If an astronaut travels at 0.8c relative to Earth, which consequence follows concerning the observed frequency of radio signals she transmits, according to an observer on Earth?

💡 Explanation

The observed frequency decreases because of the relativistic Doppler effect, a consequence of length contraction and time dilation. Observers on Earth measure a lower frequency; therefore, the decrease happens due to relative motion rather than an invariant or unpredictable shift.

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