Inverse square law confusion

  • #1
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i have heard how our broadcasts will be seen by aliens far away or whatever.
but i realize those signals are going to "attenuate" by d^-2

anyway...
how come in astronomy we can see light sources millions of light years away? shouldn't those signals be far too weak to detect after such a long distance?


furthermore i am confused about the concepts of "power" and "intensity"

say we have a 100kw isotropic radio antenna
so at, say, 5 meters away, what is our power? what is our intensity? (assume vacuum)
 

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  • #2
Dale
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how come in astronomy we can see light sources millions of light years away? shouldn't those signals be far too weak to detect after such a long distance?
They start off as really really big signals.
 
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  • #3
FactChecker
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Power is energy per unit time. Intensity is power per unit area (where the area is measured perpendicular to the direction the light is coming from).
 
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Janus
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i have heard how our broadcasts will be seen by aliens far away or whatever.
but i realize those signals are going to "attenuate" by d^-2

anyway...
how come in astronomy we can see light sources millions of light years away? shouldn't those signals be far too weak to detect after such a long distance?
1. We use telescopes with large apertures that can gather much more light than our eyes can.
2. We spend a lot of time gathering light from what our telescope is pointed at. The Hubble telescope can spend weeks looking at a single spot in the sky, slowly collecting light in order to build up an image.
 
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