What is a black body and why do black body radiations prove that classical physics is wrong?


What is a black body and why do black body radiations prove that classical physics is wrong?

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An ideal black body absorbs all radiation that comes into contact with it, there is no transmission or reflection. This also means that any radiation it does emit is exclusively due to internal thermodynamics of the body. Classical physics predicts that this type of radiation source has an intensity which approaches infinity as the wavelength decreases. This topic is referred to as the Ultraviolet Catastrophe, named so because it is around wavelengths below the ultraviolet where the predictions run away from reality. Quantum physics was necessary to predict an intensity vs. frequency profile that matched observations, and didn’t break physics.

A black body is just a hypothetical object that absorbs all electromagnetic energy that contacts it and then emits it back into the world. When it radiates that energy back out into the world, there is a very specific curve that dictates what frequencies of light (visible and non visible) and how much it would emit based only on the objects temperature. (Ie things glowing red hot, white hot, etc) this is what gives stars their color and what makes hot metal glow. The only difference with this happening in the real world is that whatever material the body is made of will inherently absorb specific frequencies again, making dips in the expected curve.

That black body curve stumped scientists because classical physics had no way to explain its shape. The way classical physics predicted a black body to act said it should produce much more high energy light (UV for example) but the curve always drops off sharply at a certain point, leaving a peak depending on the temperature. This lead scientists to make a different model, which was really good at predicting the higher energy light, but it had a similar problem in that this model predicted a near infinite amount of lower energy light. Together the two models could actually predict the emissions of a black body, but since they were built on different assumptions, they were incompatible with each other.

There is one thing those two models had in common. They made the assumption that light was just a continuous stream of energy that could be arbitrarily spilt into any smaller divisions as small as possible, but this is incorrect. Light cannot be broken down past a photon, which is a discrete packet of energy. Discrete meaning distint, or separate. Not discreet as in secret.

Max Planck, who you may of heard of, in 1900, wanted to make a better model of the black body problem. He wanted a single formula to describe the curve without stitching two together. So he decided “what if I assumed light could only be emitted in discrete packets of energy?” And made a model that perfectly matched the expected black body curve. He saw this as nothing more than a “mathematical trick” to get it to work, but it turns out that his assumption was correct, and he actually spent the rest of his life trying to disprove it because he hated the idea.

This discovery was the founding on quantum mechanics, and actually reopened the debate of whether light was a wave or a particle again, which was thought to be settled by the double slit experiment in 1801.

The black body is just iron. Yes, it’s wrong, but let me explain: German scientists/engineers back in the days wanted to study the temperature-spectrum relation of hot glowing metals. Because if you could know how exactly hot it is only by looking how bright it is, then it could be very useful for metallurgy.

To study such relation, one guy suggested a theoretical framework by introducing an object that absorbs 100% of any electromagnetic wave, so that we’re guaranteed any light coming out of it is emitted purely thermodynamically by itself – thus a theoretically ideal body to try out physical models for temperature-spectrum relation. This, we call the blackbody.

The black body radiation didn’t prove any classical theory wrong. But no classical theory could correctly explain the black body radiation. But quantum mechanical concept, retrospectively speaking, could. This is the very beginning of QM.