> No, “collecting” photons transfers momentum so at best if you have 100% conversion rate you won’t slow down, so you need a separate source for acceleration.

> No, “collecting” photons transfers momentum so at best if you have 100% conversion rate you won’t slow down, so you need a separate source for acceleration. Given 100% conversion rate you should getting energy via photovoltaic effect, so you use that to accelerate them. > Whilst photons hit your ship from all direction the photons incoming from the direction of travel would be shifted towards higher frequencies so their momentum would be higher than the photons hitting the ship from the opposite direction so unless there is a sufficient difference in the number of photons hitting you from the back to compensate for it you would still not be able to extract energy to move. But that's just "friction". Assuming a photonically uniform environment at rest, just buffer energy at rest, accelerate for a bit, goto 10. Eventually you'll get anywhere. (Assuming a non-uniform environment, you're probably close to a star, in which case gravity outweighs photon pressure anyway.)