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NEVER MIND THE EOS
George Pappas
I had just finished with my PhD defense on the subject of the properties of the spacetime around Neutron stars. We had developed an analytic model for the spacetime around Neutron stars that was based on the relativistic multipole moments and had shown that it is a very good approximation of the "actual" spacetime (constructed numerically). To make the comparison we had calculated several models for several realistic equations of state (EoS), enough to show that the analytic spacetime was a good fit, but in truth they were only sequences representing almost distant parts of the parameter space. Scattered lines in the parameter space.
I had just heard that I had gotten a short fellowship to go to Germany for a few months, a nice opportunity to hunt for the next thing. In the meantime we were discussing with my supervisor, considering what to do next. One of us put the idea on the table,
"Let's see how the models look like in the multipole moments parameter space."
"Yes. Let's plot the mass reduced moments. I bet we can tell apart the different EoSs from a plot like that."
"That would be great, to have the different EoSs occupy different parts of the parameter space."
And that is how it started. We were hoping that neutron stars constructed with different EoSs would separate enough to be distinguishable between them. That would have been a very nice result, a way to tell EoSs apart. It was not meant to be though.
A few weeks later, I had enough models to cover the parameter space, but only a couple of EoSs. I plotted the models for the first EoS and they formed a surface in the parameter space of the mass reduced spin, quadrupole and spin octupole.
"That's a nice surface", I thought. "Let's see where the other EoS will lay."
We never thought beforehand that all the models, regardless the EoS, would fall on the same surface. Neutron stars are not black holes and there are too many degrees of freedom to have something like a no-hair theorem. The rest though is history. As soon as I moved to Tuebingen, I broadened the collection of EoSs that we had and still all the models were falling on the same surface.
That was the birth of the neutron star few hair idea for us, in contrast to our initial expectations. The gravitational aspect of neutron stars turned out to be simpler than what we had thought.
This is one of the interesting properties of science. You don't always get what you expect and it is fun to be surprised. And often the surprising result is better.
The serendipity of the thing was that around the same period other people had stumbled on the related effect of I-Love-Q. The idea of universal properties of neutron stars was here to stay and people were converging to it from all sides.
And that is another of the interesting properties of science, when the time has come for an idea, it will definitely emerge.
George Pappas is a research fellow at The University of Nottingham - School of Mathematical Sciences.
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Neutron stars in general relativity: simpler than expected
