<ul><li>Using light waves (electromagnetic radiation) with a wavelength of 21 centimeters is a practical way to communicate with alien civilizations.</li><li>The 21cm radiation is tied to the hydrogen spin flip, a fundamental property of hydrogen atoms.</li><li>21cm waves can cut through interstellar dust, making them ideal for long-distance interstellar communication.</li><li>If astronomers detect a directional surge of 21cm radiation, it could be a signal from an alien civilization.</li></ul>

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