Just as the speed of light is an unattainable limiting speed for a physical system, there is also a lower unattainable limit for the thermodynamic temperature of a physical system introduced by Kelvin: absolute zero. On the degree Celsius scale, it is exactly -273.15°C, by convention. In classical physics it corresponds to the absence of motion on the microscopic scale for a physical system such as a gas. In reality, due to quantum mechanics and the Heisenberg inequalities, there is a residual state of motion corresponding to the minimum energy for a physical system. At this level of energy the temperature and even the entropy of the system are then zero.
For entropy, this is only rigorously accurate for a perfect crystal, and it is the variations in the entropy of a system that tend to zero with the thermodynamic temperature according to Nernst's theorem. The current lowest temperature attained is of the order of 450 ±80 °K in 2003. Note that some systems, such as those made up of nuclei having spins without interactions show a negative temperature when placed in a magnetic field. Based on the direction in which heat transfer occurs in these systems in thermal contact with another system at a positive temperature , the result is that the negative temperatures are higher than the positive ones!