The Elusive Nature of Time: Feynman’s Struggle to Define It

What is time? This is the deceptively simple question that Richard Feynman wrestles with in his Lectures on Physics, only to find that a satisfying definition remains elusive. In his typical style, Feynman dismantles our intuitive understanding of time, revealing the difficulties in pinning down something so fundamental. His exploration, filled with humour and vivid imagery, does not provide a rigid definition but instead offers a pragmatic approach: time is what we measure it to be.

The Challenge of Defining Time

Feynman begins by highlighting the inadequacy of dictionary definitions. To say that “a time” is “a period” and that a “period” is “a time” is to go in circles. He jokingly suggests an alternative: “Time is what happens when nothing else happens.” But this, too, proves unhelpful. The problem is that time is such a fundamental concept that any attempt to define it merely restates what we already assume it to be. In the end, Feynman concedes that time is simply how long we wait.

Yet, this does not mean time is beyond our grasp. Even if we cannot define it in absolute terms, we can at least measure it. This shift—from defining to measuring—allows Feynman to explore time through the regularity of natural events.

The Imagery of Periodic Events

To illustrate the measurement of time, Feynman turns to one of the most familiar and ancient timekeeping methods: the day. The daily cycle of sunlight and darkness provides a natural rhythm, but even this is not as straightforward as it seems. He notes that summer days feel longer than winter days—both physically and, subjectively, through human experience. A particularly dull day, for example, can seem interminable.

This subjective distortion of time’s passage leads Feynman to a more rigorous question: Are days really periodic? That is, do they truly mark off equal intervals of time? To test this, he introduces the image of an hourglass—an object that seems to provide a consistent method of tracking time. But even here, difficulties arise. If we count the number of times the hourglass is turned over from one morning to the next, we find that the total varies from day to day. This discrepancy forces us to reconsider our trust in either the sun or the hourglass itself.

The solution, Feynman suggests, is to measure from noon to noon, using the highest position of the sun in the sky as a reference. By doing this, we find a more consistent pattern. Yet even this is not proof of an absolute periodicity—what if some unseen force were slowing the flow of sand at night and accelerating it during the day? We cannot truly prove that any given event is periodic; all we can do is establish consistency between different phenomena.

The Nature of Time: A Practical Approach

Ultimately, Feynman’s lesson is that time is best understood not as an abstract philosophical entity but as a practical tool for describing the world. We do not need to define it in an absolute sense; we only need a reliable method for measuring it. Time, as we understand it, emerges from the repetition of periodic events—whether the movement of celestial bodies or the steady flow of sand through an hourglass.

Through playful but profound reasoning, Feynman leaves us with a perspective that is both humbling and liberating. We may never truly define time, but by recognising and measuring its patterns, we gain enough understanding to make sense of our world. In the end, time is not something we grasp—it is something we use.