Synopsis: Constraining the Photon Lifetime

Photons could conceivably decay, but new analysis of the cosmic microwave background shows that a visible wavelength photon is stable for at least 1018 years.

Can a photon decay? It’s hard to imagine, especially considering how long starlight travels to reach us. Still, if photons happen to have a small, imperceptible mass, then they could decay into lighter particles. A search for signs of these decays uses the oldest light in the universe—the cosmic microwave background or CMB. In Physical Review Letters, Julian Heeck of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, shows that the blackbody spectrum of the CMB rules out decays and thus sets a lower limit on the photon lifetime.

For a photon to decay, it must have a mass—otherwise there’d be nothing lighter for it to decay into. A photon with nonzero mass is not ruled out by theory, but experiments with electric and magnetic fields constrain the mass to less than 10-54 kilograms. Heeck assumed this upper limit and worked through a generic model in which photons decay into even lighter particles, which could potentially be neutrinos or some more exotic particles.

As a constraint, Heeck considered the CMB, the relic emission from the hot, opaque plasma that persisted for several hundred thousand years after the big bang. The CMB spectrum matches very closely a perfect blackbody, which implies very few, if any, of the CMB photons decayed on their 13 billion year journey. Heeck calculated that the minimum lifetime is 3 years in the photon’s rest frame. This might seem ridiculously small, but the photons are extremely relativistic. When time dilation is taken into account, a visible wavelength photon in our reference frame would be stable for 1018 years or more. – Michael Schirber


More Announcements »

Subject Areas

OpticsParticles and Fields

Previous Synopsis


Circuit Ready

Read More »

Next Synopsis

Atomic and Molecular Physics

Watching a Quick Shift

Read More »

Related Articles

Viewpoint: Connecting the Higgs Mass with Cosmic History

Viewpoint: Connecting the Higgs Mass with Cosmic History

A theoretical proposal ties the puzzling light mass of the Higgs particle to a hypothetical new particle that plays an important role during the big bang. Read More »

Synopsis: LHC Data Might Reveal Nature of Neutrinos
Particles and Fields

Synopsis: LHC Data Might Reveal Nature of Neutrinos

A long-standing question over whether the neutrino is its own antiparticle might be answered by looking at decays of W bosons. Read More »

Synopsis: Enter the Metacage

Synopsis: Enter the Metacage

An array of equally spaced nanowires, dubbed a metacage, could block optical radiation from entering or escaping a region of arbitrary shape. Read More »

More Articles