What Is Special About Absolute Zero That Makes It Impossible?

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Absolute zero is the scientific term used to define the lower limit of temperature (temperature has no upper limit). In other words, it is theoretically the lowest temperature possible for any substance. This is done by extrapolating the ideal gas law. The value for absolute zero as per international agreement is 0 Kelvin (0 K) or -273.15 degrees Celsius (-273.15°C). In case you are wondering about the relation between the Kelvin and Celsius systems, or why there is a lower limit for temperature, but no upper limit, I’ve got you covered in this article.

Remember when I mentioned that absolute zero is a theoretical value? Well, practically speaking, it turns out to be impossible to achieve. In this article, I will be exploring the question of why absolute zero is practically impossible to reach. After answering this question, I will also be exploring the pragmatic question of why we are interested in absolute zero in the first place. To start, let us establish the fundamental ground principles of physics that we need to answer these questions.

This essay is supported by Generatebg

The Basics of Warmth

We perceive temperature as the hotness or coldness of a body. But as far as physics is concerned, the concept of ‘coldness’ does not exist. In this sense, what we perceive as cold turns out to be the absence of warmth. In physics, warmth is measured using temperature. Temperature, in turn, is a measure of the average kinetic energy of the substance or body being studied.

Whenever a body heats up, its particles vibrate. The hotter the body gets, the faster its particles vibrate; the colder the body gets, the slower its particles vibrate. At certain threshold points, the vibration causes the body to lose stability and change its state. For example, the freezing point and the boiling point of water represent such threshold points of particle vibration.

Since temperature is a measure of the average kinetic energy of a substance, we could also view temperature as a measure of the average vibration speed of the particles in the substance. If all that information was too much to process for you, just take this one point forward: temperature is a measure of particle-vibration of a body (hotter body = more particle vibration; colder body = lesser particle vibration). With temperature out of the way now, there is one more roadblock we need to clear before we can answer our main questions.

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The Difference Between Temperature and Thermal Energy

We just looked at temperature. For the temperature of a substance to go up or down, it has to gain or lose thermal energy (respectively). Thermal energy (in this context) is a measure of the total kinetic energy of the body. Temperature, on the other hand, is the average kinetic energy of the body.

For ease of understanding, consider a pot of boiling water and the ocean. The temperature of the boiling water is higher than that of the ocean. But the ocean has more thermal energy than the boiling water (largely because of its scale). These terms are easy to mix up, but are key to understanding how we reach absolute zero. Now, onward to our questions, we march!

Approaching Absolute Zero

Like we saw previously, to reduce the temperature (average kinetic energy) of a body, we have to make the body lose thermal energy (total kinetic energy). According to the laws of physics, energy can neither be created nor be destroyed. So, the thermal energy that we take away from the body has to go somewhere.

Insert Thermodynamics

Consider an enclosed system that contains two bodies with different thermal energies in contact with each other; one body is hotter than the other. According to the laws of thermodynamics, (global) thermal energy flows from the body with higher thermal energy into the body with the lower thermal energy. This transfer goes on until an equilibrium is reached.

As an advanced application of this concept, consider your refrigerator. The thermal energy of the food stored inside is actively reduced by transferring the energy through the coils to the surroundings. But how is the thermal energy able to continuously flow from the colder body (food) to the hotter body (the surrounding)? Does this not break the laws of thermodynamics that we just covered? It actually does not. The flow appears to be reversed, but we pay for this using ‘electric’ energy. I chose this example for a specific reason; it ties in with what we wish to do: achieve absolute zero.

Cooling Things Down Further

The question now becomes: can we use a refrigerator of sorts to keep cooling a body until it reaches absolute zero? The trick that the refrigerator uses is local compression and expansion of a substance called refrigerant. It turns out that fluids heat up when compressed and cool down when expanded. The refrigerant is a substance that particularly suits the needs of efficient and rapid temperature changes upon compression and expansion. So, the fundamental laws of thermodynamics stay true: thermal energy flows from the hotter body to the colder body.

Let us now simplify the refrigerator to a body that is constantly colder than the food (which is essentially how it keeps the food cool/cold). Unlike the refrigerator that holds the temperature constant, we need to keep lowering the temperature until it reaches absolute zero. How do we do this? The simple answer is: we need something that is progressively or comprehensively colder than the (already cold) body we are trying to cool further. Do you remember that I mentioned fluids cooling down upon expansion? This principle is actually used to super-cool substances towards absolute zero.

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The Catch with Absolute Zero

“If the only way to cool something down further is to bring it in contact with something even colder, we need something colder than absolute zero to reach absolute zero in the first place.” — The Author.

It is important that you understand this point. So, if you have not done so already, I suggest that you read the above sentence again, and ponder upon its meaning. It is a philosophical as well as a scientific catch that makes it impossible to reach absolute zero.

What we are currently able to achieve are temperatures asymptotically close to absolute zero. The coldest recorded temperature was 100 picokelvin (100 trillionths of 1 Kelvin above zero) at the University of Aalto in Finland (a university that offered me a Masters’ seat once).

We are about to get to the question of why absolute zero interests us in the first place. But before that, it is worth asking why there is nothing in the observable universe that is colder than absolute zero. There is, unfortunately, no satisfying answer, but quantum physics gets close. Quantum mechanics establishes that zero kinetic energy cannot exist (for technical details, refer to Quantum Degeneracy). Since zero kinetic energy is impossible, any object or substance must, by this very fact, have a non-zero, positive temperature (in the Kelvin scale).

For the Love of Science, Why Absolute Zero?

Any sane person would wonder why we should go through all this trouble in the first place. Curiosity is one thing, but at what cost? Cooling things down to near absolute zero temperatures consumes a lot of energy. Aren’t there more important problems for humanity to solve?

Well, it turns out that humanity’s curiosity rewards it with unusual gifts. At temperatures close to absolute zero, the wavelengths of vibrating particles seem to align at a new ground state. Different substances behave differently in their respective ground states, and some lead to very interesting effects.

Such effects include Bose-Einstein condensate, superconductivity and superfluidity. Technologies taking advantage of these properties are used in applications like hyper-fast transportation, particle acceleration, magnetic resonance imaging (medical field), etc.

In science, this scenario plays out more often than you would expect it to. Someone tries to solve an abstract science problem out of curiosity and discovers an interesting side-effect by accident. Someone else matches the side effect with a practical problem for humanity and solves it. So, all in all, the pursuit of absolute zero was worth it, after all!

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Further reading that might interest you: Why Does Temperature Have No Upper Limit? and How Easy Is It Really To Predict The Future?