This Week’s Hype

Symmetry, the FNAL/SLAC run online magazine funded by the DOE, today is running a piece of multiverse mania entitled Is this the only universe?. It’s a rather standard example of the pseudo-scientific hype that has flooded the popular scientific media for the last 10-15 years.

Besides the usual anthropic argument for the size of the CC, the evidence for the multiverse is string theory:

For further evidence of a multiverse, just look to string theory, which posits that the fundamental laws of physics have their own phases, just like matter can exist as a solid, liquid or gas. If that’s correct, there should be other universes where the laws are in different phases from our own—which would affect seemingly fundamental values that we observe here in our universe, like the cosmological constant. “In that situation you’ll have a patchwork of regions, some in this phase, some in others,” says Matthew Kleban, a theoretical physicist at New York University.

No mention is made of the fact that there is no evidence for string theory, with the multiverse given as the usual argument for why this is so.

Kleban also claims the theory is testable in this way:

it may be possible to experimentally induce a phase change—an ultra-high-energy version of coaxing water into vapor by boiling it on the stove. You could effectively prove our universe is not the only one if you could produce phase-transitioned energy, though you would run the risk of it expanding out of control and destroying the Earth. “If those phases do exist—if they can be brought into being by some kind of experiment—then they certainly exist somewhere in the universe,” Kleban says.

No word on how to do that.

Nomura says the way to test the idea is by looking at the universe’s spatial curvature (which, according to Planck is zero within the experimental uncertainties). According to Nomura, the implications for the multiverse of possible more accurate measurements of the spatial curvature are

  • If it remains zero, that’s consistent with the multiverse.
  • If it is negative, that’s “strong evidence of a multiverse”.
  • If it is positive, that’s a problem for some multiverse models, but not evidence against the multiverse, because you can’t have evidence against the multiverse:

    a positively curved universe would show that there’s something wrong with our current theory of the multiverse, while not necessarily proving there’s only one. (Proving that is a next-to-impossible task. If there are other universes out there that don’t interact with ours in any sense, we can’t prove whether they exist.)

Kleban and Nomura are quite excited by this, because the multiverse has the wonderful implication that we can all give up on trying to find a better fundamental theory and do something more useful with our lives:

If there were different universes with different phases of laws, we might not need to seek fundamental explanations for some of the properties our universe exhibits.

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16 Responses to This Week’s Hype

  1. Scott Church says:

    Peter, I find it interesting that Kleban says, “a positively curved universe would show that there’s something wrong with our current theory of the multiverse, while not necessarily proving there’s only one…” His own research has shown that positive curvature at the 10^-4 level would rule out slow roll and false vacuum eternal inflation, and negative curvature on the same scale would rule out slow roll eternal inflation ( Guth and Nomura reported similar findings ( Current Planck results give a flat universe to a confidence interval of only +/- 10^-3 so more refined observational data could verify either possibility.

    It seems to me that eternal inflation is… forgive the expression… “the only game in town” for generating a multiverse, and the vacua of string/M-theory are needed if it’s to have the widely varying physical constants required by anthropic constraints. The only alternatives I see are Tegmark’s Level-4 multiverse or Many Worlds QM (both of which to me seem downright bizarre), or purely speculative new physics of some sort for which there isn’t a shred of evidence. So in the absence of this sort of wishful thinking/special pleading, if future refinements in observed curvature do rule out eternal inflation this way, how exactly have we failed to prove that there’s only one universe? Am I missing something here?

  2. Peter Woit says:

    Scott Church,
    The thing you quote I think comes from Nomura, not Kleban. I’ve seen the Guth/Nomura paper you mention, but I’ve never been convinced that you can’t get positive curvature out of this kind of scenario. In particular, look at the conclusion of this paper
    which I’ve never seen refuted.

    It seemed to me that all that Nomura was doing was stating the obvious: multiverse proponents will claim victory if the curvature is negative, come up with a different model if it is positive. You’d have to ask him though about this, it’s quite possible the writer mangled some subtlety of what he was trying to say.

  3. Scott Church says:

    Thanks Peter! I’ve downloaded a copy of that paper and will give it a read asap. Have you seen the Kleban & Schillo paper as well (the first link)? It was published the same year as Guth and Nomura’s, and appears to have taken a somewhat different path to the same results. In any event I imagine you’re right… whether eternal inflation is ever falsified or not, multiverse proponents will find ways to keep it alive. They have 10^500 vacua to play with… and who couldn’t get what they want out of that? 🙂

  4. Peter Woit says:

    Scott Church,
    I’ve seen it, but haven’t looked at it carefully, beyond noticing that they don’t even refer to the earlier paper I mentioned that reaches opposite conclusions. I just don’t see how the sort of arguments they’re making can possibly be strong enough to rule out all mechanisms for making extra universes.

  5. Peter Woit says:

    Maybe another comment I should have made in the posting is that, as anyone who watched “Particle Fever” knows, Nomura was saying that the multiverse predicts a 140 GeV Higgs. When that was falsified, there seems to have been no impact on his promotion of the multiverse (rather the opposite…), so I see no reason to believe that falsification by discovery of a positive spatial curvature would cause him to abandon the idea. It was interesting in this article that he seems to be explicitly acknowledging that what he is arguing for cannot be falsified.

  6. Scott Church says:

    Yeah… so I saw from some of your previous posts about LHC’s failures to date to turn up a gluino. Now that LHC Run 2 is under way it’ll be interesting to see how he (and others) respond if no superpartners are discovered by next summer.

    Btw, on another note, I just finished your book yesterday, and I cannot tell you how much I enjoyed it! Sadly, I’m in a bit of withdrawal now… what to read next that could be as enjoyable and informative…? 🙂

  7. verruckte says:

    Well, I’m disappointed in Symmetry for running that piece. Their stuff is usually better than that.

  8. Low Math, Meekly Interacting says:

    It’s not all bad. Quanta has an interesting recent piece on Navier-Stokes that might help with pop-sci-induced depression.

  9. nicola says:


    that 140GeV Higgs was obviously meant for some other Universe, not ours ;).

  10. Chris Kennedy says:

    The magnitude of multiverse nonsense that continues to be perpetuated is really unfortunate. I saw part 1 of the Uranium documentary on PBS last night. Sure it was very basic for a general audience but when you look at what real scientific discoveries and accomplishments were decades ago it shows by comparison just how tragically hollow the community’s current goals and purpose have become.

  11. Anonyrat says:

    “…we might not need to seek fundamental explanations for some of the properties our universe exhibits.”

    IMO, given planetary orbit sizes, orbital periods, masses, planetary diameters, rotational periods, etc., theory eventually tells us that the sizes of orbits and orbital periods can be related easily and follow as a consequence of a fundamental law, the other things do not. To be useful, multiverse theory has to tell us what features of the Standard Model follow readily as a consequence of some fundamental theory, and which do not.

  12. Chris W. says:

    “…we might not need to seek fundamental explanations for some of the properties our universe exhibits.”

    Too bad nobody suggested the idea of a multiverse to Isaac Newton. It might have saved him and his successors a lot of work. Never mind that the work actually bore fruit…    🙂

  13. Giotis says:

    Well on theoretical level and since String theory is currently the only reliable theory of everything and Quantum gravity to work with someone must answer first the following question:

    What is the correct vacuum of String theory?

    To answer this question you must have a background independent, non perturbative definition of the theory and (besides AdS/CFT) we are still lacking of such definition. String Field Theory for example if further developed has the potential to answer such deep questions in a background independent manner (Ashoke Sen’s conjectures in OSFT is a clear indication that it can do so).

    What we have so far in the String landscape are the low energy effective vacua derived from the perturbative/semiclassical String theory but combining these with eternal inflation and the seemingly (so far at least) unnaturalness of the SM and of the Cosmological Constant the case of the multiverse becomes quite appealing.

    But again these are mostly bottom up considerations, if we don’t have a complete Cosmology provided by String theory via its background independent, non perturbative definition I don’t think we can give a definitive answer. I know for example that David Gross strongly advocates this PoV.

    The point to take away is that String theory has not given yet the final OK for the Multiverse picture.

  14. al says:

    Hypothetically true = Real
    This is the philosophical equation of strings-cum-multiverse theories.
    The hassle is who should bear the burden of proof when rejecting the substitution
    noncontradictoriness= evidence.

  15. Peter Woit says:

    “the case of the multiverse becomes quite appealing”

    As far as I can tell, the only thing “appealing” about it is that it allows string theorists to explain away the failure of the idea of string theory unification.

  16. Richard Noth says:

    re. Giotis,

    I’m sure I’m not the only reader here who is having trouble wrapping their head around something being ‘the only reliable theory of everything’ whilst simultaneously sufficiently lacking in definition to be able to make any concrete statements about any of the ‘everything’ around us!

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