And use that energy to sustain the acceleration. Sounds like a perpetual motion machine!

Unfortunately the photons hitting the solar panel at the front of your spaceship will slow you down by at least as much as you could turn them into thrust by shooting them out behind you (conservation of momentum) :(

Sorry for my ignorance, but if photons do not have mass, how do them impacting the solar panel have any effect on the ship's velocity?

What we call "mass" is actually "rest mass" - ie. the mass when the thing is not moving. Photons don't have rest mass because they don't rest.

What photons do have is momentum, and so due to conservation of momentum, hitting a photon will slow you down.

Photons do have mass, because energy has mass & photons carry energy. They have no rest mass, which is why they are usually described as massless, but this isn't the whole story!

Photons don't have mass, but they do have momentum, which is the reason that solar sails work.

Radiation pressure. Remember, solar sails are a thing. At some point, you're blazing through spacetime so fast all that energy you're trying to share the same physical location with is impacting and ablating your hull material.

Relative velocity is a hell of a drug.

Could you collect photons from all around you and accelerate them out the back? There's no reason you have to place the solar panel at the front.

Force would be negligible of course. But if you are in space, you are surrounded by energy in the form of starlight; it would be weird if that energy could not be used for work.

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.

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.

This means that even say a solar sail has a limit on acceleration at a certain speed the photons hitting the front of the sail would be blue shifted enough to counteract the acceleration of photons hitting you from the back, even if you convert them to useful energy it wouldn’t matter since you’ll reach equilibrium.

> 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.)

They're gonna slow you down anyway, you might as well recoup some energy from the process.

I remember reading about a design that basically works like this. It’s sucks up all the atoms in front of the ship and uses them for acceleration. I think there is also a book about this where such a ship has a problem which doesn’t allow it to slow down. So they just keep accelerating. Wish I could remember the name.

You're likely talking about a Bussard ramjet. The book should be Tau Zero.

You are thinking of Tau Zero by Poul Anderson. It's well worth reading even though the cosmology is dated.

You need mass to accelerate. Not just electricity.

Just turn the energy into mass, duh.

> You need mass to accelerate.

* momentum , impulse , force

To boldly go where no one has gone before!

Or, simply move your frame of reference.

To boldly reframe your frame of reference where no man has reframed that frame of reference before! (cue music...)

Why does the road cross the chicken?