Krauss on Boltzmann Brains

Lawrence Krauss has a piece this week in New Scientist about the latest hot topic in theoretical physics, Boltzmann brains. It’s entitled String Theory’s Latest Folly, and starts off:

THOMAS AQUINAS may never have actually wondered how many angels can dance on the head of a pin, but his tortured musings about metaphysical issues associated with the non-corporeality of angels (and the related issue of whether there is excrement in heaven) stretched the limits of reasonable rational inquiry so far that later scholars invented the phrase to mock him.

My thoughts turned to Aquinas last week as I sat through a lengthy seminar on the subject of Boltzmann brains. The speaker decided his ruminations were so important that he needed 90 minutes rather than the customary hour. To my surprise, many in the room seemed to agree with him.

He goes on to explain what this is all about:

The problem is that statistical arguments suggest that in long-lived universes, far more Boltzmann-brain consciousnesses will develop than intelligences like our own, which have evolved over billions of years. That would mean we are far from typical, so anthropic explanations of our universe fall by the wayside.

Some theorists have therefore tried to develop constraints that would force all inflating universes like our own to decay well before Boltzmann brains can infect them. The bad news here is that in this case our universe must be unstable, and heading for a catastrophic end. But at least anthropic arguments from string theory would not be undermined. You can decide for yourself which you would prefer.

and to conclude:

If debating angels dancing on pins marked the intellectual low point of medieval theology, then we may similarly question the merits of debating problems that require hand-waving arguments involving unknown quantities that differ by billions and billions of orders of magnitude. Let’s focus on other issues, at least until better theories come along.

Update: At Lubos Motl’s blog, there’s a comment from Krauss noting that the title was chosen by an editor, not by him, and that he agreed that it was misleading, since the piece was not specifically about string theory.

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18 Responses to Krauss on Boltzmann Brains

  1. Tim R. Mortiss says:

    From the article: “If debating angels dancing on pins marked the intellectual low point of medieval theology”.

    I thought that was a later invention, used to smear scholasticism. If not, I would actually like to read that discussion!

  2. M says:

    maybe this “String Theory’s Latest Folly” can be interpreted as evidence for the prediction of String Anthropic Cosmology: Boltzmann brains are among us, and they will dominate the Universe

  3. DB says:

    Have you read Krauss’ latest book? It looks like he’s taking a leaf out of Smolin and yours:

  4. fh says:

    I thought the idea of the debate was that a) if a theory predicts Boltzmann Brains we are misapplying it and its wrong (that is its origin, right? it meant that Boltzmanns idea about the universe as a statistical fluctuation was wrong because it predicted Boltzmann Brains) and b) it’s actually a subtle question that teaches us something about the proper use of statistics to investigate whether it does.

  5. Xerxes says:

    While I think contemplation of Boltzmann Brains is probably philosophy and not physics, I really don’t see what is so bad about thinking about them or (to flip the argument around) what is to be gained by purposefully not thinking about them. The argument starts from assumptions that virtually every physicist would agree with and uses a pretty straightforward argument to come to a paradox. Clearly, there’s something that we don’t understand here, and resolving it might teach us something interesting. I’d consider it at least as interesting as, say, the Fermi paradox. Perhaps someday, we’ll consider it more like the Olbers paradox, where the resolution follows from a critical property of the universe.

  6. oohay says:

    [text of comment anonymously attacking Krauss deleted. Please don’t do this here, take it to one of the blogs that encourages anonymous attacks on others.]

  7. Moshe says:

    Funny piece, full of rhetorical flourishes, as usual with Krauss. The timing is puzzling though, Krauss must have been sitting in this kind of talks, given by his fellow cosmologists, for about 25 years. Why complain now?

    Oh, just saw the title, got it.

  8. Yatima says:

    It’s Friday afternoon:

    Botanist sues to stop CERN hurling Earth into parallel universe
    A colourful American botanist, teacher, former biologist and sometime physicist says (in outline) that the LHC may rip a hole in the fabric of the space-time continuum and so destroy the Earth. He wants the US government to act now and delay the LHC’s startup while a new safety review is carried out.

    These 14 TeV are gonna burn, man.

  9. Coin says:

    A little confused here since as far as I’m aware Boltzmann Brains aren’t an originally or specifically String-Theory-related idea.

    Since I can’t read the article itself, I just want to be sure: Would I be correct in interpreting that Krauss is not saying that Boltzmann Brains are “String Theory’s Latest Folly”, but rather that the “folly” is that string theorists have backed themselves into a corner via anthropic arguments such that they now feel compelled to seriously attempt to refute the Boltzmann Brains idea for fear it would pose a threat to string ideas if allowed to stand?

  10. Peter Woit says:

    Moshe and Coin,

    I think that Krauss is just properly laying at string theory’s door the phenomenon of serious physicists giving research talks devoted to anthropic computations of numbers of universes. Before the anthropic string theory landscape nonsense started up a few years ago, despite what Moshe seems to think, this wasn’t the kind of thing cosmologists gave talks about, at least not if they hoped their talk not to be drowned out by snickering from the audience. Now instead this kind of thing is covered in the New York Times and many talks about it are scheduled, given, and, amazingly enough, taken seriously.

  11. Moshe says:

    Oh, there were plenty of talks counting universes, comparing probabilities, talking about the proper measure etc. etc., I sat through some of them as a grad student in the mid-1990s, and it was already a mature subject by then. There is also a paper trail- for an entry point to this vast literature just look at the early papers of any of the main actors in this research area (quite a distinguished list of cosmologists). They start around the early 1980s, when people realized there are new and fascinating issues eternal inflation forces upon us. Even if Krauss gets his “better theories”, those would have to deal with precisely the same issues.

  12. Greg Egan says:

    I’m largely persuaded by the paper by Hartle and Srednicki, Are We Typical?, which concludes (roughly speaking) that we have no grounds for discriminating between cosmological theories on the basis of the proportion of conscious entities in the history of spacetime that human beings happen to comprise.

    We really can’t choose values for the cosmological constant, etc., on the basis that a “bad” choice would allow 10^10^10 Boltzmann brains to come into existence at some time in the future, thereby making us atypical observers. If we’re atypical, we’re atypical; Copernicus won’t really spin in his grave, and it won’t be the end of science. Nobody had to pull us out of a barrel containing all observers before we were allowed to make an observation.

  13. Zathras says:

    OT, but this article blew my mind in terms of the journalist’s sensationalism. It deals with a lawsuit to close the LHC because it will create black holes that will devour everything. The journalist made it look at least very possible

  14. Peter Woit says:


    Sure, eternal inflation and discussion of what its significance is have been around for a while, including a certain level of anthropic nonsense. As far as I remember though, the anthropic nonsense only started getting widespread and infecting other areas of physics with Susskind The attention being paid to the Boltzmann brain silliness is definitely a recent development.

    Xerxes and fh,

    Despite what you might read on certain well-known blogs, the notion that one can sensibly assign probabilities to different universes and then use this to make scientific predictions is not one shared by most theorists. This is exactly what Krauss is objecting to.

  15. Moshe says:

    Peter, as a matter of personal choice of research direction, I’d probably agree with everything Krauss says in the piece, including his summary – I don’t think we have enough knowledge of quantum gravity at the moment to ask the questions raised by eternal inflation, and attempts to proceed anyhow result in low signal to noise ratio. One interesting line of research is then to develop the tools needed, and lots of smart people are thinking about it, not likely to make it to the New Scientist pages anytime soon.

    However, Krauss’ choice of venue for this message, and timing, and emphasis, and tone, are all in things I disagree with. Oh well…

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  18. Randal says:

    If it’s your time and your mind, I guess you can spend it thinking about anything you want, but I think Krauss is essentially right: this is unlikely to prove a fruitful line of reasoning – much like Acquinas’ pin head. Future generations will likely view them similarly. Speculation makes better press than developing real understanding because it’s more fun.

    I understood the “latest folly” of the string theorists to be introducing the notion of universe decay as a constraint on the propagation of brains throughout their models; they sacrifice the stability of our universe to save their theories from this absurdity. (Some here don’t seem to like the tone, but I thought this was funny.)

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