Asked by: Peter Cole, New South Wales
Light is made up of packets of energy known as photons. So by Einstein’s famous equation linking energy and mass, E = mc2, photons do have mass-like properties such as momentum.
What they don’t seem to have is so-called ‘rest mass’, the type most of us think of as ‘real’ mass, measured when an object is stationary relative to us. If they did, the fact that light is an electromagnetic phenomenon means that electric and magnetic forces would get weaker with distance faster than expected.
Electromagnetic signals would also travel at different speeds depending on their frequency. However, attempts to detect such effects have so far drawn a blank. The evidence to date suggests that if the photon does have mass, it’s less than a billion-billionth that of the electron.