Eli5 Why 80 degree Celsius is not four times as hot as 20 degrees celsius?

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Because we chose an arbitrary point to be zero. The freezing point of water.

If 0C was the coldest thing anything could possibly be, then 80C would be 4x hotter than 20C. But we know that the real zero (absolute zero) is 0Kelvin or -273.15 Celsius. Your example in Kelvin (which has the same degrees as Celsius but with a different starting point) would be 353K vs 293K

I’m not a maths person. The difference between 20 and 80 is 60, so, isn’t it fair to say it’s 3 times hotter, not 4?

Celsius scale (and Farenheit, btw) does not measure absolute temperature. Both scales use an arbitrary point as zero. Kelvin (and Rankine) take the zero as the state of the lowest energy possible, therefore, it makes no sense to speak of negative temperatures. With this scales, you actually have that 20K is twice as hot as 10K. Given the example you’ve chosen, we’d have 80ºC=353K and 20ºC=293K, so their quotient would be roughly 1.21, just 21% hotter

Because the true zero point is at -273°C. 0°C is just a random point chosen for convenience.

So if you double the true absolute temperature (wich is measured in Kelvin) at 0°C you get 273°C.

In Kelvin the temperature is a direct measure of how much energy there is. 0K would mean absolutely no energy (but that’s impossible to reach)