# B The ultraviolet catastrophe

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1. Dec 23, 2016

when I read about the ultraviolet catastrophe on the internet I get a severe headache, and what I read in it doesn't stick in my mind, and my text book says: "The classic physics failed in explaining the black body radiation because as a classical point of view the radiation is electromagnetic waves so the intensity of the radiation increases as the frequency increases." So, I would like someone give me a simple explanation of the UV catastrophe. Another thing I don't understand is how to measure the intensity of the radiation, I did some research and found that intensity is power per unit area, but still I don't know how to measure the power of the radiation, and I don't think that I understand the concept "intensity of the radiation" well.

2. Dec 23, 2016

### phinds

There have been many threads here on the UV Catastrophe. I suggest a forum search starting with the links at the bottom of this page.

Last edited: Dec 23, 2016
3. Dec 23, 2016

### Staff: Mentor

Well unless you know the math of statistical mechanics it cant be explained except to say if you apply the usual rules it gives the so called Rayleigh-Jeans law (see equation (2c2) which gives the energy density of a black body (the start of the article explains without any math what is being considered ie a cubical cavity filled with radiation):

As the frequency increases note the energy density increases without bound ie to infinity which is physically impossible.

You would measure it by say putting a radiation source in a black box with a small hole in it to measure what radiation escapes. Obviously the formula is wrong, and was found experimentally to be wrong.

Along come Planck who actually made a mistake in his derivation and got the right answer by assuming the energy comes in discrete packets. This was one of the first indications classical mechanics was wrong.

Actually there are others such as the Lorentz Dirac equation which requires QED to fix up:
https://arxiv.org/pdf/gr-qc/9912045v1.pdf
'The root of the problem resides with the fact that we are trying to describe the motion of a point particle within a purely classical theory of electromagnetism.This cannot be done consistently. Indeed, a point particle cannot be taken too literally in a classical context; it must always be considered as an approximation to a nonsingular, and extended, charge distribution. Essentially, the difficulties of the Lorentz-Dirac equation come from a neglect to take this observation into account.'

Thanks
Bill

Last edited: Dec 23, 2016