Search FQXi

If you are aware of an interesting new academic paper (that has been published in a peer-reviewed journal or has appeared on the arXiv), a conference talk (at an official professional scientific meeting), an external blog post (by a professional scientist) or a news item (in the mainstream news media), which you think might make an interesting topic for an FQXi blog post, then please contact us at with a link to the original source and a sentence about why you think that the work is worthy of discussion. Please note that we receive many such suggestions and while we endeavour to respond to them, we may not be able to reply to all suggestions.

Please also note that we do not accept unsolicited posts and we cannot review, or open new threads for, unsolicited articles or papers. Requests to review or post such materials will not be answered. If you have your own novel physics theory or model, which you would like to post for further discussion among then FQXi community, then please add them directly to the "Alternative Models of Reality" thread, or to the "Alternative Models of Cosmology" thread. Thank you.

Forum Home
Terms of Use

Order posts by:
 chronological order
 most recent first

Posts by the blogger are highlighted in orange; posts by FQXi Members are highlighted in blue.

By using the FQXi Forum, you acknowledge reading and agree to abide by the Terms of Use

 RSS feed | RSS help

Wilhelmus Wilde: on 2/6/14 at 15:35pm UTC, wrote Sean Carroll's idea is based on our causal way of thinking that in OUR...

Zeeya Merali: on 1/9/14 at 18:11pm UTC, wrote More audio from the FQXi meeting is now in. Those of you that read Ian...


jany watson: "At we have a long list of similar buy law essay online on diverse..." in Koalas, Quantum Mechanics...

Eckard Blumschein: "Lorraine, Thank you for addressing basics of mathematics. You refer to..." in FQXi Essay Contest 2016:...

Eckard Blumschein: "Georgina, I apologize. Lorraine, Must "an economic system be found that..." in FQXi Essay Contest 2016:...

James Putnam: ""The math time = 1/nu defines time ... " Nu is cycles/second. 1/nu is..." in Alternative Models of...

Steve Agnew: "Your work has shown how normal physics like S&Z mass, length, and time have..." in Alternative Models of...

kab: "The word is enternal magic. Hope all secret never has answer will be out..." in Does Quantum Weirdness...

Robert McEachern: ""According to QBism, the wavefunction is no longer to be thought of as an..." in Painting a QBist Picture...

Robert McEachern: ""We’re interested in what distinguishes the behaviour of particles in the..." in Bohemian Reality:...

click titles to read articles

Bohemian Reality: Searching for a Quantum Connection to Consciousness
Is there are sweet spot where artificial intelligence systems could have the maximum amount of consciousness while retaining powerful quantum properties?

Quantum Replicants: Should future androids dream of quantum sheep?
To build the ultimate artificial mimics of real life systems, we may need to use quantum memory.

Painting a QBist Picture of Reality
A radical interpretation of physics makes quantum theory more personal.

The Spacetime Revolutionary
Carlo Rovelli describes how black holes may transition to "white holes," according to loop quantum gravity, a radical rewrite of fundamental physics.

Riding the Rogue Quantum Waves
Could giant sea swells help explain how the macroscopic world emerges from the quantum microworld? (Image credit: MIT News)

March 27, 2017

CATEGORY: Blog [back]
TOPIC: Sean Carroll Fluctuates on Boltzmann Brains and Eternal Inflation [refresh]
Bookmark and Share
Login or create account to post reply or comment.

FQXi Administrator Zeeya Merali wrote on Jan. 9, 2014 @ 18:11 GMT
Sean Carroll
More audio from the FQXi meeting is now in. Those of you that read Ian Durham's summary of Day 2 will already know that cosmologist Sean Carroll, at Caltech, gave (in his own words) an "inflammatory" talk (with the deliberately boring title "Quantum Fluctuations in de Sitter Space'). With an eye to stirring up controversy, we present the talk for you to listen to, in its entirety.

Carroll has been thinking deeply about the what quantum fluctuations really are, and has come to the somewhat surprising (and contentious) conclusion that they are less fluctuation-y (that they fluctuate less) than we might have assumed. His argument (I think) roughly boils down to saying that even though we often talk about "quantum fluctuations" in a vacuum, those fluctuations happen because there's something out of equilibrium to generate them--essentially an observation.

This has implications for the Boltzmann brain problem: the idea that given long enough, fully formed conscious entities could pop out of the vacuum. This is an unsettling thought because, if Boltzmann brains become increasingly likely, it could be argued that we ourselves are more likely to be disembodied consciousnesses that have just been created, and--as Carroll says--we wouldn't then be able to trust our own thoughts about our history, or our conclusions about the workings of the universe. (I never trust my own thoughts, so I'm not too alarmed by this prospect.) But, at any rate, BBs would be more typical observers than normal humans, which calls into question a lot of our assumptions about cosmology.

But Carroll is now questioning whether this really is such a problem. Thinking specifically about de Sitter space, he argues that there won't be any quantum fluctuations in the vacuum to generate BBs: "If there are no Boltzmann brains now, you won't see Boltzmann brains tomorrow." He does clarify that you could see BBs pop into existence if you were there to look--your experimental BB-detecting-apparatus would be out of equilibrium with the vacuum and would interact with it, producing the fluctuations needed to generate BBs. But since you aren't out there observing, BBs shouldn't arise. Phew!

Of course, quantum fluctuations aren't always thought of as an enemy to cosmologists. Without them in the early universe, we wouldn't have the density perturbations needed to give rise eventually to structure in the cosmos. If Carroll apparently eradicates quantum fluctuations from the vacuum, then the obvious question (that Carroll asks and answers) is do we have to rethink the early history of the universe? No, and yes, is what he says. He claims that we *will* still get the required quantum fluctuations in the inflationary scenario. In standard cosmology, after this early burst of inflation--a rapid cooling expansion in the infant cosmos--inflation ends with a period of "reheating" (which does what it says on the tin, and makes the universe hotter). In Carroll's words this produces a "butt-load of entropy" (which I can't help thinking sounds like a very unpleasant affliction). This dramatic change acts like an observation of the universe, collapsing wavefunctions left, right and center, and generating the necessary perturbations in the cosmic microwave background.

However, it's not clear that, if Carroll is right, you would get the required quantum fluctuations needed to set up eternal inflation--the process that would create multiple big bangs, and give rise to a multiverse of neighbouring universes. More work needs to be done, etc. etc. to see what the implications are for eternal inflation.

Anyway, as Ian Durham blogged yesterday, it's by no means agreed that Carroll's correct to say that these quantum fluctuations are not happily fluctuating away as physicists have long assumed. But listen to Carroll in his own words on the podcast, and see what you think.

Suddenly popping into existence is not fun...

this post has been edited by the forum administrator

report post as inappropriate

Wilhelmus de Wilde wrote on Feb. 6, 2014 @ 15:35 GMT
Sean Carroll's idea is based on our causal way of thinking that in OUR universe everything must have a cause (reason). Maybe our universe is in "touch" with another "dimension" or is emerging from another source (field, maybe not measurable by us), and from this "source" emerge also quantum fluctuations.

It was also Sean Carroll who said : "Everything is Field".


this post has been edited by the author since its original submission

report post as inappropriate

Login or create account to post reply or comment.

Please enter your e-mail address:

And select the letter between 'X' and 'Z':

Note: Joining the FQXi mailing list does not give you a login account or constitute membership in the organization.