Why is a temperature of absolute zero unachievable?
by 
saviorknights
ANSWERS: 2
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Temperature corresponds to kinetic energy per molecule. Apart from the practical considerations, there is a quantum mechanical limit to how low temperature can go. In quantum mechanics, Heisenberg's uncertainty principle gives a physical limit to how accurately a pair of physical quantities can be measured. Position and momentum are two such quantities. The uncertainty in position multiplied by the uncertainty in the momentum must be greater than Plank's constant (h bar). Take one of the atoms in the container you are cooling. As zero momentum for the atom is approached, the uncertainty principle means that the uncertainly in the position becomes such that we aren't sure the particle is in the container, which is a contradiction since we chose a particle in the container. This implies that there is a limit to how cold the container can get. There is "quantum fluctuation" which prevents particles becoming absolutely still. Even if the container is empty, there is residual energy due to the quantum fluctuation of the vacuum. This is called the Zero Point Energy.
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The simple answer is that heat always flows from a hot object to a cold object. When you try to remove heat from a system you must transfer it to a colder object. Since absolute zero is the temperature at which molicules stop moving it is the absolute bottom limit. The problem is that as an object approaches absolute zero you must still transfer its energy to an even colder object, which is impossible.
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