Search FQXi


If you have an idea for a blog post or a new forum thread, then please contact us at forums@fqxi.org, with a summary of the topic and its source (e.g., an academic paper, conference talk, external blog post or news item).
Forum Home
Introduction
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
RECENT POSTS IN THIS TOPIC

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...


RECENT FORUM POSTS

Georgina Parry: "Hi Lorraine, yes the Daniel Amen talk is really good. Most amazing is the..." in Your Invitation to FQXi's...

John Cox: "Tom, Okay, Minkowski sets his working limits and qualifies them such that..." in Melting Spacetime

John Merryman: "Steve, I don't know that it has to be entirely due to usefulness. Lots of..." in Why Quantum?

John Merryman: "Peter, Further up this thread, Aug. 25, 2014 @ 17:38 GMT, I posted a..." in Why Quantum?

Lorraine Ford: "Georgina, The TED talk was interesting: Daniel Amen made a good case for..." in Your Invitation to FQXi's...

Akinbo Ojo: "Vladimir, I am getting paranoid from what's in the news lately. Is World..." in How Should Humanity Steer...

Thomas Ray: ""You won't believe anything I say.. I really, really wish I could,..." in Melting Spacetime

Thomas Ray: "By the way, I think there is some significant relation between the..." in Quantum theory escapes...


RECENT ARTICLES
click titles to read articles

Quantifying Occam
Is the simplest answer always the best? Connecting Medieval monks to computational complexity, using the branch of mathematics known as category theory.

Heart of Darkness
An intrepid physicist attempts to climb into the core of black hole.

Why Quantum?
Entropy could explain why nature chose to play by quantum rules.

Reality's NeverEnding Story
A quantum version of Darwinian natural selection could enable the universe to write itself into being.

The Quantum Dictionary
Mark Van Raamsdonk is re-writing how we define the shape of our universe. Can such translations help to unite quantum theory and gravity?


FQXi BLOGS
August 29, 2014

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".

Wilhelmus

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