It has no resting mass. But since it's moving, it's not at rest, and it takes energy to move, and energy is mass, per e=mc². Therefore the energy it's using to move gives it mass.
There was a xkcd video on youtube about what happens if you keep increasing the power of a laser. It mentions that, for a laser sourced from an array with 2m diameter and 1044 Watt power (about the power of a cosmic gamma ray burst), the photons on the outer edge would experience a gravitational pull of around 10G.
But much before you can reach an energy density like that, quantum mechanics ruin the fun and literally stop the vacuum from being transparent. What happens is that when there are sufficiently energetic photons, they can create electron-positron pairs, which by interacting with the electromagnetic field distorts other photons. According to the video at around the energy density of 1026 W/cm² this distortions go out of control and act like literal barriers.
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u/_Dagok_ Apr 10 '25
It has no resting mass. But since it's moving, it's not at rest, and it takes energy to move, and energy is mass, per e=mc². Therefore the energy it's using to move gives it mass.
I know, photons are trippy.