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Comment by MontyCarloHall | Hacker News Reader

I post this on every Fermi paradox thread.

Non-directional radio transmissions (e.g. TV, broadcast radio) would not be distinguishable from cosmic background radiation at more than a light year or two away [0]. Highly directional radio emissions (e.g. Arecibo message) an order of magnitude more powerful than the strongest transmitters on Earth would only be visible at approximately 100 light years away [1], and would only be perceptible if the detector were perfectly aligned with the transmission at the exact time it arrived.

Space is really big, and radio emissions are comparatively really weak.

[0] https://physics.stackexchange.com/a/245562

[1] https://arxiv.org/pdf/astro-ph/0610377.pdf

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I feel the same way. How do the SETI people explain this away and justify their search? One light year is tiny, I think it would take an incredibly generous Drake equation estimate to make that contain intelligent life. Same for 100 light years too. But I lean towards pessimistic Drake equation values. Life is even more rare and precious the more I dig into the equation.

> The true stellar density near the Sun is estimated as 0.004 stars per cubic light year, or 0.14 stars pc−3.

> There are around 60,000 stars within 100 light-years, and the same again with 100-200 light-years, and then the numbers fall quickly. By the time we're 1,000 light-years out, there are fewer than 8,000 stars and, at 1,300 light-years, fewer than 5,000 stars.

There are other techno-signatures to look for: Dyson swarms (marked by irregular dimming). Infinitely more powerful transmitters - i.e. irregular pulsars. Oumuamua type fly-bys. "Unnatural" gasses in atmospheres.

All purely theoretical, of course. As far as we know, we might not even notice life on Alpha Centauri planets 4ly away.

I think a big Orion drive powered spaceship flight profile would be widely noticeable. Like, nature rarely produces hundreds of sequential nuclear detonations in deep space in quick succession, possibly perfectly matching high energy transfer orbits.

Just not sure what would the other party thing about builders of such craft.

On the other hand, its clear what would they think about anyone running a Nuclear Salt Water Rocket - eq. riding continuous fission detonation for minutes or hours! They would definitely think they are batshit insane! :D

I would have to assume that they know this and are in fact only looking for messages that are purposefully sent at very high power. It's also why they favor listening at the hydrogen line since transmissions at that frequency stand the best chance of penetrating through cosmic dust, i.e. you would only use that frequency if you were intentionally trying to transmit across interstellar distances.

I think they don't popularize the downsides of a radio search since it could threaten their already tenuous funding. Basically, they have to be optimistic in order to maintain the pittance of a budget given to them which allows us an extremely small chance at detecting a purposefully sent signal if we happen to be in the right place at the right time and listening on the right frequency pointed in the right direction, etc.

In order to do SETI right we really need radio telescopes in solar orbit far away from the Earth and enough of them so that they can cover all of the nearby stars or other likely directions for signals to originate from. The only way we're going to get there is if space launches continue to become a lot cheaper. Fingers crossed.

https://en.wikipedia.org/wiki/Hydrogen_line

For me it boils down to- would an advanced civilization beam out a very strong locating signal on purpose? They would have to be very certain they were the baddest strongest civilization in the galaxy. I can’t think of a way intelligence and that idea go together.

With one single data point, we presume that life is ubiquitous in the universe and that intelligent civilizations probably exist in more than one place, yet we assume that everyone else is scared to give away their position, even though we aren't. If we can presume intelligent life is out there even though up to now we only know of ourselves, why not presume that that life behaves like us in at least this one way? Seems paradoxical to assume differently, no?

I don’t think we are that intelligent obviously. When I watch the news this is confirmed. And when we send out a “look here and come plunder” beacon it is confirmed as well. We just exited the campfire stage a hundred years ago or so. An alien civilization could be millions or billions of years ahead of us with weapons that we can’t even imagine.

> Non-directional radio transmissions (e.g. TV, radio) would not be distinguishable from cosmic background radiation at more than a light year or two away

this is going to come across as sarcastic in text, I genuinely mean this.

Why don't the authors know this? Why doesn't someone tell them?

Because I suspect that under focused analysis the signal won’t quite look like cosmic radiation background. Now I also haven’t done the analysis, but information, and entropy are funny things. Will they get to watch a human TV show ? Hell no. Would it look “random” versus showing that it once contained some structured information ? I think that’s a very open question. There are probably more serious papers that deal with the matter and how long and far “techno-signatures” survive in the noise.

An old NTSC video signal has a clear periodic structure. Simply counting lines is going to reveal a lot about the structure and if anyone gets the idea of plotting lines they will figure out how to decode it.

An efficient digital signal is going to maximize the use of bandwidth if it looks like white noise. That said, a signal like that needs a carefully calibrated receiver, so part of the signal in time-frequency space is going to be allocated for something that is easy to acquire and will tell to how to decode the main signal. You might see a periodic structure: if you want to run a "signal frequency network" with multiple transmitters with the same signal, have to deal with a lot of reflected signals, or listen from a mobile terminal that suffers from the Doppler Effect you will probably turn off the signal for part of the cycle so that reflections can die down and you can reset your echo cancellation algorithms.

Thus digital signals aren't going to be invisible but they are going to spend maybe 5% of the energy in a way that is easy to interpret as opposed to all of it.

Makes me wonder if alien life will actually not find our messages simply because we're using such "ancient" techniques. Perhaps they invented a TV like object and used a NTSC like signal, but like 100k years ago and no one in their race knows to look for our old signals. Maybe our comms look like hieroglyphics to them ?

Periodicity is a clear signal that is useful for natural objects like pulsars. I think it would be very clear that analog videos are periodic at the line level and periodic at the frame level. If somebody gets the idea of plotting signals on a grid they will get a sequence of frames. Even creature that don’t see the way we do might develop some kind of raster basted signal processing.

On top of that TV signals would have additional periodicity because the transmitters will be hidden behind the Earth once every 24 hours.

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