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ok so I've read this passage, over and over again... but I'm still really confused. What is a black body? and what is black body radiation? And how does this lead to the idea that things are quantized? I can probably find the passage come monday since I'll be going down to the faculty anyway, it's in a book called "In Search of Shroedinger's Cat".

Just a side question: Is Black Body something that actually exists? Or is it a theoretical object?

A perfect blackbody is a spherical cow.

A perfect black body is assumed to simplify things, a bit like the old spherical cow joke. I don't know of anything that is a perfect black body. That said, black bodies do exist to various degrees. Anything that can be modelled as a black body [to a good approximation] has a temperature-wavelength relationship as shown below.

They are not always "black" because they can give off radiation in the optical region. Stars are modelled [albeit poorly] as black bodies, for example.


The peak of the graph is given by Wiens Law which relates the temperature of the blackbody to the peak wavelength/frequency of light emitted.

Edits: Oh, you need the quantized nature otherwise the spectrum should be the dashed line, the "Ultraviolet catastophe" already mentioned. Quantizing the light gives the more accurate model of the radiation.

Firstly, everything behaves like a black body to some degree.

A black body, as it's name suggests, absorbs all light (electromagnetic waves) that shines on it. No reflection takes place at all. As a result, the body heats up and re-emits the energy as electromagnetic waves. A black body is a body that absorbs all light incident upon it and re-emits it.

But here's the thing. A black body is supposed to emit the light at all wavelengths available to it. According to classical mechanics, there are an infinite amount of possible wavelengths (for every wavelength, there would be a wavelength of half, or quarter, or 1/8th, or 1/38276th its length, an infinite amount of wavelengths) , so they should technically be emitting an infinite amount energy.

When you turn on a lightbulb, it's pretty obvious that it doesn't emit an infinite amount of light, so there's obviously something wrong with classical mechancis. Enter quantisation, which restricts the wavelengths to discrete values, so you no longer have to worry about an infinite number of possible wavelengths.