‘Solar Twins’ Reveal the Consistency of the Universe

Sometimes we must look to the heavens to understand our own planet. In the 17th century, Johannes Kepler’s insight that planets move in elliptical orbits around the sun led to a deeper understanding of gravity, the force that determines Earth’s tides. In the 19th century, scientists studied the color of sunlight, whose distinctive properties helped reveal the quantum structure of the atoms that make up the star—and all matter around us. In 2017, the detection of gravitational waves showed that much of the gold, platinum, and other heavy elements on our planet are forged in the collisions of neutron stars. 

Michael Murphy studies stars in this tradition. An astrophysicist at 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 the way 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 the electron are. By measuring the fine structure constant in many settings, Murphy is challenging this assumption. 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 totally random. No other part of physics theory explains why it is this value, and thus, why the electromagnetic field is the strength that 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 puts it this way: “We don’t really understand where these numbers come from, even though they’re in the back of textbooks.”