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From: Doug McDonald on 18 Jul 2010 16:05
On 7/18/2010 1:03 PM, Robert Coe wrote:
> On Sun, 18 Jul 2010 08:18:24 -0500, Doug McDonald
> <mcdonald(a)scs.uiuc.edu.remove.invalid> wrote:
> : On 7/17/2010 8:19 PM, Peter wrote:
> :> "Robert Coe"<bob(a)1776.COM> wrote in message news:mmj4469l67f7hcf6lvijf880vbglh61io1(a)4ax.com...
> :>> On Sat, 17 Jul 2010 19:58:02 -0400, "Peter"<peternew(a)nospamoptonline.net>
> :>> wrote:
> :>> : "Robert Coe"<bob(a)1776.COM> wrote in message
> :>> : news:98p346hl4t9752r9774j6db5ioq0b1brjf(a)4ax.com...
> :>> :
> :>> :> Rich, did you sleep through high school physics? An object isn't black
> :>> :> because it absorbs IR; it's black because it absorbs all *other* wavelengths
> :>> :> and *radiates* in the IR band. That's why if you lay different-colored cloth
> :>> :> squares on snow, the black square sinks into the snow fastest and the
> :>> :> white square sinks hardly at all. Canada gets plenty of snow in the
> :>> :> winter, so they must have showed you that in the seventh grade. Did you
> :>> :> play hooky that day?
> :>> :
> :>> : Uhm! Black is the absence of color.
> :>>
> :>> Which is another way of saying that a black object absorbs all visible
> :>> wavelengths (and reflects none). Which is what I said. Your point is ???
> :>
> :>
> :> Since when does black radiate in any band?
> :>
> :
> : Since the Big Bang.
> :
> : Anything radiates if it is hot.
> :
> : However, a "red hot" piece of silver or aluminum will not radiate
> : as much as a piece of tungsten at the same temperature, in the visible
> : region, because they are "whiter", that is, reflect more.
> :
> : At room temperature the same thing applies, only they radiate
> : around 10 microns in the IR. This is really true and is why,
> : for example, the old-fashioned glass Thermos bottles are
> : silvered on the inside.
> :
> : The thermodynamic proof of this is one of the classic examples of
> : physics, and is the obvious (to a physicist) fact that led Hawking
> : to his Nobel Prize: Black Holes are black, therefore the MUST
> : radiate.
>
> If you're right about that, the Nobel Committee should stop giving prizes in
> physics until they get someone on the Committee who knows any. (But we knew
> that when they gave one to Albert Gore.)
>
> Black holes are not "black" in the classical sense. They were a theoretical
> construct *defined* as emitting nothing at all, not even radiation. They're
> called "black" ("unable to radiate in the visible spectrum") because the
> English language has no common term for "unable to radiate at all". To turn
> around and use that definition to conclude that black holes must radiate
> because other "black" objects do is absurd.

Indeed they ARE black bodies. Any photon that crosses the horizon is
sucked in.

>
> What little I know (and it is very little) about the putative radiation of
> black holes suggests that the argument is far more subtle and well-reasoned
> than what you suggest.

I actually know quite lot. It's on the edges of my business.

>The argument may also, by the way, be wrong.

No, it rests on one of the absolute pillars of Physics:
thermodynamics.

> No one,
> Hawking included, fully understands the ramifications of black holes.

That's quite true, of course. But the theory of their radiating
is actually quite simple in its essence. What Hawking did was
to generate a QUANTITATIVE quantum relativistic field theory that
gave the correct radiation spectrum, which agreed with (relativistic)
thermodynamics. No one previously had done that.

Doug
From: Doug McDonald on 18 Jul 2010 19:12
John A. wrote:

>>
>> The human body is an excellent black-body radiator. Put a person in a sealed,
>> lightless room, and the most you'll see is the luminous dial (if any) on his
>> watch. But look at him through IR-sensitive glasses, and he lights right up.
>> That phenomenon is the bane of terrorist guerillas sneaking up in the dark to
>> blow up a target.
>
> Let's hope they don't train polar bears to do it.

There are poisonous snakes that can indeed do it.


Doug McDonald
From: Wolfgang Weisselberg on 18 Jul 2010 20:05
Robert Coe <bob(a)1776.COM> wrote:
> On Sun, 18 Jul 2010 15:51:22 +0200, Wolfgang Weisselberg
>: John A <john(a)nowhere.invalid> wrote:
>: > On Sat, 17 Jul 2010 21:19:05 -0400, "Peter"

>: >>Since when does black radiate in any band?

>: > You've never heard of black body radiation?

>: Apparently not.

>: http://www.google.de/search?q=black+body+radiation+definition

>: Think of a carbon filament lamp. It's black, and if heated
>: (e.g. electrically) it radiates off heat and some light.

> What makes that a poor example is that black-body radiation doesn't have to be
> associated with visible light.

Aeh, and why did I write "HEAT and SOME light" (emphasis added)?

> The human body is an excellent black-body radiator.

Actually, no. Especially light skin reflects quite a bit of
light and is thus not a black body radiator.

> Put a person in a sealed,
> lightless room, and the most you'll see is the luminous dial (if any) on his
> watch. But look at him through IR-sensitive glasses, and he lights right up.

Try it with IR photography instead of thermography, then
report back.

> That phenomenon is the bane of terrorist guerillas sneaking up in the dark to
> blow up a target.

Can you please decide between terrorists and guerillas?

-Wolfgang
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