There has been a lot of coverage of infrared (IR) astronomy recently and, with the release of the first JWST image, this should continue. One of the most common things I've come across in this area is the assertion that IR radiation is heat.
It isn't.
To explain why very knowledgeable people say this and why it is wrong will take a bit of explanation.
First, let's talk about light. For the average person that's a really familiar topic. Light is the stuff we see. But we can do better than that. It has been known since 1865, when James Clerk Maxwell derived the speed of electromagnetic (EM) waves, that the light our eyes can see is just one kind of electromagnetic wave. Let's call that visible light. The study of electricity and magnetism (E&M) is very mature and, without going into any detail here, one of the very well understood aspects of E&M is that energy can be sent through space as a wave of intertwined electric and magnetic fields. These all travel at the same speed and the speed derived from Maxwell's Equations is the measured speed of visible light.
It was noticed by William Herschel in 1800 that a prism produces more than just the colors that we can see. If you take light from the Sun and pass it through a prism, as was done by Newton, it is split into a fan of color in the familiar pattern of a rainbow, Red-Orange-Yellow-Green-Blue-Indigo-Violet (or not so familiar since indigo isn't a color we encounter often). Herschel was interested in the amount of energy in different colors of light. Thermometers with blackened bulbs were placed so that different parts of the spread-out sunlight would heat up the thermometers. He also noticed that a thermometer placed past the red light coming out of the prism would also heat up. In fact, the thermometer beyond the red got even warmer than the one in red light. This showed that there is something coming from the Sun that transferred energy, was bent by a prism, and was not visible. We have since learned that there are a wide range of EM waves that physicists call "light". Everything from radio waves to gamma rays. Only a tiny fraction can be seen with our eyes and are called "visible light". This is even true for sunlight. Less than half of the energy emitted from the Sun is in visible light. A bit more, still less than half, is IR. Most of the rest is ultraviolet (UV).
Next, let's talk about heat. All matter is made of molecules. These molecules are moving around, even in a solid. That kinetic energy, the energy of motion, is what heat actually is. When an object absorbs more energy it gets warmer. This is true no matter how hot the object is.
What kind of energy is emitted by an object at a given temperature? You probably remember that heat is transferred in three different ways: Convection, conduction, and radiation. Since we're going to be talking about objects that aren't touching anything, only radiation matters. What determines the properties of the radiation given off by an object? This is another fascinating topic but the most important things to know are:
1) The radiation given off is EM waves
2) (almost) All objects made of (almost) any material give off (almost) the same radiation when at the same temperature. (The "almosts" will be ignored from now on.)
3) All objects (at a nonzero temperature) give off the most radiation at a wavelength that gets shorter as the temperature gets higher.
4) If object A is at a higher temperature than object B, object A will give off more radiation than object B at ALL wavelengths.
For objects that are a few thousand degrees that peak is in visible light. For temperatures in the millions of degrees it is in X-rays. For the coldest objects warmed only by the cosmic background radiation it is microwaves. All of these types of radiation tell you (something) about the temperature of the object giving off that radiation.
What happens when various forms of EM waves are absorbed by an object? When the energy in that wave is absorbed it heats the object up. This is true of all forms of EM waves. This highlights the first misconception caused by saying that "Infrared is heat". Most people think that IR is particularly good at heating things. It is, but not because IR is heat. It is because most of the materials we deal with our day-to-day lives absorb IR quite strongly. This is not true in general.
The association of IR with heat is mostly an accident of the way it was discovered and the temperatures we normally experience.
IR is useful in astronomy NOT because of this association. Here are a few of the reasons that an IR telescope is useful
We think of space as empty, and compared to what we usually experience it is. But there are large volumes that contain lots of dust. That dust absorbs visible light. It absorbs IR far less. This allows us to see inside these clouds or even through them. This has allowed us to track individual stars as they orbit the supermassive black hole at the center of our galaxy. This work won a recent Nobel Prize in Physics.
As the universe expands light that is traveling expands with it. The earliest stars that we think exist have most of their visible light shifted into IR. That means that we need to be able to detect IR light to see them.
Different molecules absorb light in characteristic patterns in many different parts of the EM spectrum. Many of the most interesting constituents of planetary atmospheres have their most distinctive absorption features in IR light.
Many of the asteroids in our solar system reflect very little of the light that illuminates them. This means that they are very faint. But absorbing that light means that they warm up and emit light, much of it is IR light.
Being able to explore the universe in IR with JWST is sure to teach us a LOT. But it isn't because IR is heat.
