One of the first people in history to discuss the possibility of an absolute minimum temperature was Robert Boyle. His 1665 New Experiments and Observations touching Cold articulated what was known as the “primum frigidum”. Then, in 1848, William Thomson, 1st Baron Kelvin tested the hypothesis that heat is just the result of molecules moving around in matter. He wanted to get stuff as cold as he could get it. So, Kelvin conducted experiments that drew heat from a warm substance toward a cooler one and found that the kinetic energy could be drained from the warm substance. Surprisingly, this temperature wasn’t like melting points or boiling points which change for every substance. Instead, it was the same for everything! As a result of this, Kelvin created a thermodynamic temperature scale that measured the amount of kinetic energy within any given material, and we still use it to this day. As part of this, the kelvin is the base unit of temperature in the International System of Units (SI), having the unit symbol K.
In the most technical sense, absolute zero is the lowest limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin (0 K, not 0°K). The theoretical temperature is determined by extrapolating the ideal gas law, so by international agreement, absolute zero is taken as −273.15° on the Celsius scale, which is equal to −459.67° on the Fahrenheit scale. The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition. Absolute temperature measurement is uniquely determined by a multiplicative constant which specifies the size of the degree, so the ratios of two absolute temperatures are the same in all scales. The thing is that, even though scientists know exactly how cold it really is, absolute zero is still the holy grail of temperatures. Reaching it has alluded mankind for centuries and will continue to allude people for generations, if not forever.
Absolute zero is commonly thought of as the lowest temperature possible, but it is not the lowest enthalpy state possible, because all real substances begin to depart from the ideal gas when cooled as they approach the change of state to liquid, and then to solid. The sum of the enthalpy of vaporization from gas to liquid and enthalpy of fusion from liquid to solid exceeds the ideal gas’s change in enthalpy to absolute zero. In the quantum-mechanical description, solid-state matter at absolute zero is in its ground state, the point of lowest internal energy. The laws of thermodynamics indicate that absolute zero cannot be reached using only thermodynamic means, because the temperature of the substance being cooled approaches the temperature of the cooling agent asymptotically, and a system at absolute zero still possesses quantum mechanical zero-point energy, the energy of its ground state at absolute zero.
The kinetic energy of the ground state cannot be removed, because things never totally stop moving at the smallest scales of the local space-time continuum. Scientists know that absolute zero doesn’t mean a complete absence of motion in a substance at the quantum mechanical level. This is completely impossible to achieve, at least according to the known laws of physics. Instead, absolute zero marks the state of minimum motion of the particles in a given substance. In the strictly mathematical sense, this is the result of the uncertainty principle, which dictates that for every particle in the universe it is impossible to know both its momentum and its exact position at the same time. As such, subatomic motion can never completely cease because then one would know both the particle’s position and momentum which would both be zero. Again, measuring this, is impossible, because it’s forbidden by the uncertainty principle.
Absolute cold and absolute hot only exist theoretically. It may be that the birth of a universe is as hot as anything ever gets, so maybe the death of a universe is as cold as anything ever gets. Presumably, nothing ever gets down to 0 K. As far as we know, the coldest natural place in the known universe is the Boomerang Nebula which is 5,000 light-years away from Earth in the constellation Centaurus. The astronomical object has been spitting out gas for so long that it’s cooled down to only about 1 K. Whereas, the coldest synthetic place in the known universe is in laboratories right here on Earth. So far, the use of laser cooling has produced temperatures less than a billionth of a kelvin. This is important because, at very low temperatures, matter exhibits unusual properties. For instance, below about 30 K certain substances can become superconductive, meaning that they can carry an electrical current with no resistance. This is incredibly useful in making particle accelerators and powerful electromagnets to put in MRI machines. The applications are seemingly endless, and that’s really cool. The coolest, in fact.