[HN Gopher] Diffractive Solar Sail
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       Diffractive Solar Sail
        
       Author : Savely
       Score  : 39 points
       Date   : 2023-11-03 15:39 UTC (7 hours ago)
        
 (HTM) web link (en.wikipedia.org)
 (TXT) w3m dump (en.wikipedia.org)
        
       | jspann wrote:
       | I'm always caught off guard whenever I see Rochester Institute of
       | Technology (RIT) or the City of Rochester in general mentioned
       | anywhere. I just don't expect it. I know Rochester is a former
       | U.S. Boomtown and has a major historical presence in the U.S. but
       | with the great leaps other cities made in Rochester's decline I
       | don't jump to the idea of seeing Rochester mentioned unless it is
       | about lasers.
       | 
       | That being said, Rochester is an up and coming city and I say
       | that more emphatically with each passing year. Pittsburgh is a
       | great case study, but when combined with a Great Lake, quality of
       | life, infrastructure for big business (including the people and
       | schools who are still there), and the talent that closely
       | surrounds the city on all sides (Boston, NYC, Toronto, Chicago,
       | Pittsburgh), future growth is inevitable.
        
         | actionfromafar wrote:
         | And Eastman Kodak!
         | 
         | https://newyorkphotonics.org/2019/12/04/kodaks-enduring-lega...
        
       | kurthr wrote:
       | It's interesting, but there's no reason you can't do both
       | reflection and diffraction. A refractive grating allows most
       | light to pass through, but reflective gratings use smaller
       | (sometimes less than half wavelength) steps/spacings to diffract
       | different wavelengths.
       | 
       | What is interesting to me is that you should be able to stretch
       | the grating (as well as rotate relative to perpendicular
       | incidence) to change the effective periodicity/wavelength, and
       | also what effect that would actually have (on angle and
       | efficiency).
       | 
       | Perhaps one advantage of refracting diffraction is that blue
       | light (with the largest momentum) might bend to larger angles?
       | But that would also depend on the grating size/angle. A rotating
       | grating with a blaze angle might allow the most flexibility.
        
       | mitthrowaway2 wrote:
       | The Wikipedia article lists several advantages relative to
       | reflective solar sails, but there's also a major disadvantage not
       | mentioned. A reflected photon imparts twice its momentum to the
       | solar sail, whereas a diffracted photon imparts only a small
       | portion of its momentum. The overall thrust would therefore be
       | much lower than a reflective solar sail. The fact that "photons
       | can be reused... by passing through a second diffraction grating
       | for more thrust" is not an advantage but rather a testament to
       | the lower efficiency of the diffractive sail.
       | 
       | Perhaps that's offset by the other advantages, but this is not
       | clear.
        
         | monocasa wrote:
         | I'm not sure that's necessarily the case.
         | 
         | A reflected photon can only impart half of its momentum before
         | it's barreling off away from you.
         | 
         | Several layers of diffractive sails would seem to be able to
         | (at least theoretically) exceed that.
        
           | AnimalMuppet wrote:
           | A reflected photon imparts _twice_ its momentum.
        
             | monocasa wrote:
             | Momentum was the wrong word, but it feels like layers of
             | diffraction could red shift the light more, absorbing more
             | energy out of each photon.
        
               | jiggawatts wrote:
               | ... which just heats up the sail.
               | 
               | To _move_ , you want to absorb momentum, not energy.
        
               | woleium wrote:
               | Correct as per Newtonian physics, bit shit gets weird
               | when you go quantum :)
        
               | monocasa wrote:
               | At the scale of individual particles, heat _is_ movement.
        
               | ianburrell wrote:
               | Heat is random movement. Infrared photons go in random
               | directions, not the direction wanting to go. Some go in
               | the wrong direction.
               | 
               | Reflection means solar sail goes in direction you want to
               | go.
        
               | mitthrowaway2 wrote:
               | The light reflected from a mirror is already red-shifted
               | due to the velocity of the mirror.
        
               | monocasa wrote:
               | Which is why I said red shift _more_.
        
       | littlestymaar wrote:
       | Stupid question, where does the energy comes from?
       | 
       | With reflection, I see how you can harvest energy through red-
       | shift (the reflected photon ends up with a lower frequency after
       | it bounces due to Doppler effect, which means it lost energy and
       | gave it to the sail as kinetic energy), but here I don't really
       | understand were the energy comes from.
        
         | kurthr wrote:
         | It's momentum transfer from deflecting (diffracting) the light
         | through an angle. Similarly, any photon would be slightly red-
         | shifted after hitting the surface whether it goes through or
         | reflects off.
        
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       (page generated 2023-11-03 23:02 UTC)