1/137

1/137, an odd fraction to be sure but it seems it's rather important as it defines how charged particles interact with the electromagnetic force and, as researchers note, no one knows where it came from. 

A seemingly harmless, random number with no units or dimensions has cropped up in so many places in physics and seems to control one of the most fundamental interactions in the universe.

Its name is the fine-structure constant, and it's a measure of the strength of the interaction between charged particles and the electromagnetic force. The current estimate of the fine-structure constant is 0.007 297 352 5693, with an uncertainty of 11 on the last two digits. The number is easier to remember by its inverse, approximately 1/137. 

If it had any other value, life as we know it would be impossible. And yet we have no idea where it comes from.

The fine-structure constant is a seemingly random number with no units or dimensions, which has cropped up in many places in physics, and seems to control one of the most fundamental interactions in the universe. (Image credit: Wikimedia)

In this Feynman diagram, an electron (e−) and a positron (e+) annihilate, producing a photon (γ, represented by the blue sine wave) that becomes a quark–antiquark pair (quark q, antiquark q̄), after which the antiquark radiates a gluon (g, represented by the green helix).

The mystery deepens ...

The introduction of a constant wasn't all that new or exciting at the time. After all, physics equations throughout history have involved random constants that express the strengths of various relationships. Isaac Newton's formula for universal gravitation had a constant, called G, that represents the fundamental strength of the gravitational interaction. The speed of light, c, tells us about the relationship between electric and magnetic fields. The spring constant, k, tells us how stiff a particular spring is. And so on.

But there was something different in Sommerfeld's little constant: It didn't have units. There are no dimensions or unit system that the value of the number depends on. The other constants in physics aren't like this. The actual value of the speed of light, for example, doesn't really matter, because that number depends on other numbers. Your choice of units (meters per second, miles per hour or leagues per fortnight?) and the definitions of those units (exactly how long is a "meter" going to be?) matter; if you change any of those, the value of the constant changes along with it.

The Tao beckons ...

So why does it have the value it does? Remember, that value itself is important and might even have meaning, because it exists outside any unit system we have. It simply … is.