This book represents a journey through the history of science in regards to the concept of time, specifically, the question as to whether it is absolute, relative, or irreversible.

The best-known contribution, or at least the most popular one, came from Einstein. He took the illusion that time was universal, a concept dating back, essentially, to Galileo and Newton, and shattered it, both within and without the scientific community.

Thermodynamics teaches us that time has a preferential direction, i.e., forward, and is irreversible, as shown by Prigogine and his theories on dissipative structures and complex systems. Time is not only an external spectator to what happens, but assiduously takes part in making it happen.

This book will be a valuable read for students, researchers, and interested laypersons alike.

After spending a research period at the University of Houston, TX, USA, Prof. Grassi has been involved in experimental and theoretical studies in thermo-fluid-dynamics. As part of this activity, he has engaged in several experimental campaigns in microgravity on parabolic flights, sounding rockets, and satellites funded by the ESA (European Space Agency). He has also been a member of the Physical Science Working Group and chair of a Topical Team of the European Space Agency (ESA).

Prof. Grassi has also performed a broad number of activities on Energy Saving and Management in collaboration with the ENEA (Italian Department of Energy) and has been responsible for several energy contracts. In this context, he has been the president of the Regional Energy Agency (no longer extant) of Tuscany and of the Italian Union of Geothermics.

Prof. Grassi has received in 2023 the Max Jakob Memorial Award in Los Angeles, one of the highest international recognitions for thermal sciences, awarded jointly by the American Society of Mechanical Engineers (ASME) and the American Institute of Chemical Engineers (AIChE).