On 18/02/16 09:20, Jos Bergervoet wrote:
OK, we look at:
https://dcc.ligo.org/public/0122/P150914/014/LIGO-P150914_Detection_of_GW150914.pdf
There in Fig. 1, the predicted curves are called "Numerical
relativity" and they *still* have differences in shape, that
are not inversion or shift transformations! It is becoming
more and more intriguing.
Possible explanations:
1) For some reason (to make the curve look more "natural"?)
someone decided to add random noise to the computed results.
And they added *different* noise for Hanford and Livingston.
To me this seems a silly eplanation.
2) The results are different polarization componentsa (after
all you only need a 45 degree tilt to see the independent
other polarization for a spin-2 field.)
3) The numerical routines generate some numerical errors
visible as small random looking "ripples" in the computed
result. This seems likely since complex curved space-time
will enforce a complicated non-uniform grid in the 4
coordinates.
A combination of 2) and 3) seems most likely to me (I didn't
find any proof for it in the text, but I may have overlooked
it in the extensive list of papers that have accumulated.)
As far as I understand as a non-expert in the field of GWs, in the PRL
they write that the numerical-relativity and the sine-Gauss wavelet fits
are ploted within the "detector filter". That's the usual way to compare
calculations with data. Also in our field (relativistic heavy-ion
collisions) you have to run your theoretical results for cross sections
and related observables through the detector-acceptance filter.
Sometimes that's a simple cut but often it's also a numerical routine
developed by the experimentalists taking into account details of the
detector. I guess, that's the same here. The analysis of the GW signals
out of the detector noise is far from trivial, and I cannot understand
this in all details, of course. Note that there are a lot of papers by
the LIGO+VIRGO collaboration on the arXiv, where you can find many more
details. Among them are some papers about the analysis of the detector
noise and GW signal reconstruction:
http://arxiv.org/abs/1602.03843
http://arxiv.org/abs/1602.03845
http://arxiv.org/abs/1602.03844
--
Hendrik van Hees
Goethe University (Institute for Theoretical Physics)
D-60438 Frankfurt am Main
http://fias.uni-frankfurt.de/~hees/
[Mod. note: quoted text trimmed --mjh]