science!
It is all heat, until a temperature of absolute “zero” is reached. Absolute zero is a theoretical state in which all molecular motion ceases.
**Cold is the lack of heat. Or, more accurately, cold is when it’s not hot: in physics, heat refers to a transfer of energy between two systems. What we talk about when we talk about hot and cold is temperature.
And temperature can and is measured absolutely. But what does it measure? The agitation of the molecules within a system. To be slightly more specific, temperature is more or less proportional to the average square of speed at which molecules move within a body. The faster they move, the hotter the body is, and the higher the temperature.
So, while temperature has no theoretical maximum, it has a minimum: the state in which all the molecules within the body have no movement at all. Scientists refer to it as the absolute zero, and in practice it is impossible to attain: we’ve only been able to get really, really, really close to it in labs (think a tenth of a billionth of a degree).
The absolute zero is equal (small approximation) to −273.15 °C (degrees Celsius), or −459.67 °F (degrees Fahrenheit). And there exists a third temperature unit, widely used in science, called Kelvins (not degrees Kelvin, just Kelvins). It’s very simple: the Kelvin scale is just the Celsius scale shifted so that the absolute zero is equal to 0 K. So 0 °C is equal to + 273.15 K, 100 °C (temperature at which water boils in the usual conditions) is + 373.15 K, etc. For example, in outer space it’s about 3 K hot. So, not very hot…**
**Anything higher than that point is heat. The calculation of heat, expressed as a “number” is entirely arbitrary, based on assumptions and needs of the people doing the measuring. But all of it eventually refers to molecular activity, in which “faster” is “hotter.”
In a simplified analogy, we might compare temperature to speed, and ask if “slow” is the lack of “speed,” or if “speed is the lack of slow.” In truth, as long as something is moving, it has speed, until there is no speed, at all. Everything above 0 is speed, however small it may be.**
So, it’s all a koan. :)