When you consider the range of appliances you have in your home, as well as thinking of industry and machinery in general, you’ll notice that mankind has put a lot of effort in to finding different ways to heat things up. From ovens and microwaves to stoves, boilers, and kettles, we have a wide array of tools at our disposal to increase temperature.

In contrast, the options for cooling are far more limited – in most homes the only appliances at your disposal for cooling are typically just a fridge and a freezer. This discrepancy raises an interesting question: why is it that we have so many more heating appliances than cooling ones?

As warm-blooded animals, humans have historically prioritized finding ways to heat themselves rather than cool down. Throughout our evolution, the methods for cooling have remained relatively simple: seeking shade from the sun, using moving air to enhance sweat evaporation, swimming in cooler water, or drinking refreshing liquids. While these techniques have served us well for centuries, our modern advancements in cooling technology have not expanded significantly beyond air conditioning.

Although air conditioning has transformed our ability to regulate indoor temperatures, it still reflects a limited range of options compared to the numerous ways we can generate heat. This historical context helps explain why our homes are filled with heating appliances while cooling devices remain sparse. The focus on heating is deeply rooted in our survival instincts and biological needs, making it a more pressing concern for humans throughout history.

How do we make things hot?

Early hominids likely mastered fire as far back as a million years ago, a pivotal development that played a crucial role in their survival. Not only did fire allow them to cook food, making it more nutritious and easier to digest, but it also provided warmth during cold nights. This ability to create heat was essential for humans as they migrated to the colder climates of the world.

The process of using fire to generate heat involves releasing the energy stored in the molecular bonds of fuel. When burned, this energy is converted into heat, which is the primary benefit humans seek. While combustion does produce waste products, many of these are in gaseous form, allowing for relatively efficient energy use. This mastery of fire fundamentally changed the human experience, enabling not only survival in diverse environments but also paving the way for advancements in technology and culture.

When you use gas-powered appliances like your stove or boiler, natural gas is burned to produce heat. This heat is either used directly to cook food, as in the case of a stove, or indirectly, such as in a boiler, where it heats water that circulates through your home to warm various rooms.
Although there are other methods of generating heat, like exothermic chemical reactions, they aren’t widely used because they are inefficient. These reactions often leave behind waste products, either in solid or liquid form, which can be not only wasteful but also harmful. Gas-powered appliances remain more efficient and practical for everyday heating needs.

About 300 years ago, humans discovered how to harness the power of electricity, which opened the door to countless innovations. One often unintended byproduct of using electricity, however, is the generation of heat in many applications.
Conductive materials have a property known as resistance, which is their tendency to resist the flow of electricity. When energy is supplied to one point in a conductive material, some of it is lost before it reaches the other another. This lost energy is typically dissipated as heat. While this can be problematic for delicate electronics like camera sensors, it’s ideal for devices designed to generate heat.

Today, we can use electricity to directly generate heat with no waste products—except for those produced at power stations during electricity generation, which is another, though related, issue.
With this capability, we now have at least three fundamental methods of producing heat. After centuries of practice, we’ve become quite skilled at it. In fact, we can generate extreme levels of heat, with the hottest temperature ever produced by humans reaching around 5.5 trillion degrees Celsius, though this wasn’t for any practical heating purpose. But while we’ve mastered heat, the question remains:

How do we make things cold?

For a long time, we relied on two basic methods to cool things down. The first involved exposing objects to something already cold. This approach had its limitations—it required a readily available cold source, which would eventually warm up as it absorbed the heat from whatever you were cooling. Using a steady flow of water was another option since water absorbs heat efficiently, but it still only gets you close to zero degrees Celsius, making it less effective for deeper cooling.
The second method was to pass moving air over the object to remove heat. Dry air, however, isn’t as effective at absorbing heat as water, so the cooling effect is often minimal, like opening a window or standing in the breeze. To improve this, you could wet the object, as moving air promotes evaporation, which removes heat from the surface. This method works much like sweating does for humans. While more effective than just air, it still doesn’t achieve significantly low temperatures.

It wasn’t until about 100 years ago that powered refrigeration—and eventually air conditioning—was invented. These systems work by pumping a substance called a refrigerant, that easily absorbs heat, into the area you want to cool. The refrigerant absorbs the heat and is then pumped out to another area where the energy is released.

Ironically, this process produces heat as a byproduct, meaning we are essentially using energy to move heat around, resulting in some energy waste. Despite this inefficiency, powered refrigeration and air conditioning revolutionized how we control indoor temperatures.

We now have some significantly more advanced methods to cool down substances. Lasers and magnets are used in advanced techniques to achieve extremely low temperatures, nearing absolute zero. Laser cooling works by using the precise interaction of photons with atoms to slow their movement, effectively reducing their thermal energy.

Laser cooling can be understood as using photons to resist the movement of atoms caused by thermal energy, which is a form of kinetic energy. As the atoms move or “wiggle” due to their thermal energy, photons interact with them, opposing their motion and slowing them down. This process reduces their movement and brings them into a lower-motion state. Essentially, the photons absorb the kinetic energy from the atoms or molecules and carry it away from the substance, resulting in significant cooling.

Meanwhile, magnetic cooling, or adiabatic demagnetization, involves aligning atomic spins in a magnetic field and then gradually reducing the field, causing the system to cool as the spins randomize.

Both of these methods are crucial in fields like quantum mechanics, where ultra-cold conditions are needed to observe and manipulate atomic behaviors. They are, however, limited to these ultra-small-scale applications as they have a very small effective range and require an enormous amount of energy, relatively speaking.

When you think about the temperatures we experience daily, we’re actually at the colder end of the possible scale. While there are things in the universe that can reach billions or even trillions of degrees, nothing can be colder than -273.15°C, the absolute lower limit – where particles are completely still.

Temperature is essentially a measure of how much movement energy the particles in a substance have. We can add as much energy as we want to make those particles move faster, increasing the substance’s overall temperature. However, absorbing that movement energy to slow particles down is much more difficult. Reaching absolute zero at -273.15°C is nearly impossible because there will almost always be some residual energy that keeps the particles in motion, preventing them from coming to a complete stop.