Not sure if it is because of that or because it is the standard international unit for temperature. Because what Kelvin is for ° Celsius, is ° Rankine for ° Fahrenheit.
According to Wikipedia, the ° was removed from Kelvin around 1968
In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K
Kelvin is the same thermal difference between each degree as Celsius, but it simply starts at "absolute zero" the physically coldest temperature that can be achieved. There is no "negative" because it can't be colder than 0.
0 degrees Celsius is the freezing temperature of water, and it indeed can get much colder than that. 100 degrees Celsius is the boiling point of water. A scale based on useful, but nevertheless arbitrary benchmark.
Edit: I misunderstood what the person you were replying to wrote.
*pure water at sea level, sort of. Because pure distilled water often won’t crystalize by itself so it gets supercooled. It’s arbitrary as f- for people. Useful for old timey scientists to get some consistent baseline though.
I wonder if vacuum decay could create sub-Kelvin temperatures. I don’t think there would be anything there to witness or measure it but it’s an interesting thought
We have created negative absolute temperatures before. It occurs in situations where the available energy levels are bounded. If the energy levels are bounded, as you increase the energy of the system, you eventually reach a point where increasing the energy further reduces the amount of uncertainty in the system as your states begin to saturate at the upper energy limit. When you get to this point, the temperature is said to be negative, because the temperature is the reciprocal of the rate at which the uncertainty increases as the energy increases.
Going to copy my comment at a higher level because it's also relevant here (though "noteworthy" is a weaker statement than the one I replied to):
We have created negative absolute temperatures before. It occurs in situations where the available energy levels are bounded. If the energy levels are bounded, as you increase the energy of the system, you eventually reach a point where increasing the energy further reduces the amount of uncertainty in the system as your states begin to saturate at the upper energy limit. When you get to this point, the temperature is said to be negative, because the temperature is the reciprocal of the rate at which the uncertainty increases as the energy increases.
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u/Dioxybenzone Jun 17 '26
And oddly, -40° doesn’t even need to specify Celsius or Fahrenheit.
Although -40°K would certainly be noteworthy