We have a new upper limit for the mass of light.
According to measurements of pulsating stars scattered throughout the Milky Way and mysterious radio signals from other galaxies, a particle of light—called a photon—cannot be heavier than 9.52 × 10.-46 kilograms.
It’s a small limit, but finding that light has any mass at all would significantly affect how we interpret the universe around us and our understanding of physics.
Photons are typically described as massless particles. These discrete amount of energy float through spacetime at a constant speed, unable to speed up or slow down in a vacuum. This constant velocity implies weightlessness and there is no evidence to the contrary.
However, we do not know with absolute certainty that photons are massless.
Non-zero mass would have profound consequences. It would contradict Einstein’s special theory of relativity and Maxwell’s electromagnetic theory, possibly leading to new physics and possibly answering some giant questions about the universe (though raising many more in the process).
If a photon had mass, it would have to be extremely small to not have a fundamental effect on the way the universe appeared, which means we simply don’t have the tools to measure it directly.
But we can make indirect measurements that give us an upper limit for this hypothetical mass, and that’s 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 flashes from a range of sources to determine how massive the light might be.
The Pulsar Timing Array is an array of neutron star monitoring radio telescope antennas that beam pulsed beams of electromagnetic radiation at extremely precise millisecond pulsars. Fast radio bursts are extremely powerful bursts of light of unknown origin that are detected across the vast intergalactic gulfs of space.
The property the researchers investigated is known as the dispersion rate, one of the key attributes of pulsars and fast radio bursts. It refers to how much a strongly pulsating beam of radio light is scattered by the free electrons between us and the light source.
If photons have mass, their propagation through the non-vacuum space populated by the plasma would be affected by both the mass and the free electrons in the plasma. This would lead to a delay time proportional to the photon mass.
The pulsar timing field looks for delays in the timing of the pulsar pulses relative to each other. Especially within ultra-wide bandwidth, scattering effects can be minimized, allowing researchers to calculate how much delay a hypothetical photon mass might contribute.
Meanwhile, dedispersion of signals from fast radio bursts can also reveal delays proportional to the photon mass.
By carefully studying this data, the team was able to derive its upper limit of 9.52 × 10-46 kilograms (or in equivalent energy 5.34 × 10-10 electron volts c-2). Note that this does not mean that the photon has mass; it just means we have a new boundary where matter could fall if it existed.
“This is the first time,” the authors write, “that the interaction between the non-zero photon mass and the plasma medium has been taken into account and calculated as the photon propagates through the plasma medium.”
It’s not much lower than the measurement published in 2023, but it’s a refinement. This means that scientists investigating the effects of hypothetical photon mass have a more precise range in which to work.
The study also shows, astronomers say, the need for high-precision radio telescopes. It’s unlikely that we’ll be able to measure the photon anytime soon, but getting consistently better quality data will allow us to further narrow down the measurements, and thus its potential effects on the universe around us.
The research was published in The Astrophysical Journal.