Why was it specifically 40 years and how does a radio signal get back to you being sent out in space?
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The Altair star system is a little less than 20 light-years from here, so a one-way radio signal (which travels at the speed of light) would take a little less than 20 years to arrive. If we assume that someone from that system heard the message, took some time to decode it and formulate their reply, and sent that reply back, it would be reaching us right about now.
That’s part of the key: they’re not necessarily expecting to get the same signal back. It *is* possible to reflect signals off of objects in space -ham radio operators like to bounce their signals off the Moon, for example- but Altair is too far away for that to work: too much of the signal would be lost over that kind of distance. It’s not impossible that people in that star system might just copy the message and send it back, but even if they do, any signal strong enough for us to hear would have to be a new transmission. It’s hoped, however, that a completely reply will come back instead.
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