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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 streng

by yannyu 8y ago

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.

prerok 8y ago
Well, that's true, but that is why we use dishes to direct the signal: https://en.m.wikipedia.org/wiki/Parabolic_antenna 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/1997/fcc97001.pdf https://www.fcc.gov/Bureaus/Engineering_Technology/Orders/19...
jerf 8y ago
"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.
- hughes 8y ago
  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.
- abdullahkhalids 8y ago
  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.
  - blacksmythe 8y ago
    Not true. See beam divergence: https://en.wikipedia.org/wiki/Gaussian_beam

by yannyu 8y ago

prerok 8y ago

Well, that's true, but that is why we use dishes to direct the signal: https://en.m.wikipedia.org/wiki/Parabolic_antenna 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/1997/fcc97001.pdf https://www.fcc.gov/Bureaus/Engineering_Technology/Orders/19...

jerf 8y ago

"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.

hughes 8y ago
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.
abdullahkhalids 8y ago
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.
- blacksmythe 8y ago
  Not true. See beam divergence: https://en.wikipedia.org/wiki/Gaussian_beam