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This question already has an answer here:

A lot of the papers I've been reading say that's the case, so we needed VIRGO to analyze the polarization of the GWs, but not a single one of them explains the reasoning behind this decision. Why weren't both LIGO detectors aligned differently in the first place?

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marked as duplicate by Kyle Kanos, Jon Custer, rob Apr 2 '18 at 23:32

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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It appears that the alignment increases sensitivity to some events, at a cost of being able to determine polarization alone.

From https://physics.aps.org/articles/v10/110

LIGO’s two L-shaped interferometers are aligned “back-to-back,” pointing away from each other. This arrangement enhances the sensitivity of the pair for waves coming from some directions but prevents a direct test of polarization. The different orientation of the Virgo interferometer allowed the team to study polarization and show that the data strongly favor a pure tensor waveform over a pure vector or pure scalar form.

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