Not to be confused with Physician, a person who practices medicine.
Albert Einstein, a key physicist of the 20th century who developed the theory of relativity and parts of early quantum theory.
A physicist is a scientist trained to understand the interactions of matter and energy across the physical universe. Physicists study a wide range of phenomena in many branches of their field, spanning all length scales: from sub-atomic particles of which all ordinary matter is made (particle physics), to molecular length scales of chemical and biological interest, to cosmological length scales encompassing the Universe as a whole. Physicists generally are interested in the root or ultimate causes of phenomena, and usually frame their understanding in mathematical terms.
The term "physicist" was coined by William Whewell in his 1840 book The Philosophy of the Inductive Sciences.
The three major employers of career physicists are academic institutions, government laboratories, and private industries, with the largest employer being the last. Many trained physicists, however, apply their skills to other activities, in particular to engineering, computing, and finance, often quite successfully. Some physicists take up additional careers where their knowledge of physics can be combined with further training in other disciplines, such as patent law in industry or private practice. In the United States, a majority of those in the private sections having a physics degree actually work outside the fields of physics, astronomy and engineering altogether.