# The fine structure constant

I have a question about the really meaning of the fine structure constant. Actually, my doubt is why the physicist look to a meaning for this constant. I understand the difference between this constant and the ordinary constant, that is, it carry no units with it. But is not this just like pi or euler number? It sometimes appears in physics situations too, without units, but nobody looks for its "meaning" as much as looking for that of the fine structure constant (As Far As I Know). As we can just sit and accept the importance of pi, the importance of euler number, why we need to find a meaning to the fine structure and not just sit and accept its importance too?

• People totally look into $\pi ,$ $e ,$ etc.. Guessing you're probably trying to ask why mathematicians focus on such constants more while physicists are more concerned with physical constants? – Nat Sep 19 '20 at 18:24

Mathematical constants such as $$\pi$$ and $$e$$ have values that are determined by their definitions. For example, you can define $$\pi$$ as the ratio of the circumference of a circle to its diameter, and then use this definition to calculate $$\pi$$ to as many decimal places as you want. Constants like this cannot have any other value than the value they have.

By contrast, we know of no reason why the fine structure constant has the value it has. And we can imagine a universe in which it has a different value. So this leads physicists to wonder why it is approximately $$1/137$$ instead of, say, $$1/140$$, or something much smaller or much larger.

In short, dimensionless physical constants are not mathematical constants. Or at least they aren’t in our current physical theories. Instead they are simply parameters of our models.

• "[...] at least they aren’t in our current physical theories" - I like this sentence. Maybe we will get some progress done in the next 50 years. – user224659 Sep 20 '20 at 18:21
• @TheoreticalMinimum A great example would be the $\sigma$ in $P=\epsilon\sigma AT^4$, which with the advent of quantum mechanics can be nondimensionalised to $\pi^2/60$. To a 19th century worker in thermodynamics, only the existence of such a $\sigma>0$ can be inferred. – J.G. Sep 20 '20 at 18:35

Distinct from $$\pi$$ and $$e$$, the fine structure constant does not have a meaning defined simply in terms of mathematical formulae, which are valid without reference to physics or indeed to anything outside of mathematics. In quantum electrodynamics the likelihood of an interaction between a photon and an electron, as represented by the Feynman diagram

is determined by the coupling constant, usually designated by $$e$$ (clearly not to be confused with Euler's number). The fine structure constant is given (in natural units) by

$$\alpha = \frac{e^2}{4\pi},$$

but, distinct from $$e$$, the fine structure constant is defined to be dimensionless. Thus if $$e$$ is measured in units of charge, the fine structure constant is defined as

$$\alpha = \frac{e^2}{4\pi\hbar c\epsilon_0}$$

so as to be dimensionless. The fine structure constant is a measure of the probability, or frequency, of interactions between photons and electrons, contingent on the distribution of matter. It is dimensionless for the same reason that any probability is dimensionless.

Just because something doesn't have units, doesn't make it inherently interesting enough to "sit with." The ratio of my height to the length of a football doesn't have units, but it also isn't a very fundamental or interesting quantity.

The parameter that controls the strength of a force is something we can derive from measurements, and it has many implications for how things around us behave, but at the end of the day if it was a slightly different number, QED wouldn't be a very different theory.

Also once you take into account the running of couplings, the fine structure constant looks even less fundamental. If we measure the strength of the electromagnetic interaction at a different every scale, we get a different number. The fine structure constant can only be defined as the strength of the EM interaction at a specific energy scale.

First, I would argue that $$\pi$$, in fact, does have a meaning, and an explanation as to its value. The ratio of a circle's radius to its diameter is dependent on the curvature of the space that the circle is occupying. The $$\pi$$ that you're used to is the value for space with zero curvature; if the space is strongly postively or negatively curved, the value of that ratio will be a different number. Try drawing a circle on a globe - the circumference of that circle will be smaller than $$\pi$$ times the distance from one side of the circle to the other.

This means that measuring the ratio of a circle's circumference to its diameter tells you something about the curvature of the space you're living in. So the value of $$\pi$$ does have a specific meaning and explanation - the measured value is explained by the fact that the space we live in is locally well-approximated by a "flat" space, without any strong curvature.

Euler's number also has a meaning and explanation: we first discovered its value when we started playing around with compound interest. It was eventually discovered that if you started compounding the interest faster and faster, in smaller and smaller increments, the amount of money you had was described by an exponential function with $$e$$ as the base. Since then, $$e$$ has been associated with continuous processes, and often when converting a discrete process into a continuous one, you'll encounter $$e$$ somewhere. Its specific value depends on our definition of the derivative, but the fact that we see it so often in our measurements means that natural processes are often well-approximated by continuous processes.

Getting back to the main point: for a long time in physics, there's been the expectation that the numbers that we come across in our measurements are not arbitrary. When we measured the emission line spectrum of hydrogen, we did not stop there and say, "Well, we have these arbitrary frequencies that hydrogen emits, things must just be like that." We looked for a model to explain where those frequencies came from. Eventually, we came up with a simple formula that predicted all of the spectral lines using only a single constant, the Rydberg constant.

We could have also stopped there, saying, "Well, the Rydberg constant has this arbitrary value, and that's the end of discussion," but we didn't. We expected, again, that the value of that constant had some deeper explanation, and wasn't arbitrary. It turned out that we were correct again: the Rydberg constant has the value it does because its value is set by a quantum-mechanical model of the hydrogen atom, which only depends on such fundamental things like $$\hbar$$ (the conversion factor between energy and angular frequency), the fundamental quantum of free charge (i.e. the charge on the electron), the mass of the electron, the permittivity of free space, and the speed of light.

Even then, we continued to expect that those constants had some deeper explanation. Some of them are fairly shallow - for example, the value of $$\hbar$$ is simply the result of the fact that our "human" units of energy and time are not really compatible with each other. In more "natural" units, the value of $$\hbar$$ can be set to one. Similarly, the value of the speed of light, and the permittivity of free space, are artefacts of our units for distance and time not really being compatible, and their values can also be set to one with a more "natural" choice of units, where mass, energy, time, and distance are all the same unit and intercompatible with each other.

But the other constants in that formula do have a deeper explanation. Quantum electrodynamics dictates that the electron charge that we measure is actually the "dressed" electron charge; the "natural" or "bare" electron charge is roughly 1, but interactions between the electron and the quantum vacuum induce a "vacuum polarization" that reduces the apparent charge that we see. So the value of the measured electron charge is determined by the strength of the electromagnetic interaction, which is parametrized by the fine structure constant $$\alpha$$. And, according to the Higgs mechanism, the mass of the electron is determined by the coupling strength of the Yukawa coupling between the electron and the Higgs field.

So now, in the present day, we have some constants, like the above Yukawa coupling strength and $$\alpha$$, which might seem arbitrary. But every time we encountered a seemingly arbitrary constant in the past, there was always some deeper explanation for why it had the value it did. Is it possible that there just isn't such a meaning for these constants this time? Yes, it's definitely possible. But based on our past experience in science, there is no reason to expect that to be true.

• Note that CGS units don’t have $\epsilon_0$. The actual numerical value of $\alpha$ has some anthropic significance: see Barrow, J. and Tipler, F., 1986. The cosmological anthropic principle. – ZeroTheHero Sep 20 '20 at 4:30
• Your first paragraph is misleading. In a curved space, the ratio between radius and circumference is not constant, and depends on the radius (so in particular it tends to pi at low radii). – Emilio Pisanty Sep 20 '20 at 11:26
• The ratio of circumference to diameter of a ball varies depending on the curvature of the space, but $\pi$ does not. $\pi$ itself is a constant that is independent of any concept of geometry. For example, if one considers the differential equation $y'' + y =0$ one finds that the solutions are functions that necessarily have period $2\pi$.The fact that $\pi$ is the circumference-diameter ratio in a flat space is of course related to this, but it is not fundamental to the definition of $\pi$. Your claims about $e$'s value depending on the “definition of the derivative” are similarly misguided. – Mark Dominus Sep 20 '20 at 15:07
• Also, your claim that “we first discovered [$e$'s] value when we started playing around with compound interest” is false as a matter of historical fact. The $e$ constant was first discovered in connection with the tabulation of logarithms. – Mark Dominus Sep 20 '20 at 15:08
• Richard Feynman explains in Chapter 22 of his Lectures how the constant $e$ naturally appears in numerical analysis. This Math SE post explains how it actually did appear historically, not in connection with compound interest but in the construction of tables of logarithms. For a discussion of the naturality of $\pi$ and its appearance in connection with simple differential equations, see G. Simmons Differential Equations with Applications and Historical Notes, p.115–118. – Mark Dominus Sep 20 '20 at 15:36