Is absolute zero empirically certain? And why is there nothing colder?

An object consists of many atoms. Each of those atoms does not necessarily sit still, but can vibrate back and forth about its equilibrium position. The warmer the object is, the more the atoms vibrate. From a physical point of view, ‘temperature’ is a measure of the vibration of the atoms.

Well, if we keep getting the same object colder and colder, we’ll reach a point where the atoms don’t move anymore, they just sit still. This happens at a temperature of -273.15C. You can’t get any colder, because atoms sitting still cannot be made to vibrate any less.

One is theoretically sure of this, empirically one has not yet (but almost) been able to reach this temperature in a laboratory. It is very difficult to extract the last breath of heat from an object.

It is empirically certain that we have never seen anything colder than -273.15C. The darkest corners of the universe have a temperature of around -270, a few degrees warmer than the absolute minimum. This is believed to be another ‘afterglow’ of the big bang. Far in the universe, the absolute minimum indeed also applies.

There is no upper limit like an absolute boiling point or anything. One can always make the atoms vibrate harder and harder. After a while, they will vibrate so fast that all objects will disintegrate (melt) and eventually evaporate. Heating it even further causes the atoms themselves to vibrate apart, and then you get a plasma like in the sun.