Jacob Lund Fisker

Jacob Lund Fisker (born 1975) is a Danish astrophysicist and writer. He is known as the author of a philosophy of extreme early retirement that has inspired a lifestyle movement. Fisker's book Early Retirement Extreme discusses how to become financially independent with a median income. His philosophy has similarities to LeanFIRE within the FIRE movement. The New York Times described him as often thought of as the father of the FIRE movement.

Life Fisker holds a cand.scient. degree in physics and mathematics from Aarhus University, and a PhD in theoretical physics from the University of Basel in Switzerland. While at Aarhus, he received Statens Uddannelsesstøtte, a state education grant that provides Danish university students with a stipend to cover living expenses while enrolled. Even after completing his degrees, Fisker continued to live on a budget corresponding to the SU stipend he received as an undergraduate, although his income increased over time. As a postdoc, he saved 80% of his income After becoming financially independent, he briefly came out of retirement to work as a quantitative analyst for three years, motivated by personal interest. Fisker is a permanent resident of the United States and lives in Chicago.

Personal finance As Fisker's income increased, the surplus was initially set aside in a savings account. Later it became clear to Fisker that the money could be invested in the stock market and generate a larger return.

The book contains a mathematical study of savings rate and its impact on a person's ability to retire. With conventional personal finance advice, it is typically recommended to save 10–15% of income for retirement. With this savings rate, neglecting investment returns and compound interest, it takes between five and 10 years to save enough to cover one year's expenses. However, if 75% of income is saved, it is possible to cover three years' expenses for each year worked. The high savings rate allows for a much earlier retirement compared to the conventional savings rate.

In order to achieve low expenses, Fisker applies systems theory to personal finance. For example, instead of searching for the cheapest housing and the cheapest car, one may find an even cheaper solution with inexpensive housing within walking distance of work and a supermarket, eliminating the cost of a car as it is no longer needed for transportation. This built on his ideas about the 'Renaissance Man' as introduced in the Early Retirement Extreme book, focusing on re-skilling and understanding emergent societal systems.