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Someone posted the following on the article: "This's never made sense to me. When you consider the inverse square law for electromagnetic transmission in ligh

Someone posted the following on the article: "This's never made sense to me. When you consider the inverse square law for electromagnetic transmission in light of the distances we're talking about, any signal big enough for us to receive would have completely cooked anything at the point of origin." A quick Wikipedia search says that the inverse square law does apply to radio waves. Can someone that understands this better than I explain why this may or may not apply? It seems like a total deal killer for the project.

blhack 15y ago
Here: http://www.seti.org/page.aspx?pid=225 http://www.seti.org/page.aspx?pid=225 They address this under "No More TV for Aliens". CTRL-F for it. Also: yes the inverse square law does apply to radio waves.
- AgentConundrum 15y ago
  Direct link, courtesy of Firebug: http://www.seti.org/page.aspx?pid=225#PC754_dgStories_ctl18_tableStory http://www.seti.org/page.aspx?pid=225#PC754_dgStories_ctl18_...
ceejayoz 15y ago
Considering we can detect radio sources more than 10 billion light years away, I suspect ours can probably be spotted a few tens of thousands away.
- tedunangst 15y ago
  I can see the sun from more than 90 million miles away, but that does not mean I can see your flashlight from a few tens of miles away.
  - ceejayoz 15y ago
    With good optics and a vacuum in between, sure you can.
russell 15y ago
http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html In the middle of the table is an interesting chart that shows the distance that radio can be detected. You will notice that for large distance you need terawats of power and a very low band widths, fractions of a Hz. A big TV station doesnt make it out of the Solar System, while a very powerful, low bandwidth, focused source can be detected for hundreds of light years. Unless someone is pointing at us at the same time we are pointing at them, we will probably miss them. We can detect quasars billions of light years away because they are big mothers, but really hard to modulate, and turned off by now.
philwelch 15y ago
A quick Wikipedia search says that the inverse square law does apply to radio waves I don't understand why not. Aside from the difference in wavelength, there's absolutely no physical difference between radio waves and physical light.
demallien 15y ago
Can someone that understands this better than I explain why this may or may not apply? That's a good choice of wording :) It may or may not apply, it depends on the antenna. The physical basis for all inverse square laws - gravity, electrical charge attraction, radio signal strength, is actually geometric. They are all measurements of some kind of flux, so you have a certain quantity of signal, which then has to be divided by the surface area of the sphere surrounding the source. As the distance from the source increases, the surface area of the sphere increases as the square of the radius. So yes, if the source is in fact a point source radiating equally in all directions, the signal drops off as per the inverse square law. But, if you have a very tight beam, then the signal strength is maintained for a lot longer. In that case, signal loss is caused by diffusion, and by attenuation due to hitting matter that is in the beam. A highly parallel laser beam of a not unreasonable power level, preferably generated from a satellite orbiting outside any atmosphere, should be capable of being received pretty much anywhere in the galaxy. Alternatively, if you have a big enough dish, you could do the same thing with radio waves. Indeed, it is estimated that Arecibo is capable of communicating with any other Arecibo-style dish in a roughly 100 hundred light year diameter around the Earth, if the two happen to be pointed at each other, and one is transmitting whilst the other is receiving... http://www.cplire.ru/html/ra&sr/irm/limitations.html http://www.cplire.ru/html/ra&sr/irm/limitations.html And that's with a measly 1000kW radio, weaker than most primary radars used today.

blhack 15y ago

Here: http://www.seti.org/page.aspx?pid=225 http://www.seti.org/page.aspx?pid=225 They address this under "No More TV for Aliens". CTRL-F for it. Also: yes the inverse square law does apply to radio waves.

AgentConundrum 15y ago
Direct link, courtesy of Firebug: http://www.seti.org/page.aspx?pid=225#PC754_dgStories_ctl18_tableStory http://www.seti.org/page.aspx?pid=225#PC754_dgStories_ctl18_...

ceejayoz 15y ago

Considering we can detect radio sources more than 10 billion light years away, I suspect ours can probably be spotted a few tens of thousands away.

tedunangst 15y ago
I can see the sun from more than 90 million miles away, but that does not mean I can see your flashlight from a few tens of miles away.
- ceejayoz 15y ago
  With good optics and a vacuum in between, sure you can.

russell 15y ago

http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html In the middle of the table is an interesting chart that shows the distance that radio can be detected. You will notice that for large distance you need terawats of power and a very low band widths, fractions of a Hz. A big TV station doesnt make it out of the Solar System, while a very powerful, low bandwidth, focused source can be detected for hundreds of light years. Unless someone is pointing at us at the same time we are pointing at them, we will probably miss them. We can detect quasars billions of light years away because they are big mothers, but really hard to modulate, and turned off by now.

philwelch 15y ago

A quick Wikipedia search says that the inverse square law does apply to radio waves I don't understand why not. Aside from the difference in wavelength, there's absolutely no physical difference between radio waves and physical light.

demallien 15y ago

Can someone that understands this better than I explain why this may or may not apply? That's a good choice of wording :) It may or may not apply, it depends on the antenna. The physical basis for all inverse square laws - gravity, electrical charge attraction, radio signal strength, is actually geometric. They are all measurements of some kind of flux, so you have a certain quantity of signal, which then has to be divided by the surface area of the sphere surrounding the source. As the distance from the source increases, the surface area of the sphere increases as the square of the radius. So yes, if the source is in fact a point source radiating equally in all directions, the signal drops off as per the inverse square law. But, if you have a very tight beam, then the signal strength is maintained for a lot longer. In that case, signal loss is caused by diffusion, and by attenuation due to hitting matter that is in the beam. A highly parallel laser beam of a not unreasonable power level, preferably generated from a satellite orbiting outside any atmosphere, should be capable of being received pretty much anywhere in the galaxy. Alternatively, if you have a big enough dish, you could do the same thing with radio waves. Indeed, it is estimated that Arecibo is capable of communicating with any other Arecibo-style dish in a roughly 100 hundred light year diameter around the Earth, if the two happen to be pointed at each other, and one is transmitting whilst the other is receiving... http://www.cplire.ru/html/ra&sr/irm/limitations.html http://www.cplire.ru/html/ra&sr/irm/limitations.html And that's with a measly 1000kW radio, weaker than most primary radars used today.