Because of Reasons™, an organization in my world needs to design and construct a transmitter capable of continuously pumping out a looping radio transmission that can be recognized and deciphered out to about sixty thousand light-years.

Roughly how much energy would said transmitter need to dump into the signal to accomplish this? How the energy is acquired is of no concern - this organization can daisy-chain Dyson swarms if that's what it requires.

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    $\begingroup$ At radio frequencies, or is modulating a laser OK? Or are you talking spherical transmission not point-to point? Related. $\endgroup$ Sep 18, 2021 at 3:28
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    $\begingroup$ While it's true that the Stack allows real-world questions with a worldbuilding context, that context really should be something more than "I'm building a world, it's a lot like the real world, none of the rules that make it fictional apply." This is a plain physics problem (propagation of radio waves through a vacuum) that can be solved with some basic Googling or, if that doesn't work (and you're expected to actually try), then by asking on Physics. $\endgroup$
    – JBH
    Sep 18, 2021 at 3:51
  • $\begingroup$ How complicated is the message? $\endgroup$
    – Mary
    Sep 18, 2021 at 3:51
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    $\begingroup$ @Mary Generally speaking, If the receiver is sensitive enough and if the transmitter were in space, the complexity of the message is irrelevant (Voyager 1's transmitter is about as powerful as a refrigerator light bulb). What would be relevant (much more relevant than this question) is the encryption or composition of the message as it's unlikely aliens speak French and even less likely that they understand MP4 video encoding. $\endgroup$
    – JBH
    Sep 18, 2021 at 3:55
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    $\begingroup$ What are you aiming at? If you want to transmit to one specific solar system on the far side of the galaxy that is a completely different proposition to blanketing half the galactic rim. $\endgroup$
    – Ash
    Sep 18, 2021 at 4:23

2 Answers 2



The jansky (symbol Jy, plural janskys) is a non-SI unit of spectral flux density,[1] or spectral irradiance, used especially in radio astronomy. It is equivalent to 10e−26 watts per square metre per hertz.

It all in some complicated relationships with other stuff, funny and with pictures https://science.nrao.edu/facilities/vla/docs/manuals/oss/performance/sensitivity

Which also includes exposure, which defines bitrate as well, and defines detection range.

Looking at Radio telescope sensitivity calculator and understanding not so much from it, but having some fantasy and gaming expirience from playing with it, totally ignoring antenna gain, and just taking System Equivalent Flux Density (Jy) == 1743Jy, which I understood as antenna own noise it has at 50K etc.

We probably ready to estimate lower limit for signal detection, but warranty is void at inception.

Energy required to arrive to a detector looks like 3e-23 W/m2/Hz (that Hz thing is not that important for our estimations it can be 100MHz band or monohromatic source, things are not necessarily that simple there but let's ignore for simplicity)

So omnidirectionl source emitter should be about 1e20 W, if a reciever is 1m2 antenna. So bigger antenna can slash the number proportionally to its area, 1km2 as an example, maybe by 6 orders of magnitude. A dyson worthy big one, 10'000'000 km square, probably can slash it down to a 1W, eh.

If it is a more directional emitter, it also can help reduce power requirements, getting there 4 orders magnitude reduction is reasonable enough with usual phase array, and more with some laser based solutions, but there will be limits defined by all kinds of factors.

Bitrate also affects required power, but it hard for me to tell, but for current numbers, as it defined as detection with 1s exposure(which comes from watts in Jy), it seems that number has to be multiplied by desired bit rate, but I can be wrong about that.

So 1MW emitter, 10'000 by 10'000 km reciever, modem bit rate from 90's can be a valid setup, for directional connection. And few GW's for omidirectional signal.

Hm, seems someone needs to make bigger antennas, eh, because it seems if we need to waste more than few GW's they aren't worthy to listen to our radio station with best hits from 60'000 BCE


The most powerful transmitter in a solar system is ... the star. We know that we can detect a star out to 60k light years because we know of stars at that distance.

So if we want to broadcast across the galaxy, it may be cheaper/easier to modulate the output of your star than to build a terawatt-scale omnidierctional transmitter.

How can we modulate a stars output?

  • Controllable mirrors/lenses on a dyson sphere.

  • Use a gravity lensing to direct light at/away from a NULL target

  • If you tap the star (eg sci-fi constructs penetrating into the stellar atmosphere) maybe you can alter the fusion rates directly.

  • Temporarily reduce the effects of gravity within the star. This will permit the star to expand and slow down it's fusion. Given time this would dim the star. I don't expect much bandwidth with this method.


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