system
Given the following post, respond as if you're commenting on Reddit:
human
How can a photon have a mass of zero without violating the mass - energy equivalence?

Photons are particles without mass. But whenever a system emits one, it loses a small fraction of its energy, and therefore its mass. However, the photon lacks mass and only transports energy, so conservation of energy is given, while conservation of mass is violated. If the photon is added to another system, the other system gains (although very little) mass.
So is a photon transporting mass without actually having any? I'm really confused right now, and might be I just miss something simple here.

Thanks in advance for any answers!

EDIT: Thank you all for taking the time to write these answers down. They did indeed clarify a lot, concerning my question and giving a lot of further, interesting information!
gpt
Yes, everything you've said is correct (for a particular choice of "system" versus "surroundings"). It's because mass is not exactly what you think it is. Mass isn't simply "the amount of stuff" in the system like you might've learned in introductory science classes.

Mass is just energy **not** due to the overall motion of your system as a whole. Since a photon has no mass, all of its energy is energy of motion (you could call it kinetic energy).

The mass of a system of particles is not simply the sum of the masses of the particles. So while photons have no mass, adding or removing photons from your system can change the total mass of the system.