Zero kelvin, or absolute zero, is the lowest possible temperature on the thermodynamic scale. It is the point where all molecular motion stops and the entropy of a system reaches its minimum value. However, reaching zero kelvin is impossible for several reasons.
One reason is that cooling a system requires transferring heat to another system with a lower temperature. To cool something to zero kelvin, we would need something that is cooler than zero kelvin, which does not exist. Therefore, we can only approach zero kelvin asymptotically, but never reach it.
Another reason is that quantum mechanics prevents atoms from having zero energy. Even at zero kelvin, atoms still have some residual motion due to the uncertainty principle, which states that we cannot know both the position and momentum of a particle with absolute precision. Therefore, atoms must have some minimum amount of energy, called the zero-point energy, that cannot be removed.
A third reason is that some physical phenomena, such as superconductivity and superfluidity, occur at very low temperatures and prevent further cooling. These phenomena involve quantum effects that make the system behave as a single entity, rather than a collection of particles. For example, superfluids have no viscosity and can flow without friction, which means they cannot lose heat by conduction or convection. Similarly, superconductors have no electrical resistance and can carry currents without dissipation, which means they cannot lose heat by radiation or Joule heating. Therefore, these systems have no mechanism to transfer heat to a colder system and cannot be cooled further.
In summary, zero kelvin is impossible to reach because of the limitations of thermodynamics, quantum mechanics, and quantum phenomena. The lowest temperature ever achieved in a laboratory is about 100 picokelvin, or 0.0000000001 kelvin, which is still above absolute zero.