# Inverse square law confusion

• I
• Clay

#### Clay

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)

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.

sophiecentaur, russ_watters and Clay
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).

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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