Theory Carnival: Phenomenological Quantum Gravity

[Geek Mommyprof from Academic Jungle is hosting a carnival on real people's work in theoretical or computational sciences, and what that work entails. She asked me to contribute some lines about what I do for a living, so here we go.]

I am a theoretical physicist and I work on the phenomenology of quantum gravity. Phenomenology is the part of theory that makes contact with experiment. (For more read my earlier post On the Importance of Phenomenology). Quantum gravity is the attempt to resolve our problems in formulating a common treatment for the quantum field theories of the standard model and Einstein's general relativity. Quantum gravity has for a long time been dominated by theory, and it's only been during the last decade or so that more effort has been invested into phenomenology.

I like working in this area because it offers interesting and still unexplored topics, and if there will ever be an experimentally confirmed theory of quantum gravity there's no way around phenomenology. My work requires keeping track of what the theorists are doing and what the experimentalists are planning and trying to find a way to connect both. Since gravity is a very weak interaction, finding evidence for its quantum effects is difficult to do, and so far there has been no signature. In fact, it can be quite frustrating if one puts in the numbers and finds the effect one considered is 40 orders of magnitude too small to be measurable, which is the normal state of affairs. I've joked on occasion I should write a paper "50 ways you can't measure quantum gravitational effects," just so all my estimates will finally be good for something. But there are areas, early universe and high energy densities, high energies and large distances, where it doesn't look completely hopeless.

Lacking a fully established theory of quantum gravity, phenomenology in this area requires developing a model that tests for some specific features, may that be extra dimensions, violations of Lorentz Invariance, antigravitation or faster-than-light travel. Model building is like having a baby. While you work on it, you have an idea of how it will be and what you can do with it. Yet, once it's come into life, it starts crying and kicking and doesn't care at all what you wanted it to do. Mathematical consistency is a very powerful constraint that is difficult to appreciate if one hasn't made the experience: You can't just go and, for example, introduce antigravitating masses into general relativity. It sounds easy enough to just put in stuff that falls up, but once you look into the details the easy ways are just not compatible with the theory, and it turns out to be so easy not. (I should know, since I spent several years on that question and out came a paper that I doubt anybody read.)

You might ask now, well, what has antigravitation got to do with phenomenological quantum gravity? Nothing actually. It's just that people always ask me what I work on and I used to say: A little bit of particle physics and a little bit of cosmology and my recent paper was about this-and-that and I'm also interested in the foundations of quantum mechanics and organizational design, and then I wrote this paper on the utility function in economics and so on. But I figured that what they actually wanted was a three word answer, so that's why I work on phenomenological quantum gravity. On the institute's website it says I work on "high energy and nuclear physics," which isn't too far off, still, 5 is larger than 3.

But no matter what the headline, what my work looks like is like this: I start with an idea and try to build a model that incorporates it while maintaining mathematical consistency, after all that's what I sat through all these classes for. In addition, the model should be compatible with available data and ideally predict something new. The failure rate is high. But there's the occasional idea that turns out not to be a failure. It gets written up and submitted to a journal and, if all goes well, gets published. I usually publish in Classical and Quantum Gravity, Physics Letters B or Physical Review D.

In the process of working on a paper, I almost always have an ongoing exchange with some people who work on related topics. If the finances allow it, I might visit them or invite them to come here. I might also attend a workshop or conference, or organize one myself. In addition, my work brings the usual overhead like writing or reviewing grant proposals, attending or giving seminars, coming up with a thesis topic, reading applications, reviewing papers, attending faculty meetings and so on. I presently work at a pure research institute, the Nordic Institute for Theoretical Physics in Stockholm, and have no teaching duties, which has advantages and disadvantages. And if you are following this blog you know that I'm only just back from parental leave.

For more on what my work is like, see also What I am is what I am and One day. You can also follow me on Twitter, or Google+.