Hacker News Reader

Comment by jeletonskelly | Hacker News Reader

So, aren't we sending out radio signals in a spiral as the planet rotates that would be massively spread out over a distance of 3 billion light years? If I were broadcasting something into space constantly some alien civilization would only be exposed to perhaps a very short portion of the signal until Earth had fully rotated again and my signal lined up perfectly to their planet again.

View on HN

> aren't we sending out radio signals in a spiral as the planet rotates that would be massively spread out over a distance of 3 billion light years?

We started sending radio signals in an appreciable amount somewhere around the 1920-1930s, and have since made our radios so efficient (also with the help of satellites) that by now they are basically too weak to be realistically noticable by distant civilizations.

Given that we started about 100 years ago, those radio signals only traveled 100 light years (enough to reach quite a few solar systems, but only about 0.1% of the radius of the milky way).

We don't know quite how good alien civilizations are at signal processing, but beyond 1000 light years or so it will be seriously hard to seperate our broadcasts from random noise. So not quite 3 billion light years.

For some clarity, the signals we have been broadcasting (mostly things like TV and radio) are MUCH weaker than the radio waves that the sun produces in the same spectrum. So our signals do get "washed out".

EM waves are spherical in nature (analogous to ripples in a pond) rather than directional. So the rotation of the earth wouldn't significantly affect the strength/reception of the signal by a distant planet. The bigger factors would be signal degradation because distance and other kinds of interference.

"EM waves are spherical in nature (analogous to ripples in a pond) rather than directional."

No, they aren't. This is an approximation taught to you in high school to make certain problems more tractable. You've got at least two methods of generating non-spherically-expanding EM waves sitting in your pocket right now; neither your phone's screen nor its flashlight are perfectly spherical in emission. This is trivially observable by the fact that the orientation of the phone impacts your perception of its brightness.

It's true that you can't make a "fully" directional EM wave with no spread whatsoever, but you can make it arbitrarily small with engineering effort, and they're trivially not spherical.

Any EM waves created to be highly directional at earth-scale distances quickly lose that directionality at galactic-scale distances. Doesn't have to be spherical, but it will quickly spread out over distances much greater than typical solar system sizes.

Not true. See beam divergence:

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

Regardless of the shape of the beam pattern, the signal strength scales with distance equally as with a sphere, which is the inverse square law.

I don't think anything from Earth will be detectable beyond more than a few light years.

Well, that's true, but that is why we use dishes to direct the signal:

https://en.m.wikipedia.org/wiki/Parabolic_antenna

I think we generally believe that the dishes are used just for receivers but they are also used in directing transmitters.

In fact, there are also heavy regulations for high-speed long distance directed microwave antennas to not spread the signal too broadly:

https://www.fcc.gov/Bureaus/Engineering_Technology/Orders/19...

Yes we are sending radio light out, but it certainly has not traveled 3 billion light years. Also, our weak transmission would be nearly invisible. FRBs are interesting because they are super high-energy bursts over short time durations.

Indeed, hell, our earliest broadcasts have barely made it past the closest star systems.

If we can control the amount of energy as these broadcasts, we'd basically be gods for all intents and purposes.

Reply on news.ycombinator.com