Witten on Dark Energy

Commenter Shantanu pointed to a web-site with talks available on-line from a symposium about Dark Energy now going on at the Space Telescope Science Institute. Yesterday Witten gave a talk entitled “Models of Dark Energy”, where he lays out very clearly the conventional wisdom of the string theory community about the dark energy problem and its implications for string theory.

Witten describes how the problem of a huge number of possible vacua has always been an embarrassment for string theory. Until about 10 years ago his attitude towards most constructions of string vacua was “who needs this mess”, thinking that once one figured out the vacuum energy problem, such constructions would all go away. He explains how the discovery of a small positive CC has changed his attitude, that he’s no longer sure that one can find a distinguished vacuum state, and thus maybe the anthropic landscape/multiverse crowd is right. He describes this possibility as involving both good news and bad news:

The good news (such as it is) then is that if we are really living in a “multiverse”, it may be that the theory as we know it is pretty close to the truth.

But there’s a hefty dose of bad news… If the vacuum of the real world is really a needle in a haystack, it is hard to see how we are supposed to be able to understand it. In other words, if an unimaginably large number of approximate “vacuum” states are realized in different parts of the Universe, none of them with any special meaning, and with the details of particle physics depending on where one happens to live, then what sort of understanding of particle physics can we hope to get? I don’t have an answer to this question, although we might learn something from the LHC that will help…

The crucial point of course is this last one: how can you ever test these ideas, making them real science and not metaphysics? At the end of his talk, Rachel Bean tried to pin him down on this question, leading to this exchange:

Bean: “If we have this landscape, this multiverse, … can we learn nothing, or is there some hope, do you have some hope, that if you were to find a universe that had remarkably small CC you could also make some allusion to the other properties of that universe for example the fine structure constant, or are we saying that all of these things are random variables, uncorrelated and we’ll never get an insight.”

Witten: “Well, I don’t know of course, I’m hoping that we’ll learn more, perhaps the LHC will discover supersymmetry and maybe other unexpected discoveries will change the picture. I wasn’t meaning to advocate anything.”

Bean: “I’m asking your opinion.”

Witten (after a silence): “I don’t really know what to think has got to be the answer…”

Besides the landscape problem, Witten also described attempts to model dark energy as an aspect of some differerent sort of physical field, saying that he has been working on this with a student, but that the problem is the strong experimental bounds on the existence of light fields coupling to ordinary matter.

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20 Responses to Witten on Dark Energy

  1. Aaron Bergman says:

    I believe Witten was referring to his previous work with Peter Svrcek who graduated a few years ago.

  2. Peter Woit says:

    Thanks Aaron,

    I think you’re right, he seemed to be referring to work with Svrcek, although that paper didn’t seem to mention anything about the dark energy problem, which was what Witten’s talk was about.

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  4. Ethan Siegel says:

    At the very least, I’ve always found Witten to be remarkably balanced on the utility of string theory for the real world. This points towards that as well.

    He recognizes the limitations and pitfalls of the idea, as well as the difficulties inherent in attempting to find what we can learn from string theory if the models are non-predictive. I think that’s a very reasonable answer to Rachel’s question.

  5. Bill says:

    At the risk of embarrassing myself I have to admit that my knowledge of string theory is about at the level of that discussed in Zwiebach’s introductory book, so I’m an idiot. But the site’s mention of Witten and Moataz Emam’s proposed “future” of a failed string theory motivates this layperson’s following questions: One, if by adding a single dimension Witten can collapse five theories into one, can we not just add lots more dimensions to eliminate all the other inconsistencies (like the 10^500 vacuua) in the theory? (Of course, I’m thinking about von Neumann’s reputed curve-fitting comment regarding squeezing an elephant into some given model, and of course I’m also being facetious.) Two, if no one other than a post-doc math major can possibly appreciate this stuff, what good is it (recall Eddington’s comment about the dozen or so people who could understand general relativity at the time), particularly if it doesn’t deliver? And three, what right has the legitimate physics community to even consider taking a falsified or failed theory and turning it into another “discipline” (read: “religion”) on the sole basis that it is too beautiful to ignore (or, much worse, because so much time and effort has already been expended that we have to press on regardless)? This is the very definition of fanaticism.

  6. Kea says:

    Witten’s talk was clear, and surprisingly noncommittal regarding the existence of /\, but I preferred Steinhardt’s talk, which discusses the need for a range of complementary tests and possible JDEM exclusions of large numbers of models, both stringy and not. As Witten also stressed, more accurate measurements of w will be important in ruling out classes of model, especially /\ varying ones.

  7. Chris W. says:

    Kea (and anyone else who is interested):

    The HTML character entity for Λ is Λ.

    The HTML character entity for λ is λ.

    (Note the variation is case of the mnemonic, ie, the entity name.)

    For greek letters (and other characters) see this reference or Wikipedia, among others.

  8. Chris W. says:

    From Peter: “The crucial point of course is this last one: how can you ever test these ideas, making them real science and not metaphysics?”

    In the preceding paragraph (within the quote) Witten asks “then what sort of understanding of particle physics can we hope to get?”

    Note the implicit identification of understanding and testable explanation. It seems that those who are attracted to the idea of a multiverse—in the form Witten has in mind—have effectively endorsed breaking down this identification, while continuing to assert that such an explanation could be scientific. What Witten is acknowledging is the untenability, even incoherence, of this assertion.

    Untestable explanations may fulfill a desire for a putative understanding of the world that can assume various cultural roles, but in science they are at best preludes to the development of an explanation that can be tested, and stands up under testing. It does not seem that the multiverse can be such a prelude, except as part of an attempt that is frankly admitted to be a failure and thereby helps point the way to crucial insights. Of course for now, as Witten is also acknowledging, the way forward remains shrouded in obscurity.

  9. mike harney says:

    I like Witten’s coments that the LHC might give some clues – at least he is in touch with experimental reality.

  10. Arun says:

    In fact .. I think he’s going to be at CERN on a sabbatical when the machine turns on 🙂

  11. Shantanu says:

    Peter and others,
    Mario Livio’s talk was also very good and worth a look
    and discusses anthropic principle.
    right now John Peacock is giving a talk (which can be viewed live)

  12. egbert says:

    There’s talk on CNN about the LHC “test” of string theory being a religious experience for Nina Arkani-Hamed.

    When I signed up to be a scientist, religion was considered a negative influence on science. How times change.

  13. Chris Oakley says:

    My favourite quote from the CNN article:

    “From the point of view of the big experiments at the LHC, there is no amount of money or craftsmanship that would produce the kind of insight that comes from sharing LHC data with a true visionary like Nima Arkani-Hamed,” Tully said.

  14. fh says:


    Ludwig Boltzmann, quoting Goethe on Maxwells equations:

    “War es ein Gott, der diese Zeichen schrieb?”
    (Was it a God that penned these signs?)

    A religious experience does not a religion make and in many forms it has a long and positive tradition in fundamental science.

  15. D R Lunsford says:


    He was paraphrasing – no, QUOTING – Goethe. You can find it for yourself.

    Ok, I give in – it’s in the movie “Faust Times at Egmont High”.


  16. Peter Woit says:


    I think he was just referring to visiting the LHC as a “religious experience”. The claims made in the article that the LHC may see strings and test string theory involve a different and more dubious sort of religious belief….

  17. I read an LA Times article about the passing of Robert Wilson (1st director of Fermilab). He was a sculptor, & desired an aesthetic looking building for Fermilab. Something to the effect of a cathedral, where physicists would try to unlock the secrets of the atom.

    “It [ theoretical physics ] is very religious”
    — xx, Caltech CS professor

    My take on HEP & cosmology is that of ill-conditioned problem: insufficient data.

    “Insufficient facts always invite danger.”
    — Spock, Space Seed, Stardate 3141.9, Episode

  18. Ethan Siegel says:


    I’m sure you’ve seen this posting today:


    But I’m really curious as to your take on it; especially on sections 5 and 6. Does he get it right?


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