By Sascha Brodsky
Publication Date: 2026-10-27 00:00:00
According to Al-Khalili, one of the biggest obstacles to understanding time is that physicists still lack a single explanation of what it actually is.
“It’s a real thing. It’s a tangible part of our physical reality,” Al-Khalili said. “It’s not our invention; it’s not just in our minds; it’s not just that thing that clocks measure.”
Al-Khalili’s book explores three descriptions of time that physicists have yet to reconcile. In many equations, time is little more than a number that tracks how things change. Einstein’s theory of relativity tells a different story, treating time as a fourth dimension woven into the fabric of space. Thermodynamics, the study of heat and energy, adds another complication: time has a direction, an arrow pointing from the past toward the future.
Einstein also showed that time doesn’t pass at the same rate everywhere. A clock’s rate relative to another depends on motion and gravity. According to the National Institute of…
