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Re: starship-design: Casimir-Foreward balloon

It seems like any surface that is reflective on one side and absorptive
on the other could work as a light sail in deep space.  If photons
coming from one direction are transmitted or absorbed while photons
coming from the opposite direction are reflected, then this surface will
pick up velocity in the direction away from the reflective side because
reflection imparts more momentum to the surface than absorption.

This process would be very slow if you're using the very diffuse
microwave background radiation as the source of photons, but in
principle it should still work.

If the surface actually absorbs photons on one side rather than being a
one-way mirror, then there is a velocity limit.  As the surface picks up
speed, photons striking the forward absorptive surface will impart more
backward momentum because they are blue-shifted, and the reflected
photons on the rear surface will start imparting less forward momentum
as they become more red-shifted.  As the surface picks up speed the
forces will approach equality and the acceleration will go to zero.

If the surface is selectively reflective for only certain photon
energies, then again Doppler shifting will eventually shift the incident
photons out of the reflection band.  This is probably what will kill the
acceleration of your "balloon", as eventually the incident photons will
no longer be in the wavelengths that your balloon material will handle.