Are Boltzmann brains predicted by our current cosmological theories?

grassyourhorse
Messages
4
Reaction score
0
TL;DR
Reading papers on boltzmann brains seem to suggest our current models predict an infinite number of boltzmann brains that are produced by thermal fluctuations in a de sitter causal patch. While carroll has argued that such fluctuations dont occur, it seems that the majority still believe boltzmann brains are real due to reasons such as coherent histories or finite hilbert spaces
Dyson kleban susskind showed that if their idea of horizon complementarity is correct then boltzmann brains should vastly outnumber normal observers like us. However, there is some caveats, namely hilbert space needs to be finite dimensional. We currently assume that Hilbert space is infinite dimensional so our universe reaches a bunch davies vacuum. Sean carroll argues that in this state there can be no fluctuations since there are no measuring devices. However, cant it be argued that the environment system split is arbitrary so you could in principle see how decoherent branches with Boltzmann brains will appear even in a stationary state by choosing a basis. See seth lloyd paper (https://arxiv.org/pdf/1608.05672). Additionally, isnt it impossible to define a time invarient vacuum when you have massless fields such as gravity or an inflaton field so is the assumption that our universe approachs the bunch davies vacuum even legit? Lastly, does the horizon in bunch davies vacuum serve as an measuring device since it effectively cuts off causal connection to degrees of freedom that was once entangled?
 
Space news on Phys.org
I felt a bit like a Boltzmann brain yesterday, but not so much today!

:welcome:
 
Reply
  • Haha
Likes   Reactions: berkeman
Welcome to these forums.
You have created an interesting stew of physics topics.

I'll start with your last question. I don't believe there is resolution to what happens when one particle of an entangled pair becomes inaccessible (such as dropping through an event horizon). The way I usually look at it (my default interpretation) is that the eventual measurements are a necessary prerequisite for the creation of an entangled pair. It's possible that there is some kind of experiment that could disprove this (and thus it would not be an "interpretation"), and I am ready to back down when and if that happens.

Clearly, in order to compare the likelihood of a "Boltzmann Brain" (which, as a key function, is a non-observer) and a "normal observers brain", you would need to determine what, at its minimum, is a Boltzmann Brain. Then you would need to consider that normal Darwinian-developed brains are both more persistent (lasting decades, not milliseconds or femtoseconds) and will proliferate. Clearly, you don't need an entire brain if the Boltzmann brain will only persist for nanoseconds - you only need the part that is hallucinating - which may be only scores of entangled molecules. Such brains might be widespread.

But the point is trite. Concluding that you are a Boltzmann Brain and predestined to instant expiration could be accurate but pointless.
 
.Scott said:
Concluding that you are a Boltzmann Brain and predestined to instant expiration could be accurate but pointless.
I read that when you posted it and I'm still here, apparently.
 
PeroK said:
I read that when you posted it and I'm still here, apparently.
For an instant, I had this odd hallucination about a "PeroK".
 
Reply
  • Haha
Likes   Reactions: PeroK
.Scott said:
Welcome to these forums.
You have created an interesting stew of physics topics.

I'll start with your last question. I don't believe there is resolution to what happens when one particle of an entangled pair becomes inaccessible (such as dropping through an event horizon). The way I usually look at it (my default interpretation) is that the eventual measurements are a necessary prerequisite for the creation of an entangled pair. It's possible that there is some kind of experiment that could disprove this (and thus it would not be an "interpretation"), and I am ready to back down when and if that happens.
What would you consider as a measurement though? Does the loss of info and perhaps coherence when one of the entangled pair is produced and pulled away by faster than light expansion count as a measurement?
 
grassyourhorse said:
What would you consider as a measurement though?
I would say that it's not a "measurement" unless the quantum parameter of interest interacts with something persistent.
(Of course, is anything really persistent enough to irrevocably force a measurement result?)

grassyourhorse said:
Does the loss of info and perhaps coherence when one of the entangled pair is produced and pulled away by faster than light expansion count as a measurement?
No. Both of the entangled particles are destined to be "measured". But transiting expanding empty space won't be what does it.
 
grassyourhorse said:
What would you consider as a measurement though? Does the loss of info and perhaps coherence when one of the entangled pair is produced and pulled away by faster than light expansion count as a measurement?
An entangled system of two particles, as it is usually presented, is a state associated with non-relativistic QM. How it is understood in terms of an expanding universe in not clear. You may, at least, need a theory of quantum gravity.

That said, there is nothing inherently contradictory about two eventual measurements being correlated, even if they are causally disconnected. In fact, a critical element of entanglement is that the measurements can be spacelike separated (hence causally disconnected). The only difference is that the two outomes can be compared and checked for correlation.
 
Reply
  • Like
Likes   Reactions: PeterDonis
.Scott said:
I would say that it's not a "measurement" unless the quantum parameter of interest interacts with something persistent.
(Of course, is anything really persistent enough to irrevocably force a measurement result?)


No. Both of the entangled particles are destined to be "measured". But transiting expanding empty space won't be what does it.
Wdym both particles are destined to be measured?
Furthermore the reason i asked about the stretching beyond causal contact is because isnt that what inflation looks at. Modes are stretched beyond hubble radius and thus act as a classical fluctuation rather than a quantum one? Is there more to the mechanism?
 
Last edited:
  • #10
grassyourhorse said:
Wdym both particles are destined to be measured?
The alternative would be for that spin (or whatever is being measured) to be preserved forever. The particles will eventually hit something that is not reflective. Even if they are captured, how would you keep them from interacting with anything forever?
 
  • #11
.Scott said:
The alternative would be for that spin (or whatever is being measured) to be preserved forever. The particles will eventually hit something that is not reflective. Even if they are captured, how would you keep them from interacting with anything forever?
What if you have an empty bunch davies vacuum. I suppose if you have an interacting field theory it would be harder but you have people like sean carroll arguing the conventional wisdom of things appearing in a bunch davies vacuum is wrong since nothing ever gets measured again.