If you have a dyson swarm around a star, you can temporarily alter how much of the star’s light escape in a particular direction by tilting the solar sails on the desired part of the sphere.
If you have dyson swarms around a significant percentage of a galaxy’s stars, you can do the same for a galaxy, by timing the directional pulses from the individual stars so they will arrive at the same time, when seen from the desired direction.
It then just becomes a matter of math, to calculate how often such a galaxy could send a distinctive signal in your direction:
Nm (number of messages)
The surface area of a sphere at 1 AU is about 200,000 times that of the area of the sun’s disc as seen from afar.
Lm (bit length of message)
The Aricebo message was 1679 bits in length.
Db (duration per bit)
Let’s say a solar sail could send a single bit every hour.
We could expect to see an aricebo length message from such a galaxy once every Db x Lm x Nm = 40 millennia.
Of course messages could be interleaved, and it might be possible to send out messages in multiple directions at once (as long as their penumbra don’t overlap). If they sent out pulses at the points of a icosahedron and alternated sending bits from the longer message with just a regular pulse to attract attention, 200 years of observation should be enough to peak astronomer’s interest.
But would such a race really be interested in attracting the attention of species who couldn’t pay attention for at least a few millennia? It isn’t as if they’d be in a rush to get an answer.
If you have a dyson swarm around a star, you can temporarily alter how much of the star’s light escape in a particular direction by tilting the solar sails on the desired part of the sphere.
If you have dyson swarms around a significant percentage of a galaxy’s stars, you can do the same for a galaxy, by timing the directional pulses from the individual stars so they will arrive at the same time, when seen from the desired direction.
It then just becomes a matter of math, to calculate how often such a galaxy could send a distinctive signal in your direction:
Nm (number of messages)
The surface area of a sphere at 1 AU is about 200,000 times that of the area of the sun’s disc as seen from afar.
Lm (bit length of message)
The Aricebo message was 1679 bits in length.
Db (duration per bit)
Let’s say a solar sail could send a single bit every hour.
We could expect to see an aricebo length message from such a galaxy once every Db x Lm x Nm = 40 millennia.
Of course messages could be interleaved, and it might be possible to send out messages in multiple directions at once (as long as their penumbra don’t overlap). If they sent out pulses at the points of a icosahedron and alternated sending bits from the longer message with just a regular pulse to attract attention, 200 years of observation should be enough to peak astronomer’s interest.
But would such a race really be interested in attracting the attention of species who couldn’t pay attention for at least a few millennia? It isn’t as if they’d be in a rush to get an answer.