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The coldest theoretical temperature, and why it can never be achieved - Absolute Zero

It turns out that there is indeed a theoretical coldest temperature!

I know the name can be seem a bit confusing... After all, I'm sure that everyone is aware that temperatures can very easily go below zero degrees Celsius or Fahrenheit.

But not below zero Kelvin...

(Also, just note that Kelvin isn't used with "degrees")

Contrary to what you might have previously thought, there are 3 units of temperature, two of which I'm sure you're already very familiar with, but the 3rd is the most commonly used one throughout the sciences:

  • Celsius

  • Fahrenheit

  • Kelvin

0 Kelvin equates to -273 (3 s.f) degrees Celsius, and -460 (3 s.f) degrees Fahrenheit.

It's the lowest theoretical temperature, and I say it's only theoretical because it has never and likely will never be achieved.

So that's the lowest temperature, otherwise known as absolute zero, but why can it never be achieved?

Heisenberg's Uncertainty Principle

It's all to do with Heisenberg's uncertainty principle, which, if you haven't heard of before, lies in that territory of physics where things can get very complicated! Or at least definitely too complicated for this article, but stick with me and I'll briefly summarise it below.

Heisenberg's uncertainty principle essentially says that we cannot predict the exact velocity and position of an electron with 100% certainty.

We can predict the velocity at any given time with 100% certainty, but not the position at that same time, and visa versa.

This relationship between velocity (or more specifically momentum) and position is an example of what Neil's Bohr coined as "complementarity", and it's as simple as saying that some properties compliment each other, and can't be measured simultaneously to 100% accuracy.

In gaining information about a particle's position, you lose information about its momentum!

Hopefully that's pretty straight-forward to understand if you just assume, for now at least, that it's correct, but let's see what implications this has for trying to achieve absolute zero....

Say you take a piece of aluminium and you decide that you're going to attempt to cool it to absolute zero.

As you do so, the atoms that make up the aluminium begin to have less and less Ke (Kinetic energy) as you reduce the temperature, and hence their vibrational oscillations reduce, in terms of the amount of distance that they cover.

Kinetic energy is proportional to mass and velocity, meaning that a decrease in Ke will result in a decrease in velocity (since the rest mass can't change).

You keep doing this until the atoms barely have any energy at all, at which point you might have successfully cooled the aluminium down to -270 degrees Celsius or so, but you now hit a hurdle...

As you've been lowering the amount of energy that these atoms have, the velocities of the electrons that make up the atoms (along with neutrons and protons) have been getting closer and closer to 0, since the amount of Ke that they have has been reducing.

With their velocities closer to 0, you're ability to predict their positions and velocities has been becoming increasingly probable, but remember, Heisenberg's uncertainty principle tells us that we can never predict the exact velocity and position of a particle with 100% certainty.

If you were to cool the aluminium down to absolute zero, the electrons would have no kinetic energy whatsoever, and hence would be perfectly still.

Therefore, we could predict their velocities (zero), and hence their positions with absolute certainty, which is a direct violation of the uncertainty principle.

Such violation is not acceptable, except the repercussions of which aren't dictated by a school headmaster/mistress or government as you're probably used to them being, rather these rules are governed by the universe, and cannot be broken!

At least in theory they can't...


There is however, another angle that you could approach this scenario with, that involves the laws of Thermodynamics.

Briefly, this suggests that an infinite amount of "work" would be required in order to achieve absolute zero.

That sounds confusing taken by itself, but it's important to understand that you have to put in some form of "work" in order to reduce the temperature of a substance.

After all, temperature is just a measure of the energy of atoms, and you can't just remove energy from an atom without doing anything.

So, essentially, the laws of thermodynamics suggest that you would have to do an infinite amount of "work" in order to get the energy level of an atom down such that the temperature is absolute zero.

You could get very close with a lot of work, but never quite achieve absolute zero.

Don't be confused by the term "work" here, as it's just a term that's used generally in physics to describe the action of doing something. You'll often here the word being associated with topics like temperature, pressure, forces etc.... as they always involve some kind of action.

I suppose the easiest example would be to take a refrigerator.

They require a certain amount of "work" in order to reduce the temperature of the container, which we provide in the form electrical energy. We all know how much energy those things need, and they only cool down to 5 degrees Celsius or so!

In order to obtain -273 degrees Celsius (absolute zero to 3 s.f), you'd need an infinite amount of energy, whether that be in the form of electricity as with a fridge, or something else.

If I asked you, "Do you think that we could ever produce an infinite amount of electricity?", I'm assuming you'd reply with no...

So there's 2 different concepts that both support the idea that absolute zero can never be achieved.

If you found them interesting, I'd recommend sticking around as we have tons of other articles here at Expansive that are all designed to educate in a hopefully interesting way!

Thanks for reading!

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