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A key constant’s new measurement hints ‘dark photons’ don't exist

This is the most precise look yet at the fine-structure constant

2:00pm, April 12, 2018
Weicheng Zhong

CONSTANT QUANTIFED A team of physicists, including Weicheng Zhong of the University of California, Berkeley (pictured) has performed the most precise measurement yet of the fine-structure constant, which governs the strength of electromagnetic interactions.

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An ultraprecise new measurement has given some weird particle physics theories a black eye.

By measuring one of nature’s most fundamental constants more precisely than before, scientists have tested proposed tweaks to the standard model, the theory governing fundamental particles. The result, reported April 13 in Science, casts doubt on hypothetical particles called dark photons and other potential oddities.

The quantity in question is the fine-structure constant, a number that governs the strength of electromagnetic interactions (SN: 11/12/16, p. 24), such as those that confine electrons within atoms. Previously, the most precise measurement of the constant was indirect, relying on a measurement of the electron’s magnetic properties and using complex theoretical calculations to infer the constant’s value.

Now, physicist Holger Müller of the University of California, Berkeley and colleagues have measured the constant more directly. The team fired lasers at cesium atoms to create a quantum superposition — a bizarre state in which each atom is in two places at once — and watched how the atoms interfered with themselves as they recombined. This interference reveals how fast the atom moved when hit by the laser, which scientists then used to calculate the fine-structure constant.

The answer: The fine-structure constant is approximately 1/137.035999046.

If the new measurement disagreed with the earlier one, that might be an indication of new particles. But the two agree reasonably well, which confirms that the electron is probably not composed of smaller particles and disfavors the possibility of dark photons. These hypothetical particles are similar to run-of-the-mill photons, or particles of light, but unlike normal photons would have mass and interact very weakly with known particles.

But while close, the two measurements didn’t match perfectly, a result which leaves some wiggle room for physicists to think up other types of strange new particles.


R.H. Parker et al. Measurement of the fine-structure constant as a test of the Standard Model. Science. Vol. 360, April 13, 2018, p. 191. doi: 10.1126/science.aar7432.

Further Reading

E. Conover. Proton size still perplexes despite a new measurement. Science News. Vol. 192, November 11, 2017, p. 14.

E. Conover. Units of measure are getting a fundamental upgrade. Science News. Vol. 190, November 12, 2016, p. 24.

G. Popkin. New limit placed on physics constant. Science News. Vol. 184, November 16, 2013.

M. Rosen. Notorious 'Big G' gets a little larger. Science News Online, September 12, 2013.

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