From the meter to the second to the joule, the units we use to describe the universe are the legacy of human convenience, historical accident, and Earth-bound intuition. They were not born out of the fundamental laws of physics, but rather constructed to fit the scale and needs of human activity. As a result, much of modern physics is burdened by a clutter of conversion constants—Planck’s constant, the gravitational constant, the Boltzmann constant—used not because nature demands them, but because our units are misaligned with reality. What if we could strip away these artifacts and build a system of measurement grounded not in tradition, but in the universe itself?
The Arbitrary Nature of Human Units
Consider the origins of the meter: once defined as a fraction of the Earth’s circumference, then later based on a metal bar in Paris, and now tied to the speed of light in a vacuum. The second began as 1/86,400th of a day, and the kilogram was literally a chunk of metal stored in a vault until 2019. These units were necessary for commerce, navigation, and engineering—but they are not rooted in any deeper physical truth.
Because of this arbitrariness, we need constants like Planck’s constant h, the speed of light c, and the gravitational constant G to bridge the gap between our human-made units and the real behavior of nature. These constants act like currency exchange rates between the abstract world of physics and the parochial one of meters, seconds, and kilograms. But what if we eliminated the middlemen?
Natural Units: Physics Without the Clutter
In theoretical physics, it’s common to work in natural units where the fundamental constants of nature—such as the speed of light and Planck’s constant—are set to 1. This isn’t just a mathematical trick. It reflects a profound truth: these constants aren’t numbers handed down by the universe—they are artifacts of our unit choices.
By setting c = 1, time and space become interchangeable: distances can be measured in seconds, and time in meters. When \hbar = 1, energy and frequency are the same thing, as are mass and inverse length. Suddenly, the equations of quantum mechanics and general relativity simplify, constants disappear, and we’re left with a view of physics that is cleaner, more symmetric, and closer to nature’s true language.
Planck Units: The Universe’s Native Code
Max Planck, the founder of quantum theory, went one step further. He proposed building a unit system using only the most fundamental constants of nature: G, \hbar, and c. This gives rise to Planck units:
Planck length (~1.6 \times 10^{-35} m): the smallest meaningful unit of distance. Planck time (~5.4 \times 10^{-44} s): the smallest possible unit of time. Planck energy: the scale at which gravity, quantum mechanics, and relativity all converge.
These are not arbitrary—they emerge from the architecture of the universe. A kilogram is a human artifact; a Planck mass is a cosmic truth.
Why This Matters
So what’s the harm in using messy units? For engineers, maybe not much. But in theoretical physics, the clutter of constants obscures structure. Equations become longer, dimensional analysis becomes harder, and intuition is clouded by bookkeeping. Worse, it can lull us into thinking these constants are physical substances, rather than conversion tools.
By adopting natural units as a default—especially in education and theoretical research—we free ourselves to see the form of physical laws, not their formatting. The equations become cleaner, symmetries become obvious, and connections between different areas of physics (like thermodynamics and quantum field theory) emerge more naturally.
Conclusion
The joule, the meter, and the second are powerful tools for navigating human-scale problems. But they are not the language of the cosmos. If we aim to understand the universe at its deepest level, we must be willing to shed our anthropocentric scaffolding and adopt a system of measurement that speaks in nature’s native tongue. That path leads through Planck units, natural units, and a vision of physics unburdened by its own past. To move forward, we may first need to let go of the tape measures we’ve outgrown.