View source for Philosophy:Boltzmann brain

{{Short description|Philosophical thought experiment}}
[[File:Boltzmann2.jpg|thumb|Ludwig Boltzmann, after whom Boltzmann brains are named]]
The '''Boltzmann brain''' [[Thought experiment|thought experiment]] suggests that it might be more likely for a single brain to spontaneously form in a void, complete with a memory of having existed in our universe, rather than for the entire universe to come about in the manner cosmologists think it actually did. Physicists use the Boltzmann brain thought experiment as a ''[[Philosophy:Reductio ad absurdum|reductio ad absurdum]]'' argument for evaluating competing scientific theories.

In contrast to [[Philosophy:Brain in a vat|brain in a vat]] thought experiments which are about perception and thought, Boltzmann brains are used in [[Astronomy:Cosmology|cosmology]] to test our assumptions about [[Physics:Thermodynamics|thermodynamics]] and the development of the universe. Over a sufficiently long time, random fluctuations could cause [[Physics:Particle|particles]] to spontaneously form literally any structure of any degree of complexity, including a functioning human brain. The scenario initially involved only a single brain with false memories, but physicist [[Biography:Sean M. Carroll|Sean M. Carroll]] pointed out that, in a fluctuating universe, the scenario works just as well with entire bodies, even entire galaxies.<ref>{{cite podcast |url=https://www.preposterousuniverse.com/podcast/2019/06/17/episode-51-anthony-aguirre-on-cosmology-zen-entropy-and-information/ |title=Sean Carroll's Mindscape |website=preposterousuniverse.com |publisher=Sean Carroll |host=[[Biography:Sean M. Carroll|Sean Carroll]] |date=17 June 2019 |time=1:01.47 |access-date=2 March 2019}}</ref><ref name= MS200>{{cite podcast |url=https://www.preposterousuniverse.com/podcast/2022/06/06/200-solo-the-philosophy-of-the-multiverse/ |title=Sean Carroll's Mindscape |website=preposterousuniverse.com |publisher=Sean Carroll |host=[[Biography:Sean M. Carroll|Sean Carroll]] |date=6 June 2022 |time=1:47:20|access-date=7 June 2022}}</ref>

The idea is named after the physicist [[Biography:Ludwig Boltzmann|Ludwig Boltzmann]] (1844–1906), who, in 1896, published a theory that tried to account for the fact that the universe is not as chaotic as the budding field of [[Physics:Thermodynamics|thermodynamics]] seemed to predict. He offered several explanations, one of them being that the universe, even after it had progressed to its most likely spread-out and featureless state of [[Physics:Thermal equilibrium|thermal equilibrium]], would spontaneously fluctuate to a more ordered (or low-[[Physics:Entropy|entropy]]) state such as the universe in which we find ourselves. Boltzmann brains were first proposed as a ''[[Philosophy:Reductio ad absurdum|reductio ad absurdum]]'' response to this explanation by Boltzmann for the low-entropy state of our universe.<ref>{{cite web|last1=Carroll|first1=Sean|date=29 December 2008|title=Richard Feynman on Boltzmann Brains|url=http://www.preposterousuniverse.com/blog/2008/12/29/richard-feynman-on-boltzmann-brains/|access-date=24 June 2019}}</ref> 

The Boltzmann brain gained new relevance around 2002, when some cosmologists started to become concerned that, in many theories about the universe, human brains are vastly more likely to arise from random fluctuations; this leads to the conclusion that, statistically, humans are likely to be wrong about their memories of the past and in fact be Boltzmann brains.<ref>{{cite journal |last1=Dyson |first1=Lisa |last2=Kleban |first2=Matthew |last3=Susskind |first3=Leonard |title=Disturbing Implications of a Cosmological Constant |journal=Journal of High Energy Physics |date=2002 |volume=2002 |issue=10 |page=011 |doi=10.1088/1126-6708/2002/10/011 |arxiv=hep-th/0208013 |bibcode=2002JHEP...10..011D |s2cid=2344440 |url=http://jhep.sissa.it/archive/papers/jhep102002011/jhep102002011.pdf |access-date=28 April 2022}}</ref><ref>{{cite news |last1=Overbye |first1=Dennis |title=Big Brain Theory: Have Cosmologists Lost Theirs? |url=https://www.nytimes.com/2008/01/15/science/15brain.html?smid=url-share |access-date=28 April 2022 |work=New York Times |date=15 January 2008}}</ref> When applied to more recent theories about the [[Physics:Multiverse|multiverse]], Boltzmann brain arguments are part of the unsolved [[Astronomy:Measure problem (cosmology)|measure problem of cosmology]].

== "Boltzmann universe" ==
In 1896, the mathematician [[Biography:Ernst Zermelo|Ernst Zermelo]] advanced a theory that the [[Physics:Second law of thermodynamics|second law of thermodynamics]] was absolute rather than statistical.<ref>Brush, S. G., ''Nebulous Earth: A History of Modern Planetary Physics'' (Cambridge: {{wipe|Cambridge University Press}}, 1996), [https://books.google.com/books?id=vTBZyesS08kC&pg=PA129 p. 129].</ref> Zermelo bolstered his theory by pointing out that the [[Poincaré recurrence theorem]] shows statistical entropy in a closed system must eventually be a [[Periodic function|periodic function]]; therefore, the Second Law, which is always observed to increase entropy, is unlikely to be statistical. To counter Zermelo's argument, Boltzmann advanced two theories. The first theory, now believed to be the correct one, is that the universe started for some unknown reason in a low-entropy state. The second and alternative theory, published in 1896 but attributed in 1895 to Boltzmann's assistant Ignaz Schütz, is the "Boltzmann universe" scenario. In this scenario, the universe spends the vast majority of eternity in a featureless state of [[Astronomy:Heat death of the universe|heat death]]; however, over enough eons, eventually a very rare thermal fluctuation will occur where atoms bounce off each other in exactly such a way as to form a substructure equivalent to our entire observable universe. Boltzmann argues that, while most of the universe is featureless, humans do not see those regions because they are devoid of intelligent life; to Boltzmann, it is unremarkable that humanity views solely the interior of its Boltzmann universe, as that is the only place where intelligent life lives. (This may be the first use in modern science of the [[Astronomy:Anthropic principle|anthropic principle]]).<ref name="carroll2017">[[Biography:Sean M. Carroll|Carroll, S. M.]],  [https://arxiv.org/abs/1702.00850 "Why Boltzmann brains are bad"] (Ithaca, New York: [[ArXiv|arXiv]], 2017).</ref><ref>{{cite book|last1=Bostrom|first1=Nick| title=Anthropic Bias: Observation Selection Effects in Science and Philosophy|date=2002|publisher=Psychology Press|isbn=9780415938587|language=en|chapter=Introduction}}</ref>

In 1931, astronomer [[Biography:Arthur Eddington|Arthur Eddington]] pointed out that, because a large fluctuation is exponentially less probable than a small fluctuation, observers in Boltzmann universes will be vastly outnumbered by observers in smaller fluctuations. Physicist [[Biography:Richard Feynman|Richard Feynman]] published a similar counterargument within his widely read ''[[Physics:The Feynman Lectures on Physics|Feynman Lectures on Physics]]''.<ref>{{cite book |last1=Feynman |first1=Richard P. |title=The Feynman lectures on physics |date=1963–1965 |publisher=Addison-Wesley Pub. Co |location=Reading, Mass. |url=https://www.feynmanlectures.caltech.edu/I_46.html#Ch46-S5 |access-date=2 October 2022 |chapter=Order and entropy}}</ref> By 2004, physicists had pushed Eddington's observation to its logical conclusion: the most numerous observers in an eternity of thermal fluctuations would be minimal "Boltzmann brains" popping up in an otherwise featureless universe.<ref name="carroll2017"/><ref name="Albrecht & Sorbo">{{cite journal |last1=Albrecht |first1=Andreas |last2=Sorbo |first2=Lorenzo |date=September 2004 |title=Can the universe afford inflation? |url=http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:hep-th/0405270 |journal=[[Physics:Physical Review|Physical Review D]] |volume=70 |issue=6 |page=063528 |bibcode=2004PhRvD..70f3528A |doi=10.1103/PhysRevD.70.063528 |access-date=16 December 2014 |arxiv=hep-th/0405270|s2cid=119465499 }}</ref>

== Spontaneous formation ==
In the universe's eventual state of [[Ergodicity|ergodic]] "heat death", given enough time, every possible structure (including every possible brain) will presumably get formed via random fluctuation, the timescale of which is related to the Poincaré recurrence time.<ref name="carroll2017"/><ref name=nbc/><ref name=linde/> A Boltzmann brain (or body or world) need not fluctuate suddenly into existence, argue Anthony Aguirre, Sean M. Carroll, and Matthew C. Johnson. Rather, it would form in a sequence of smaller fluctuations that would look like the brain's decay path run in reverse.<ref>{{cite podcast |url=https://www.preposterousuniverse.com/podcast/2023/07/03/ama-july-2023/ |title=Sean Carroll's Mindscape |website=preposterousuniverse.com |publisher=Sean Carroll |host=[[Biography:Sean M. Carroll|Sean Carroll]] |date=3 July 2023 |time=47:36 |access-date=19 July 2023}}</ref><ref>{{Cite journal |last1=Aguirre |first1=Anthony |last2=Carroll |first2=Sean M. |last3=Johnson |first3=Matthew C. |date=2012-02-17 |title=Out of equilibrium: understanding cosmological evolution to lower-entropy states |journal=Journal of Cosmology and Astroparticle Physics |volume=2012 |issue=2 |pages=024 |doi=10.1088/1475-7516/2012/02/024 |arxiv=1108.0417 |bibcode=2012JCAP...02..024A |s2cid=5156103 |issn=1475-7516}}</ref> Boltzmann-style thought experiments generally focus on structures like human brains that are presumably self-aware observers. However, smaller structures that minimally meet the criteria are vastly and exponentially more common than larger structures; a rough analogy is how the odds of a single real English word showing up when one shakes a box of ''[[Engineering:Scrabble|Scrabble]]'' letters are greater than the odds that a whole English sentence or paragraph will form.<ref name=nytimes/> The average timescale required for the formation of a Boltzmann brain is vastly greater than the current age of the universe. In modern physics, Boltzmann brains can be formed either by [[Physics:Quantum fluctuation|quantum fluctuation]], or by a [[Physics:Thermal fluctuations|thermal fluctuation]] generally involving [[Physics:Nucleation|nucleation]].<ref name="carroll2017"/>

=== Via quantum fluctuation ===
By one calculation, a Boltzmann brain would appear as a quantum fluctuation in the vacuum after a time interval of <math>10^{10^{50}}</math> years. This fluctuation can occur even in a true [[Physics:Minkowski space|Minkowski vacuum]] (a flat spacetime vacuum lacking [[Physics:Vacuum energy|vacuum energy]]). Quantum mechanics heavily favors smaller fluctuations that "borrow" the least amount of energy from the vacuum. Typically, a quantum Boltzmann brain would suddenly appear from the vacuum (alongside an equivalent amount of virtual antimatter), remain only long enough to have a single coherent thought or observation, and then disappear into the vacuum as suddenly as it appeared. Such a brain is completely self-contained, and can never radiate energy out to infinity.<ref name=davenport>{{cite arXiv|author1=Matthew Davenport |author2=Ken D. Olum|title=Are there Boltzmann brains in the vacuum? |year=2010|class=hep-th|eprint=1008.0808}}</ref>

=== Via nucleation ===
Current evidence suggests that the vacuum permeating the observable universe is not a [[Physics:Minkowski space|Minkowski space]], but rather a [[De Sitter space|de Sitter space]] with a positive [[Astronomy:Cosmological constant|cosmological constant]].<ref>[[Biography:Viatcheslav Mukhanov|Mukhanov, V.]], ''Physical Foundations of Cosmology'' (Cambridge: Cambridge University Press, 2005), [https://books.google.com/books?id=OqIgAwAAQBAJ&pg=PA30 p. 30].</ref>{{rp|30}} In a de Sitter vacuum (but not in a Minkowski vacuum), a Boltzmann brain can form via nucleation of non-virtual particles gradually assembled by chance from the [[Physics:Hawking radiation|Hawking radiation]] emitted from the de Sitter space's bounded [[Astronomy:Cosmological horizon|cosmological horizon]]. One estimate for the average time required until nucleation is around <math>10^{10^{69}}</math> years.<ref name="davenport"/> A typical nucleated Boltzmann brain will cool off to absolute zero and eventually completely decay, as any isolated object would in the vacuum of space. Unlike the quantum fluctuation case, the Boltzmann brain will radiate energy out to infinity. In nucleation, the most common fluctuations are as close to thermal equilibrium overall as possible given whatever arbitrary criteria are provided for labeling a fluctuation a "Boltzmann brain".<ref name="carroll2017"/>

Theoretically a Boltzmann brain can also form, albeit again with a tiny probability, at any time during the matter-dominated early universe.<ref name="bousso">Bousso, R., Freivogel, B., & Yang, I. S. (2008). Boltzmann babies in the proper time measure. ''Physical Review D'', 77(10), 103514.</ref>

==Modern reactions to the Boltzmann brain problem==
The consensus amongst cosmologists is that some yet to be revealed error is hinted at by the surprising calculation that Boltzmann brains should vastly outnumber normal human brains.<ref name="carroll2017"/> [[Biography:Sean M. Carroll|Sean Carroll]] states "We're not arguing that Boltzmann Brains exist—we're trying to avoid them."<ref name=nbc>{{cite news |last1=Choi |first1=Charles Q. |title=Doomsday and disembodied brains? Tiny particle rules universe's fate |url=https://www.nbcnews.com/sciencemain/doomsday-disembodied-brains-tiny-particle-rules-universes-fate-8C11144331 |access-date=25 March 2020 |work=NBC News |date=13 September 2013 |language=en}}</ref> Carroll has stated that the hypothesis of being a Boltzmann brain results in "cognitive instability". Because, he argues, it would take longer than the current age of the universe for a brain to form, and yet it thinks that it observes that it exists in a younger universe, this shows that memories and reasoning processes would be untrustworthy if it were indeed a Boltzmann brain.<ref>{{cite news |last1=Ananthaswamy |first1=Anil |title=Universes that spawn 'cosmic brains' should go on the scrapheap |url=https://www.newscientist.com/article/mg23331133-200-universes-that-spawn-cosmic-brains-should-go-on-the-scrapheap/ |access-date=25 March 2020 |work=New Scientist |date=2017|quote=That discrepancy means that if we truly are Boltzmann brains in an old universe, then our perceptions are befuddled, too. "We'd have no reason to believe that our memories of the past are accurate," says Carroll.}}</ref> [[Biography:Seth Lloyd|Seth Lloyd]] has stated, "They fail the Monty Python test: Stop that! That's too silly!" A ''New Scientist'' journalist summarizes that "The starting point for our understanding of the universe and its behavior is that humans, not disembodied brains, are typical observers."<ref>{{cite news |last1=Grossman |first1=Lisa |title=Quantum twist could kill off the multiverse |url=https://www.newscientist.com/article/mg22229692-600-quantum-twist-could-kill-off-the-multiverse/ |access-date=25 March 2020 |work=New Scientist |date=2014}}</ref>

Some argue that brains produced via quantum fluctuation, and maybe even brains produced via nucleation in the [[De Sitter space|de Sitter vacuum]], do not count as observers. Quantum fluctuations are easier to exclude than nucleated brains, as quantum fluctuations can more easily be targeted by straightforward criteria (such as their lack of interaction with the environment at infinity).<ref name="carroll2017"/><ref name="davenport"/>

Carroll believes that a better understanding of the [[Physics:Measurement problem|measurement problem]] in quantum mechanics would show that some vacuum states have no dynamical evolution and cannot support nucleated brains, nor any other type of observer.<ref>Boddy, Kimberly K., Sean Carroll, and Jason S. Pollack. "Why Boltzmann brains do not fluctuate into existence in the de Sitter vacuum." The philosophy of cosmology (2017): 288-240.</ref> Some cosmologists believe that a better understanding of the degrees of freedom in the [[Physics:Quantum vacuum state|quantum vacuum]] of [[Physics:Holographic principle|holographic string theory]] can solve the Boltzmann brain problem.<ref>Garriga, J., & Vilenkin, A. (2009). Holographic multiverse. ''[[Physics:Journal of Cosmology and Astroparticle Physics|Journal of Cosmology and Astroparticle Physics]]'', 2009(01), 021.</ref>

[[Biography:Brian Greene|Brian Greene]] states: "I am confident that I am not a Boltzmann brain. However, we want our theories to similarly concur that we are not Boltzmann brains, but so far it has proved surprisingly difficult for them to do so."<ref name="guardian">{{cite news|last1=Sample|first1=Ian|date=8 February 2020|title=Physicist Brian Greene: 'Factual information is not the right yardstick for religion'|work=The Observer|url=https://www.theguardian.com/science/2020/feb/06/brian-greene-theoretical-physicist-interview-until-the-end-of-time|access-date=25 March 2020}}</ref>

=== In single-universe scenarios ===
In a single [[Astronomy:De Sitter universe|de Sitter universe]] with a cosmological constant, and starting from any finite spatial slice, the number of "normal" observers is finite and bounded by the heat death of the universe. If the universe lasts forever, the number of nucleated Boltzmann brains is, in most models, infinite; cosmologists such as [[Biography:Alan Guth|Alan Guth]] worry that this would make it seem "infinitely unlikely for us to be normal brains".<ref name=nytimes>{{cite news |last1=Overbye |first1=Dennis |title=Big Brain Theory: Have Cosmologists Lost Theirs? |url=https://www.nytimes.com/2008/01/15/science/15brain.html?pagewanted=all |access-date=26 February 2018 |work={{wipe|The New York Times}} |date=2008}}</ref> One caveat is that if the universe is a [[Physics:False vacuum decay|false vacuum]] that locally decays into a Minkowski or a [[Astronomy:Big Crunch|Big Crunch]]-bound [[Anti-de Sitter space|anti-de Sitter space]] in less than 20 billion years, then infinite Boltzmann nucleation is avoided. (If the average local false vacuum decay rate is over 20 billion years, Boltzmann brain nucleation is still infinite, as the universe increases in size faster than local vacuum collapses destroy the portions of the universe within the collapses' future [[Astronomy:Light cone|light cones]]). Proposed hypothetical mechanisms to destroy the universe within that timeframe range from superheavy [[Physics:Gravitino|gravitinos]] to a heavier-than-observed top quark triggering "[[Physics:Higgs boson#Nature of the universe, and its possible fates|death by Higgs]]".<ref>{{cite news |title=Death by Higgs rids cosmos of space brain threat |url=https://www.newscientist.com/article/mg21929333-200-death-by-higgs-rids-cosmos-of-space-brain-threat/ |access-date=26 February 2018 |work=New Scientist |date=15 February 2017}}</ref><ref>Boddy, K. K., & Carroll, S. M. (2013). Can the Higgs Boson Save Us From the Menace of the Boltzmann Brains?. arXiv preprint arXiv:1308.4686.</ref><ref name=linde>Linde, A. (2007). Sinks in the landscape, Boltzmann brains and the cosmological constant problem. ''Journal of Cosmology and Astroparticle Physics'', 2007(01), 022.</ref>

If no cosmological constant exists, and if the presently observed [[Physics:Vacuum energy|vacuum energy]] is from [[Physics:Quintessence|quintessence]] that will eventually completely dissipate, then infinite Boltzmann nucleation is also avoided.<ref>Carlip, S. (2007). Transient observers and variable constants or repelling the invasion of the Boltzmann's brains. ''Journal of Cosmology and Astroparticle Physics'', 2007(06), 001.</ref>

=== In eternal inflation ===
{{Further|Astronomy:Measure problem (cosmology)}}

One class of solutions to the Boltzmann brain problem makes use of differing approaches to the measure problem in cosmology: in [[Infinity|infinite]] [[Physics:Multiverse|multiverse]] theories, the ratio of normal observers to Boltzmann brains depends on how infinite limits are taken. Measures might be chosen to avoid appreciable fractions of Boltzmann brains.<ref name=DeSimoneEtAl2010>{{cite journal |author1=Andrea De Simone |author2=Alan H. Guth |author3=Andrei Linde |author4=Mahdiyar Noorbala |author5=Michael P. Salem |author6=Alexander Vilenkin |title=Boltzmann brains and the scale-factor cutoff measure of the multiverse |journal=Physical Review D |date=14 September 2010 |doi=10.1103/PhysRevD.82.063520 |arxiv=0808.3778 |bibcode=2010PhRvD..82f3520D |volume=82 |issue=6 |page=063520|s2cid=17348306 }}</ref><ref name=LindeVanchurinWinitzki2009>{{cite journal |author1=Andrei Linde |author2=Vitaly Vanchurin |author3=Sergei Winitzki |title=Stationary Measure in the Multiverse |journal=Journal of Cosmology and Astroparticle Physics |date=15 January 2009 |issue=1 |doi=10.1088/1475-7516/2009/01/031 |arxiv=0812.0005 |bibcode=2009JCAP...01..031L |volume=2009 |page=031|s2cid=119269055 }}</ref><ref name=LindeNoorbala2010>{{cite journal |author1=Andrei Linde |author2=Mahdiyar Noorbala |title=Measure problem for eternal and non-eternal inflation |journal=Journal of Cosmology and Astroparticle Physics |date=9 September 2010 |issue=9 |doi=10.1088/1475-7516/2010/09/008 |arxiv=1006.2170 |bibcode=2010JCAP...09..008L |volume=2010 |page=008|s2cid=119226491 }}</ref> Unlike the single-universe case, one challenge in finding a global solution in eternal inflation is that all possible string landscapes must be summed over; in some measures, having even a small fraction of universes permeated with Boltzmann brains causes the measure of the multiverse as a whole to be dominated by Boltzmann brains.<ref name=linde/><ref>{{cite journal |last1=Bousso |first1=R. |last2=Freivogel |first2=B. |year=2007 |title=A paradox in the global description of the multiverse |journal=[[Physics:Journal of High Energy Physics|Journal of High Energy Physics]] |volume=2007 |issue=6 |page=018 |doi=10.1088/1126-6708/2007/06/018|arxiv=hep-th/0610132 |bibcode=2007JHEP...06..018B |s2cid=14462665 }}</ref>

The measurement problem in cosmology also grapples with the ratio of normal observers to abnormally early observers. In measures such as the [[Proper time|proper time]] measure that suffer from an extreme "youngness" problem, the typical observer is a "Boltzmann baby" formed by rare fluctuation in an extremely hot, early universe.<ref name="bousso"/>

==Identifying whether oneself is a Boltzmann observer==
In Boltzmann brain scenarios, the ratio of Boltzmann brains to "normal observers" is astronomically large. Almost any relevant subset of Boltzmann brains, such as "brains embedded within functioning bodies", "observers who believe they are perceiving 3 K microwave background radiation through telescopes", "observers who have a memory of coherent experiences", or "observers who have the same series of experiences as me", also vastly outnumber "normal observers". Therefore, under most models of consciousness, it is unclear that one can reliably conclude that oneself is not such a "Boltzmann observer", in a case where Boltzmann brains dominate the universe.<ref name="carroll2017"/> Even under "[[Philosophy:Anti-individualism|content externalism]]" models of consciousness, Boltzmann observers living in a consistent Earth-sized fluctuation over the course of the past several years outnumber the "normal observers" spawned before a universe's "heat death".<ref>{{cite journal |last1=Schwitzgebel |first1=Eric|title=1% Skepticism |journal=Noûs |date=June 2017 |volume=51 |issue=2 |pages=271–290 |doi=10.1111/nous.12129}}</ref>

As stated earlier, most Boltzmann brains have "abnormal" experiences; Feynman has pointed out that, if one knows oneself to be a typical Boltzmann brain, one does not expect "normal" observations to continue in the future.<ref name="carroll2017"/> In other words, in a Boltzmann-dominated universe, most Boltzmann brains have "abnormal" experiences, but most observers with only "normal" experiences are Boltzmann brains, due to the overwhelming vastness of the population of Boltzmann brains in such a universe.<ref>{{cite journal |last1=Dogramaci |first1=Sinan |title=Does my total evidence support that I'm a Boltzmann Brain? |journal=Philosophical Studies |date=19 December 2019 |volume=177 |issue=12 |pages=3717–3723 |doi=10.1007/s11098-019-01404-y|s2cid=214109116 |url=https://philpapers.org/rec/DOGDMT-2 }}</ref>

== See also ==

* {{Annotated link|Anthropic principle}}
* {{Annotated link|Bekenstein bound}}
* {{Annotated link|China brain}}
* {{Annotated link|Dream argument}}
* {{Annotated link|Evolution of biological complexity}}
* {{Annotated link|Heat death of the universe}}
* {{Annotated link|Heat death paradox}}
* {{Annotated link|Infinite monkey theorem}}
* {{Annotated link|Matrioshka brain}}
* {{Annotated link|Observer (physics)}}
* {{Annotated link|Omphalos hypothesis}}
* {{Annotated link|Poincaré recurrence theorem}}
* {{Annotated link|Simulation hypothesis}}
* [[Philosophy:Swampman|Swampman]] - philosophical thought experiment

== References ==
{{Reflist|30em}}

== Further reading ==
{{Refbegin}}
* "Disturbing Implications of a Cosmological Constant", Lisa Dyson, [[Biography:Matthew Kleban|Matthew Kleban]], and [[Biography:Leonard Susskind|Leonard Susskind]], ''[[Physics:Journal of High Energy Physics|Journal of High Energy Physics]]'' 0210 (2002) 011 ([https://arxiv.org/abs/hep-th/0208013 at arXiv])
* "Is Our Universe Likely to Decay within 20 Billion Years?", Don N. Page, ([https://arxiv.org/abs/hep-th/0610079 at arXiv])
* "Sinks in the Landscape, Boltzmann Brains, and the Cosmological Constant Problem", [[Biography:Andrei Linde|Andrei Linde]], ''[[Physics:Journal of Cosmology and Astroparticle Physics|Journal of Cosmology and Astroparticle Physics]]'', 0701 (2007) 022 ([https://arxiv.org/abs/hep-th/0611043 at arXiv])
* "Spooks in Space", Mason Inman, ''New Scientist'', Volume 195, Issue 2617, 18 August 2007, [https://www.newscientist.com/channel/fundamentals/mg19526171.100-spooks-in-space.html pp. 26-29].
* "Big Brain Theory: Have Cosmologists Lost Theirs?", Dennis Overbye, [https://www.nytimes.com/2008/01/15/science/15brain.html 15 January 2008], ''{{wipe|The New York Times}}''
{{Refend}}

== External links ==
* [https://www.youtube.com/watch?v=nhy4Z_32kQo "Are You a Boltzmann Brain?"], ''PBS Space Time'' (2017)
* [https://www.sciencealert.com/wildly-fun-new-paper-compares-the-human-brain-to-the-structure-of-the-universe Human Brain and Universe - Similar Shapes?], ScienceAlert (2020)

[[Category:Philosophy of thermal and statistical physics]]
[[Category:Physical cosmology]]
[[Category:Thought experiments in philosophy of mind]]

{{Sourceattribution|Boltzmann brain}}