Synopsis

Constraining the Photon Lifetime

Physics 6, s96
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


Subject Areas

OpticsParticles and Fields

Related Articles

Axion Clouds Enveloping Pulsars
Particles and Fields

Axion Clouds Enveloping Pulsars

Axions—theorized particles that could account for dark matter—could accumulate around rapidly rotating neutron stars to the point that they become detectable. Read More »

Cleaning Intense Laser Pulses with Plasma
Optics

Cleaning Intense Laser Pulses with Plasma

When two laser beams converge on a volume of gas, their interference creates a diffraction grating made of plasma that can divert and shape a third beam. Read More »

Positronium Cooled to Record Low Temperature
Atomic and Molecular Physics

Positronium Cooled to Record Low Temperature

A short-lived combination of an electron and an antielectron has been cooled with lasers to near absolute zero—a step toward tackling fundamental questions about matter and antimatter. Read More »

More Articles