Doctor of Medicine (MD, from the Latin Medicinæ Doctor meaning "Teacher of Medicine") is a doctoral degree for physicians granted by medical schools. It is a professional doctorate / first professional degree (qualifying degree) in some countries, including the United States and Canada, although training is entered after obtaining from 90 to 120 credit hours of university level work (see second entry degree) and in most cases after having obtained a Bachelors Degree. In other countries, such as India, the United Kingdom, Germany, and Sri Lanka, the MD is an advanced academic research degree more similar to a PhD. In India, Britain, Ireland, and many Commonwealth nations, the medical degree is instead the MBBS i.e., Bachelor of Medicine, Bachelor of Surgery (MBChB, BM BCh, MB BCh, MBBS, BMBS, BMed, BM) and is equivalent to the MD and DO degrees in the U.S. system.

According to Sir John Bagot Glubb, Syed Faride and S. M. Imamuddin, the first medical schools to issue academic degrees and diplomas were the teaching Bimaristan (Hospitals) of the medieval Islamic world. The first of these institutions was opened in Baghdad during the time of Harun al-Rashid. They then appeared in Egypt from 872 and then in Islamic Spain, Persia and the Maghreb thereafter. Physicians and surgeons at these hospital-universities gave lectures on Medicine to medical students and then a medical diploma or degree was issued to students who were qualified to be practicing physicians.

Albert Einstein ( /ˈælbərt ˈaɪnstaɪn/; German: [ˈalbɐt ˈaɪnʃtaɪn] ( listen); 14 March 1879 – 18 April 1955) was a German theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics. While best known for his mass–energy equivalence formula E = mc² (which has been dubbed "the world's most famous equation"), he received the 1921 Nobel Prize in Physics "for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect". The latter was pivotal in establishing quantum theory within physics.

Near the beginning of his career, Einstein thought that Newtonian mechanics was no longer enough to reconcile the laws of classical mechanics with the laws of the electromagnetic field. This led to the development of his special theory of relativity. He realized, however, that the principle of relativity could also be extended to gravitational fields, and with his subsequent theory of gravitation in 1916, he published a paper on the general theory of relativity. He continued to deal with problems of statistical mechanics and quantum theory, which led to his explanations of particle theory and the motion of molecules. He also investigated the thermal properties of light which laid the foundation of the photon theory of light. In 1917, Einstein applied the general theory of relativity to model the structure of the universe as a whole.