(Science: radiobiology) wave packets (pulses) which have been reflected or otherwise returned to the detector, which are sufficiently delayed and retain sufficient magnitude so that they are perceived as a signal distinct from the one transmitted directly. (in other words, just like sound echoes, only for analogous phenomena with other waves.)
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... Unless your aliens are so good that they can change the laws of physics, it is improbable that they could come here. If they are quite far, the echoes of human activities would come to their planet with a delay equal to the number of light years between our worlds. Then they would have to come ...
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