Thursday, October 11, 2018

More Problems For Sting Theory

In string theory, a paradigm shift could be imminent. In June, a team of string theorists published a conjecture which sounded revolutionary: String theory is said to be fundamentally incompatible with our current understanding of 'dark energy'. A new study has now found out that this conjecture seems to be incompatible with the existence of the Higgs particle.
From Science Daily discussing the following paper:
According to a conjecture recently put forward in [1], the scalar potential V of any consistent theory of quantum gravity satisfies a bound |∇V|/V≥O(1). This forbids de Sitter solutions and supports quintessence models of cosmic acceleration. Here, we point out that in the simplest models incorporating the standard model in addition to quintessence, with the two sectors decoupled as suggested by observations, the proposed bound is violated by 50 orders of magnitude. However, a very specific coupling between quintessence and just the Higgs sector may still be allowed and consistent with the conjecture.
Frederik Denef, Arthur Hebecker, Timm Wrase. "de Sitter swampland conjecture and the Higgs potential." 98 (8) Physical Review D (August 7, 2018). DOI: 10.1103/PhysRevD.98.086004

These papers don't by themselves, entirely rule out string theory, but they do take, what was just a year ago a "landscape" of string theories too vast to sort though, and rule out almost all of those possibilities. 


Marnie said...

typo ?


neo said...

"Wrase was able to show that the Higgs field also has properties that should actually be forbidden by Vafa's conjecture—and the Higgs field is considered an experimentally proven fact. For its discovery, the 2013 Nobel Prize in Physics was awarded."

Read more at:

how sound is Vafa's conjecture?

could 2018 be the end of string theory?

what's the most promising non-string QG?

Mitchell said...

Vafa's conjecture is highly controversial. Remember that this is ultimately about whether de Sitter space is possible in string theory. Vafa suspects not, he doesn't have a fundamental reason why, but he has guessed this constraint on the behavior of scalar fields, as a step closer to a fundamental reason. He has thrown this proposition out there - maybe scalar fields can't do this - and in effect he's dared string theorists to prove him wrong by finding a counterexample.

Now this paper is saying that the Higgs field when at its maximally unstable point (zero vev), plus a condition on the quintessence field that Vafa proposed as an alternative explanation for dark energy (alternative to de Sitter soace), together violate his conjecture. I don't follow this stuff closely at all, for reasons I will explain in a moment, but I guess they're saying that the resulting combination of energy densities would temporarily create a de Sitter geometry? Temporarily, because the Higgs field ought to quickly move towards the nonzero vev stable state...

So it's not clear to me that a *temporary* de Sitter geometry violates Vafa's conjecture about scalars. OK, they found a place in the space of field values where that conjecture is violated, but if the dynamics immediately carries the fields into values that *do* satisfy it - maybe the conjecture just needs to be amended, e.g. |del V|/V must not be less than 1 (that's the existing conjecture) for longer than a certain duration (that would be the amendment).

I would also want to look at existing string models that contain an implementation of the standard model Higgs mechanism, to see whether they go anywhere near violating Vafa's constraint. But there's already a long queue of models, whose builders have lined up to accost Vafa, saying "my string model has de Sitter space, what's wrong with it?"

But you shouldn't regard my commentary as very informed. The main reason I don't follow this much, is that it's all still very preliminary, and it doesn't immediately affect my own attempts at model-building with strings and branes. It's obvious that string theory can at least mimic the standard model in a variety of ways. What I care about is whether it can give you Koide and MOND on top of that. I'll let the mainstream worry about Vafa's idea for now.

neo said...
This comment has been removed by the author.
neo said...


If Vafa's proposal stands scrutiny, and Wrase paper on the Higgs, does this imply that since the Higgs exist, string theory is falsified?

also, Urs has a post that the KKLT proposal, the only known proposal to get deSitter space out if strings, is incorrect.

where does that leave us?

in the event Vava and Wrase papers hold, is there a nonstring QG that seems more promising

andrew said...

"how sound is Vafa's conjecture?"

Very plausible, but since it is new, not as rock solid as strong conjectures like the Reimman Hypothesis that have not been disproved for a long period of time. It hasn't been definitively proven.

"could 2018 be the end of string theory?"

No. There is no way that an argument will convince the entire string theory community in the next three months.

"what's the most promising non-string QG?"

Probably asymptotic safety. But, there is really no TOE or even GUT lined up to replace Sting Theory.

andrew said...

See, e.g.,
"How perturbative is quantum gravity?"
Astrid Eichhorn, Stefan Lippoldt, Jan M. Pawlowski, Manuel Reichert, Marc Schiffer
(Submitted on 5 Oct 2018)
We explore asymptotic safety of gravity-matter systems, discovering indications for a near-perturbative nature of these systems in the ultraviolet. Our results are based on the dynamical emergence of effective universality at the asymptotically safe fixed point. Our findings support the conjecture that an asymptotically safe completion of the Standard Model with gravity could be realized in a near-perturbative setting.

neo said...


Let me clarify,

could 2018 be the beginning of the end of string theory, depending on Vafa's conjecture, and now Higgs, along with nondetection of SUSY.

Eva Silverstein called Vafa's conjecture "false", as related in the news.

I grew up listening to Braine Greene Michio Kaku and Stephan Hawking promoting string theory. I can't imagine physic news after strings.

does lqg imply asymptotically safe fixed point?

andrew said...

"could 2018 be the beginning of the end of string theory," It could be, but it won't be the end until its big name proponents are all dead. Honestly, it may be the climate, but the beginning of the end was really more like a decade ago or so after which progress stalled.

"does lqg imply asymptotically safe fixed point?" I don't think anyone knows for sure, and there are various sub-types of lqg that could have different conclusions on that point. My current intuition is that lqg will not work out.

"Eva Silverstein called Vafa's conjecture "false", as related in the news." That is a gut reaction, not a credible statement of the reality.

neo said...

i'm thinking that 2018 could be the beginning of the end for string theory as SUSY not being found just means wait for the next 100TEV collider.

if 2018 with Vafa's conjecture + now Wrase Higgs, string theory is "debunked:

What would physics departments, undergrads, graduate students, and current string theorists do next? everything from courses to textbooks, if string theory is found to be incorrect?

what would Eva Silverstein, Ed Witten, Briane Green et al, do in a post-string world, will there still be graduate students still attemping phD's

ever heard of a grad student named Aaron Wall? he's currently trying to earn a phD in string theory.

he was an undergrad at saint john's college and wanted to study string theory, and now is at Princeton studying strings. (not sure who)

how would all this change if string theory was debunked? will physics departments still hire string phD's like Aaron Wall? will Nova continue to make string theory documentaries?

what's Michio Kaku going to talk about in a post-string world?

I'm aware of Urs and Mitchell's objection to LQG, i'm surprised that this issue wasn't raised among Loopers themselves like Rovelli.

Mitchell said...

May I point out that Wrase is evidently a skeptic with respect to Vafa's conjecture? His second most recent paper, with Christopher Roupec, proposes that a certain class of de Sitter critical points might be the simplest way to falsify the conjecture (and even mentions an amended version of the conjecture that may be more plausible). His most recent paper, with Renata Kallosh, blithely goes ahead and proposes still more de Sitter string vacua.

No doubt it is interesting, and maybe it's even important, that a certain kind of minimal BSM theory (SM, plus a dark-energy quintessence field that is maximally segregated from the SM sector) is severely in tension with Vafa's conjecture. This is all directing people's thoughts along new channels and *that* is certainly welcome in string phenomenology.

neo said...


i understand you work on string theory.

if Vafa's conjecture holds true, and string theory is in conflict with observation, including Wrase claims on the Higgs, building on Vafa,

what would you and other string theorists you know work on next?

Mitchell said...

Field theory.

Marnie said...

"what's the most promising non-string QG?"

What about gravity and decoherence?

Joseph Samuel
Gravity and decoherence: the double slit experiment revisited

Admittedly, not a complete QG, but to me, thought provoking.

Andrew, why do you think that "lqg will not work out" ?

Marnie said...

How about Claus Kiefer's recent talk on decoherence in quantum mechanics and quantum cosmology?

To me, Claus' work looks promising.

Unknown said...

String Theory will shortly die because M theory has been derived from an alternative approach to quantum gravity. Game over.

neo said...


if Vafa's conjecture stands the test of time and therefore string theory is falsified

as a quantum field theorist, what's your next most promising QG, is it asymptomatic safety?

Mitchell said...

If I am trying to explain all the matter fields too? Modified N=8 supergravity.

Mitchell said...

But it's more likely that string theory will force a modification of Vafa's conjecture, by providing a counterexample. I am particularly struck by the fact that Wrase finds, not just that Higgs plus quintessence creates problems for the conjecture, but *pions* plus quintessence creates problems. It seems like it should be really easy to 'design' a consistent stringy toy model that has 'pions' arising from a stack of branes, and 'Vafa quintessence' coming from a Calabi-Yau handle. I might post about this in Urs's thread at PF...

Mitchell said...

Sorry, it wasn't Wrase who made the point about pions, it was three Korean authors.

Mitchell said...

P.S. And I only just realized that in the comments above, Marnie (engineer and amateur prehistorian?) is a different person to Marni (actual quantum gravity researcher). That was confusing.

Marnie said...

Hello Mitchell

from Marnie (engineer and integrated circuit designer)

I do have an undergraduate degree in math and physics and di take quantum mechanics at the graduate level at the University of British Columbia.

Anyway, I'm not really interested in whether string theory can somehow maintain itself. I notice that Claus Kiefer refers to as an "alternate formulation" in some of his slides.

Can you point us to some papers that describe non-string approaches to "Modified N=8 supergravity" ?

Thank you.

Marnie said...

Hello Marni,

Nice to run across you on the web.

This paper (your recent paper), is a very nice:

Neutrino Oscillations with Hopf algebras

Kind regards,


neo said...

Hello mitchell,

yes i found marni and marnie to be a bit confusing to.

to clarify, Vafa conjectures that string theory is incompatible with standard dark energy, so the question is about quintessence which can also explain expansion. Wrase and the korean authors say quintessence is not compatible with higgs and even pions, is this correct?

LHC hasn't found evidence of SUSY so what makes N-8 supergravity promising?

Unknown said...

Hello, Marnie. Nice to meet you too. Yes, and I have done a lot of work since that paper was written (when I was homeless in Auckland last year).

Mitchell said...

Wrase and the Korean authors say that Vafa's conjecture is incompatible with certain forms of "quintessence + other scalars" (Higgs and pions are both scalars). Vafa and coauthors already put out a new paper amending the conjecture, I haven't read it yet.

N=8 supergravity was a candidate for theory of everything before strings. Its combination of fermions and gauge groups has some peculiar similarities to the standard model. Nowadays N=8 supersymmetry is generally regarded as a sign of 11-dimensional microphysics, and N=8 supergravity as arising from string theory.

However, Marni has expressed the idea that the extra 'dimensions' might be obtained non-geometrically (cf Alain Connes), and some of Hermann Nicolai's recent work (eg arXiv:1804.09606) could be viewed in this vein. Even Peter Woit is a fan of modified N=8 supergravity.