How Heavy Can a Particle of Light Be? Scientists Just Figured It Out

We have a new upper limit for the mass of light.

According to measurements of pulsing stars scattered throughout the Milky Way and mystery radio signals from other galaxies, a particle of light – called a photon – can be no heavier than 9.52 × 10^-46 kilograms.

It's a tiny limit, but finding that light has any mass at all would significantly impact how we interpret the Universe around us, and our understanding of physics.

Photons, typically, are described as massless particles. These discrete quantities of energy zip through space-time at a constant speed, unable to accelerate or slow down in a vacuum. This constant velocity implies masslessness, and there isn't evidence to the contrary.

However, we don't know for absolute certainty that photons are massless.

A non-zero mass would have profound implications. It would contradict Einstein's special relativity, and Maxwell's electromagnetic theory, probably leading to new physics, and possibly answering some giant questions about the Universe (although raising many more in the process).

If a photon did have mass, it would need to be extremely small to not have major effects on the way the Universe appeared, which means that we just don't have the tools to measure it directly.

But we can take indirect measurements that will give us an upper limit for this hypothetical mass, and this is exactly what a group of astronomers did.

A team from Sichuan University of Science & Engineering, the Chinese Academy of Sciences, and Nanjing University analyzed data collected by the Parkes Pulsar Timing Array and data on fast radio bursts from a number of sources to determine how massive light can possibly be.

A pulsar timing array is an array of radio telescope antennas to monitor neutron stars that send out pulsing beams of electromagnetic radiation on extremely precise millisecond pulsars. Fast radio bursts are extremely powerful bursts of light of unknown origin that are detected across vast intergalactic gulfs of space.

The property the researchers examined is known as the dispersion measure, one of the key attributes of pulsars and fast radio bursts. It refers to how much a tightly pulsed beam of radio light is scattered by the free electrons between us and the light source.

If photons have mass, their propagation through non-vacuum space populated by plasma would be affected both by the mass and the free electrons in the plasma. This would lead to a delay time proportional to the mass of the photon.

A pulsar timing array looks for delays in the timing of pulsar pulses relative to each other. Particularly within the ultrawide bandwidth, the dispersion effects can be minimized, allowing the researchers to calculate how much delay could be contributed by the hypothetical photon mass.

Meanwhile, dedispersing the signals from fast radio bursts can also reveal a delay proportional to the photon mass.

By carefully studying this data, the team was able to derive their upper limit of 9.52 × 10^-46 kilograms (or, in equivalent energy, 5.34 × 10^-10 electron volts c^-2). Note that this doesn't mean that the photon has mass; it just means that we have a new boundary wherein the mass could fall, if it existed.

"This is the first time," the authors write, "that the interaction between a nonzero photon mass and the plasma medium has been taken into account and calculated as the photon propagates through the plasma medium."

It's not very much lower than a measurement published in 2023, but it is a refinement. This means that scientists investigating the effects of a hypothetical photon mass have a more precise range in which to operate.

The study also demonstrates, the astronomers say, the need for highly precise radio telescopes. We're not likely to be able to weigh a photon anytime soon, but obtaining consistently higher-quality data will allow us to narrow the measurement down further, and with it its potential effects on the Universe around us.

The research has been published in The Astrophysical Journal.