Ultraviolet catastrophe
The difficulty that classical physics, applied to blackbody radiation, predicts energy growing without limit at short wavelengths.
In June 1900 Lord Rayleigh applied the equipartition theorem of classical physics to light in a box in thermal balance and considered the resulting radiation formula[1]. The form that follows directly matches measurement at long wavelengths, but grows without limit as the wavelength shortens, making the total energy infinite.
The name "ultraviolet catastrophe" was coined by Paul Ehrenfest in 1911[2]. Planck, who introduced steps of energy in December 1900, was not trying to solve this problem; he was looking for the reason behind a formula that fitted the data. The story that the ultraviolet catastrophe was Planck's starting point was tidied up afterwards.
If energy can be exchanged only in steps proportional to frequency, each step is larger for high-frequency (short-wavelength) light, so such light is hardly emitted at all. That is how the infinity disappeared.
Related terms
- Blackbody radiationLight emitted by an ideal body that absorbs all light falling on it. Its spectrum depends only on temperature, not on what the body is made of.
- Equipartition theoremThe classical law that in thermal balance, every way a system can move receives, on average, the same share of energy, proportional to temperature.
- Planck constantThe fundamental constant, written h, that sets the step size of energy for light and matter. Frequency times h is the smallest unit of energy for that light.