The universe's magic number
In physics, some numbers are foundational. The fine-structure constant, denoted by the Greek letter alpha (α), is one of them. It is a dimensionless quantity, a pure number with a value close to 1/137.036. This constant quantifies the strength of the electromagnetic force, one of the four forces of nature. It dictates how charged particles like electrons interact with light and with each other, effectively defining the structure of atoms and molecules.
The idea that a fundamental constant might change over time is not new. In 1937, physicist Paul Dirac proposed the "Large Numbers Hypothesis," which suggested that some constants, including the gravitational constant, might evolve as the universe ages. While Dirac's specific theory has been largely superseded, the question of whether the constants of nature are truly constant is still being researched. If alpha's value were different, the universe would be unrecognizable. A change of just 4% would prevent stars from producing carbon, a necessary element for life as we know it.
Evidence from ancient light
To test if alpha has changed, astronomers look at light from the most distant objects in the universe: quasars. These are extremely luminous galactic cores, and their light travels for billions of years to reach Earth. On its journey, the light passes through intergalactic gas clouds. Atoms within these clouds absorb light at very specific frequencies, creating a pattern of dark lines in the quasar's spectrum. The precise spacing of these absorption lines is sensitive to the value of the fine-structure constant at the time and place the light passed through the cloud.
In the late 1990s and 2000s, teams of astronomers led by John Webb used high-resolution spectrographs on the Keck Telescope in Hawaii and the Very Large Telescope (VLT) in Chile to analyze these ancient absorption lines. Their results hinted at something extraordinary. Data from Keck suggested that in one direction of the sky, alpha was slightly smaller 10-12 billion years ago, by just a few parts per million. Conversely, VLT data, looking in a different direction, suggested alpha was slightly larger.
When combined, this data points to a potential "spatial dipole," suggesting the value of the fine-structure constant might not be the same everywhere in the universe. This result is at the absolute limit of measurement and could be due to undiscovered systematic errors. Other studies have found no evidence of variation. If confirmed, it would mean the laws of physics are not uniform, challenging one of the basic assumptions of modern cosmology.