To understand our planet, we occasionally need to gaze at the stars. The 17th century saw a greater understanding of gravity—the force that governs Earth’s tides—thanks to Johannes Kepler’s discovery that planets orbit the sun in elliptical paths. The peculiar characteristics of sunlight’s color, which scientists in 19th-century research, led to the unveiling of the quantum structure of the atoms that make up the star and all other matter in the universe. A large portion of the planet’s gold, platinum, and other heavy elements are created in neutron star mergers, as evidenced by the gravitational wave detection in 2017.
Michael Murphy studies star in this tradition. An astrophysicist at the Swinburne University of Technology in Australia, Murphy analyzes the color of the light emitted by stars similar to the sun in temperature, size, and elemental content—”solar twins,” as they are called. He wants to know what their properties reveal about the nature of the electromagnetic force, which attracts protons and electrons to form atoms—which then bind into molecules to form almost everything else.
In particular, he wants to know if this force behaves consistently across the entire universe—or at least, among these stars. In a recent paper in Science, Murphy and his team used starlight to measure what’s known as the fine structure constant, a number that sets the strength of the electromagnetic force. “By comparing the stars to each other, we can learn if their fundamental physics is different,” says Murphy. If it is, that hints that something is wrong with how we understand cosmology.
Standard physics theory, known as the Standard Model, assumes this constant should be the same everywhere—just as constants like the speed of light in a vacuum or the mass of an electron are. Murphy challenges this assumption by measuring the fine structure constant in many settings. If he finds discrepancies, it could help researchers amend the Standard Model. They already know the Standard Model is incomplete, as it does not explain the existence of dark matter.
To understand this constant, think of the electromagnetic force in analogy with the gravitational force, says Murphy. The strength of an object’s gravitational field depends on its mass. But it also depends on a number known as G, the gravitational constant, that remains the same regardless of the object. A similar mathematical law dictates the electromagnetic force between two charged objects. The two attract or repel each other based on their electric charge and their distance from each other. But that force also depends on a number—the fine structure constant—that stays the same regardless of the object.
All experiments thus far have indicated that in our universe, that constant equals 0.0072973525693, with uncertainty less than one part per billion. But physicists have long considered this number a mystery because it seems random. No other part of physics theory explains why it is this value, and thus, why the electromagnetic field is the strength as it is. Despite the word “constant” in its name, physicists also don’t know if the fine structure constant has the same value everywhere in the universe for all time. Physicist Richard Feynman famously described it as “a magic number that comes to us with no understanding.” Murphy says this: “We don’t understand where these numbers come from, even though they’re in the back of textbooks.”
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