I really hate being pedantic here, but it's ultimately using mass for thrust instead of being a pure photon rocket (which arguably is also expelling energy whic

I really hate being pedantic here, but it's ultimately using mass for thrust instead of being a pure photon rocket (which arguably is also expelling energy which has mass but in this particular case you're expelling momentum - and probably causing smarter people than me to groan in exasperation at my description).