4 ms·

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 redshift

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?

mdorazio 7y ago
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_light_lose_energy_when_reflected/ https://www.reddit.com/r/askscience/comments/4anry6/does_lig...
- Ductapemaster 7y ago
  Thanks for sharing this! Fascinating.
kmm 7y ago
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?
- azernik 7y ago
  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 https://en.wikipedia.org/wiki/Photonic_laser_thruster) and is a variant of laser propulsion (https://en.wikipedia.org/wiki/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.

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?

mdorazio 7y ago

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_light_lose_energy_when_reflected/ https://www.reddit.com/r/askscience/comments/4anry6/does_lig...

Ductapemaster 7y ago
Thanks for sharing this! Fascinating.

kmm 7y ago

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?

azernik 7y ago
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 https://en.wikipedia.org/wiki/Photonic_laser_thruster) and is a variant of laser propulsion (https://en.wikipedia.org/wiki/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.