Why is -273.15°C the lowest possible temperature in theory? : Limits in physics (1)

Michelle Ncube
Aug 29, 2025
3 min read

Updated: Oct 3, 2025

If there is no upper limit to how hot an object/substance can be, why is -273.15°C (also known as absolute zero) theoretically the lowest possible temperature anything can have?

This blog marks 1/3 of the ‘Limits in Physics’ short collection of blogs which will go through 3 different limits I researched about in physics!

In this blog, I’ll be covering what absolute zero is, why absolute zero is -273.15°C & most importantly why it’s a temperature we can never reach!

As absolute zero is a measure of temperature, knowing what temperature is, is important!

What is temperature?

A simple definition of temperature would be a measure of how hot or cold an object is. But to be more specific, temperature is a measure of the average kinetic energy that particles in an object/substance that you are measuring have. Contrary to popular opinion, it isn’t directly the amount of heat energy or thermal energy an object has.

This means that the hotter an object, the greater the average kinetic energy its particles have, vice versa. (The colder the object, the lower the average kinetic energy its particles have).

So, what is absolute zero?

Absolute zero is the temperature at which an object’s thermal energy reaches its minimum value. Below this temperature, we cannot cool an object or substance even further than absolute zero as atomic motion is seen to completely stop according to classical physics. This means the atoms have no kinetic energy so there is no energy in the system that can be transferred into thermal energy to cool the object down. Hence why -273.15°C is the limit. However in quantum physics this differs.

Why is it -273.15°C?

In the 1700s and 1800s, many scientists such as Charles and Lussac noticed that there’s a relationship between volume of gas and its temperature at a constant pressure. So, what William Thomson did (in 1948) was extrapolate the straight line, in order to find at which point the gas would reach a volume of zero.

This did not make any sense as gasses in real life can not reach a volume of zero, so Thomson thought deeply and decided to create the Kelvin scale. On this scale 0K represented the temperature at which no thermal energy was present and thus acted as a limit, as it is impossible to have ‘less than no energy’ at all.

The value of 0K corresponded to -273°C according to the extrapolated graph and this value was later refined mathematically to be -273.15°C.

So the reason as to why absolute zero is -273.15°C is simply because it is what 0K corresponds to on the Celsius scale.

Thomson was later given the title First Baron Kelvin and subsequently Lord Kelvin for his excellent work.

Can we reach absolute zero? What quantum physics says.

To have an object reach absolute zero, we would need to be able to have its thermal energy transferred out of its system. This can be done by putting a cooler object next to it and having the thermal energy transferred from the object we want to cool to another. However, this brings about a paradox as if we want to cool an object to absolute zero, we would in theory need an object that is colder than absolute zero which is physically impossible, hence why we cannot reach absolute zero.

Another reason related to why we can’t reach absolute zero or measure it, is because of the Heisenberg uncertainty principle and zero point energy (ZPE).

The Heisenberg uncertainty principle states we cannot know the speed and position of a particle with perfect accuracy. So if a particle was at absolute zero, meaning its speed is zero, we would be able to know its position as it would not be moving, which violates this principle.

Furthermore, ZPE is the lowest possible energy a quantum system can have. This lowest possible energy is always present which implies that a particle can never truly be at rest. This means an object can never reach absolute zero to reach absolute zero, the requirement is no kinetic energy at all.

Further reading/ Prompts

Whilst researching about absolute zero, there were a few interesting concepts and questions I had myself. I’ve listed them below as they might inspire you to research more into this yourself :).