Why is -273.15 °C the low temperature limit for the universe? According to Ideal Gas Law the lowest  temperature of an ideal gas can be $-273.15 °C$. This temperature is also considered the lowest temperature in the universe. But it is the lowest possible temperature for ideal gases. What is the proof that it is the lowest possible temperature for real gases and more importantly for all things in the universe? 
 A: 
Why is -273.15 °C the low temperature limit for the universe?

When they measure you for a shirt, they measure you from the waist up. Does that mean you're three feet tall? No, it just means the moved the zero point, Moving the zero point does not change your height, just the way you record it.
Likewise, the lowest temperature of the universe is zero. It's always been zero, we just didn't know what that number was. So we picked other numbers to be the zero point for purely practical reasons.
The Fahrenheit scale set its zero point at the coldest temperature where saturated salt water is still liquid. They did this because it was difficult to make really pure water, whereas anyone can make brine in their kitchen. Celsius set zero at the freezing point of pure water, because that made more sense to Celsius.
So the bigger question is "why is there a zero at all?"
Well that's obvious when you consider the definition of temperature, its the average kinetic energy of the molecules. So if there's no motion, then we would call that zero temperature.
A: Maybe it is rather the case that you define 0 K as the lowest temperature, where kinetic energy of spoke-of gas is 0, and the Celsius scale is somehow derived by it (it has the same steepness) but is shifted by 273.15 [ ]. Negative Kelvin-temperatures can be achieved in open systems, as maybe a laser or something, when you extract energy somehow, depending on how you define your temperature measurement.
