We have a new upper limit for the light mass.
According to measurements of pulsating stars scattered across 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 if we discover that light has any mass at all, it would have a significant impact on the way we interpret the universe around us, and on our understanding of physics.
Photons are usually described as massless particles. This discrete amounts of energy rushing through space-time at a constant speed, unable to speed up or slow down in a vacuum. This constant speed implies masslessness, and there is no evidence to the contrary.
However, we do not know with absolute certainty that photons are massless.
A non-zero mass would have profound consequences. It would contradict Einstein’s special theory of relativity and Maxwell’s electromagnetic theory, which would likely lead to new physics and possibly answer some huge questions about the universe (although many more would arise).
If a photon had mass, it would have to be extremely small not to have a major impact on the way the universe appeared, meaning we simply don’t have the means to measure it directly.
But we can make 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 fast radio burst data from a number of sources to determine how large a light might be possible .
A pulsar timing array is an array of radio telescope antennas to monitor neutron stars that emit pulsating beams of electromagnetic radiation at extremely precise millisecond pulsars. Fast radio bursts are extremely powerful bursts of light of unknown origin that are detected in vast intergalactic waves of space.
The property the researchers investigated is known as the dispersion measure, one of the most important characteristics of pulsars and fast radio bursts. It refers to the extent to which 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 a non-vacuum space populated by 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 mass of the photon.
A pulsar timing array looks for delays in the timing of pulsar pulses relative to each other. Particularly within the ultra-wide bandwidth, dispersion effects can be minimized, allowing the researchers to calculate how much delay the hypothetical photon mass could cause.
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 kilogram (or, in equivalent energy, 5.34 × 10-10 electron volt c-2). Note that this does not mean the photon has mass; it just means that we have a new boundary within which the masses could fall, if one existed.
“This is the first time,” the authors write, “that the interaction between a non-zero photon mass and the plasma medium has been taken into account and calculated as the photon propagates through the plasma medium.”
It is not much lower than a measurement from 2023, but it is a refinement. This means scientists investigating the effects of a hypothetical photon mass have a more precise range to operate.
According to the astronomers, the study also shows the need for highly accurate radio telescopes. It’s not likely that we’ll be able to weigh a photon anytime soon, but by obtaining consistently higher quality data we can further refine the measurement, and thus its potential effects on the universe around us.
The research was published in The Astrophysical Journal.