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Comment by remcob | Hacker News Reader

Could that radiative energy be turned into propulsion, at least theoretically?

Yes, but photons carry momentum even if they are massless, and you can't convert more than 100% of that momentum to energy for propulsion. So, if you achieve perfect efficiency, you can avoid them from slowing you down and still have to rely on internal energy sources to accelerate you .

I'm guessing you could at least use them to:

- Maneuver, like a sailboat.

- Decelerate. Shut off your thrust and dump the light-absorbing covers to reveal mirrors, for 2x the deceleration. Photobraking?

True and true, especially the decelerating part. Akin of what a light-sail would do.

Can during deacceleration all energy be absorbed and used for accelerating at a later moment?

Mmmh, I don't think so. At least efficiently. You might be able to decelerate emitting photons in front of you but they would escape to infinity unless you put them in orbit around a black hole. In that case perhaps.

If we will ever have the technology to instead create proper massive particles at scale you could eject those in orbit around some other massive object and then re-steal their angular momentum to accelerate yourself. But that I believe would be an extremely lossy process so you would lose most of the energy.

fundamentally? yes, of course. there's no physical laws or reason why you couldn't - just observe the usual caveats, such as being unable to achieve 100% efficiency, that sort of thing.

now, the question of how to design an actual working solution? i don't know; and basically, it depends. you would then also have the problem of whether it was possible to implememnt that solution, using real matter and physical objects...

unfortunately, that was probably the question you really wanted answered, right?

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