# Can acceleration be achieved without reaction mass?

In a previous question, I asked whether a flashlight can accelerate me. The answer is yes. This has led me to wonder, this is inferior to all other known propulsion methods in all aspects except one: Shooting light (photons) accelerates the shooter without ejecting any mass. This means we can space travel forever, assuming we have enough electricity.

Are there any other ways to accelerate without losing mass? Note that there are lots of way to do it with external forces like laser propulsion, but I am only interested in doing it without those. e.g. consider a spacecraft swimming in the void trying to accelerate.

• I'm not sure if your preservation of mass is all that useful, because you are still ejecting energy that you need for propulsion. Other options which "don't lose mass" would include things like solar sails, where your craft uses none of its own fuel. – webb May 15 '14 at 21:00
• Solar sails require an external source, I will edit my question to exclude that. Thanks. – Hello World May 15 '14 at 21:01
• A related question computes the mass lost by a laser-driven "rocket" after it has reached some final velocity. – rob May 15 '14 at 21:07
• Here's a nice summary of current technologies you can probably pick something from that list you could put into your swimming ship. en.wikipedia.org/wiki/Spacecraft_propulsion like a plasma engine. Until we can bend space-time at will, loosing mass of some kind is the most efficient way to accelerate. – user6972 May 15 '14 at 23:02
• Thanks, but I already searched there, all the technologies mentioned lose mass. – Hello World May 17 '14 at 14:22

That energy came from on on-board store of some kind1 and its loss to your craft making it less massive by $E/c^2$ where $E$ is the energy of the photons.