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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.

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2 Answers2

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You are shooting photons out the back. Now, they don't have (rest) mass, but they do carry energy.

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.

1 If it didn't come from an on board store you are either (1) using a light sail which is a different problem or (2) are trying to pick up the energy from an external source which is transferring momentum to your craft and generates some serious limitations of the behavior of your craft relative your energy source.

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If you don't mind waiting millions of years to accelerate, You can do CMB photon sailing: thermodynamics of a dual-face surface in space

There is also spacetime swimming: Can a deformable object "swim" in curved space-time? which is (from what we know) also really really slow, but satisfies your demands

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