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Comment by Retric | Hacker News Reader

You can see beaver dams from space, or atmospheric oxygen. Only sending radio signals is uniquely human.

In terms of space, single called organisms have traveled further from earth than people have. So calling us spacefaring is kind of misleading in the context of alien life. I doubt we’re detectable outside of a 100 light year sphere from earth and none of those stars look like they contain any habitable planets. The only way to suggest alien intelligent life is meaningfully different than beavers in this context is to raise the bar far enough we don’t qualify.

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Surely no single celled organism built a spaceship or other launch system to go into space because it wanted to do so?

The ability of humans to willingly create the technology to do it is what counts, in this case.

Neither had humans until very recently.

>I doubt we’re detectable outside of a 100 light year sphere from earth and none of those stars look like they contain any habitable planets.

Do you have a source for that?

His source is probably the amount of time we've been dabbling in radio comms.

However if the aliens had a big telescope surely that radius would expand?

> However if the aliens had a big telescope surely that radius would expand?

I think a more interesting question is how far away could we detect a civilisation equivalent to our own, using our current technology. Otherwise you can assume aliens (or our future selves) with arbitrarily sensitive detection equipment limited only by whether or not the detectors are inside the speed-of-light radius of the emitting civilisation.

IIRC our most powerful transmissions have been from NORAD radars looking for missiles coming over the north pole. In some ways we are now emitting less detectable signals, e.g. with lower power 5G cells and fibre broadband replacing good old radio.

GP's reasoning is probably: the only plausible way to detect human civilization (and not just our planet, which is already hard enough from a different star) is by it's radio signature, and the oldest radio emissions is from 100 years ago, give or take. Anyone standing outside of a sphere whose center is earth and radius is 100 light year hadn't been reached by our oldest detectable-from-space civilization artifacts yet.

It strikes me as a pretty intuitive arguemnt to make and true by default, to show it wrong you have to find or somehow plausibly argue for the existence of some physical medium or process that human civilization (or even earth life in general) emit or modulate in unique, detectable-from-space ways other than electromagnetic waves and that we have been doing this since before radio.

Something could detect oxygen at longer distances, but that isn’t a sign of intelligent life.

An unaccountably high level of carbon in the atmosphere could be such an indicator.

I think it might be considered evidence, but CO2 levels vary quite a bit over time. Picture what would happen if we measured atmospheric bass on a planet over a few hundred years and saw that kind of swing in CO2.

Decidable radio signals on the other hand are far less ambiguous. I just can’t think of much that would be both detectable and a smoking gun.

*gas not bass dam auto corrupt.

You can build better receivers but eventually only so many photons are being sent. Prior to 1921 people were sending out very little radio waves and the signals where vary weak. The you might move things back to say 1920 with the very first radio station but it really wasn’t putting out much power. Things ramped up fairly quickly though, at least assuming something built a truly massive detector at 100+light years.

For comparison look at what we need to do to detect messages from the voyager 1 at 1/400th a light year, realize that’s sent from a directional antenna and we know exactly where to look for it. A signal that’s ~1/1,600,000,000th as strong would take some serious hardware even with more advanced technology because you can only collect 100% of the photons in a radio transmission quickly you need a physically larger device.

Lots of discussion on that [1].

In general, the signal from our most powerful commercial radio transmitters will go below the background noise floor well before 100 light years.

However, a very low bandwidth pumped through something on the scale of the Arecibo dish at the transmitting end, and a helium-cooled receiver, could bridge many more light years. But then we're no longer talking about a sphere of a signal or even a big cone, but in astronomical terms an extremely tight-focused point-to-point laser that is more apt to just intersect cosmic dust and gases than an alien civilization.

[1] https://www.quora.com/How-far-do-radio-signals-travel-into-s...

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