Are the Fundamental Constants Changing?

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Matt O'Dowd (1:03)

This episode is supported by The Great Courses Plus. The laws of physics are the same everywhere in the universe. At least we astrophysicists hope so. After all, it's hard to unravel the complexities of different parts of the universe if we don't know the basic rules. But what if this is wrong? There is a hint of evidence that the fundamental constants that govern our universe may evolve over time and even from one location to another.

The laws of physics are the relationships we observe between space and time and the fields and particles that occupy it. Those relationships are often expressed as equations, but they're also governed by the constants within those equations. For example, the standard model of particle physics is comprised of equations that predict the existence and behavior of the particle building blocks of our universe.

But the standard model also depends on a set of constants that cannot be predicted by that model, only measured. Things like the mass of the electron and the relative strengths of the forces of nature.

Why the fundamental constants take the values they do is a very deep and unanswered question.

The answer lies in a deeper, underlying theory, a so-called Grand Unified Theory. But some proposed Grand Unified Theories predict something unsettling. They predict that the fundamental constants may not be constant at all and instead may vary over time and space. Now we're not going to get into the nitty-gritty of the theory today. We'll come back to it another time. Instead, we're going to look at the experiments and the evidence. Because there's a hint of evidence that at least one fundamental constant is in fact varying. To measure changes in a fundamental constant, we first need to choose the right constant. Let's try the speed of light. Its value today is 299,792,458 m per second in a vacuum.

But was it always this value? It's been suggested that a changing speed of light might be an alternative to inflation theory, or even to the apparent expansion of the universe. But this may be untestable. For one thing, our definitions of the units used to define the speed of light are arbitrary and themselves depend on that speed. The meter is officially defined as the distance light travels in 1 299,792,458th of a second. And the second is defined in terms of a particular frequency of light emitted by the cesium-133 atom.

If the speed of light changes, the rulers we use to measure that speed change also.

The speed of light defines the relationship between space and time. So is it even meaningful to talk about it changing independently to its underlying dimensions? In fact, it may be impossible to interpret changes in any physical constant that has units.

The dimensions behind, say, Newton's gravitational constant or the mass of the electron all have arbitrary human definitions. To be confident that we've seen a change in a fundamental constant, we need to study a dimensionless constant, one that has no units and therefore isn't dependent on our definitions of those units. Perhaps the most promising example is the fine structure constant. It's a dimensionless description of the strength of the electromagnetic force. In the language of quantum field theory, it's the coupling strength between the electromagnetic field and a charged field like the electron field. We use the Greek letter alpha to represent the fine structure constant and its numerical value is one of the most precisely measured quantities in physics, precise to one part in 4 billion. Its approximate value is around 1 over 137 with no units. It's a dimensionless number. No one knows how alpha ended up with this value, but if you change it by much, our universe would look very different. The first measurement of this fundamental parameter was through its effect on the fine grained structure of atomic energy levels, which is where the constant gets its name. This effect is also how we'll test whether alpha is changing, so let's look into it.