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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50 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:


    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.

  23. mr edwards says:

    Professor Hawkin said that matter can appear out of nothing for very short periods of time and then disappear (inside an atom). Also, a universe can appear from nothing due to quantum fluctuations. I understand how matter can pop out of nothing within the universe as it’s within the universe which is full of energy where space and time exist. How can quantum fluctuations exist in order that a universe can ‘pop’ into existence if time and space, never mind the laws of physics, weren’t in existence?

  24. Fizzah Khan says:

    How is it possible to decrease the entropy of the universe ?

  25. The Physicist The Physicist says:

    @Fizzah Khan
    There’s no way, even in theory, to decrease the total entropy of the universe.

  26. KeMystery says:

    So… if the heat builds up..then it will eventually “catch up with” the coldness of the universe…warming it until their temperatures are “perfect”ly even and identical…. so if the universe heating up via stars is what will eventually bring the heat death of the universe… then… if we were, theoretically, a huge alien and could “snuff out” the trillions of stars and just leave one star left floating in the universe….would that slow the heating process, thus the “aging” process/heat death of the Universe down to a tremendously slower rate? (And thus keep human life around longer?-assuming we could do this, and assuming we’d still be the life form that needed heat to survive)

  27. KeMystery says:

    And/or could we, assuming we were one enormous ass alien, add “icicles” where star used to lie and expect temperatures to continuously decrease? The balloon example above has me thinking on these considerations…

  28. KeMystery says:

    In other words…. if there were “one star left” in the universe…. and you added the “heat star”, then added two “icicle” planets to freeze up the universe, to maintain its imbalance so that the heat may still be at use and work… then when its heat “catches up” with the cold temperatures that the icicles caused… (which, could one bonfire ever truly heat up all of Alaska eventually, or would Alaska remain cold forever if the bonfires heat could never catch up? or could it eventually heat up all of Alaska?)… then keep adding icicles as necessary… again, this is all theoretical…”if we could” obviously lol So then let’s suppose the heat of the one star eventually “caught up” with the icicles… (in other words, let’s say a bonfire eventually could heat up Alaska…).. then what if our big alien hand snuffed out the last star… and allowed the icicle planets to dominate the universe and let it reach a phenomenal freezing point once again… before it plopped its one star back in the universe…. could this be a process that would defy the heat death? (in other words, we snuff out the bonfire that has finally uniformly heated up Alaska…and so it refreezes itself as if it were reset and had to start all over in its heating process… does this defy the heat death? Would love to hear comments on this 😉 🙂

  29. KeMystery says:

    Also, Kane S. Latranz, I would just like to add that I had this incredible epiphany of the exact same thing.. only MINUTES before I read what you wrote!!!!! I found that so amazing and reassuring that someone else was on the exact same page and line of thought that I was…. wow. =D =o My curiosities and theories entwine likewise with yours 😉 🙂

  30. A Person says:

    What about quantum mechanics, complete balance shouldn’t be possible with constant fluctuations in quantum fields

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  33. Marcel says:

    I was thinking about that right now. QM is a statistical process at the (sub)particle level, and therefore physics at atomic level is chaotic in the mathematical sense. That means tiny changes at the outset create enormous changes in the outcome.
    Entropy in the heat death sense requires not only that all heat has been uniformly distributed, but that all energy has. All possible fusion and fission has taken place, all particles and atoms have equal potential energy (gravity and electromagnetism), etc. even if that has happened, one single quantum event would restart all of it.
    E.g. Think of a billiards table with hundreds of balls on it, all of them rolling and bouncing against each other. That would come to rest at some point because of in elasticity of balls and cushions. That represents entropy (since we can’t envision all balls moving with equal energy.) QM would be represented by someone occasionally giving a ball a whack (meaning a change of energy state of that ball/atom/particle.)
    I would suggest that at some point QM would be as strong an influence as the distribution process of thermodynamics.

  34. Jason Young says:

    Its been 30+ years since I did physics at school, and while I try to keep up, I know I’m a bit behind. I do however think about this stuff alot. I have been able to reverse engineer a piece of special relativity for myself, but it has left me with a conclusion that I am not sure is true or false. Any help would be greatly appreciated.
    It starts with relativity, with every aspect of the universe expanding away from every other. Thus we have the ‘observable universe’, which is our visable globe within the full universe. No matter where you are, you have your ‘visable universe’. This means far away parts of the universe can appear to expand away faster than light, while from their perspective, they are sitting still, and WE are expanding away from them faster than light. Okay so far?
    (I reverse engineered this from a childhood toy, an extending arm, usually has a boxing glove or a grasping claw on the end, the extending arm represents one of the universes expanding dimensions).
    This is where I go into territory that I’m not sure is true or false.
    1) The universe CAN be infinite, our observable universe is just a sphere within that.
    2) If that is the case, the universe is not ‘truly’ expanding…
    By which I mean, the heat of the universe is being converted into dimensions, in other words, as the universe cools, spaces is created between the spaces. Thus we have the illusion of the matter of the universe flying though space, whilst actually, extra space is created between the distances, thus extending the distances.

    I have no idea if this is true or false, but it seams to make sense, whilst also answering the bothersome question “what is the universe expanding into” (it isnt).

  35. Jason Young says:

    Now that I see this written down, I realise my mistake, since the heat-energy (for want of a better term) of the universe is, instead, being locked into ever more solid forms, ie, matter, thus the cooling of the universe.
    Still, something is still niggling the back of my mind, that the head death of the universe and the expansion of space are connected.

  36. Mark Janssen says:

    There is no question that entropy can *decrease*. When you took that photograph of the rolled dice, you removed entropy from the universe by adding a dimension of memory. And when you looked at it, it changed the balance in the equation between information and disorder, forming a unit of mass in your neural net. That’s the little butterfly in the chaos that provided you this momenta to write on the topic, isn’t it?

  37. Bripat says:

    I didn’t know that memory was a dimension. Wow!

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  39. Adrian Loza says:

    Okay questions.
    If there is a heat death what would happen:
    Would the universe be so hot that it would cause elements to fission and fusion together until all work has been exhausted and reach a final temperature?
    Or would the universe be extremely cold because the heat has expanded so much?
    Also entropy has no relativistic affect on temperature so increasing the entropy in the universe wont affect of what we perceive as 0 kelvin?

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  41. Posh says:

    The Beginning of Time (Big Bang, whatever) —> The Arrow of Time —> Heat Dissipation —-> Fusion and Heavy Element Creation (Including C and O2)—> Biological Life —> Deaths and Births of Stars —-> Near-abouts Zero degree Kelvin —> Physicists considering Ending of Time…But laymen logic says forget it —> Nudge (similar to one that began time) —-> Temperature Heterogeneities —> Re-beginning of Time.

    If something began once, it can again and again. Believing any particular theory is fatalistic (Even if physicists say it). Don’t believe it anyone. Question it. Even though we may want to go to sleep.

  42. Bripat says:

    Am I wrong to assume that there is no beginning and there is no end but, unfortunately, our poor brains are incapable of assimilating this?

  43. ibz says:

    As the sun and stars are losing or emitting the heat , the same heat might be the reason for creating or activating some other star nd this way there would be an everlasting universe with a constant entropy..

  44. Andy says:

    When the universe does reach this state of maximum entropy might it be able to reset itself? Entropy only increases because it is overwhelming likely to do so, it is not guaranteed. So picture all these particles bouncing around randomly in a vast and empty universe for an infinite amount of time. Is there not an infinitesimally small chance that these random bounces might form a person, a star or perhaps an entire universe?

    It is said that a monkey typing random keys on a typewriter for an infinite amount of time will eventually produce the entire works of Shakespeare. This idea is even more unlikely but sort of in the same vein.

  45. Shikhar says:

    The limitation with the exact science is that it does not give due consideration to the invisible existence of which the visible universe is just a fraction. And that’s why all the scientific researches and theories end up modifying themselves. There is more to truth as a fact than mere theories based on assumptions. If you are interested read the all time famous work by H. P. B know as The Secret Doctrine.

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  47. David says:

    Lovely artcile. Thank you. I always thought entropy meant things breaking down into simpler constituent parts, or perhaps lower energy forms after going through some transformation that gives off energy.
    So this was a revelation. I still don’t fully understand what all this means. But hopefully I know a bit more now.

  48. Logan Ohnoutka says:

    Two thermo courses and mutiple Chem classes, have given me the bare concept of entropy and how it is affected by chemical reactions and physical processes in formulas. I still never Really fully grasped what Entropy IS in regards to the big picture! This explanation came 4 years to late, and I have never had a light bulb burn so bright in my head after I finished this article. Whomever wrote his, thank you for enlightening me. I feel 100x better now!

  49. Bob Jenkins says:

    A closed system of fixed volume and energy has a maximum possible entropy, and increasing entropy causes the system to approach that limit over time. However the universe is not of fixed volume, is has been expanding and is expected to keep expand forever, at an ever increasing rate. As the volume of the universe increases, the maximum possible entropy increases too (with the log of the volume). So yes the entropy of the universe keeps increasing, but we’re never going to run out of the ability to increase entropy in the universe, because the goalpost keeps moving.

    Now, how to turn that into useful work as the temperature approaches absolute zero, I don’t know.

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