# 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, 2014 at 21:00
• Solar sails require an external source, I will edit my question to exclude that. Thanks. May 15, 2014 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, 2014 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. May 15, 2014 at 23:02
• Thanks, but I already searched there, all the technologies mentioned lose mass. May 17, 2014 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.