Astrobiology: Message in a bottle

@article{Sullivan2004AstrobiologyMI,
  title={Astrobiology: Message in a bottle},
  author={Woodruff Turner Sullivan},
  journal={Nature},
  year={2004},
  volume={431},
  pages={27-28}
}
Extraterrestrial civilizations may find it more efficient to communicate by sending material objects across interstellar distances rather than beams of electromagnetic radiation. 
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References

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Inscribed matter as an energy-efficient means of communication with an extraterrestrial civilization
TLDR
The results suggest that initial contact with extraterrestrial civilizations may be more likely to occur through physical artefacts—essentially messages in a bottle—than via electromagnetic communication.