Friday, January 30, 2015

Two Supposedly Fun Things I'll Never Do Again

Writing a review is like pregnancy (as far as I can imagine being pregnant and with due respect to real mothers...), one starts super-excited, then begins to realize the initial expectations were too optimistic and that the entire experience is going to be much tougher than originally expected; and while the "baby" is growing, everything becomes tougher and tougher, to the point that one starts looking forward to the delivery (and THAT is probably the toughest part).

This is why finishing two reviews (here and here) in the same week is pretty much like having a twin childbirth, and now I feel like one of those exhausted mothers who stares at their babies with extreme joy.

For those who missed the reference in the title, one of the best books ever..nothing less than a must-read! 

The first work is an overview on superradiance. [If you are curious about what superradiance is... well, read the book! Meanwhile, in very few words superradiance is a broad class of phenomena related to energy amplification in dissapative systems. Because of dissipation, in special kinematic configurations the energy stored in some body/medium can be transferred to another body or to radiation, thus producing a sort of amplifier].

This project started more or less one year ago. I remember having a meeting with Richard and Vitor around Feb 2014 and discussing a problem: we were working on various problems related to black-hole superradiance  and, each time we introduced the topic in a paper, we missed a comprehensive work to refer to. This means that in many of our recent papers the introduction contains two or three paragraphs explaining and repeating what black-hole superradiance is [again very briefly, because the event horizon of a black hole behaves as a vviscousmembrane, black holes too are dissipative systems and they are prone to superradiance. This can be seen through a gedanken experiment. Consider the scattering of a wave off a rotating (Kerr) black hole: in certain situations the energy of the scattered wave can be larger than the energy of the incident radiation, the difference being extracted from the black-hole rotational energy]. Thus, we thought -Hey, why don't we write a review on superradiance ourselves?-

At the beginning, our interest was to cover only superradiance of black holes, having in mind some modern applications which we are working on (food for another post). However, it soon became clear that before discussing black-hole superradiance we had to explore in more detail what superradiance was in general, how it occurs in non-gravitational systems, and which kind of phenomena in other areas of physics are related to this effect. It turned out that superradiance is really ubiquitous in physics, as it basically hinges only on one ingredient that one can always find in nature: energy dissipation. Effects like Cherekov emission are nothing but linear superradiance and various instabilities of spinning stars are also related to this process. Thus, we started including new chapters even before we started scratching the surface of our original problems. The preliminary title changed from the original "Black-hole superradiance" to a more generic "Black-hole superradiance: from fundamental physics to astrophysics" and finally settled to the one in the draft, "Superradiance", with the table of content changing (and inflating!) accordingly. For example, while writing and studying, we also prepared a list of milestones, trying to reorganize the existing literature on the subject and keeping track of the major contributions. As it turned out, we had to dig down to 1930s to give a comprehensive historical account. The list of milestones is now at the beginning of the book and I hope it would be enjoyable also for nonexperts.

Nearly at the same time, we realized that the work was evolving from a review of existing material to something between an overview and an original work, because we needed to work out new examples, do novel computations from scratch and extended previous work in a number of directions. Nonetheless, what we were working on was not even a proper book, because it contained a sizeable part (about 2-3 chapters) of review material and very specialized applications. Thus, for quite a long time we kept working on this Frankestein without even knowing whether someone would have been interested in publishing it. Notwithstanding, after the work was nearly completed (over Christmas time), we came across Springer's Lecture Notes in Physics and, quoting from their webpage:

Books published in this series are conceived as bridging material between advanced graduate textbooks and the forefront of research [...]

This description was fitting precisely how our work was shaping! We therefore submitted a draft to Springer, and that's the short story on how "Superradiance" will see birth in the next months as a Springer's Lecture Notes in Physics.

The other work is instead a proper review on astrophysical tests of strong gravity, but its birth experienced no less troubles. The story started in January 2014, when Emanuele Berti organized this workshop at the University of Mississippi. The quality of the workshop couldn't be better, for most of the experts in tests of strong gravity attended the event (although awful whether conditions in Europe made it tough for some speakers to reach Oxford, MS in time!) The workshop brought together world-leading experts in tests of gravity with binary pulsars, cosmological tests, and especially researchers working on testing General Relativity with future gravitational-wave observations based on the dynamics of compact objects (black holes and neutron stars). Such objects are very abundant in the Universe and, especially when forming binary systems, they can reach highly-relativistic velocities and experience very strong gravitational fields. Think about two objects as big as our Sun but orbiting each others nearly at the speed of light (!), if you think you can imagine how much energy is stored in such system, I'm pretty sure you would underestimate it, as it actually is enormous! No doubt that such systems are perfect testbeds to test Einstein's theory in the most extreme settings, which was the topic of the workshop in fact.

The workshop itself was quite a success, but the toughest part was yet to come. During the last days at OleMiss, we decided to split the effort among each other's expertize and prepare a sort of White Paper on current and future tests of gravity. The main motivation was that there exist excellent reviews on tests in the Solar System, on cosmology in modified gravity, and on gravitational-wave tests (here and here), just to mention a few, but most of them are focused on a specific subfield and didn't attempt at bridging observational tests with the fundamental principles of General Relativity that can be violated in alternative theories (something done very elegantly by the Parametrized Post-Newtonian approach discussed and developed by Will). In a sense, Emanuele, who lead the effort to kick off this gigantic project, wanted to take advantage of having gathered together experts in different fields, from observations to phenomenology and to more fundamental problems, to write a simple yet comprehensive work, which was hopefully useful for different communities working in this multifaceted field.

The optimistic plan was to have a first draft in place within two months after the workshop and no one, probably not even Emanuele, really thought this would possibly happen.  Rather, it quickly became clear that organizing the work of ~50 participants and potential contributors was a formidable task, especially because the goal was not to prepare a book of proceedings (which I personally find quite useless...) but really to write a single, self-standing work as if it were written by a single person. As one can imagine, it took a lot of time, skype meetings and work even just to converge on the structure, and an equally big effort to have the review in final form. Some of us were in charge of separated chapters, but we soon realized that each chapter was deeply connected to the others, so that we ended up joining forces and working on the review as a whole. I'm mostly proud of Chapter 2 in which I worked the most (isn't Fig.1 nice?!?), together with Chapters 3 and 4, but this was really a joint effort which would have just been impossible without the hard work of many people and, as another author has put it "this would have never happened without Emanuele's legendary acting like a pain in the ass to spur us contributors".

I think that Tables 1-3, which give a comprehensive (yet inevitably biased) overview on extensions of General Relativity and their effects in black holes and neutron stars, well summarize the work under the hood and we hope they will be useful for colleagues approaching these topics.

One year after this intensive work on two fronts, as someone else has put it once, now i feel all reviewed out and, although I really enjoyed writing these works, I'm done with long writing for a while and am really looking forward for the next (hopefully short!) research project...

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