Eli5: The radio transmission sent out in space 40 years ago


Why was it specifically 40 years and how does a radio signal get back to you being sent out in space?

In: 2

>Why was it specifically 40 years

Not sure I understand this part of the question. That’s just the date when it happened, and it was Nov 1974 so very nearly 50 years ago. The Arecibo telescope sent out our strongest broadcast yet into space, with the goal of showing off humanity’s technological status.

>how does a radio signal get back to you being sent out in space

It doesn’t, it just keeps going out farther and farther into space, moving at the speed of light (radio waves are just a different frequency of electromagnetic waves, which is what light is too). We’re never going to “get this signal back”. We’re thinking someone might receive our transmission and send back *a different radio signal of their own,* because they’d know we’re radio-capable. All we’d receive is what *they* send in our direction, our own broadcast will just keep expanding outwards and onwards into space.

Here’s a page with some more info on the radio transmission, its contents, and other attempts before and since:


The hope was that aliens living near Altair, a star 17 light years away, would by now have had time to decipher the message and send a return.

The date is significant in Japanese culture: it’s Tanabata, or their “Star Festival”.

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.