Thread: Messages sent to a planet 20 Light years away

Originally Posted by Eonnn

I thought radio waves dissipate over distance? like if you go into the country you can't pick up the same radio stations as you can in the city...

That has more to do with interference than signal strength. It can be shown that the electric (or magnetic) field amplitude of a light signal degrades with only the first power of distance, ie. if you double the distance it gets cut in half. But that's actually a very slow degradation. An alien civilization with even a modest radio telescope would easily be able to detect our ambient radio noise up to pretty much any arbitrary distance (up to about 80 lightyears away, obviously).

I think they do. There's hypothetically a "shell" of radio wave millions of miles away from Earth and once you break that shell it's complete radio silence.

I'm pretty sure there'd need to be some kind of relay system in place to communicate with another planet.

Even then it would still be a ridiculous way of communicating unless is was instantaneous(in the vein of Ender's Game)

Originally Posted by Elis D.

I think they do. There's hypothetically a "shell" of radio wave millions of miles away from Earth and once you break that shell it's complete radio silence.

Except for natural sources of radio and possible extra-terrestrial sources

Think of it this way. For every single star that is visible to the naked eye, their is the possibility of a planet that is capable of seeing our radio signal with a good sized telescope.

Originally Posted by Elis D.

Even then it would still be a ridiculous way of communicating unless is was instantaneous(in the vein of Ender's Game)

Well actually, I think its about as useful as being able to send a slow signal that we're here so that anyone out there might be able to send a way to create a faster signal back to us, a la Isaac Asimov's Contact. Also, I'd like point out another point Asimov so astutely made; we've already been sending signals; just not intentionally. In reality, any purposeful signal we send has possibly been beat by about 70 years or so, since our day to day radio radiation has been strong enough to have a pretty far reach for a good long time. If a civilization has the ability to decipher any info that we send, then they have already been watching our Television for 20-40 years.

Originally Posted by Xaqaria

Well actually, I think its about as useful as being able to send a slow signal that we're here so that anyone out there might be able to send a way to create a faster signal back to us, a la Isaac Asimov's Contact.

Just a small correction: Contact was written by Carl Sagan, not Isaac Asimov.

Don't electromagnetic waves dissipate according to the inverse square law?

I think they do. There's hypothetically a "shell" of radio wave millions of miles away from Earth and once you break that shell it's complete radio silence.

This is different; 100 light years or so away there will be radio silence because nobody was sending radio signals 100 years ago.

10 light years away you'll hear radio broadcasts from 10 years ago, etcetera.

Originally Posted by Xei

Don't electromagnetic waves dissipate according to the inverse square law?

Intensity drops with the inverse square law, but electric field amplitude drops with the first power of distance because intensity is proportional to the square of electric field amplitude. Now, if we're talking about your personal radio, intensity matters more. But if we're talking about a civilization intentionally trying to listen for signals (like we are), then you have to consider that they can always make their dish arrays bigger and get more detection strength. And the point is, in a SETI-like situation, the fact that electric field amplitude only drops off linearly means that a small increase in dish size greatly increases detection distance.

A good example of this is space telescopes in other spectrums. When you look at the sky, you can't see other galaxies, but the modestly sized Hubble can take deep fields showing thousands of galaxies billions of light years away, and that's because of the electric field strength dropping off so slowly.