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.