[anon291]

Photons have no mass. Massive particles cannot travel at c. Massive particles experience time. Photons have momentum because they contain energy, but no mass. Momentum being expelled is how the mass is accelerated. You don't need a propellant, but you do need a means of generating momentum.

[api]

Photons have mass in the sense that energy and mass are equivalent. What I meant is that a ship with a photon rocket is expending mass, given that the power requirements demand some kind of mass energy conversion to get enough photons. Viable photon rockets able to move anything of any size would require petawatts of power.

If you do the rocket equation math it kind of behaves like you are throwing mass at c even though you are not. You are converting mass to energetic photons and throwing them at c.

There’s a rough breakdown I saw once on a forum about future space flight tech:

Launch is dominated by thrust. Your T/W ratio must be >1. Travel within the solar system is dominated by specific impulse. Small thrust for a long time can get you going real fast, but you can only carry so much propellant. Interstellar flight or beyond is dominated by raw energy. To get to the stars in any “reasonable” time requires you to be doing a fair amount of E=mc^2-ing.

[anon291]

> Photons have mass in the sense that energy and mass are equivalent

Mass and energy are convertible but not equivalent.

You are confusing momentum and mass, which are two different concepts.