Sciency Words: Blackbody Radiation

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


This is a thing you already know, even if you don’t know that you know it. In fact this may be one of the very first scientific discoveries you made as a child: when things get really hot, they glow. As they get hotter, they glow brighter and change colors from red to orange to yellow to white.

This concept is modeled scientifically using something called a blackbody. Imagine an object that is perfectly black, by which I mean that it reflects absolutely nothing and absorbs 100% of the light that falls upon it. No such object exists in real life (although vantablack paint comes close), but a good scientist knows how to imagine impossible things for the sake of a thought experiment.

Now if you were to observe this hypothetical perfectly black object in all wavelengths of light, you might find that it glows slightly in infrared. This is due to the blackbody’s internal heat. As the temperature goes up, the “shade” of infrared you see will start creeping towards the visible part of the spectrum.

Soon, the blackbody will glow dull red, then orange, then bright yellow. As the temperature continues to climb, green and blue will get into the mix, but when so many colors of light are mixed together you tend to see pure white. If you keep going, you’ll get into ultraviolet light and beyond.

I have to admit that I’m glossing over a lot of details here (click here or here for a little more technical info). The important thing to know is that under ideal conditions, when you’re dealing with a glowing hot substance or a glowing hot object (like a stovetop or the Sun or flowing lava), color serves as a useful gauge for temperature.

In the late 19th and early 20th Centuries, the study of blackbody radiation became a matter of paramount importance to science, in large part due to the demands of the Industrial Revolution. Industrialists really needed a way to measure the temperatures of things like molten iron or steel. Dipping a thermometer into molten metal wasn’t a practical option.

Scientists came up with ways to approximate blackbodies in real life (it’s really clever how they did this). Thing is in these laboratory experiments the light emitted by blackbodies did not behave the way it was supposed to, according to classical physics.

In 1900, physicist Max Planck came up with what must have felt like an inelegant way to model light’s weird behavior. For Planck’s model to work, he had to pretend light is sometimes a particle and sometimes a wave. That didn’t make any sense, but it worked. Then in 1905, Albert Einstein proved that no pretending was required: light really is both a particle and a wave.

And thus began the madness of quantum physics!

6 thoughts on “Sciency Words: Blackbody Radiation

  1. It’s interesting that Planck’s mathematical kludge, considering electromagnetic radiation to be composed of irreducible quantities, “quanta”, purely to solve troublesome mathematics (the notorious ultraviolet catastrophe), eventually came to be recognized as a physical reality, what we now call “photons”. It reminds me of all the people who considered Copernicus’ theory useful mathematically, but something that just couldn’t reflect reality.

    Of course, “reality” is itself just another predictive model that we use. In the end, we can only assess these models by how accurately they predict future observations. Something to remember when assessing quantum theories and interpretations.

    Liked by 1 person

    1. Quite true. I don’t really have much to add to what you’ve said. We all depend a lot on theoretical models of how we think the real world works. That’s not just a scientific truth; that’s a fact of life.

      Liked by 1 person

    1. It’s important to have both of those: theoretical and experimental physicists. Though as I understand it, Planck did a fair amount of experimental physics himself. I could be wrong about that. I’m not as familiar with his biography as I am with other physicists.


  2. Ironic at the end of the 19th century physicists thought there was little left to discover. Then along comes a small problem like blackbody radiation, and poof! Here comes quantum mechanics!

    Liked by 1 person

    1. I remember reading a 19th Century quote that the mystery of blackbody radiation was “a matter of detail” that would be worked out soon enough, but that otherwise classical mechanics had the whole universe figured out. I think of that whenever I hear that we’re close to having a grand unification theory or a “theory of everything.” I keep wondering what small matter of detail we might be overlooking.


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