Relativity

Step 1: Identify the physical principle. Since the source of the signals (Swati) is moving away from the receiver (Ravi) at a relativistic speed, we must use the relativistic Doppler effect to find the time interval between the reception of signals.
Step 2: Recall the formula for the relativistic Doppler effect (time interval). The time interval between received signals ( ) is related to the time interval between sent signals ( ) by the formula: where is the relative velocity of separation.
Step 3: Substitute the given values. - Time interval between sent signals, year. - Relative velocity, . This means Ravi receives a signal from Swati every 3 years.
Step 4: Calculate the total number of signals received. The trip lasts for 15 years from Ravi's perspective. Since he receives one signal every 3 years, the total number of signals received is: