How do we resolve the Boltzmann Brain problem

[carl_sebastian]

I think this refutes the standard picture since if ylour universe would just keep expanding we should be a Boltzmann brain.

https://en.wikipedia.org/wiki/Boltzmann_brain
https://arxiv.org/pdf/0802.0233
I would bet on the "big rip" (dark energy getting stronger) since that would also resolve the Hubble tension.

 

[PeterDonis]

I think this refutes the standard picture

What do you think refutes what standard picture?

 

[kimbyd]

I think this refutes the standard picture since if ylour universe would just keep expanding we should be a Boltzmann brain.

https://en.wikipedia.org/wiki/Boltzmann_brain
https://arxiv.org/pdf/0802.0233
I would bet on the "big rip" (dark energy getting stronger) since that would also resolve the Hubble tension.

A complete vacuum doesn't actually fluctuate, so there are no sources which could cause Boltzmann brains to come into existence:
https://arxiv.org/abs/1405.0298

 

[carl_sebastian]

A complete vacuum doesn't actually fluctuate, so there are no sources which could cause Boltzmann brains to come into existence:
https://arxiv.org/abs/1405.0298

Otherwise an infinite amount of Boltzmann brains would emerge every second if space is infinite.

 

[PeterDonis]

Otherwise an infinite amount of Boltzmann brains would emerge every second if space is infinite.

Again, what do you think refutes what standard picture?

 

[carl_sebastian]

Again, what do you think refutes what standard picture?

If our universe would just continue to expand we would run into the Boltzmann brain problem and there would not be a complete vacuum since there would always be particles.

 

[PeterDonis]

If our universe would just continue to expand we would run into the Boltzmann brain problem and there would not be a complete vacuum since there would always be particles.

Why do you think that this is the "standard picture"? The whole point of @kimbyd's post is that the "standard picture" does not have this problem because there are no "vacuum fluctuations" to produce Boltzmann brains.

 

[carl_sebastian]

Why do you think that this is the "standard picture"? The whole point of @kimbyd's post is that the "standard picture" does not have this problem because there are no "vacuum fluctuations" to produce Boltzmann brains.

That only offered a solution for absolute vacuum but our universe will never become empty in the standard picture, just accelerated expansion forever.

 

[PeterDonis]

That only offered a solution for absolute vacuum

All of the arguments for why Boltzmann brains should be produced also assume an absolute vacuum. So if an absolute vacuum does not produce Boltzmann brains, all of those arguments are wrong.

 

[kimbyd]

That only offered a solution for absolute vacuum but our universe will never become empty in the standard picture, just accelerated expansion forever.

Yes, it will. Or to be more precise, eventually every Hubble volume will contain either one or zero particles, with the Hubble volumes containing zero particles growing exponentially in number while the number containing a single particle remains static. Both types of Hubble volume will necessarily be in a ground state, and thus not fluctuate.

 

[carl_sebastian]

Yes, it will. Or to be more precise, eventually every Hubble volume will contain either one or zero particles, with the Hubble volumes containing zero particles growing exponentially in number while the number containing a single particle remains static. Both types of Hubble volume will necessarily be in a ground state, and thus not fluctuate.

Why would a volume containging a single particle always be in the ground state?

 

[PeterDonis]

Why would a volume containging a single particle always be in the ground state?

Because if there is only a single particle, it must already have given up all possible energy--if it hadn't, it would be able to emit particles like photons, which would mean more than one particle would be in that Hubble volume. "Given up all possible energy" means the particle is in its ground state.

 

[carl_sebastian]

Case0
Boltzmann brains can emerge in complete vacuum, thus both space* and time must me limited but you cannot reach the end of our universe since it's expanding at the speed of light.

*at least space where vacuum fluctuations can occur.

Case1
A Boltzmann brain can emerge from a single particle, this require our universe to only exist for a finite amount of time.

Time being limited in this context means that eventually our current universe/aeon will come to an end eventually, it's hard to tell what if anything would follow after that.

case2
A Boltzmann brain can only emerge when 2 particles are sufficiently close (such as the same Hubble volume). this is compatible with the standard picture of our universe just continuing expanding forever.

 

[PeterDonis]

Case0

Case1

case2

Where are you getting these cases from?

 

[windy miller]

A complete vacuum doesn't actually fluctuate, so there are no sources which could cause Boltzmann brains to come into existence:
https://arxiv.org/abs/1405.0298

Are you really sure about this paper? Carroll's own commentary on it says it relies heavily on the Everttian many worlds interpretation of Qm. And since we don't know that this is the correct interpretation of QM are you really sure this paper is so solid? http://www.preposterousuniverse.com...boltzmann-brains-and-maybe-eternal-inflation/

 

[carl_sebastian]

Are you really sure about this paper? Carroll's own commentary on it says it relies heavily on the Everttian many worlds interpretation of Qm. And since we don't know that this is the correct interpretation of QM are you really sure this paper is so solid? http://www.preposterousuniverse.com...boltzmann-brains-and-maybe-eternal-inflation/

I took a look and it obviously doesn't prove anything, i am pretty sure it's wrong, it does however offer a way out (case2, earlier post).

 

[kimbyd]

Are you really sure about this paper? Carroll's own commentary on it says it relies heavily on the Everttian many worlds interpretation of Qm. And since we don't know that this is the correct interpretation of QM are you really sure this paper is so solid? http://www.preposterousuniverse.com...boltzmann-brains-and-maybe-eternal-inflation/

The interpretation is irrelevant because there's no system that can perform measurements of any kind in an empty universe. All interpretations reduce to the many worlds interpretation in that limit.

 

[carl_sebastian]

The interpretation is irrelevant because there's no system that can perform measurements of any kind in an empty universe. All interpretations reduce to the many worlds interpretation in that limit.

That's incorrect
1. the universe never becomes empty and there may also be vacuum fluctuations allowing for interactions.
2. why do you bring up "measurements" if interpretation is irrelevant?

So you are already assuming "no vacuum fluctuations" and that a measurement of some kind is required to collapse the wavefunction, this isn't the case for the penrose interpretation where the wavefunction can self-collapse.

 

[carl_sebastian]

Case0
Boltzmann brains can emerge in complete vacuum, thus both space* and time must me limited but you cannot reach the end of our universe since it's expanding at the speed of light.

*at least space where vacuum fluctuations can occur.

Case1
A Boltzmann brain can emerge from a single particle, this require our universe to only exist for a finite amount of time.

Time being limited in this context means that eventually our current universe/aeon will come to an end eventually, it's hard to tell what if anything would follow after that.

case2
A Boltzmann brain can only emerge when 2 particles are sufficiently close (such as the same Hubble volume). this is compatible with the standard picture of our universe just continuing expanding forever.

This is if you count hadrons as single particles or if all hadrons are unstable.

 

[PeterDonis]

the universe never becomes empty

@kimbyd already responded to this; see post #10.

why do you bring up "measurements" if interpretation is irrelevant?

She's making the point that if there is no way to make any measurements, then all interpretations reduce to the MWI. Another way to put it would be that without measurements, all interpretations say we just have unitary evolution, and "we just have unitary evolution" is equivalent to the MWI.

 

[PeterDonis]

This is if you count hadrons as single particles or if all hadrons are unstable.

You have not answered the question I have repeatedly asked: where are you getting these cases from?

 

[PeterDonis]

the penrose interpretation where the wavefunction can self-collapse.

What is the penrose interpretation? Do you have a reference?

 

[windy miller]

The interpretation is irrelevant because there's no system that can perform measurements of any kind in an empty universe. All interpretations reduce to the many worlds interpretation in that limit.

One of the paper's own authors appears to disagree .From the link I posted above. " As far as quantum fluctuations are concerned, we readily admit that our analysis relies heavily on the Everett/Many-Worlds formulation of quantum theory. In that view, there is nothing truly random and unpredictable about quantum dynamics."
Do you have a reference for the claim that interpretation is irrelevant ?

 

[PeterDonis]

One of the paper's own authors appears to disagree .

But note carefully that Carroll says "Everett formulation", not "Everett interpretation". At the end of the article, he explains the difference:

Finally, it’s interesting to note the role of “interpretations of quantum mechanics” in this story. (I don’t like that term, since we’re not discussing “interpretations,” we’re comparing manifestly different physical theories.) In the Everett formulation, the wave function is a direct reflection of reality; when it is stationary, so is the quantum system. Other approaches take a very different tack. There are formulations of quantum mechanics where collapse of the wave function is truly random and unpredictable; there are others with hidden variables, in which the true state of the universe isn’t defined by the wave function. In any of those cases, our analysis is completely beside the point. It’s interesting to think — but perhaps unsurprising in retrospect — that the correct formulation of quantum mechanics might have crucial implications for the evolution of the universe.

These "other formulations" are not the other standard interpretations of standard QM--Copenhagen etc. They are, as Carroll says in the quote, "manifestly different physical theories". For example, random and unpredictable collapse is a formulation like the GRW model, which makes different predictions from standard QM. If something like the GRW model were actually correct, then it would be impossible for an empty de Sitter universe to stay that way forever; at any time, a random "collapse" could happen that would kick the universe into a different quantum state. All Carroll is saying is that he is not considering such models; he is only considering standard QM, where as long as no measurement is made you just have unitary evolution. And if the entire universe is in its ground state, unitary evolution just says it stays that way forever, never changing at all.

 

[kimbyd]

He's making the point that if there is no way to make any measurements, then all interpretations reduce to the MWI.

She

One of the paper's own authors appears to disagree .From the link I posted above. " As far as quantum fluctuations are concerned, we readily admit that our analysis relies heavily on the Everett/Many-Worlds formulation of quantum theory. In that view, there is nothing truly random and unpredictable about quantum dynamics."
Do you have a reference for the claim that interpretation is irrelevant ?

Basically because I'm disregarding hidden variable interpretations. All interpretations that involve unitary evolution of the wavefunction as some component of the system should act like the many worlds interpretation here. It's technically possible for something else to be going on, that the unitary wavefunction evolution is an illusion and something fundamentally different is going on under the hood.

 

[PeterDonis]

She

Sorry, fixed.

 

[windy miller]

But note carefully that Carroll says "Everett formulation", not "Everett interpretation". At the end of the article, he explains the difference:

These "other formulations" are not the other standard interpretations of standard QM--Copenhagen etc. They are, as Carroll says in the quote, "manifestly different physical theories". For example, random and unpredictable collapse is a formulation like the GRW model, which makes different predictions from standard QM. If something like the GRW model were actually correct, then it would be impossible for an empty de Sitter universe to stay that way forever; at any time, a random "collapse" could happen that would kick the universe into a different quantum state. All Carroll is saying is that he is not considering such models; he is only considering standard QM, where as long as no measurement is made you just have unitary evolution. And if the entire universe is in its ground state, unitary evolution just says it stays that way forever, never changing at all.

I follow Carroll quite a lot, read all his books, read his blog, listen to his podcasts. I think he's one of the best public intellectuals/science communicators there is. He seems to be consistent in the claiming that MWI simply is QM and any other interpretation is QM plus something else. But he also accepts this is a controversial view. Its his view and he defends it well. So the use of the word "formalism" here rather than interpretation is simply Carroll's stance on MWI. Notice he says he doesn't like the word interpretation but most other physicists do. So I don't think its appropriate to dress up Carroll's views as facts about the matter. Carroll himself doesn't do that. So I think simply stating to the original poster than the BB problem has been solved or doesn't exist is not entirely correct. This is one potential solution maybe seems better to me. Carrol himself provides a number of caveats for example "My confidence in this story about quantum fluctuations and de Sitter space is extremely high, even though it does conflict with the way many cosmologists think about the situation. The less secure part of our story is when we move from the idealization of pure de Sitter space to the messy real world..." So not only is moving the model into the real world problematic according to Carroll but also he says many cosmologists disagree with him.
In this video 27m 13 you can see Alan Guth discussing the problem of BB and it seems his stance is quite different to Carroll's .

 

[.Scott]

I think this refutes the standard picture since if your universe would just keep expanding we should be a Boltzmann brain.

Since there is no way to know you are not a Bolzmann brain, nothing is refuted. Of course, that would make me a chance memory in your Bolzmann brain.
Funny: If you are a Bolzmann brain, you were, by chance, created with an understanding of Physics sufficient to recognize the possibility of a Bolzmann brain.

 

[charters]

Yes, it will. Or to be more precise, eventually every Hubble volume will contain either one or zero particles, with the Hubble volumes containing zero particles growing exponentially in number while the number containing a single particle remains static. Both types of Hubble volume will necessarily be in a ground state, and thus not fluctuate

Because if there is only a single particle, it must already have given up all possible energy--if it hadn't, it would be able to emit particles like photons, which would mean more than one particle would be in that Hubble volume. "Given up all possible energy" means the particle is in its ground state.

But this one particle - let's say an electron - should still be receiving Hawking type thermal radiation from the de Sitter horizon, which can Compton scatter off it. Get lucky a few times where the photon is high energy, and you can pair produce some leptons and nucleons, which then happen to tunnel and bind into proteins, etc., and soon you've got a brain in a vat. It happens once every $10^{10^{10}}$ years at best, but we've got infinite time and space. To be clear, I have a suspicion this can't be correct, but what's my error?

 

[PeterDonis]

this one particle - let's say an electron - should still be receiving Hawking type thermal radiation from the de Sitter horizon

That's one of the topics treated in the Carroll paper that @kimbyd linked to earlier in the thread. Basically, in order to have horizon radiation, you have to have a detector present that measures it. But in a true de Sitter vacuum, there are no such detectors. (There's also the point that such a detector would have to be accelerated, but the one particle in the de Sitter vacuum will be in free fall.)

 

[charters]

That's one of the topics treated in the Carroll paper that @kimbyd linked to earlier in the thread. Basically, in order to have horizon radiation, you have to have a detector present that measures it. But in a true de Sitter vacuum, there are no such detectors. (There's also the point that such a detector would have to be accelerated, but the one particle in the de Sitter vacuum will be in free fall.)

But why isn't my one electron a detector? It detects the horizon radiation by Compton scattering.

 

[PeterDonis]

why isn't my one electron a detector?

Because there is no other state it can transition to. See below.

It detects the horizon radiation by Compton scattering.

No, it can't. The electron is in free fall, so the vacuum is in its ground state; that means the electron can't gain energy from the vacuum through Compton scattering, since that would imply that the vacuum could lose energy from the ground state, which is impossible since the ground state is the state of lowest energy. And the electron is in its ground state, so it can't lose energy to the vacuum through inverse Compton scattering either.

Another way of looking at it is to note that the vacuum is isotropic to the electron--the same in all directions. But Compton scattering would mean there was a preferred direction to the vacuum.

 

[charters]

Another way of looking at it is to note that the vacuum is isotropic to the electron--the same in all directions. But Compton scattering would mean there was a preferred direction to the vacuum.

Hmm I think this made it click for me. Could I equivalently say something like there is indeed some amplitude for the electron to Compton scatter off horizon radiation from the east, but also the same is true for radiation coming from the west, and these processes have to interfere destructively?

 

[PeterDonis]

Could I equivalently say something like there is indeed some amplitude for the electron to Compton scatter off horizon radiation from the east, but also the same is true for radiation coming from the west, and these processes have to interfere destructively?

As a heuristic picture, perhaps, but I don't know that there is anything corresponding to this in the math.

 

[carl_sebastian]

But this one particle - let's say an electron - should still be receiving Hawking type thermal radiation from the de Sitter horizon, which can Compton scatter off it. Get lucky a few times where the photon is high energy, and you can pair produce some leptons and nucleons, which then happen to tunnel and bind into proteins, etc., and soon you've got a brain in a vat. It happens once every $10^{10^{10}}$ years at best, but we've got infinite time and space. To be clear, I have a suspicion this can't be correct, but what's my error?

Ofc one of your assumptions must be wrong and i think the error is assuming infinite time over big rip or something else that would be the end our our (current) universe.