Are Boltzmann brains predicted by our current cosmological theories?

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Discussion Overview

The discussion revolves around the concept of Boltzmann brains and their implications within current cosmological theories, particularly in relation to horizon complementarity and the nature of entanglement in quantum mechanics. Participants explore theoretical frameworks, assumptions about vacuum states, and the nature of measurements in quantum systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants reference Dyson, Kleban, and Susskind's work suggesting that if horizon complementarity is valid, Boltzmann brains could vastly outnumber normal observers, contingent on the dimensionality of Hilbert space.
  • There is a discussion about whether the assumption of an infinite-dimensional Hilbert space is valid and its implications for the existence of fluctuations in a Bunch-Davies vacuum.
  • One participant questions if the horizon in a Bunch-Davies vacuum acts as a measuring device due to its causal disconnection from certain degrees of freedom.
  • Another participant expresses uncertainty about the nature of measurements, particularly when entangled particles become inaccessible, and whether loss of coherence counts as a measurement.
  • Some participants propose that measurements require interaction with something persistent, while others argue that entangled particles will eventually be measured regardless of their causal separation.
  • There are inquiries about the implications of inflation on quantum fluctuations and whether particles can be considered "destined to be measured" if they are never observed.
  • A participant raises the idea that in a Bunch-Davies vacuum, the nature of thermal fluctuations and Gibbons-Hawking radiation may be effectively virtual.

Areas of Agreement / Disagreement

Participants express a range of views on the nature of Boltzmann brains, measurements, and the implications of quantum entanglement. There is no clear consensus, and multiple competing perspectives remain throughout the discussion.

Contextual Notes

Participants highlight limitations in understanding measurements in quantum mechanics, particularly in relation to expanding space and entangled particles. The discussion also touches on unresolved assumptions regarding the nature of vacuum states and their implications for cosmological theories.

grassyourhorse
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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?
 
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I felt a bit like a Boltzmann brain yesterday, but not so much today!

:welcome:
 
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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".
 
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.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.
 
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.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?
 
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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.
 
  • #12
.Scott said:
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.
A follow up on this. are you basically saying that the particles only get created if it will be measured in the future. So if you have lets say an empty thermal patch in a bunch davies vacuum, would the temperature or like Gibbons hawking radiation be effectively virtual and thus not have any effects?
 
  • #13
By profession, I am a Software Engineer, not a Physicist. But these are the important points as I see them:

Photons do not follow a simple trajectory. Between the time that a baseball is pitched to the time it is hit, it always has a position. But that's not how photons work. If you adjust a Mach-Zehnder Interferometer so that one of the interference patterns it is projecting is dark in the middle, then block one of the paths, some photons will now reach the middle position. Those photons arrive there because they could have struck the block but didn't. That block presents a counterfactual condition where only because an event could have happened, it has an effect on the final outcome.

The photons reaction to a double slit or a section of a hologram also illustrate this. The photo traverses both slits and all transparent sections of the hologram before "deciding" where to land.

In a Bell test such as Aspect's experiment, the test is run many times and the statistics are collected. When the results are tallied, it is found that the measurement results from each detector depend, in part, on the measurement angle of the other detector. This rather begs the question of what if I run 10,000 of these tests but keep one of each particle pair in limbo after reporting the results from the first one. The answer is that I can use that report to predict the measurements that I will eventually make on the particles in limbo. That would suggest that the particles measured first are the cause and those measured after limbo are the effect. But when the two measurement events are space-like separated, neither one is objectively "before" the other. In fact, I could put both members of every pair in limbo, transport them to separate Schwarzschild black hole, and have the measurements done as they fall beyond the event horizons but before the singularity.

So, in most cases, it's probably better to think of "entangled particles" as "particles that entangle the measurement events".