Suppose that SETI researchers discovered a radio signal coming from a star in the direction of the centre of the Milky Way that was about 10% closer to the centre than the Sun. If humans sent a reply to this signal immediately, and the aliens replied when they got our message, how long would it take before the alien reply reached Earth?

Question

Quizplus · Accepted Answer

The Milky Way is about 100,000 light-years in diameter, and the Sun is located roughly 27,000 light-years from the Galactic Center. A star 10% closer to the center than the Sun would be about 24,300 light-years away. Sending a message there and receiving a reply would take twice that distance in time, or about 48,600 years, making "thousands of years" the correct scale.

Suppose That SETI Researchers Discovered a Radio Signal Coming from a Star