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Let's assume that a human colony was established on an exoplanet, dozens of light years from another inhabited planet, with the expectation that no one—not even the colonists' far-future descendants—could inform the outside world of their existence in any way, unless they launched a physical object off-planet. In other words, no radio signals, either because they're blocked or because they're jammed by another, more powerful radio source. Is there any pre-existing astronomical phenomenon that could provide this effect?

The first thing that comes to mind is a planet orbiting a pulsar, but the radiation would be too intense for a human colony to survive. (Perhaps if it were tide-locked, and the colony were on the dark side? But that has its own problems…)

Alternately, assuming the planet is far outside the edge of currently human-occupied space, could some kind of object, like a black hole, exist between the planet and the rest of civilization, blocking any signals? I'm not sure how plausible any of this is.

In general, it should be possible for humans to live in sealed colony structures on this planet, and to walk on the surface in spacesuits, although the planet doesn't need to be terraformed or have a breathable atmosphere.

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    $\begingroup$ Do you need the radio signals to be blocked/scrambled in all directions, or just in the direction towards the closest inhabited planet? $\endgroup$ – Brenn_ Nov 5 '15 at 6:19
  • $\begingroup$ @Brenn_ Toward civilization in general, but not in all directions, as the planet is outside of the rest of inhabited space. $\endgroup$ – Adam R. Nelson Nov 5 '15 at 13:39
  • $\begingroup$ Do they need to eventually contact the outside world? Or does "never" work? $\endgroup$ – Dan Smolinske Nov 5 '15 at 18:40
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The question assumes interstellar radio communications is possible in the first place. We actually don't know this is true, because we've never successfully contacted any artificial EMR-producing device beyond our own deep space probes in the Kuiper Belt, in the outskirts of our own Solar System (but still only about 6 light-hours from the Sun).

The big problem we have is the inverse-square law. Most of the radio transmissions we generate as a race that leave our neighborhood are more or less omnidirectional. That means that the power of the transmission, as received by an antenna, decreases on the square of distance, because all other things being equal your antenna is intercepting some fraction of the surface area of a sphere of the transmitter's broadcast, so at a constant carrier wave power produced by an antenna as essentially a point source, the surface area that energy is spread across is roughly (4/3)πr2, so as radius increases, the surface area increases on the square of radius, and therefore your antenna area as a fraction of surface area decreases on the square.

Currently, the most powerful single artificial radio signal generator we have is the Taldom Transmitter in Russia. It broadcasts on two frequencies, the higher one (261kHz) is generated at 2500kW or 2.5MW. That sounds like a lot, and it is, but the very nearest star is 4.25 light years away from us.

Radio waves can be measured in Janskys (it's a non-SI unit but based on metric measurements so it can be easily converted to the SI watt). A Jansky is a unit of wave power density equal to 10-26W/m2/Hz. The unit is common in radio astronomy because astronomical objects like stars involve massive amounts of EM flux acting across very long distances. Most astronomical objects outside our solar system have EM flux in the range of 1 to 100 Janskys.

Given the transmitter's frequency and strength, the radio flux density of the Taldom Transmitter as detected by a ship orbiting Proxima Centauri would be on the order of:

$$2500000 / \dfrac{4\pi}{3}(4.25 * 9.461^{15})^2 / 261000 * 10^{26} = 1.414 * 10 ^{-7}\text{Jy}$$

That's 141.4 nanoJanskys. And this back-of-the-envelope calculation doesn't account for free-space path loss and other phenomena involved in interstellar EMR transmission. Basically, if you're broadcasting for anyone to hear, nobody can hear you.

We could improve on that, dramaticaly, by making the signal directional. Ideally, if the Taldom Transmitter's power were directionalized with a parabolic antenna, say one similar to Arecibo (152m radius, 72583m2 area) that send the radio power laser-like out into space toward Proxima, the signal strength would be:

$$2500000/72583/261000 * 10^{26} = 13.197 * 10^{21} \text{Jy}$$

That's about a quintillion times the normal radio emissions of anything else in our neighborhood, so assuming the antenna was exactly spotlighting the receiver, your transmission would blow out a dime-store transistor radio. But, all you'd need to block it is one not-very-large asteroid along the transmission path (or a misalignment of the transmitter antenna by picoseconds of arc; trying to hit your target planet with a radio transmission perfectly focused at 305 meters width of beam, in the grand scheme of things, is like trying to hit the bullseye on a dartboard, mounted on a G-simulation centrifuge at top speed, from 3 miles away).

So, the answer is, you can very easily prevent the transmission of radio information between two communicating entities. An omni-directional "beacon" type signal, you wouldn't even have to block. A more directional signal could be physically blocked or misdirected with a solar sail or similar device somewhere directly in line between transmitter and receiver.

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  • $\begingroup$ In order to block the beam with a physical obstruction, you'd need an object roughly the size of the first Fresnel zone, $\sqrt{\lambda d}$, around the size of a gas giant. $\endgroup$ – 2012rcampion Nov 6 '15 at 6:58
  • $\begingroup$ @KeithS Is the Taldom Transmitter still operational on longwave? From what I can see, it seems to be shut down. $\endgroup$ – AndrejaKo Nov 6 '15 at 17:43
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Give them a faulty interstellar transmitter and let them think it works.

They're already pretty far out there. Any communication with a planet dozens of light-years away has a round trip time greater than 48 years (assuming immediate response of a planet 24 light years away). If you give them an interstellar communicator and they try it out, it won't actually send a message, then just have the equipment fake an incoming message in 50 years (if required).

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  • $\begingroup$ For some reason this solution really appeals to me. Probably says unpleasant things about my personality. $\endgroup$ – Ville Niemi Nov 5 '15 at 17:08
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    $\begingroup$ @VilleNiemi The fake incoming message can be this. $\endgroup$ – Samuel Nov 5 '15 at 17:10
  • $\begingroup$ Personally, I'd tell them there is an interstellar war going on and that they must, apart from the "secure laser relay communicator", maintain absolute radio silence or risk annihilation. The communicator should link to an AI programmed to produce suitable horror stories about events in the war. $\endgroup$ – Ville Niemi Nov 5 '15 at 17:14
  • $\begingroup$ This is probably the most clever solution here; unfortunately, it doesn't fit the setting of the story I'm writing (the colonists are protecting a secret that they know the outside world can't know about, they just need a guarantee that future people, a rebel faction, etc. can't break that secrecy), so I can't actually use it. :( $\endgroup$ – Adam R. Nelson Nov 5 '15 at 17:27
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    $\begingroup$ @Samuel I'm somehow upset that I didn't get Rick Rolled. $\endgroup$ – Mr. Smythe Aug 19 '16 at 17:42
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To physically block the signal, the most likely option is interstellar gas clouds in a star forming region — not as solid an obstacle as a star or black hole, but still reasonably noisy, and the planet can be inside a gas cloud, allowing for blocking in all directions. However, this would also make it very difficult to find the planet to colonize in the first place.

Without something in the way, absolute prevention of radio signals is probably impossible — you can overcome any level of jamming by adding more power, and adding more power only stops being an option when you are releasing enough energy to melt the planet. If you need that, the background energy is probably at unhealthy levels.

At a more practical level, you don't need a pulsar — the average star is plenty bright enough to drown out any nearby signals. Most of the time a very bright star would increase the energy needed to stand out, but a dimmer star may be more effective overall, as it would allow a lower orbit, and any signals would appear to be coming from the already very noisy surface of the star.

On top of that, a thick atmosphere would help a lot with blocking surface signals. As well as absorbing some wavelengths directly, it could allow a much more active ionosphere than Earth without increasing the surface radiation too much.

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  • $\begingroup$ "a dimmer star may be more effective overall, as it would allow a lower orbit, and any signals would appear to be coming from the already very noisy surface of the star" -- Can you expand on this? This is potentially interesting. Before I thought of the radio-blocking aspect of it, my original concept of the planet was that it was tide-locked and orbited extremely close to a red giant; could that be enough of a reason to assume that most radio signals from it would be indistinguishable from noise? $\endgroup$ – Adam R. Nelson Nov 5 '15 at 13:38
  • $\begingroup$ Actual numbers for stellar radio emissions are a bit hard to come by - the best I could find was that the sun is the most powerful radio souce we can see, especially during solar storms, but at interstellar distances similar stars are indistinguishable from background noise. $\endgroup$ – Quentin Clarkson Nov 6 '15 at 1:10
  • $\begingroup$ Even at that level, solar jamming is a real problem for geosync satellites when the sun passes directly behind the transmitter, usually only for a few minutes. A low orbit just means that the sun is almost always aligned with the transmitter. You may also have some flexibility in the level of noise generated - in looking for numbers I came across AB Doradus A, a mostly sunlike star that does have strong radio emissions due to much faster rotation. $\endgroup$ – Quentin Clarkson Nov 6 '15 at 1:10
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You mentioned that there exists a human colony. Do they need to retain their homo sapien nature?

It is possible that the planet is in orbit around a star and an external factor (say a distant black hole) is gradually decreasing the size of the orbit. Or the star is growing old and is becoming hotter and more powerful.

Such processes take hundreds of millions of years. Even the end of the solar system is predicted to cause the Sun to enlarge and heat up or something. It is possible that this human race would gradually evolve to the changing environment. Eventually the planet would get hotter, and get surrounded with gases from the star. This will probably lead to poor radio signal transmission. But humans may have evolved into more intelligent and climate-resistant species, and are still on the planet.

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