According to Bose and Einstein, bosons -particles having an integral value of the property termed “spin ” -are allowed to coexist locally in the same quantum energy state. When atoms have been cooled to within millionths or billionths of a degree of absolute zero, a number of important phenomena appear, such as the creation of Bose-Einstein condensates, so called because they were predicted in 1924 by German physicist Albert Einstein (1879 –1955) and Indian physicist Satyendranath Bose (1894 –1974). If a photon is absorbed, its momentum is transferred to the atom if the atom and photon were originally traveling in opposite directions, this slows the atom down, which is equivalent to cooling it.

A laser beam can cool atoms that are fired in a direction contrary to the beam because when the atoms encounter photons, they absorb them if their energy is at a value acceptable to the atom (atoms can only absorb and emit photons of certain energies). As a result, various special phenomena (e.g., Bose-Einstein condensation, superfluids such as helium II) can only be observed in materials cooled nearly to absolute zero.Ītoms may be cooled by many methods, but laser cooling and trapping have proved essential to achieving the lowest possible temperatures. The motions of particles near absolute zero are so slow that their behavior, even in large groups, is governed by quantum-mechanical laws that otherwise tend to be swamped by the chaotic atomic- and molecular-scale motions that are perceived as heat. To reach such low temperatures, procedures use specially insulated vessels called cryostats. These very low temperatures are referred to as cryogenic temperatures. Consequently, absolute zero can be approached but never attained.Īlthough the third law of thermodynamics declares that it is impossible to cool a substance all the way to absolute zero, temperatures of only a few billionths of a degree Kelvin have been achieved in the laboratory. Another way of stating this dilemma is that as the temperature of a substance approaches absolute zero, it becomes increasingly more difficult to remove heat from the substance while decreasing its entropy. At any temperature above zero, however, imperfections in the crystal lattice will be present (induced by thermal motion), and to remove them requires compensatory motion, which itself leaves a residue of imperfection. If the particles comprising a substance are not ordered as a perfect crystal, then their entropy cannot be zero. The law states that the entropy of a perfect crystal is zero at absolute zero. The third law of thermodynamics dictates that absolute zero can never be achieved. Equivalently, when the entropy of a substance has been reduced to zero, the substance is at absolute zero. It is also referred to as the temperature at which pressure is zero. Absolute zero, 0 degrees Kelvin (K), –459.67 ☏, or –273.15 ☌, is the minimum possible temperature: the state in which all motion of particles in a substance is at a minimum.

When is absolute entropy zero