Our galaxy is 150–200 thousand light years across. Andromeda is 2.5 million light years away.

Even within our own galaxy detecting others is almost impossible, what to speak of someone in an entirely different galaxy millions of light years away. Light and radio signals from an alien world would have the same problem that ours have, illustrated below. After 120 years our signals haven’t even dented the Milky Way galaxy, and every time the signal doubles in distance it becomes 4 times weaker. Already our radio signals at 120 light years away are indistinguishable from background noise.

So would the signals from alien worlds that arrive here. Suppose an alien on Andromeda sent a signal that took 2.5 million years to get here, and suppose it was heard and understood here. Would the aliens still be alive when we hear them? And if we answer will they hear the answer 2.5 million years from now?