*Question*: What is the main difference between Absolute and Arbitrary Scales?

On an Absolute scale the number '0' means actually none of the quantity concerned.

On an Arbitrary scale '0' can by chosen to mean anything.

Arbitrary scales only allow you to compare values.

This makes it only possible to discuss relative increases (ratios) on an Absolute Scale.

It makes perfect sense to talk about half as much energy or three times as much energy, but you cannot talk about having twice as much temperature.

Why not? Well, for example, what is twice as much as -10^{o}C? Obviously -20^{o}C cannot represent twice as much temperature as -10^{o}C!

Which is why arbitrary scales only allow you to say that a values is bigger or smaller than another value.