A theoretical physicist is a scientist who uses mathematical models and theories to explain natural phenomena. They develop new ideas, concepts, and principles that help us understand how the universe works at its most fundamental level. Theoretical physicists use advanced mathematics and computer simulations to make predictions about physical systems and test their hypotheses through experiments or observations. Their work often involves abstract thinking and problem-solving, as they try to uncover the underlying laws of nature that govern everything from subatomic particles to galaxies.