What Temperature Is Twice As Cold As Zero Degrees?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

23 January 2005

A reader asked this interesting question: what temperature is twice as cold as zero degrees? Zero multiplied by anything is zero; therefore the answer to the question is zero degrees. Right? Let’s go over it again.

There are many temperature scales, the most common being the Fahrenheit and Celsius. When we say zero degrees, we must state which system we are referring to because, for example, zero Fahrenheit is equal to minus 18 in celcius and zero celcius is positive 32 if Fahrenheit.

In Kenya we measure temperature in degrees celcius. In this scale, the freezing point of water is approximately zero and the boiling point is 100 degrees. However, zero degrees celcius is not the coldest temperature – we know quite well that even the domestic fridge can cool things to negative temperatures. Therefore “twice as cold as zero” cannot be zero, it must be a value lower than freezing.

This leads to another question: what is the coldest temperature possible? To answer that, we must first define temperature. Textbooks say that it the degree of coldness or hotness. A more accurate definition is that it is a measure of the (vibration) energy of the molecules of matter. Therefore, the point at which the molecular energy is zero must be the lowest temperature. This is called absolute zero and it is about negative 273 degrees celcius.

There is a temperature scale that starts from this absolute zero going up – this is known as the kelvin scale commonly used in science. The symbol is a capital K. The coldest point is thus, zero kelvin and zero degrees celcius is therefore equal to positive 273K.

Now we can attempt to answer the original question of what temperature is twice as cold as zero degrees. However, let us first answer a simpler one: what temperature is twice as cold as zero? Knowing the definition of temperature, this can be asked in another way thus, what temperature has the twice the molecular energy of zero degrees. Since zero celcius is 273K, then twice as hot must be 546K. This is equal to 273 degrees celcius.

But what would be twice as cold as zero degrees?

The phrase twice as cold is confusing. What does it mean if I said I am twice as short as you are? Is it that I am half as tall as you or that I am double your short height? If you think about it strictly, the latter meaning is probably more accurate. However, normal usage would take the former.

Taking the normal usage of the phrase, twice as cold as zero degrees celcius means half of this temperature, that is, half of 273K, which is 136.5K. This is equivalent to minus 136.5 degrees celcius.

Finally, what would happen if we cooled an object below zero kelvin? Well, at absolute zero, the molecules stop moving. Asking about lower temperatures than this is like wondering how to slow down an object that has already stopped or killing someone who is already dead!