double of temperature I'm not a physicist, but I was having a discussion with a friend and I could not convince her about the main point.
I asked her: "If I am in a place which temperature is 10ºC and she is in a place which temperature is 20ºC, can I assert that the temperature where she is the DOUBLE of the temperature where I am?".
What I need is a mathematic/physic explanation answering yes or no, and why.
Thx.
 A: If you want absolute temperatures, then the answer would be no. If you don't care about scales, but just numerical value, I'd say yes.
The Celcius scale is a 'relative' scale, based on the freezing and boiling points of water, and the temperatures in $^oC$ do not have much meaning. Whatever they tell, they only tell it with respect to freezing and boiling points of water.
The Kelvin scale, on the other hand is an absolute measure of temperature, and are better suited for comparisons. The equivalent temeratures for $10^oC $ and $20^oC$ in Kelvin scale are $283 K$ and $293 K$. The notion of 'double' temperature on Kelvin Scale would mean that the resulting temperature would have twice the effect (for a 'linear' relationship), whereas this is not the case for Celcius Temperatures.
A: Yes, you can assert it, and in one sense you are obviously correct - but only so long as you insist that Celsius is the "real" way to measure temperature. 
10 degrees C is 50 degrees Fahrenheit. 20 degrees C is 68 degrees F. 68 / 50 is 1.36. 
So, are you claiming her to be simultaneously double the temperature AND 1.36 times the temperature?
Or, more fundamentally, exactly what do you mean by "double the temperature"? Think about Stubham's answer.
A: Double of 10°C is 293.3°C
(First convert to Kelvins then double then convert back to Celsius.)
