I’ve been wondering about this for a while, but as I’ve stated before, my knowledge of physics is woeful.
We have giant dishes for detecting radiation from other planets right?
And presumably if other species exist then they can detect us by the same methods (within a certain distance).
Why don’t the waves thin out and dissipate? As the distance grows, surely they are spreading out more and more and becoming weaker and weaker?
Otherwise, how they would miss the dishes (the area they cover would be too small and the chances of hitting a dish would be infintesimal).
The intensity of the radiation decreases by the square of the distance - so if you measure an signal strength of 10 at distance A, at distance 10A the signal strength will be 1/100 as strong, or .1
That is the reason that we use huge dishes - to capture as much signal as possible.
That said, the man-made signals that are escaping from the Earth are quite powerful, when compared to natural sources in the same frequency bands. I remember hearing that the Earth looks like a fairly bright star when viewed with a radio telescope.
Our instruments can detect the radio output of very distant stars - if another planet is emitting signals of the same magnitude as we do, we’d be able to “hear” it many light years away.
After all, light follows the same inverse-square brightness law, and we can see stars 13 Billion LY away!
Yes, but the difference is that a star puts out many, many orders of magnitude more radio-frequency radiation than any terrestrial source could possibly deliver. This isn’t just a matter of power throughput from the transmitter, either; trying to deliver very large amounts of power through the atmosphere would result in it being absorbed and/or diffused by the air.
We can see radiation sources (mostly x-ray and gamma ray) at very distant sources (billions of light years) because the sources themselves are massive and powerful. Even at that, it takes some pretty phenomenal processing capability to discern individual sources. Detecting a terrestrial radio source from a planet in a nearby system would require the ability to suss out a coherent signal from the noise. A radio signal at more than a few tens of light years would be indistinguishable from the background radio-frequency noise.
Well this indicates that the farthest SETI target is around 10,000 LY away, but this assumes that the target would need to be broadcasting using an antenna aimed at us in order to be detectable.