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Rich Grise

Jan 1, 1970
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Other blackbody radiators, normalized by peak output.


As I recall, they do. The sun emits very little microwave radiation, while
the cosmic background temperature gives off copious amounts.

(I suppose it might be possible to place microwave detectors on the sun
(nevermind the physical difficulty of diodes operating at 7000K, but that's
just an engineering problem), collecting the few picowatts of CBR that pass
along, relaying that power back to a cold sink which thereby radiates more
microwave radiation. Likewise, the cold sink transforms the visible
radiation from the detector into additional microwave radiation.

It's an interesting thought, but I'm sure that, if I ran the numbers, I'd
discover that it doesn't come out over unity, which is how it ought to be,
after all.)


Infrared perhaps, but not far IR or microwave.

Consider: if this were true, then celestial x-ray and gamma sources would be
visible on telescopes (assuming there is a direct line of sight, which for
energetic radiation, need not be). Many radio and x-ray sources are only
visible due to visible-spectrum matter, as I recall.

Not that I'm qualified here, but my gut is telling me that if you pointed
a radiotelescope at the sun, its input would probably be overloaded.

I do wonder what it would sound like, however. ;-)

Cheers!
Rich
 
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Phil Hobbs

Jan 1, 1970
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Tim said:
Other blackbody radiators, normalized by peak output.


As I recall, they do. The sun emits very little microwave radiation, while
the cosmic background temperature gives off copious amounts.

Nope, sorry. Undergraduate statistical mechanics--see e.g.
http://en.wikipedia.org/wiki/Planck's_law_of_black_body_radiation.

Cosmic background radiation is very, very faint--which is why it's hard
to measure.
(I suppose it might be possible to place microwave detectors on the sun
(nevermind the physical difficulty of diodes operating at 7000K, but that's
just an engineering problem), collecting the few picowatts of CBR that pass
along, relaying that power back to a cold sink which thereby radiates more
microwave radiation. Likewise, the cold sink transforms the visible
radiation from the detector into additional microwave radiation.
It's an interesting thought, but I'm sure that, if I ran the numbers,
I'd discover that it doesn't come out over unity, which is how it
ought to be, after all.)

Right, which is one good way of proving that the curves can't cross--if
they could, you could just use an optical filter and make energy flow
spontaneously from cold to hot--which would be a perpetual motion machine.

Antennas are commonly tuned by pointing them at the sun and adjusting
for maximum signal at the desired frequency--see e.g.
http://www.jach.hawaii.edu/JCMT/telescope/pointing/SolarAlign.html and
http://www.hs0zee.com/Amateur/Quad Antenna/Quad Antenna.htm.

Infrared perhaps, but not far IR or microwave.

Not so. See above.
Consider: if this were true, then celestial x-ray and gamma sources would be
visible on telescopes (assuming there is a direct line of sight, which for
energetic radiation, need not be). Many radio and x-ray sources are only
visible due to visible-spectrum matter, as I recall.

The difference there is that their angular subtense is so very small--in
the picoradians or smaller, something like 10**-23 steradians. If you
made an antenna big enough that a quasar, say, could fill its entire
beam width, you would indeed get a big fat signal, bigger than if you
pointed the same antenna at a star. The problem is that such an antenna
would be the size of the Milky Way, or thereabouts.

Cheers,

Phil Hobbs
 
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Jasen

Jan 1, 1970
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Other blackbody radiators, normalized by peak output.


As I recall, they do. The sun emits very little microwave radiation, while
the cosmic background temperature gives off copious amounts.

the sun gives off enough to drown out the signals from satellites when it passes behind them.
empty space isn't a problem.
 
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redbelly

Jan 1, 1970
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How does it manage that? An antenna even as big as the
orbit of Earth is still small compared to that wavelength
(of 2.4 billion kilometers). The Sun is positively tiny
compared to that wavelength.

That doesn't seem to matter. Atoms are thousands of times smaller
than the wavelengths they radiate. And 60 Hz radiation has a
wavelength of about 3000 miles, yet our local house wiring manages to
radiate plenty of it.

Mark
 
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redbelly

Jan 1, 1970
0
Other blackbody radiators, normalized by peak output.


As I recall, they do.


No, they don't. People often draw different blackbody curves with a
normalized peak output = 1, which makes them appear to cross. But in
terms of actual power radiated (per source area), they do not cross.
A hotter blackbody always radiates more than a cooler blackbody, at
any wavelength.
 
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Don Bowey

Jan 1, 1970
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the sun gives off enough to drown out the signals from satellites when it
passes behind them.
empty space isn't a problem.

Even with mid 50s search radar technology, I could watch the sun rise on the
scopes.

Don
 
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