Solar Sails

**Cold Fusion** · February 9th, 2010, 04:59 AM

http://en.wikipedia.org/wiki/Solar_sail

I am baffled...how do they plan on conserving the laws of thermodynamics by reflecting light off of the sail in order to impose kinetic energy unto the craft? Am I missing something?

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**Waveman28** · February 9th, 2010, 05:14 AM

When the light reflects off the sail, it transfers some of its energy to the craft. The light reflected back has less energy than before it strikes the sail. So in doing this, the solar sail craft gains momentum.

**Cold Fusion** · February 9th, 2010, 05:21 AM

But....I was not aware that partial absorption per photon could occur, which would result in a wavelength increase of the photon. And if that is not the case, then REFLECTION is not the point, but ABSORPTION.

**Waveman28** · February 9th, 2010, 05:33 AM

Regardless of reflection or absorption, the reflected light MUST lose energy. Otherwise conservation of energy would be violated, as you initially feared.

**Cold Fusion** · February 9th, 2010, 05:40 AM

Wait...do you mean loose energy in terms of individual photon wavelength, or in terms of total quantity of photons being reflected as opposed to absorbed?

This is weirding me out...I always thought that solar sails used cosmic radiation (alpha radiation namely) for propulsion....however, if they use light, AND IT ACTUALLY WORKS.....this completely changes my view of quantum mechanics.

**Waveman28** · February 9th, 2010, 05:49 AM

I am a prominent supporter of light being a wave, not a particle (or both for that matter). So when the light is reflected off the sail, it will lose intensity and the sail will gain kinetic energy.

**Janus** · February 9th, 2010, 02:03 PM

Originally Posted by Cold Fusion

Wait...do you mean loose energy in terms of individual photon wavelength, or in terms of total quantity of photons being reflected as opposed to absorbed?

This is weirding me out...I always thought that solar sails used cosmic radiation (alpha radiation namely) for propulsion....however, if they use light, AND IT ACTUALLY WORKS.....this completely changes my view of quantum mechanics.

By shift of photon wavelength. In other words, if you point a light at a light sail and then measure the frequency of that light when it returns to you, it will have a lower frequency.

**Cold Fusion** · February 10th, 2010, 04:38 AM

Hmm.....I will need to read into this more and then get back to you guys.

**drowsy turtle** · February 10th, 2010, 01:52 PM

Originally Posted by Waveman28

I am a prominent supporter of light being a wave, not a particle (or both for that matter).

Out of interest, how do you explain the photoelectric effect?

**Cold Fusion** · February 18th, 2010, 07:57 PM

Ok...did some thinking. It appears to me, that because the emission of light coming off of the solar sail and hence the reflection of the incident photons, must be red shifted due to the wave nature of matter....because the light is red shifted, energy must be given up. Due to that, energy is 'artificially' transmitted as kinetic energy into the solar sail. Hilarious.....