The nature of heat

March 2025

Summer is approaching, and soon we can enjoy wonderful temperatures again. Yet many people do not stop to think about what temperature actually is. How does science view heat and cold, other than the difference between sweltering and shivering? I already wrote an essay on thermodynamics addressing two forms of energy, work and heat, but today I want to give another perspective on the latter.

Heat is not a substance, but that was believed for quite some time. In the late 17th century, it was assumed that heat was a substance called phlogiston. When you burnt something, captured phlogiston escaped, and that was the heat you felt. However, combustion makes matter heavier, which suggested that phlogiston would have negative mass, which was a problematic consequence. That enigma was solved by Lavoisier in the 18th century. Heat would be a "subtle liquid" aptly named calorique. This stuff would "flow" right through matter from warm to cold places. In the meantime, oxygen was discovered, that finally explained weight gain by combustion. All this settled the problems posed by phlogiston - or did it?

On a side note - you occasionally see miraculous substances like calorique come and go in physics. For example, it was only a century ago that the ether, an omnipresent substance through which light or radio waves would travel, turned out to not exist at all. Modern variants are elusive dark matter and dark energy that, according to astronomers, help explain the structure of the Universe – does this really exist, or is this our lack of knowledge once again? Only time will tell.

So what is heat, according to modern insights? To explain this, we have to zoom in really far on matter, with our microscope. You probably know that everything you see consists of building blocks such as molecules and atoms. The warmer matter gets, the more intensively those building blocks move. Temperature is a measure of that kinetic energy. Fast "hot" particles cool down when they collide with slow "cold" particles that heat up, thereby distributing kinetic energy evenly until you are left with lukewarm soup.

Let us discuss a few facts about heat. To start, can you cool something down so much that nothing moves at all, to zero temperature so to speak? Could you freeze an atom to a standstill? That theoretical absolute zero was determined to be -273.15°C, which proved to be practically unattainable, and this impossibility is a fundamental law in physics.

Also, objects radiate light because their constituent particles move. You know of course that you can feel warm things from a distance due to invisible infra-red light. Think of taking an evening walk after a summer day, when you still feel brick walls and tarmac glowing with heat. When stuff gets blazing, you can actually start seeing it going from red hot to white hot, just by heating it further and further.

That radiation argument also works the other way - if something emits radiation, it has a temperature. For example, you have probably heard of black holes in the Universe – collapsed stars or crumbled gas clouds that suck up everything, including light. Yet even those give off a very small amount of radiation and are therefore not completely black, Hawking discovered. But when a black hole emits radiation, it also has a temperature, very close to absolute zero, but still. Think about that for a moment.

Physicists have long pondered the radiation of a black body. That is an ultimately opaque, non reflective object, that will radiate light since it has a temperature. If you would build a box with black inner walls, a pinhole in that box would emit black body radiation. Scholars have long worked on a theory of how that radiation varies with temperature. Planck solved it, at the beginning of the 20th century. He had to make a radical assumption however - a particle could only attain an energy value that is a discrete multiple of some basic amount. Energy would be quantized, opposed to continuous, so to speak. That assumption, dear reader, eventually led to quantum mechanics. But that is a different story altogether.

With these philosophical thoughts in mind, I go prepare for upcoming beach barbeques – wishing you all a nice, warm summer!

Published in category Essays. More on Education, Science or Thermodynamics.