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Comment by hairytrog | Hacker News Reader

If a photon hits the sail and is reflected, it must transfer two photon momentums to the sail. The energy should be less than before it the sail. Is it redshifted?

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Correct. Consider an initially stationary sail. Even though this isn't physically what is happening, you can imagine the reflection from the point of view of the sail as a photon getting absorbed and a photon of exactly the same frequency being emitted immediately after in the opposite direction. Now, to a stationary observer, the sail will have started moving after the absorption, so the emmitted photon will be redshifted due to the Doppler effect.

It's only be a very tiny amount of course at normal velocities, probably smaller than the energy wasted due to a mirror not being 100% efficient. To extract all the kinetic energy of a bundle of light, you'd have to bounce it between two mirrors many times. Maybe that's a possibility for shorter distances, perhaps during take off?

Reflection can indeed be used! However, it's only useful for pushing apart two objects that you've built - e.g. a large station with a mirror and a laser, and a small probe with just a mirror.

This is called a photonic laser thruster (https://en.wikipedia.org/wiki/Photonic_laser_thruster) and is a variant of laser propulsion (https://en.wikipedia.org/wiki/Laser_propulsion). Like a mass driver, it gets around the rocket equation by separating the propulsion system from the spacecraft.

Yes, but by a tiny amount. This reddit comment [1] explains in better detail than I can.

[1] https://www.reddit.com/r/askscience/comments/4anry6/does_lig...

Thanks for sharing this! Fascinating.

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