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Theheat death of the universe (also known as theBig Chill orBig Freeze)[1][2] is ascientific hypothesis regarding theultimate fate of the universe which posits theuniverse will evolve to a state of nothermodynamic free energy and, having reached maximumentropy, will therefore be unable to sustain any furtherthermodynamic processes. The hypothesized heat death does not imply any particularabsolute temperature; it only requires that temperature differences or other processes may no longer be exploited to performwork. In the language ofphysics, this is when the universe reachesthermodynamic equilibrium.
If thecurvature of the universe ishyperbolic or flat, or ifdark energy is a positivecosmological constant, the universe will continue expanding forever, and a heat death is expected to occur,[3] with the universe cooling to approach equilibrium at a very low temperature after a long time period.
The theory of heat death stems from the ideas ofLord Kelvin who, in the 1850s, took thetheory of heat asmechanical energy loss in nature (as embodied in the first twolaws of thermodynamics) andextrapolated it to larger processes on a universal scale. This also allowed Kelvin to formulate theheat death paradox, which disproves an infinitely old universe.[4]
The idea of heat death stems from thesecond law of thermodynamics, of which one version states thatentropy tends to increase in anisolated system. From this, the hypothesis implies that if the universe lasts for a sufficient time, it willasymptotically approach a state where allenergy is evenly distributed. In other words, according to this hypothesis, there is a tendency in nature towards thedissipation (energy transformation) ofmechanical energy (motion) intothermal energy; hence, by extrapolation, there exists the view that, in time, the mechanical movement of the universe will run down as work is converted to heat because of the second law.
The conjecture that all bodies in the universe cool off, eventually becoming too cold to supportlife, seems to have been first put forward by the French astronomerJean Sylvain Bailly in 1777 in his writings on the history of astronomy and in the ensuing correspondence withVoltaire. In Bailly's view, all planets have aninternal heat and are now at some particular stage of cooling.Venus, for instance, is still too hot for life to arise there for thousands of years, whileMars is already too cold. The final state, in this view, is described as one of "equilibrium" in which all motion ceases.[5]
The idea of heat death as a consequence of the laws of thermodynamics, however, was first proposed in loose terms beginning in 1851 by Lord Kelvin (William Thomson), who theorized further on the mechanical energy loss views ofSadi Carnot (1824),James Joule (1843) andRudolf Clausius (1850). Thomson's views were then elaborated over the next decade byHermann von Helmholtz andWilliam Rankine.[6]
The idea of the heat death of the universe derives from discussion of the application of the first twolaws of thermodynamics to universal processes. Specifically, in 1851,Lord Kelvin outlined the view, as based on recent experiments on the dynamicaltheory of heat: "heat is not a substance, but a dynamical form of mechanical effect, we perceive that there must be an equivalence between mechanical work and heat, as between cause and effect."[7]
In 1852, Thomson publishedOn a Universal Tendency in Nature to the Dissipation of Mechanical Energy, in which he outlined the rudiments of the second law of thermodynamics summarized by the view that mechanical motion and the energy used to create that motion will naturally tend to dissipate or run down.[8] The ideas in this paper, in relation to their application to the age of theSun and the dynamics of the universal operation, attracted the likes of William Rankine and Hermann von Helmholtz. The three of them were said to have exchanged ideas on this subject.[6] In 1862, Thomson published "On the age of the Sun's heat", an article in which he reiterated his fundamental beliefs in the indestructibility of energy (thefirst law) and the universal dissipation of energy (the second law), leading to diffusion of heat, cessation of useful motion (work), and exhaustion ofpotential energy, "lost irrecoverably" through the material universe, while clarifying his view of the consequences for the universe as a whole. Thomson wrote:
The result would inevitably be a state of universal rest and death, if the universe were finite and left to obey existing laws. But it is impossible to conceive a limit to the extent of matter in the universe; and therefore science points rather to an endless progress, through an endless space, of action involving the transformation ofpotential energy intopalpable motion and hence intoheat, than to a single finite mechanism, running down like a clock, and stopping for ever.[4]
The clock's example shows how Kelvin was unsure whether the universe would eventually achievethermodynamic equilibrium. Thomson later speculated that restoring the dissipated energy in "vis viva" and then usable work – and therefore revert the clock's direction, resulting in a "rejuvenating universe" – would require "a creative act or an act possessing similar power".[9][10] Starting from this publication, Kelvin also introduced theheat death paradox (Kelvin's paradox), which challenged the classical concept of an infinitely old universe, since the universe has not achieved its thermodynamic equilibrium, thus further work andentropy production are still possible. The existence of stars and temperature differences can be considered an empirical proof that the universe is not infinitely old.[11][4]
In the years to follow both Thomson's 1852 and the 1862 papers,Helmholtz andRankine both credited Thomson with the idea, along with his paradox, but read further into his papers by publishing views stating that Thomson argued that the universe will end in a "heat death" (Helmholtz), which will be the "end of all physical phenomena" (Rankine).[6][12][unreliable source?]
Proposals about the final state of the universe depend on the assumptions made about its ultimate fate, and these assumptions have varied considerably over the late 20th century and early 21st century. In a theorized"open" or "flat" universe that continues expanding indefinitely, either a heat death or aBig Rip is expected to eventually occur.[3][13] If thecosmological constant is zero, the universe will approachabsolute zero temperature over avery long timescale. However, if the cosmological constant ispositive, the temperature will asymptote to a non-zero positive value, and the universe will approach a state of maximumentropy in which no furtherwork is possible.[14]
Thermodynamic heat death is a classical-physics concept, but has been extended to quantum physics. Even when thermodynamic entropy has reached its maximum, other quantities such as quantum circuitcomplexity continue to increase, eventually also reaching a maximum value.[15]
The theory suggests that from the "Big Bang" through the present day,matter anddark matter in the universe are thought to have been concentrated instars,galaxies, andgalaxy clusters, and are presumed to continue to do so well into the future. Therefore, the universe is not inthermodynamic equilibrium, and objects can do physical work.[16]:§VID The decay time for asupermassive black hole of roughly 1 galaxy mass (1011 solar masses) because of Hawking radiation is in the order of10100 years,[17] so entropy can be produced until at least that time. Some largeblack holes in the universe are predicted to continue to grow up to perhaps 1014M☉ during the collapse ofsuperclusters of galaxies. Even these would evaporate over a timescale of up to 10106 years.[18] After that time, the universe enters the so-calledDark Era and is expected to consist chiefly of a dilute gas ofphotons andleptons.[16]:§VIA With only very diffuse matter remaining, activity in the universe will have tailed off dramatically, with extremely low energy levels and extremely long timescales. Speculatively, it is possible that the universe may enter a secondinflationary epoch, or assuming that the currentvacuum state is afalse vacuum, the vacuum may decay into a lower-energy state.[16]:§VE It is also possible that entropy production will cease and the universe will reach heat death.[16]:§VID
It has been hypothesized that, over vast periods of time, a spontaneousentropydecrease could eventually occur via thePoincaré recurrence theorem,[19][additional citation(s) needed]thermal fluctuations,[20][21][22] andfluctuation theorem.[23][24] Through this, another Big Bang could possibly create a new universe similar to the current one byquantum fluctuations andquantum tunnelling in roughly years.[25]
Max Planck wrote that the phrase "entropy of the universe" has no meaning because it admits of no accurate definition.[26][27] In 2008, Walter Grandy wrote: "It is rather presumptuous to speak of the entropy of a universe about which we still understand so little, and we wonder how one might define thermodynamic entropy for a universe and its major constituents that have never been in equilibrium in their entire existence."[28] According toLászló Tisza, "If an isolated system is not in equilibrium, we cannot associate an entropy with it."[29]Hans Adolf Buchdahl writes of "the entirely unjustifiable assumption that the universe can be treated as a closed thermodynamic system".[30] According toGiovanni Gallavotti, "there is no universally accepted notion of entropy for systems out of equilibrium, even when in a stationary state".[31] Discussing the question of entropy for non-equilibrium states in general,Elliott H. Lieb andJakob Yngvason express their opinion as follows: "Despite the fact that most physicists believe in such a nonequilibrium entropy, it has so far proved impossible to define it in a clearly satisfactory way."[32] In Peter Landsberg's opinion: "Thethird misconception is that thermodynamics, and in particular, the concept of entropy, can without further enquiry be applied to the whole universe. ... These questions have a certain fascination, but the answers are speculations."[33]Julian Barbour said: “It’s because entropy does not apply to the universe. It’s just naïve extrapolation from what is perfectly true in a box. … Heat death. This has been a horrendous sort of nightmare for the universe. But it could be just a complete, fundamental mistake in thinking that what happens in a box is true of what happens in the whole universe.”[34]
A 2010 analysis of entropy states, "The entropy of a general gravitational field is still not known", and "gravitational entropy is difficult to quantify". The analysis considers several possible assumptions that would be needed for estimates and suggests that theobservable universe has more entropy than previously thought. This is because the analysis concludes that supermassive black holes are the largest contributor.[35]Lee Smolin goes further: "It has long been known that gravity is important for keeping the universe out of thermal equilibrium. Gravitationally bound systems have negative specific heat—that is, the velocities of their components increase when energy is removed. ... Such a system does not evolve toward a homogeneous equilibrium state. Instead it becomes increasingly structured and heterogeneous as it fragments into subsystems."[36] This point of view is also supported by the fact of a recent[when?] experimental discovery of a stable non-equilibrium steady state in a relatively simple closed system. It should be expected that an isolated system fragmented into subsystems does not necessarily come to thermodynamic equilibrium and remain in non-equilibrium steady state. Entropy will be transmitted from one subsystem to another, but its production will be zero, which does not contradict thesecond law of thermodynamics.[37][38]
The discovery of the principle of the heat death of the universe led to philosophical reevaluation of the place of man in the universe. In 1856,Hermann von Helmholtz suggest that it compelled humans to complete a moral destiny.Philipp Mainländer, another philosopher whose 1875 workDie Philosophie der Erlösung has been associated with the philosophy ofpandeism, saw the expected heat death of the Universe as "a different, redemptive destiny for humanity: an end of suffering".[39]
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In Isaac Asimov's 1956 short storyThe Last Question, humans through the ages repeatedly wonder how the heat death of the universe can be avoided.
In the 1981Doctor Who story "Logopolis", the Doctor realizes that the Logopolitans have created vents in the universe to expel heat build-up into other universes—"Charged Vacuum Emboitments" or "CVE"—to delay the demise of the universe. The Doctor unwittingly travelled through such a vent in "Full Circle".
In the 1995 computer gameI Have No Mouth, and I Must Scream, based onHarlan Ellison'sshort story of the same name, it is stated that AM, the malevolent supercomputer, will survive the heat death of the universe and continue torturing its immortal victims to eternity.
In the 2011 anime seriesPuella Magi Madoka Magica, the antagonistKyubey reveals he is a member of an alien race who has been creatingmagical girls for millennia in order to harvest their energy to combat entropy and stave off the heat death of the universe.
In the last act ofFinal Fantasy XIV: Endwalker, the player encounters an alien race known as the Ea who have lost all hope in the future and any desire to live further, all because they have learned of the eventual heat death of the universe and see everything else as pointless due to its probable inevitability.
The overarching plot of theXeelee Sequence concerns the Photino Birds' efforts to accelerate the heat death of the universe by accelerating the rate at which stars become white dwarves.
The 2019 hit indie video gameOuter Wilds has several themes grappling with the idea of the heat death of the universe, and the theory that the universe is a cycle of big bangs once the previous one has experienced a heat death.
In "Singularity Immemorial",[40] the seventh main story event of the mobile gameGirls' Frontline: Neural Cloud, the plot is about a virtual sector made to simulate space exploration and the threat of the heat death of the universe. The simulation uses an imitation of Neural Cloud's virus entities known as the Entropics as a stand in for the effects of a heat death.
Since we have assumed a maximum scale of gravitational binding—for instance, superclusters of galaxies—black hole formation eventually comes to an end in our model, with masses of for up to 1014M☉ ... the timescale for black holes to radiate away all their energy ranges ... to 10106 years for black holes of up to 1014M☉
The importance of Planck's Vorlesungen über Thermodynamik (Planck 1897) can hardly be [over]estimated. The book has gone through 11 editions, from 1897 until 1964, and still remains the most authoritative exposition of classical thermodynamics.
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