[ExI] Boltzmann brains

[Stathis Papaioannou]

On Mon, 11 May 2020 at 15:02, Rafal Smigrodzki via extropy-chat <
extropy-chat at lists.extropy.org> wrote:

>
>
> On Tue, May 5, 2020 at 4:21 AM Stathis Papaioannou <stathisp at gmail.com>
> wrote:
>
>>
>>
>> On Tue, 5 May 2020 at 16:47, Rafal Smigrodzki via extropy-chat <
>> extropy-chat at lists.extropy.org> wrote:
>>
>>>
>>>> ### Which cosmological models make Boltzmann brains more likely, and
>>> how?
>>>
>>
>> Here is a paper co-authored by several eminent cosmologists:
>>
>> “The simplest interpretation of the observed accelerating expansion of
>> the universe is that it is driven by a constant vacuum energy density ρΛ,
>> which is about three times greater than the present density of
>> nonrelativistic matter. While ordinary matter becomes more dilute as the
>> universe expands, the vacuum energy density remains the same, and in
>> another ten billion years or so the universe will be completely dominated
>> by vacuum energy. The subsequent evolution of the universe is accurately
>> described as de Sitter space.
>> It was shown by Gibbons and Hawking [1] that an observer in de Sitter
>> space would detect thermal radiation with a characteristic temperature TdS
>> = HΛ/2π, where
>> HΛ =��8πGρΛ (1) 3
>> is the de Sitter Hubble expansion rate. For the observed value of ρΛ, the
>> de Sitter temperature is extremely low, TdS = 2.3 × 10−30 K. Nevertheless,
>> complex structures will occasionally emerge from the vacuum as quantum
>> fluctuations, at a small but nonzero rate per unit space-time volume. An
>> intelligent observer, like a human, could be one such structure. Or, short
>> of a complete observer, a disembodied brain may fluctuate into existence,
>> with a pattern of neuron firings creating a perception of be- ing on Earth
>> and, for example, observing the cosmic mi- crowave background radiation.
>> Such freak observers are collectively referred to as “Boltzmann brains” [2,
>> 3]. Of course, the nucleation rate ΓBB of Boltzmann brains is extremely
>> small, its magnitude depending on how one defines a Boltzmann brain. The
>> important point, however, is that ΓBB is always nonzero.
>> De Sitter space is eternal to the future. Thus, if the accelerating
>> expansion of the universe is truly driven by the energy density of a stable
>> vacuum state, then Boltzmann brains will eventually outnumber normal
>> observers, no matter how small the value of ΓBB [4, 7, 5, 8, 9] might be.
>>
>> https://arxiv.org/pdf/0808.3778.pdf
>>
>> There are other models, such as eternal inflation, where Boltzmann brains
>> may predominate.
>>
>> Most physicists see it as a problem with their theories, but on its own
>> it doesn’t seem to be enough to dismiss a theory, unlike, say, an
>> astronomical prediction that turns out to be wrong.
>>
>>
>>
> ### The theories assume that the quantum vacuum fluctuations sample the
> space of all possible arrangements of matter in an inverse-size dependent
> manner. All structures are created by fluctuations but the larger the
> structure the lower the density of such structures in the De Sitter space.
> Since De Sitter space is infinitely growing, Boltzmann brains at some point
> outnumber evolved brains, for some choices of measurement basis. I agree
> that this assumption is not enough to dismiss those theories.
>
> As I mentioned before, Wolfram's approach dispenses with randomness and
> imposes structure on vacuum, thus allowing (but not necessarily forcing)
> universes without Boltzmann brains. This is of course not a sufficient
> reason to choose his approach over conventional ones. However, if his
> research program generates theories that have an equal explanatory power to
> conventional theories, then the potential absence of Boltzmann brains might
> be a factor in his favor, for Occam's razor reasons.
>

Randomness is not required, in general, to explore the possibility space.
We see this with deterministic Many Worlds compared with probabilistic
Copenhagen Interpretation.

> --
Stathis Papaioannou
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