Physicspunks

[Tim May]

(The physics cited by Anonymous is hand-waving. I regret spending even 
10 minutes composing this reply. First, the physics is naive. Second, 
this has nothing to do with any conceivable topic of interest.  Third, I 
expect Choate will be jumping in with his Choate Prime version of 
physics.)

On Thursday, August 16, 2001, at 05:03 PM, Anonymous wrote:

> The photon discussion a few weeks back got me reading about cavity
> radiation.  I'm puzzled.  Perhaps somebody can point me in the right
> direction.
>
> For those who don't already know, cavity radiation is a surprising
> phenomenon which required quantum theory to model.  Metals radiate
> energy in the form of light.  Each metal has a characteristic
> "radiance" for each temperature, the amount of energy it radiates.

Planck solved the black body radiation problem by figuring out that 
energy levels are quantized (heading off the "ultraviolet catastrophe," 
which we knew wasn't happening, but not why it wasn't).


>
> If a block of a metal is hollowed out and a small port is drilled to
> see in, the radiance of the cavity is substantially higher than that
> of the surface of the metal.  As if that weren't shocking enough, it
> turns out that the radiance of cavities is the same no matter what
> kind of metal is used.  (This is so counterintuitive that I almost
> don't believe it!)

It is not correct to say the radiance is either higher or lower than 
some other material: the radiance approximates that of a perfect black 
body.

These are not mystical objects. I used them in my lab a few decades ago. 
"Integrating spheres" is the lab supply store name. Looked at from 
another point of view, a hollowed-out sphere with a small hole. Light 
entering the sphere bounces around and is absorbed, reflected, bounced 
around, etc., until it "thermalizes." The integrating sphere thus 
"integrates" the light hitting the hole.

(A more mundane example is the ordinary house with a glass window. 
Nearly all of the light passing through a house window is "thermalized.")

Take the above physics and play it BACKWARDS IN TIME: the light exiting 
the hole in an integrating sphere is the result only of the TEMPERATURE 
of the sphere.

The sphere behaves pretty much like a perfect black body radiator. 
Geometry is the key.

Nothing mystical at all.

> So far so good, but this argument should also apply to surface
> radiance, which we know to be different:
>
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
> XXXXXXXXXXXXXXXXXXXX YYYYYYYYYYYYYYYYYYYY
>
> If Y has a higher level of surface radiance than X, then one would
> expect X to grow hotter, also making possible a perpetual motion
> machine.
>
> Clearly, something went wrong somewhere.  Can anybody clue me in?


The "perpetual motion" part is a non sequitor.

There are many cases where a material of higher radiance, e.g. the 
surface of the earth, is "looking at" (in the sense of the drawing 
above) a material or thing of lower radiance, e.g., deep space.

And guess what: the earth radiates more energy toward deep space than 
deep space radiates toward the earth. This is one reason deserts get so 
cold so fast at night.

Absent other sources of energy input (heating by the sun, geological 
energy, etc.), the earth would indeed eventually reach thermal 
equlibrium with deep space. So? Nothing surprising, and no perpetual 
motion.

You need to spend about an hour reading up on some basic thermodynamics, 
especially about why all objects in a furnace (a good approximation to 
the cavity radiator) look to be the same "color" no matter their 
composition.

Think in terms of the time-reversed model, with light going into a 
furnace and bouncing around, if it helps you to see the intuitive reason 
for the equilibrium solution.

Why would someone ask for help on a physics problem using anonymous 
methods? I suspect a troll.


--Tim May