Abstract
We answer frequently asked questions (FAQs) about the Hellings and Downs
correlation curve -- the "smoking-gun" signature that pulsar timing arrays
(PTAs) have detected gravitational waves (GWs). Many of these questions arise
because of intuition based on how ground-based interferometers like LIGO
respond to GWs. These have arms that are short (km scale) compared to the
wavelengths of the GWs that they detect (hundreds to thousands of km). In
contrast, PTAs respond to GWs whose wavelengths (tens of light-years) are much
shorter than their arms (a typical PTA pulsar is thousands of light-years from
Earth). To elucidate this, we calculate the exact response of a öne-arm,
one-way" detector to a passing GW, and compare it in the "short-arm"
(LIGO-like) and "long-arm" (PTA) limits. This provides qualitative and
quantitative answers to many questions about the Hellings and Downs
correlation. The resulting "FAQ sheet" should help in understanding the
"evidence for GWs" recently announced by several PTA collaborations.
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