Gravitational-wave phenomenology at PennState

Nils Andersson, University of Southampton, UK
During a few surprisingly warm days in early November 2001, 86 keen gravitational-wave scientists gathered at the State College Days Inn for the first annual Gravitational Wave (GW) phenomenology workshop. The event was an excellent celebration of the recently funded Physics Frontier Center -- the talks were of extremely high standard, and there were numerous fruitful discussions.

Talks were given on topics covering most areas relevant for gravitational-wave physics. In fact, I think the meeting is well described as a serious attempt at figuring out what ``gravitational-wave phenomenology'' might actually mean... The Tuesday morning started off with a session on astrophysics. Peter Meszaros provided an update on gamma-ray bursts and the possible connection to GWs. Particularly interesting here is the fact that there no longer seems to be an ``energy crisis'': The energy released generally seems to be within one order of magnitude of $10^{51}$ ergs. Tony Mezzacappa told us about the most recent simulations of core collapse supernovae. He easily convinced us that this is a very hard problem, involving plenty of ``dirty'' and not very well understood physics. Although he was not very optimistic about being able to do fully relativistic calculations in the near future, he indicated significant progress on understanding the stabilizing influence of multidimensional radiation transport. Basically there now seems to be a consensus in the collapse community: Supernovae simply don't explode! Joan Centrella rounded off the morning session with a nice talk covering the range of issues from source modeling to data analysis. She highlighted recent results that indicate that the dynamical bar-mode instability in rapidly spinning neutron stars may be much longer lived than was thought a couple of years ago. This could be very good news for observations!

The afternoon session was focussed on key problems in relativity. Saul Teukolsky discussed whether numerical relativity was ``on the right track''. The question was motivated by the fact that LIGO (and other detectors) are due to come online and there still are no ``accurate'' template signals for black-hole mergers. However, as Saul made clear, there has been a lot of progress recently. In particular, our understanding of the fundamental lack of stability of the ADM formalism has been much improved. The one issue of major importance that must ultimately be faced was also discussed. Namely, how to formulate ``astrophysical'' initial data. This is a very difficult problem, which requires serious attention. Abhay Ashtekar discussed recent work on isolated black-hole horizons. He outlined an exciting scheme wherein the properties of individual black holes may be evaluated in the vicinity of the horizon. If this idea could be implemented numerically, it could prove of tremendous use for black-hole excision etcetera. During the following coffee break, the main topic of discussion was Saul Teukolsky's slides: The general consensus was that they must have come out in the wash, and that he would be well advised to use permanent ink next time... The day ended with a talk by Eanna Flanagan on the radiation reaction problem. The main challenge still concerns the general binary orbital evolution in the Kerr spacetime. How are we supposed to deal with the Carter constant? Kip Thorne commented that the issue is becoming crucial as LIGO is only 5 months away. But when he then asked what people in the community were doing about it Eanna was saved by the bell (as the firealarm went off!).

In the evening Kip gave one of his vintage public lectures on gravitational waves. It was extremely well attended and clearly a very popular event.

The Wednesday began with another astrophysics session. Vicky Kalogera discussed the constraints that the several different binary pulsars pose on the general stellar population, and how this relates to observable GWs from inspiraling binaries and gamma-ray bursts. She pointed out that GW observations could challenge current stellar evolution models, eg. by finding black holes with masses above $15-20M_\odot$. Steinn Sigurdsson followed this with a discussion of stochastic GW backgrounds, both primordial and astrophysical. He discussed the fact that GW has an ``Olber's paradox'' in that the summed strain from all sources is not divergent. Recent estimates by Sterl Phinney were discussed at length. These suggest that it is because LIGO has difficulty seeing point sources that there can't be a significant astrophysical background. If you see plenty of sources, the background will swamp the detector. Given the number of galactic stellar binaries this could provide a severe problem for LISA, and people are now thinking hard about how accurately one can hope to filter out the strongest binary signals from the LISA data stream in order to unveil the primordial background. The morning session finished by Alex Wolszczan describing how the radio technology is improving towards the point where one should be able to detect GWs from relativistic binaries. This would require measurements of the pulse arrival time to $\mu$s precision. Alex suggested that this might a serious possibility on the 5 year timescale.

The afternoon was mysteriously labeled ``interface''. First, Joel Tohline gave an overview of hydrodynamical simulations of various relevant scenarios; close binary merger, bar-mode and r-mode instabilities. These simulations provide an impressive demonstration of large scale numerics leading to new insights about the detailed physics. Various codes have now reached the level of reliability where one can probe the truly nonlinear regime for quite realistic scenarios. I find that extremely exciting! Finally, Sam Finn gave the last proper talk of the day. He discussed how GW observations could provide tests of general relativity. As he put the question: ``Is there any value added by testing the theory in the dynamical sector?''. Sam provided three cases that would provide very useful information: Binary inspiral for inferring the mass of the graviton as well as mapping the actual black-hole spacetime, and black-hole ringdowns for providing unequivocal evidence of the presence of black holes. The day ended with a round-table discussion of GW phenomenology. The consensus seemed to be that we should take this to mean ``the use of GW to explore astrophysics'', which makes a lot of sense.

In the evening we were treated to a banquet at the Nittany Lion Inn. It was a memorable event, with several entertaining speeches describing Richard Isaacson's role in supporting gravity research (in view of his retirement from NSF).

The talks on the final day concerned detector technology. Massimo Cerdonio summarized that status of existing bar detectors and described possible future advances in technology. Most exciting here is the prospect for dual spheres, which would in principle provide acoustic broadband instruments with sensitivity in the kHz regime. Alessandra Buonnanno provided a peak into the future of advanced interferometers. A main issue for future generations concerns beating the standard quantum limit. Alessandra described this problem and discussed how one could hope to how to beat it. Robin Stebbins described the modeling plan for the LISA mission. Appropriately he ended the talks of the meeting on a high note by pointing out that 75% of all large scale NASA mission have failed. This is a thought too terrible to contemplate...

Kip Thorne closed the meeting with a succinct summary of the various talks, thus putting everything in context. He concluded his overview by wishing the new PFC luck in the future, and I would like to second that here. The meeting was an exciting one, and the organizers deserve a lot of credit. Not only did they provide a pleasant atmosphere, they also left plenty of time for discussions in the busy program. This should serve as a good example for organizers of future meetings: Get people debating and you will have a great couple of days.

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Jorge Pullin 2002-02-11