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The Pioneer probes brought plaques (and later the Voyagers two golden discs) with a "pulsar map" imprinted on them. The purpose of the map is to make the position of the Sun clear for an alien intelligence, in the case they encounter the probes in the far future, by creating a frame of reference of known galactic pulsars with specified periods and their distance relations to the Sun.

Pulsars have nice properties for this goal; they can be detected from large distances (which gives a higher chance of some of them been known by both sides), their period is very specific, easily measured with clocks and can be unambiguosly matched to a specific pulsar (if known), and their period changes over time in a predictable manner (which allows for aliens to trace back the date of origin in which the map was made).

But I have a question. If pulsars are detected on Earth when their beams point towards us (the "lighthouse"/"becon" analogy) and there might be many pulsars that are not known by us due to us being ouside of the cone where the beam can point, then wouldn't the 14 pulsar used in the "pulsar map" only be evident for aliens in the immediate solar neighbourhood? Maybe a few pulsars can be observed by them in other parts of the galaxy but the 14 of them would cenrtainly not be the same set of prominent pulsars we know of, right?

What am I missing?

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    $\begingroup$ They're also essentially dark silent machines being cast into interstellar space in an effectively random direction. If anything ever recovers the Pioneer/Voyager probes again, it's almost certainly going to be because Humanity or Humanity's descendants either decided to chase them down, or told someone else where to find them. Either way, the salvagers will have a pretty good idea where Earth is already. $\endgroup$ – notovny Oct 24 '20 at 21:33
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You aren't missing anything. The pulsar map is fairly useless unless the spacecraft is found within the next million years, when it won't have got too far from the Sun (the probes will have travelled about 30 light years in that time).

The problem is not so much that the set of pulsars won't be visible, because they almost certainly would be if you change your vantage point by a few light years, but that the pulsar activity might cease. Pulsars have a limited lifetime and most likely will have stopped being pulsars in a million years or will not be identifiable from their pulse periods. In addition most pulsars have very high velocities, so they themselves will have moved by $\sim 10$ times as much as the probes.

For something of use beyond timescales of a million years they could have used a network of globular clusters, although again the motions of the globular clusters would need to be accounted for. It's a tricky problem when everything that's close enough that that their positions would change if you move of order hundreds of light years are themselves in motion. And of course the Sun is moving around the Galaxy too.

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