It's using related but somewhat different meaning of the concept.
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?
Negative temperature is a perfectly normal and well-studied phenomenon with lots of common examples. Temperature is defined as the rate of change of entropy wrt internal energy, usually more energy means more entropy, but it's not a given.
> A system with a truly negative temperature on the Kelvin scale is hotter than any system with a positive temperature. If a negative-temperature system and a positive-temperature system come in contact, heat will flow from the negative- to the positive-temperature system
Wait what?!! That feeds my daily “oh oooh” moment.
Energy flow wants to maximize entropy. Usually things flow from hot to cold because a cold thing will "make better use" of the energy to create entropy (i.e. permit microstates). Something with negative temperature will increase entropy by losing energy, and the positive temperature thing will increase entropy by gaining it, so it's (thermodynamically) a no-brainer.
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.
Okay, can we use it for cooling like in Sundiver by David Brin[0], where people (IIRC) used lasers to keep themselves cool while, as per title, diving their ship into the Sun?
Lasers are often used to cool atoms & molecules to extremely low temperatures[1]. Bose-Einstein condensates are made via laser cooling, and it's commonly used in quantum entanglement experiments since it can get the involved particles closer to absolute zero than other methods.
Interesting I wasn't aware of this definition but I would consider that moving the goal posts (not by you to be clear). Since that's using a different definition of temperature, the original poster was referring to the more common definition of temperature where at 0K nothing moves anymore.
> [...] the original poster was referring to the more common definition of temperature where at 0K nothing moves anymore.
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.
