A blackbody is a theoretical idealized object described as something "absorbing all incident radiation" commonly pictured as a cavity or empty bottle/box in which waves/photons are bouncing back and forth between walls at a certain temperature defining the temperature of the cavity. The bottle has a little peephole through which radiation is escaping to be observed, as indicated in the above common illustration of a blackbody.
A blackbody is supposed to capture an essential aspect of the radiation from a real body like the visible glow from a lump of iron at 1000 C, the Sun at 6000 C or the invisible infrared faint glow of a human body at 37 C.
But why is a lump of iron, the Sun or a human body thought of as an empty bottle with a peephole?
Yes, you are right: It is because Planck used this image in his proof of Planck's Law of blackbody radiation based on statistics of energy quanta/photons in a box. Planck's mathematical proof required a certain set up and that set up came to define the idealized concept of a blackbody as an empty bottle with peephole. But to actually construct anything near such a blackbody is impossible.
It is natural to ask if with another proof of Planck's Law, the concept of blackbody would be different, possibly closer to reality?
In Mathematical Physics of Blackbody Radiation I give a different proof of Planck's Law with a different concept of blackbody as a lattice of vibrating atoms absorbing and emitting radiation as electromagnetic waves, which models a real body like a lump of iron, and not a fictional empty bottle with a peephole.
This shows the role of mathematics in the formation of concepts of the World:
- With a strange mathematical proof the World may appear strange and incomprehensible.
- With a natural mathematical proof the World my become comprehensible.
Inlägg
"But why is a lump of iron, the Sun or a human body thought of as an empty bottle with a peephole? "
SvaraRaderaYour misunderstandings get ever more spectacular. A black body is not defined as a cavity. It's simply defined as a body which absorbs all the radiation falling on it. A cavity with a very narrow entrance happens to be one close approximation of it.
This bottle with peephole (or rather two almost opposite peepholes) can be used to show the secrets behind the SB-law. Suppose the inside temp is 1000 deg C, i.e. the inner walls are glowing (look into the peepholes). This means that all parts of the inside emit radiation, acc to the SB-law, R=sigmaxTexp4, but they are also absorbing the same radiation energy (i.e. the net transferred energy is zero). Also a case with a background temp of zero deg K will give the same radiation (in this case the net radiation.) That´s to say: a blackbody surface emits energy acc to R=sigmaxTexp4, independent of the background temp.
SvaraRaderaThis described case clearly shows that the one way heat transfer theory is wrong.
What is the interest of defining a strange concept of blackbody as a cavity with peephole, when there is a natural concept of greybody to define as a real radiating body like a lump of iron? Answer: Planck's strange proof!
SvaraRaderano one but you has ever defined a black body as a cavity with peephole.
SvaraRaderaa cavity with a peephole is just the simplest object that behaves, with a very good approximation, as a blackbody and can be used to _experimentally_ study blackbody radiation.
...and it was first proposed by kirchhoff, not planck, i believe.
Claes wrote:
SvaraRaderaWhat is the interest of defining a strange concept of blackbody as a cavity with peephole
Now you got it completely backwards.
That is not the definition of a blackbody, it's to a large extent a realisation of the definition of a blackbody . Huge differense... ;-)
Sincerely,
Dol
So if a blackbody is not a cavity with peephole, what is it then?
SvaraRaderaClaes wrote
SvaraRaderaSo if a blackbody is not a cavity with peephole, what is it then?
It is an idealization, a mathematical model or whatever you want to call it.
If you want to find a fully realized blackbody in nature you need a Hawking radiating black hole...
Never the less, the cavity works pretty well as a proxy. For studying blackbodies that is, not black holes ;-)
Sincerely,
Dol
Claes, vad säger du om den här? På Chalmers har de lyckats skapa fotoner ur vakuum! Inte illa. Vad säger du som KTH-representant om det? Kanske du kan kontra med att skapa svartkroppsstrålning ur vakuum? :-)
SvaraRaderahttp://www.chalmers.se/sv/nyheter/Sidor/Chalmersforskare-skapar-ljus-ur-vakuum.aspx
Det är inte svårt att skapa ur intet: bara att skriva 0 = 1 - 1. Det är också detta som skall rädda EU ur den finansiella krisen.
SvaraRaderaClaes wrote:
SvaraRaderaDet är inte svårt att skapa ur intet: bara att skriva 0 = 1 - 1. Det är också detta som skall rädda EU ur den finansiella krisen.
Can you claim that you understand the dynamical Casimir effect Claes? Or the "ordinary" Casimir effect?
One of the contact persons in that press release, Göran Johansson, is the person I earlier recommended you to contact to see if he could help you understand the modern view of photons. He's really nice to discuss with in my opinion. I'm sure he will help you if ask him.
Sincerely,
Dol