Negative temperatures refer to a condition where a system's temperature is measured below absolute zero, which is 0 Kelvin or -273.15 degrees Celsius. In classical thermodynamics, this concept is not possible for ordinary matter, as it implies a state where particles have less than zero thermal energy. However, in certain quantum systems, such as spin systems or laser-cooled atoms, negative temperatures can occur, indicating a population inversion where more particles occupy higher energy states than lower ones. In these systems, negative temperatures are not colder than absolute zero; instead, they represent a unique state of matter. When a system reaches a negative temperature, it can exhibit unusual properties, such as the ability to transfer heat to systems at positive temperatures. This phenomenon challenges our traditional understanding of temperature and thermodynamics, opening new avenues for research in fields like condensed matter physics and quantum mechanics.