There is now a new upper limit on the mass of light.
Measurements of pulsating stars scattered throughout the Milky Way and mysterious radio signals from other galaxies suggest that particles of light called photons are no heavier than 9.52 × 10.-46 kilogram.
It’s a small limit, but knowing that light has mass would have huge implications on how we interpret the universe around us and our understanding of physics.
Photons are typically described as massless particles. They Discrete quantities of energy It moves through space-time at a constant speed, and in a vacuum it cannot accelerate or decelerate. This constant speed would imply that it has no mass, but there is no evidence to the contrary.
However, it is not absolutely certain that photons have no mass.
The fact that the mass is non-zero has important implications: it contradicts Einstein’s special theory of relativity, Maxwell’s electromagnetic theoryPerhaps it will lead to new physics and answer some big questions about the universe (but likely raise many more questions along the way).
If photons had mass, they would have to be extremely small to not have a significant effect on the emergence of the universe, which means we don’t have the tools to measure them directly.
However, it is possible to make indirect measurements that provide an upper limit to this hypothetical mass, and this is exactly what a group of astronomers has done.
A research team from Sichuan University of Technology, the Chinese Academy of Sciences, and Nanjing University analyzed data collected by the Parkes Pulsar Timing Array, as well as data on fast radio bursts from various sources, to determine how massive light could be.
The Pulsar Timing Array is an array of radio telescope antennas to monitor neutron stars that emit pulsed beams of electromagnetic radiation at extremely precise millisecond pulsars. Fast Radio Bursts are extremely intense bursts of light of unknown origin detected deep within the vastness of intergalactic space.
The characteristics that the researchers looked at were: Dispersion MeasurementIt’s one of the key properties of pulsars and fast radio bursts: it describes the extent to which an intense pulsed beam of radio light is scattered by free electrons between us and the source.
If a photon has mass, when it propagates through the non-vacuum space where the plasma exists, it is affected by both the photon’s mass and the free electrons in the plasma, which causes a delay time proportional to the photon’s mass.
Pulsar timing arrays look for delays in the timing of pulsar pulses relative to one another, especially within ultra-wideband bands where dispersion effects can be minimized, allowing researchers to calculate how much of a delay would be caused by a hypothetical photon mass.
Meanwhile, dispersing the signal from a fast radio burst also reveals a delay proportional to the photon’s mass.
By carefully studying this data, the team was able to derive an upper limit of 9.52 × 10.-46 kilograms (or, in energy equivalent, 5.34 × 10-Ten Electron volt c-2). Note that this does not mean that photons have mass, only that there are new boundaries into which mass could fall, if they existed.
“This is the first time,” The author writes“The interaction between the non-zero photon mass and the plasma medium is taken into account and calculated as the photon propagates through the plasma medium.”
Not much lower than the measured value Published in 2023But this is an improvement: it means scientists studying the effects of the virtual photon’s mass can work with greater precision.
Astronomers say the study also highlights the need for more accurate radio telescopes: While they’re unlikely to be able to measure the weight of a photon anytime soon, consistently obtaining high-quality data could help further refine the measurement’s range and reveal its possible effects on the universe around us.
This study Astrophysical Journal.