String phenomenology 2025 (https://indico.global/event/1291/) just ended. Overall, it was a very fun conference. Kudos to the organizers.
String phenomenology is a romantic and ambitious program in that this program attempts to connect string theory (or any reasonable quantum gravity) to phenomenology. However, this goal still largely remains aspirational, because, we don't fully understand the rules of the game of string theory, and even when we understand rules of the game in the limited situation the rules are often too hard to execute.
In light of these obvious problems, many activities in the string phenomenology community in the broad sense can be categorized into three activities. (Of course, it is a very rough categorization. And most of the work span over many)
Figuring out new rules of the game.
Beating the known rules to death. (e.g, make calculations efficient, exhausting search of vacua in small corners of the landscape, etc..)
Making educated guesses, with reasonable assumptions attached, to take a leap to study phenomenology.
String pheno conference this year, arguably, was a healthy mix of all three activities which made me rather happy.
Of the talks, some were particularly memorable to me.
Quantum tunneling in the geometric landscape by Yinan Wang
Despite that the concept of string landscape is decades old, and it is rather natural to think about the tunneling rate between different vacua, the study on vacuum transition has been somewhat limited. It was good to see that some people are coming back to this important question.
¡The R7-Brane is Stable! by Jonathan Heckman
Recently, there has been a flurry of activities to understand non-supersymmetric extended objects in string theory. Obviously, we don't have many tools to analyze them, and that makes understanding such objects a worthy goal. Seems like Heckman and friends figured out a way to study the stability of a particular class of non-supersymmetric brane that has a connection to type IIA/type IIB domain wall. Their result is rather thought-provoking and invites a direct investigation into a non-perturbative domain wall on the worldsheet.
Vanishing Terms in Heterotic Effective Theories by James Gray
This was a very interesting result in that James Gray found that many terms in the effective action seem to vanish despite that there is no apparent symmetry to protect them. Maybe a better understanding of this example may lead to a better understanding of symmetry/organizational principles.
Riemann-flat manifolds + Casimir energies + flux = dS maximum in M-theory by Miguel Montero
As Eva has been often emphasizing, there are many more choices than Calabi-Yau manifolds to compactify string/m-theory. Being creative to expand the boundary of the landscape, to look for interesting playgrounds, is a very worthy goal. Seems like Miguel and friends found yet another very interesting place to study de Sitter (maxima) quantum gravity.
Mapping Moduli Spaces Across Geometric Transitions by Lara Anderson
One of the most important lessons of string theory, arguably, is that there can be many descriptions of the same physics, which go by the name of dualities. When we have the help of symmetries, e.g. SUSY, we can understand the aspects of dualities rather well. But, when the number of supercharges drops below or equal to 4, despite the fact that there are many suspected dualities, it has been very difficult to get precise handles on them. Lara and her collaborators have made one more stride in understanding dualities with four supercharges, which is quite impressive.
G_2-manifolds from Diophantine equations by Jakob Moritz
Jakob seems to have found that the type IIB flux vacua me and my collaborators found in 2019 are actually dual to G_2 compactifications of M-theory. This is a very thought-provoking result. I should very carefully study his paper to digest the results. This result likely opened a new window to study many questions that were previously inaccessible.
Holography with Null Boundaries by Sav Sethi
Although AdS/CFT is an amazing conjectured duality, as the name suggests, its utility is limited. An admirable goal is to find a holographic description of quantum gravity with the boundary condition different than that of AdS. Sav seems to have found a toy model to study the null boundary condition. Probably I will have to understand the paper better.
An index for flux vacua by Severin Lust
Given that the structure of string landscape is so intricate, one may hope to find some exact measures to count the number of supersymmetric vacua. This idea is not new, as Michael Douglas did foundational work with continuous flux approximation. However, going beyond what was already done appeared to be very difficult, especially the problem of counting vacua solely with the boundary data of the moduli space. I think there is still much to understand, but I would like to see how this development unfolds in the future.
Overall, string pheno conference was rather exciting. And, frankly, it lifted my rather demoralized spirit. Can't wait to see what people will talk about at the next installment of string pheno.
July 12, 2025
Saw on the rumor mill that a senior colleauge who also interviewed for imperial accepted the offer, as the rumor mill says "Imperial Hires." It was a great experience, and I honestly felt quite honored to be seriously considered as a candidate. Maybe next year, I will be more lucky. Also, it feels silly to worry about my job while the world is burning
June 22, 2025
Soon after the news I wrote below, I heard back that I am still in the game. Oh well, hope I will learn the final outcome soon.
Besides the job prospects, physics has been quite fun recently. The work I have been doing is in no sense ground-breaking, but laying the way to the problems that I have been obsessed with for almost 9 years. Seems like I am quite close to solving them. So, very excited. In a way, it is embarrassing that it took me almost 9 years to reach the point, but still very happy.
I have still been thinking about what would be a very promising research direction for the next 5-10 years to come.
On the more formal side, string field theory is great, but its practical use is too limited due to the technicalities of the calculations involved. It would be very important to find a way to organize string field theory such that either one can make at least perturbative calculations very efficient or can perform non-perturbative calculations. For both of the problems, it is hard to imagine making any progress without having a better understanding in the fundamentals of moduli spaces of super Riemann surfaces. This is a topic that I am almost ignorant about, so I should probably start studying this topic. Or maybe, there is a way to understand the symmetry structure of superstring theory non-perturbatively?
More on the pheno side, it is becoming more and more clear that the limitation on the tadpole available in the weakly coupled type II compactifications is a huge hurdle towards finding a well-controlled semi-realistic cosmological solution. Two formidable ways out seem to be either better understand and control genuine F-theory or understand how to study stringy geometry without the help of supersymmetry better. Either way, we need to understand strongly interacting physics much better. It might take a while to get to the bottom of them.
Regarding the future of my career, it seems quite unclear how many academic positions will be available in the US, if there is any. The best I can do now is to enjoy physics while I can, and potentially try applying to places out of the US even more vigorously. But, moving to a different country with a different language at this stage of life is not something that is easy to decide on. Also, I have been becoming less and less upset about the idea of not continuing in academia if I cannot find any position.
May 19, 2025
It's been a while since the last time I wrote something here.
I heard today that I did not get the job from Imperial College. It is certainly disappointing, but also relieving in some other ways. I will probably say more.
May 6, 2025