Michael Hartl

Michael Hartl is an American physicist, author, and entrepreneur. He is best known as the creator of the Ruby on Rails Tutorial,  founder of Tau Day,    and author of The Tau Manifesto. In The Tau Manifesto, Hartl proposes replacing pi ($&pi;$) with tau ($2&pi;$).

Education
In the 1990s, Hartl attended Harvard University for his undergraduate studies, graduating with a bachelor's degree in physics. Hartl obtained his PhD from the California Institute of Technology (Caltech) in 2003, where he researched black hole dynamics. His dissertation was titled Dynamics of Spinning Compact Binaries in General Relativity.

Career
After finishing his PhD, Hartl served as Caltech's editor on a corrected and expanded version of The Feynman Lectures on Physics at the request of Kip Thorne. Explaining in the preface why he chose Hartl for the task, Thorne noted that "Hartl understand physics deeply, he is among the most meticulous physicists I have known, and like Feynman he is an outstanding pedagogue." Thorne also noted that Hartl is the only Caltech graduate student to be granted a "lifetime achievement award for excellence in teaching" by Caltech's undergraduates.

As an entrepreneur, Hartl participated in the Y Combinator program in 2008. In 2010, he published the first edition of the Ruby on Rails Tutorial book and screencasts, which teach web development using the Ruby on Rails web application framework. The Ruby on Rails Tutorial quickly became both a critical and commercial success, eventually going through seven editions. In 2011, Hartl received a Ruby Hero Award for his service to the Ruby community, with the citation mentioning both the Ruby on Rails Tutorial and his first book, RailsSpace. In 2013, Wikipedia co-founder Jimmy Wales described the Ruby on Rails Tutorial as his "favorite book."

Hartl later co-founded the online education company Learn Enough to expand on the Ruby on Rails Tutorial, adding tutorials on other computer technologies and including online courses. In partnership with Pearson Education, Hartl authored or co-authored five books published under the brand Learn Enough to Be Dangerous. Learn Enough was acquired by a tech private equity group in 2022.

The Tau Manifesto
In 2010, Hartl published The Tau Manifesto, in which he proposed using the Greek letter tau to represent the circle constant $&tau; = C/r = 2&pi;$, the first time tau was publicly proposed for this purpose. The Tau Manifesto proved popular, and a revised edition was published in 2019, followed by a print edition in 2021. With the initial publication of The Tau Manifesto in 2010, Hartl also founded Tau Day as a mathematical celebration and to promote adoption of the new constant. Observed annually on June 28, or 6/28 (in analogy with the celebration of Pi Day on 3/14), Tau Day has become a widely celebrated mathematical holiday. Hartl's constant has also seen significant adoption, including support for tau in the official Google calculator and inclusion in programming languages such as Microsoft.NET, Java, and Python.