihilistic boogieman of science and the media, especially when no mention is made of the apparent fact that whatever else this boogieman may be, he seems to facilitate our existence. Are we the heat death in progress?

    EXCERPT “Organic cells form, reproduce, and die within complex organisms that are born, reproduce, and die, within species and civilizations that eventually segue into new species and civilizations or simply carry on until they become extinct. All because stars are born, age, die, and are replaced, some former and current stars facilitating habitable planets and life.”

    As the late Dr. Carl Sagan famously said “We are star stuff.” How might this relate to the theoretical heat death? All of these issues are addressed in the title essay in this short book, and there are several poems pondering aspects of science and/or spirituality, often from a Panentheist perspective, one of which appeared in Philosophy Now Magazine.

    “…Latranz is a burgeoning Albuquerque author with obvious skill at his craft.”~Local i.Q. newspaper.
    CreateSpace eStore: https://www.createspace.com/4324969

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  17. slick rick says:

    would heat death effect a black hole? Do we know the entropy of a black hole and if nothing can escape from an black hole does that mean heat itself? If thats the case is a black hole a perfect closed system and does this violate the laws of thermodynamics then?

  18. Philip Moseman says:

    Slick Rick. Thermodynamics claims that entropy can never decrease.
    Based on your assumptions, the entropy never changes.
    Entropy increases because a non-reversible process occurs.

    Take a balloon and the air inside as a closed system. It is the same temperature as the room. When the air is released from inside the balloon, the boundary of the closed system changes and the air heats up. This heat is dissipated.

    To put the air back into the balloon requires some external process. Pushing the air back into the balloon heats the air up, and this heat is dissipated.

    Now the room and the balloon are again equal in temperature, but the room is slightly warmer. If you think of the room as you would the universe, as an isolated system, then there is no thermodynamic process capable of decreasing the temperature.

  19. Philip Moseman says:

    I should also add, the black hole is not a closed system.

  20. Brian says:

    Can one consider the universe as being a closed system? It’s just that I have this inkling that the universe has no limits, never had, never will have, neither in time nor space. I have trouble considering entropy as having once been zero, even at a very distant point in the past. I mean, I guess we can equate entropy to zero at some point in space/time, but that only means that it was relatively negative before that, or somewhere else.

  21. Wil says:

    Life is a physical expression of entropy, in all closed systems entropy must increase according to probabilistic permutation. There are more ways for things to be mixed than sorted. It is the purpose of life that things should be mixed into more and more complex organizations until an asymptote is approached, when all things are mixed together that would be considered the one ultimate sorted unit, and as such – by probability, it will be approached by never reached.

    If consciousness is a byproduct of expressing this entropy and becoming these mixed units, then it may lead forward to higher and more complex manners of perceiving and experiencing until yet another asymptote is reached, that in which all manners of experiencing are simultaneously experienced, and as such – by probability, it will be approached but never reached.

    I would imagine approaching a ‘heat death’ scenario would be accompanied with non-corporeal experiences. Furthermore, as entropy increases toward infinity, the fact that the universe has less and less heat and work would be less interesting anyway.

    The author of Dilbert has a book called “God’s Debris”. Check it out.

  22. M Nauman Bin Zakir says:

    In a layman term what is entropy it is simply a measure of disorderness in any system. More the disorder more is the entropy.Lets look at the each and every thing in the universe you will find your answer why the net entropy of the universe always increases. Under thermodynamic subject , Entropy mainly depends on temperature and it is directly proportion to it.
    Main reason for increase in temperature is losses (may be frictional/process/mechanical etc). And Our daily life system is always accompanied with it.
    therefore this increase eventually affects gain in entropy.
    if some one control heat transfer or remove temperature gradient/Loss then we can control entropy but this is not a game of mankind. This is a game of Allah(God) Who is able to do whatever He wants.

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