3 ms·

Boltzmann brains are extremely ephemeral. An analogy is that of stirring a vat of alphabet soup and noticing that there is a fair number of single-letter words

Boltzmann brains are extremely ephemeral. An analogy is that of stirring a vat of alphabet soup and noticing that there is a fair number of single-letter words popping into view ("A", "I"), a smaller number of two-letter words, an even smaller number of three-letter words ... a very very small chance of a twenty-letter word ... and a vanishingly small chance of the 189819-letter monster <https://en.wiktionary.org/wiki/Appendix:Protologisms/Long_words/Titin https://en.wiktionary.org/wiki/Appendix:Protologisms/Long_wo...> popping into view. The stirring doesn't stop just because a multiletter word appears, so multiletter words are quickly broken up and even valid single-letter words get hidden behind the "B"s and "Q"s and other letters in the soup. Boltzmann brains will fluctuate out of existence on the order of a small multiple of the light-crossing time of the brain-matter that fluctuated into existence. As the brains are human, they won't even have a chance to react. Although their false memories are encoded however true memories exist in our own brains, they'll have no time to have a reminiscence or notice their lack of sensory organs. (Which is probably good, since they would quickly suffer and die from lack of pressure and oxygen). A Boltzmann-brain with a full encoding of a life worth of false memories (from never-existing sensory input) is a much larger number of letters. Also, in a cold universe, the stirring is slower, and the letters sparser. Boltzmann brains are tremendously unlikely except in a verrrrrrrrry big volume of spacetime. But with a sufficiently big volume of spacetime, or one with an energetic false vacuum, one should expect a lot of Boltzmann brains. This view puts some limits on our own cosmos's vacuum, since we don't see lots of Boltzmann brains (or even much less complicated but RADAR-detectable and/or eclipsing strucures) fluctuating into brief existence in our solar system. Boltzmann brains are low-entropy. A persisting Boltzmann brain (fluctuating into existence and staying in existence for a long time) is much lower entropy still. This poses problems for hypotheses that the entire early universe fluctuated into existence and then evolved into the structures we see now. Here there are human brains attached to sensory apparatus, whose memories correlate fairly well with their history of input (and recordings by ancestors, and fossil records, and so on): a system with much much lower entropy than Boltzmann brains, so what suppresses relatively high-entropy structures (including Boltzmann brains) from dominating (by count) our neighbourhood? Also, if the universe supports large low-entropy fluctuations, galaxies that briefly (~ hundred thousand years) fluctuated in and out of existence should be much more common than galaxies with a history consistent with billions of years of galactic evolution, and you'd expect random variations in morphology, chemistry, and so forth; that's not what we see. This is a bit annoying, as it would be handy to point to Boltzmannian fluctuation theory as the source of the tremendously low entropy in the very early universe, i.e., it could have arisen spontaneously in a less precisely ordered space. Oh well. > why would ... a universe appear to evolve from a low entropy past following a small set of laws Thermodynamics. The issue is: where did the low entropy past come from? Once you have that, evolving into a higher entropy structure-filled present is not too hard -- that's essentially what we have with the standard cosmology from about the electroweak epoch onwards. So in summary: > sequences of Boltzman brains who all subjectively experience time as sequential whatever these might be, they aren't Boltzmann brains, since the latter don't subjectively experience anything as objectively they fluctuate out of existence in something like a nanosecond. Very briefly, the short existence is driven by interacting fields and the need to keep entropy (relatively) high: if your starting point just before the appearance of the brain is a region that is high quality vacuum, you have to come up with protons, calcium nuclei, ... and all that requires very careful aim to have one split-second "movie frame" of brain. You need much better "aim" which really drives down the entropy (which corresponds a much larger fluctuation) to go from vacuum to a Boltzmann brain that doesn't disintegrate starting in the very next frame thanks to overshoots of momentum. The higher the entropy of the Boltzmann brain, the clearer the stat mech argument. (If one gets stuck thinking about human brains, C. elegans apparently develop memories and store them in their nerve ring. Why isn't the outer space of our solar system full of those Boltzmann-C.-elegans brains fluctuating in and out of existence with each possessing false memories of sensory stimuli? Smaller fluctuations, so there should be many more of those than human Boltzmann brains).

hackinthebochs 2y ago
I agree with all that. Bringing up Boltzman brains was just an alternate way of explaining how inhabitants of a block universe could experience time as sequential without a real sequential ordering of universe states. Presumably if one can conceptualize a Boltzman brain coming into existence to experience one instant of a virtual life with virtual memories, you can imagine a long sequence of them experiencing the entirety of this virtual life. But the order in which this sequences comes into existence doesn't alter the directionality of subjective time evolution for the Boltzman brains.
- JohnMakin 2y ago
  This is well said - this is exactly how I understood your comment as well and you put it very succinctly and in an understandable way and has been something that I've been pondering for a while now. Thanks.
- raattgift 2y ago
  > inhabitants of a block universe could experience time as sequential without a real sequential ordering of universe states tl;dr: I don't think Boltzmann brains count as "inhabitants" because their worldlines are so short. Considering together a select set of available Boltzmann brains does not really admit something that looks like a long but complicated worldline. By virtue of being a fluctuation in a thermal bath in equilibrium a BB does not affect the wider universe; a Boltzmann flashlight can't blink out a message in Morse code. The herd of elephants in the room is the exp(- \Delta S) suppression of fluctuations of size \Delta S out of equlibrium. I think you are saying that we can imagine a set of some billion billion ephemeral Boltzmann brains each having memories associated with a unique fraction of a false life. I agree we can imagine that, but at the cost of having exact duplicates and many many more ephemeral Boltzmann brains with corrupted and even wholly unrelated false memories. The true causality is the history of the thermal bath and not the memories of the brains. In principle we can distinguish between a chosen set of same-false-life-at-different-stages Boltzmann brains and a real human with a very complicated FTL-and-time-travel worldline because at each point on the worldline the latter gets stress-energy ("signals", if you like) from the predecessor point, and also from (and to) each point's neighbours not on the same worldline. That is, our real traveller can detect the thermal bath temperature (which matters in an expanding cosmology) and leaks out metabolism photons to infinity. Ephemeral Boltzmann brains do neither. The "virtual life" Boltzmann brains -- as you note -- do not have to be ordered in any way. I would go further: brains with immediately neighbouring fractions of the virtual life's false memories can be totally causally disconnected, not just causally disordered. So I don't think the thought experiment says anything other than the Poincaré recurrence theorem admits states that are close (but not arbitrarily close or exact) to the initial state. That is, BB_final will recur, but so will BB_final-minus-one-nanoseconds-of-false-memory, BB_final-minus-two-nanoseconds-of-false-memory, ... but in some arbitrary point in the system's evolution. I don't think that's surprising. There will also be brief ephemeral fluctuations into (and out of) mouse brains, cockroach brains, microchips, Jeep Wranglers, brains with memories of having lived lives as little green men from Mars, and so on and so forth. If you have BBs full of false human memories, without some unknown suppression mechanism you will also have BBs full of false nonhuman memories, and nonhuman BBs, especially smaller and less complicated ones (\Delta S being much smaller in those cases). I also don't think it says anything about our universe, since we simply do not know enough about dark energy to make confident guesses about the very far future (i.e., does it really asymptote to de Sitter with the thermal bath from a dS horizon?). We also don't know if protons are stable that far into the future. However, with what we do know (which is not enough), the far future looks pretty empty. If there aren't RQFT interactions at GUT-scale energies that allow for violations of baryon and lepton numbers, maximum entropy in the far future (>> 10^33 years) still fairly low, but also the path to new nuclei from the (photon-dominated) thermal bath probably means no BBs at all (effectively all baryons are behind horizons by ~10^{10^10} years from now, and significant numbers BBs in our future light cone are expected not much earlier than ~10^{10^50}, although of course if BBs can happen at all, the very occasional individual BB may have its ephemeral moment at any time including today.) So if there are BBs with false memories of human lives, they are so far in the future that an entire 2020s-style solar system fluctuating into existence and persisting (or a time-traveller who reaches that future) could not recover anything like our present cosmology. At best, via a strong Lorentz boost, they might detect relic and horizon photons, and maybe splatter their windscreens with the occasional BB. The fluctuated-and-persisting-solar-system scientists would quickly realize its false memories of skies full of stars and galaxies were not true memories. A real time-traveller would probably just try to measure what's left of the CMB or find another marker of the cosmological scale factor. (The time travel can have been to-the-future-only by something as boring as spending a lot of time moving close to the speed of light.) Not discussed yet: what to do about fluctuations in fields which obey conservation laws that result in antiparticles. My fast answer would be: that's one reason BBs are so short lived; they are ripped apart by matter/antimatter annihilations.

hackinthebochs 2y ago

I agree with all that. Bringing up Boltzman brains was just an alternate way of explaining how inhabitants of a block universe could experience time as sequential without a real sequential ordering of universe states. Presumably if one can conceptualize a Boltzman brain coming into existence to experience one instant of a virtual life with virtual memories, you can imagine a long sequence of them experiencing the entirety of this virtual life. But the order in which this sequences comes into existence doesn't alter the directionality of subjective time evolution for the Boltzman brains.

JohnMakin 2y ago
This is well said - this is exactly how I understood your comment as well and you put it very succinctly and in an understandable way and has been something that I've been pondering for a while now. Thanks.
raattgift 2y ago
> inhabitants of a block universe could experience time as sequential without a real sequential ordering of universe states tl;dr: I don't think Boltzmann brains count as "inhabitants" because their worldlines are so short. Considering together a select set of available Boltzmann brains does not really admit something that looks like a long but complicated worldline. By virtue of being a fluctuation in a thermal bath in equilibrium a BB does not affect the wider universe; a Boltzmann flashlight can't blink out a message in Morse code. The herd of elephants in the room is the exp(- \Delta S) suppression of fluctuations of size \Delta S out of equlibrium. I think you are saying that we can imagine a set of some billion billion ephemeral Boltzmann brains each having memories associated with a unique fraction of a false life. I agree we can imagine that, but at the cost of having exact duplicates and many many more ephemeral Boltzmann brains with corrupted and even wholly unrelated false memories. The true causality is the history of the thermal bath and not the memories of the brains. In principle we can distinguish between a chosen set of same-false-life-at-different-stages Boltzmann brains and a real human with a very complicated FTL-and-time-travel worldline because at each point on the worldline the latter gets stress-energy ("signals", if you like) from the predecessor point, and also from (and to) each point's neighbours not on the same worldline. That is, our real traveller can detect the thermal bath temperature (which matters in an expanding cosmology) and leaks out metabolism photons to infinity. Ephemeral Boltzmann brains do neither. The "virtual life" Boltzmann brains -- as you note -- do not have to be ordered in any way. I would go further: brains with immediately neighbouring fractions of the virtual life's false memories can be totally causally disconnected, not just causally disordered. So I don't think the thought experiment says anything other than the Poincaré recurrence theorem admits states that are close (but not arbitrarily close or exact) to the initial state. That is, BB_final will recur, but so will BB_final-minus-one-nanoseconds-of-false-memory, BB_final-minus-two-nanoseconds-of-false-memory, ... but in some arbitrary point in the system's evolution. I don't think that's surprising. There will also be brief ephemeral fluctuations into (and out of) mouse brains, cockroach brains, microchips, Jeep Wranglers, brains with memories of having lived lives as little green men from Mars, and so on and so forth. If you have BBs full of false human memories, without some unknown suppression mechanism you will also have BBs full of false nonhuman memories, and nonhuman BBs, especially smaller and less complicated ones (\Delta S being much smaller in those cases). I also don't think it says anything about our universe, since we simply do not know enough about dark energy to make confident guesses about the very far future (i.e., does it really asymptote to de Sitter with the thermal bath from a dS horizon?). We also don't know if protons are stable that far into the future. However, with what we do know (which is not enough), the far future looks pretty empty. If there aren't RQFT interactions at GUT-scale energies that allow for violations of baryon and lepton numbers, maximum entropy in the far future (>> 10^33 years) still fairly low, but also the path to new nuclei from the (photon-dominated) thermal bath probably means no BBs at all (effectively all baryons are behind horizons by ~10^{10^10} years from now, and significant numbers BBs in our future light cone are expected not much earlier than ~10^{10^50}, although of course if BBs can happen at all, the very occasional individual BB may have its ephemeral moment at any time including today.) So if there are BBs with false memories of human lives, they are so far in the future that an entire 2020s-style solar system fluctuating into existence and persisting (or a time-traveller who reaches that future) could not recover anything like our present cosmology. At best, via a strong Lorentz boost, they might detect relic and horizon photons, and maybe splatter their windscreens with the occasional BB. The fluctuated-and-persisting-solar-system scientists would quickly realize its false memories of skies full of stars and galaxies were not true memories. A real time-traveller would probably just try to measure what's left of the CMB or find another marker of the cosmological scale factor. (The time travel can have been to-the-future-only by something as boring as spending a lot of time moving close to the speed of light.) Not discussed yet: what to do about fluctuations in fields which obey conservation laws that result in antiparticles. My fast answer would be: that's one reason BBs are so short lived; they are ripped apart by matter/antimatter annihilations.