He was born in La Crosse, Wisconsin, to D. D. MacMillan, who was in the lumber business, and Mary Jane McCrea. His brother,
John H. MacMillan, headed the
Cargill Corporation from 1909 to 1936. MacMillan graduated from La Crosse High School in 1888. In 1889, he attended
Lake Forest College, then entered the
University of Virginia. Later in 1898, he earned an A.B. degree from
Fort Worth University, which was then a Methodist university in Texas. He performed his graduate work at the University of Chicago, earning a master's degree in 1906 and a PhD in astronomy in 1908. In 1907, prior to completing his PhD, he joined the staff of the University of Chicago as a research assistant in geology. In 1908, he became an associate in mathematics, then in 1909, he began instruction in astronomy at the same institution. His career as a professor began in 1912 when he became an assistant professor. In 1917, when the U.S. declared war on Germany, Dr. MacMillan served as a
major in the U.S. army's ordnance department during
World War I. Following the war, he became associate professor in 1919, then full professor in 1924. MacMillan retired in 1936. In a 1958 paper about MacMillan's work on cosmology, Richard Schlegel introduced MacMillan as "best known to physicists for his three-volume
Classical Mechanics" that remained in print for decades after MacMillan's 1936 retirement. MacMillan accepted that the radiance of stars came from then unknown processes that converted their mass into
radiant energy. This perspective suggested that individual stars and the universe itself would ultimately go dark, which was called the
"heat death" of the universe. MacMillan avoided the conclusion about the universe through a mechanism later known as the "
tired-light hypothesis". He speculated that the light emitted by stars might recreate matter in its travels through space. MacMillan's work on cosmology lost influence in the 1930s after Hubble's law became accepted.
Edwin Hubble's 1929 publication, and earlier work by
Georges Lemaître, reported on observations of entire galaxies far from the earth and its galaxy. The further away a galaxy is, the faster it is apparently moving
away from the earth. Hubble's law strongly suggested that universe is expanding. In 1948, a new version of a
steady-state cosmology was proposed by
Bondi,
Gold, and
Hoyle that was consistent with the measurements on distant galaxies. While the authors were apparently not aware of MacMillan's earlier work, substantial similarities exist. In an
Associated Press report, MacMillan speculated on the nature of interstellar civilizations, believing that they would be vastly more advanced than our own. "Out in the heavens, perhaps, are civilizations as far above ours as we are above the single cell, since they are so much older than ours." The
crater MacMillan on the Moon is named in his honor. ==Selected publications==