# How cold can cold be? What is the lowest possible temperature?

Is there a limit to how cold something can become? Does a minimum temperature exist? The Kelvin temperature scale was designed with this in mind, where 0 Kelvin (equal to -273.15 degrees Celsius) would be the lowest possible temperature but is 0 Kelvin even possible?

To explore this idea we first have to state what temperature is. Temperature is simply a measure of a system’s internal kinetic energy. The system in question is comprised of particles each moving randomly with their own momentum, thus giving them their own kinetic energy. The greater the intensity of this random motion, the higher the system’s temperature is. Theoretically 0 Kelvin means no internal kinetic energy, the Kelvin temperature scale was set with this definition. So, this means that the particles in the system would cease to move or even vibrate. This is perfectly fine for classical mechanics but the particles that we are talking about are not classical particles they are quantum particles. A quantum particle is a particle governed by the laws of quantum mechanics which apply at the atomic/sub-atomic level, these are the particles that every system we could consider will be comprised of. So given our particle’s are quantum in nature then at 0 Kelvin each particle would have a fixed position and 0 momentum, but this idea breaks the Heisenberg Uncertainty Principle, one of the most fundamental laws of Quantum Mechanics.

The Heisenberg’s Uncertainty Principle states that the product of the uncertainty of position and the uncertainty of momentum must be greater than or equal to a set number (Planck’s constant divided by 4 pi), put simply the Heisenberg’s Uncertainty Principle gives us a lower limit to how precisely we can measure an objects momentum and position. This means the more precisely we define a quantum particles position the less we are able to define its momentum. So if we fix a quantum particles position, its momentum becomes unknown. Its momentum will fluctuate with the possibility of very high values.

These fluctuations in momentum give rise to a little kinetic energy. This concept is the zero-point energy, the theoretically minimum energy a system can have which is NOT absolute zero. So due to quantum mechanics specifically the Heisenberg Uncertainty Principle it is impossible for an object to be cooled to absolute zero Kelvin because it will maintain a small amount of kinetic energy due to fluctuations in each particles momentum.

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