The primary motivation for solving the binary black hole problem in numerical
relativity is to supply waveforms for gravitational wave detectors.
Cross-validation of waveforms between different groups (and codes) and
comparison with post-Newtonian predictions will be essential for numerical
waveforms to be used in the computationally expensive searches conducted by
the international gravitational wave community.
The importance of cross-validation of numerical relativity results as a
community effort has prompted the formation of the Apples with Apples Alliance,
whose purpose is to coordinate and bolster the efforts of the various groups
developing numerical relativity codes.
The major activities of the Alliance are to formulate standardized tests for
code comparison, document and archive the test results and analyze their
implications for the design of improved codes.
Establishing a paradigm for standardized testbeds for numerical relativity is a
formidable task in itself. First of all, it is important to realize that the
numerical relativity community is small, with very limited available manpower.
In contrast to the size of the field, we are trying to solve many difficult
problems at the same time. Numerical methods are being developed in parallel
with the formulation of the continuum problem, with the construction of
physically relevant initial data sets, and with the unraveling ofthe physical
processes involved in the systems under investigation. All of this is, so far,
without the help of comparison with experiments. Groups working in the
field are faced with many fundamental questions in designing their approaches,
and codes are in a state of flux that makes careful documentation easy to postpone.
We propose to build up a suite of standardized testbeds for comparing
approaches to the numerical evolution of Einstein's equations, that are
designed to both probe their strengths and weaknesses, and to separate out
different effects, and their causes, seen in the results.
We distinguish two fundamentally different types of testbed: The first type
compares different codes and methods in the treatment of a physically
interesting set of solutions. In the context of the binary black hole problem,
a detailed comparison of nonspinning equal-mass inspiral would be a natural
example. The second type are idealized situations, such as the shock tube
test in computational fluid dynamics.
The ideas for this program originated in informal
discussions at the numerical relativity workshop at
Krugersdorp in South Africa, 2001. An initial meeting
was held at UNAM, Mexico City, in May 2002.
These web pages are a resource for a number of numerical
relativists from different groups, who intend to participate
in this project of code-comparison. The intent of the web site
is to facilitate the exchange of information and
results. It also forms a central repository of useful
notes, links, and tips, for working relativists, and we
welcome everyone in the community to make use of these
pages and hopefully even
contribute their own
results and ideas.