-40K are impossible AFAIK
Kelvin is the standard unit for temperature, 0 Kelvin is the absolute zero point. Each step is the same size as a degree Celsius, but there are no "degrees".
Edit: remove assertion that the scale does not go into negative, because physics.
Now that I think about, what's the etymological reason we use "degrees" to talk about temperature? We don't measure force in "degrees Newton" or electric current in "degrees Ampère". Why do we do it for Celsius and Fahrenheit?
The reason you can do it for Celsius or Fahrenheit is that those are scales but not units. The energy in two bodies (with constant heat capacity) at 100° is not the same as in a single body at 200°, but if you use kelvins then it works out.
Not for force, no, but: https://en.wikipedia.org/wiki/Newton_scale
Wait what?!! That feeds my daily “oh oooh” moment.
Basically, coldness is 1/temperature but makes more theoretical sense:
> Though completely equivalent in conceptual content to temperature, β is generally considered a more fundamental quantity than temperature owing to the phenomenon of negative temperature, in which β is continuous as it crosses zero whereas T has a singularity.[6]
> In addition, β has the advantage of being easier to understand causally: If a small amount of heat is added to a system, β is the increase in entropy divided by the increase in heat. Temperature is difficult to interpret in the same sense, as it is not possible to "Add entropy" to a system except indirectly, by modifying other quantities such as temperature, volume, or number of particles.
But we tend not to talk about the 'temperature' of electron energy levels. If we did, then they would have negative temperature.
That's a common simplification, but not really the definition. At the minimum, quantum mechanical effects make this complicated.
But you are right in a sense. From Wikipedia:
> The absolute temperature (Kelvin) scale can be understood loosely as a measure of average kinetic energy. Usually, system temperatures are positive. However, in particular isolated systems, the temperature defined in terms of Boltzmann's entropy can become negative.
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It's how these things go. You and me think of "temperature" as something simple, like it all started. You know, you find some material that noticeably expands or contracts when warmed or cooled, you turn it into an indicator along what will be a scale, and you make a mark for e.g. "roughly when water starts freezing over" and "when water starts boiling"[0]. You make a regular gradation in between, let it spill over below and above your "min" and "max" anchors, you put some numbers on this, and "Bob's your uncle".
This of course is super useful, so others refine your "thermometer", as they realize just how stable and broadly-applicable this "temperature" is as a measure. Eventually some start asking how it all works, and you hear some grumbling about average kinetic energy of particles, which still kinda makes sense. I mean, lots of tiny things buzzing around, and there's that German guy saying you can't possibly measure each of the tiny things individually, but whatever - the average, low-frequency part is stable.
But then those theorists push and push, and break through the barriers of sanity. They enter R'lyeh and start transcribing the Eldritch tablets found inside. Suddenly, you see temperature redefined using something else, something you recall should also be a statistical phenomenon but suddenly isn't[1]. You see words like entropy and enthalpy thrown around, and then some American mathematicians get involved, and suddenly there's also negtropy and fractional bits, and nothing makes any kind of sense anymore - temperatures going below absolute zero, the "nothing actually moves anymore" point, leading to negative temperatures where heat flows from colder to hotter...
...really that's one of the least weird thing in hard sciences these days.
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[0] - In the process discovering that this was a really good choice of anchors, as water behaves in strange ways at exactly those two temperatures.
[1] - Or is it? Are you sure what "statistical" means these days? Are you sure what "means" means these days?