Hint: it’s not 50 degrees Fahrenheit.
100 ℉ = 311 K, half of which is 105.5 K = −180℉
Yup — half of 100℉ is −180℉.
The difference between the Kelvin scale ℝ⁺ and the Fahrenheit scale is like the difference between a linear scale and an affine scale.
You were taught in 6th grade that
y = mx + b is a “linear” equation, but it’s technically affine. The
+b makes a huge difference when the mapping is iterated (like a Mandelbrot fractal) or even when it’s not, like in the temperature example above.
Abstract algebraists conceive of affine algebra and manifolds like projective geometry — “relaxing the assumption” of the existence of an origin.
(Technically Fahrenheit does have a bottom just like Celsius does. But I think estadounidenses conceive of Fahrenheit being “just out there” while they conceive of Celsius being anchored by its Kelvin sea-floor. This conceptual difference is what makes Fahrenheit : Celsius :: affine : linear.)
It’s completely surprising and rad that mere linear equations can describe so many relevant, real things (examples in another post). Affine equations — that barely noticeable
+b — do even more, without reaching into nonlinear chaos or anything trendy sounding like that.