<p>While radio signals have a somewhat limited range due to <a href="https://iopscience.iop.org/article/10.3847/1538-4357/acdee0" rel="nofollow noreferrer">scintillation in the interstellar medium where plasma refracts them</a>; obviously some radio emissions get through fine since we do extragalactic radio astronomy, but the directed narrowband signals we might send still lose power and coherence. Still, <a href="https://arxiv.org/abs/1305.4684" rel="nofollow noreferrer">one can design signals that get around this</a> and use the frequencies where the galaxy is fairly transparent.</p>
<p>The basic problem is that the diffraction limit makes the beam broaden with angle <span class="math-container">$\approx 1.22 \lambda/D$</span>, where <span class="math-container">$\lambda$</span> is wavelength and <span class="math-container">$D$</span> the transmitter diameter. This means that the flux declines with the square of the distance (<span class="math-container">$\propto 1/\theta d^2$</span>) and eventually reaches the noise level of the background, but also becomes too small to detect with a telescope. This is why optical signals (laser) looks promising: <span class="math-container">$\lambda$</span> is much smaller, and the galaxy is fairly transparent in some optical and IR wavelengths. But you also need to be brighter than your star, and that produces a big power requirement.</p>
<p>Here is a graph from <a href="https://www.sciencedirect.com/science/article/abs/pii/S0094576509005116?via%3Dihub" rel="nofollow noreferrer">(Shostak 2011)</a>:
<a href="https://i.sstatic.net/QsbMcANn.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/QsbMcANn.png" alt="enter image description here" /></a></p>
<p>However, note that this (slightly simplified model) implies that we can in principle send radio signals over thousands of lightyears with current technology. The caveat is that it assumes a transmitter with a size enough to target a solar system. So if the target is r=2 AU across and at distance <span class="math-container">$d$</span>, given the above beam you need <span class="math-container">$D&gt;r/1.22 d \lambda$</span>. If <span class="math-container">$D$</span> is the earth radius (using an interferometer setup) you can reach 77 lightyears at 70 MHz, and 31 million lightyears at 1<span class="math-container">$\mu$</span> (but the later laser diameter sounds a bit implausible; a 1 m aperture gives 2.55 lightyears).</p>

CC BY-SA 4.0
2024-05-29T08:13:14.723Z
2024-06-02T02:04:49.687Z
2024-06-02T02:04:49.687Z
Anders Sandberg
<p>There's no theoretical limit to distance. Bandwidth is limited by signal to noise ratio, but, given enough time, you can send any message. Search on Shannon-Hartley Theorem.</p>

CC BY-SA 4.0
2024-05-28T13:00:56.443Z
Reception period is limited by signal to noise issues.
Jon Custer
CC BY-SA 4.0
2024-05-28T14:34:47.020Z
I'm sceptical. Consider sending signals back & forth over 100 parsecs to a planet ~1 au from its star. Separating the signal from the star noise wouldn't be easy.
PM 2Ring
CC BY-SA 4.0
2024-05-28T22:34:56.283Z
But the strength of the signal goes down over distance and background noise doesn't.
in-tension
CC BY-SA 4.0
2024-05-29T23:04:50.700Z
All of the comments are correct, but overlook "given enough time." Think of sending a brief message over and over for millennia and averaging the received message. Whoever downvoted my answer: you must have some new insights into the problem. Please enlighten me.
stretch
CC BY-SA 4.0
2024-05-30T19:34:50.750Z
@in-tension and with appropriate coding and demodulation, the strength of the noise goes down over integration time, but the strength of the signal doesn't.
hobbs
CC BY-SA 4.0
2024-05-28T11:29:34.153Z
2024-06-01T09:40:43.727Z
2024-06-01T09:40:43.727Z
stretch
<p>I'm interested in both what we can do with things we've already built as well as things we could build (including with serious, but not insane amounts of funding).</p>
<p>This would be a deliberate signal sent in a specific direction and actively received from a specific direction (like you don't have to measure the whole sky, just one spot).</p>
<p>What technologies would we use?</p>

<telescope><seti>
Given our current level of a technology, how far could we send a signal that we could detect?
in-tension
# Given our current level of a technology, how far could we send a signal that we could detect?

(Asked by: username_0 on 2024-05-28)

I'm interested in both what we can do with things we've already built as well as things we could build (including with serious, but not insane amounts of funding).

This would be a deliberate signal sent in a specific direction and actively received from a specific direction (like you don't have to measure the whole sky, just one spot).

What technologies would we use?

**Tags:** telescope, seti


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## 2 answers

### Answer by username_1 on 2024-05-29. Score: 6

While radio signals have a somewhat limited range due to [scintillation in the interstellar medium where plasma refracts them](https://iopscience.iop.org/article/10.3847/1538-4357/acdee0); obviously some radio emissions get through fine since we do extragalactic radio astronomy, but the directed narrowband signals we might send still lose power and coherence. Still, [one can design signals that get around this](https://arxiv.org/abs/1305.4684) and use the frequencies where the galaxy is fairly transparent.

The basic problem is that the diffraction limit makes the beam broaden with angle $\approx 1.22 \lambda/D$, where $\lambda$ is wavelength and $D$ the transmitter diameter. This means that the flux declines with the square of the distance ($\propto 1/\theta d^2$) and eventually reaches the noise level of the background, but also becomes too small to detect with a telescope. This is why optical signals (laser) looks promising: $\lambda$ is much smaller, and the galaxy is fairly transparent in some optical and IR wavelengths. But you also need to be brighter than your star, and that produces a big power requirement.

Here is a graph from [(Shostak 2011)](https://www.sciencedirect.com/science/article/abs/pii/S0094576509005116?via%3Dihub):
[![enter image description here](https://i.sstatic.net/QsbMcANn.png)](https://i.sstatic.net/QsbMcANn.png)

However, note that this (slightly simplified model) implies that we can in principle send radio signals over thousands of lightyears with current technology. The caveat is that it assumes a transmitter with a size enough to target a solar system. So if the target is r=2 AU across and at distance $d$, given the above beam you need $D>r/1.22 d \lambda$. If $D$ is the earth radius (using an interferometer setup) you can reach 77 lightyears at 70 MHz, and 31 million lightyears at 1$\mu$ (but the later laser diameter sounds a bit implausible; a 1 m aperture gives 2.55 lightyears).


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### Answer by username_2 on 2024-05-28. Score: -1

There's no theoretical limit to distance. Bandwidth is limited by signal to noise ratio, but, given enough time, you can send any message. Search on Shannon-Hartley Theorem.

> **Comments:**
> **username_3**: Reception period is limited by signal to noise issues.  
> **username_4**: I'm sceptical. Consider sending signals back & forth over 100 parsecs to a planet ~1 au from its star. Separating the signal from the star noise wouldn't be easy.  
> **username_0**: But the strength of the signal goes down over distance and background noise doesn't.  
> **username_2**: All of the comments are correct, but overlook "given enough time." Think of sending a brief message over and over for millennia and averaging the received message. Whoever downvoted my answer: you must have some new insights into the problem. Please enlighten me.  
> **username_5**: @username_0 and with appropriate coding and demodulation, the strength of the noise goes down over integration time, but the strength of the signal doesn't.  

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