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Not an optics expert but I consult for a company that makes lasers. Afaik not currently possible. The beam divergence is only one of many limiting factors

by 6d6b73 3y ago

wombatpm 3y ago
I am not a physicist. Does beam divergence decrease in a vacuum? Could we put the lasers in space at L5 to power the swarm? Or L5 for Venus for the increased solar flux?
- selecsosi 3y ago
  Beam divergence, while happening at greater rate not in a vacuum, is a property of how photons propagate, so it's not something that can be eliminated
- jiggawatts 3y ago
  It's almost entirely a function of wavelength and aperture diameter. It's basic wave mechanics, and is almost impossible to improve upon. I just recently watched a great video explaining how lithography machines push the limits of what's possible: https://www.youtube.com/watch?v=rdlZ8KYVtPU&t=1151s https://www.youtube.com/watch?v=rdlZ8KYVtPU&t=1151s Note that the best ASML machines can eke out an improvement on the order of a factor of two at best, not two thousand, which is what would be likely needed to power space probes at light-year distances.
- kragen 3y ago
  most laser systems get close to diffraction-limited divergence, which is governed by the size of your optics. vacuum vs. air affects it, but the effect is so small that you need sensitive instruments to detect the effect https://en.wikipedia.org/wiki/Gaussian_beam#Beam_waist https://en.wikipedia.org/wiki/Gaussian_beam#Beam_waist the divergence angle is inversely proportional to your aperture diameter https://en.wikipedia.org/wiki/Airy_disk https://en.wikipedia.org/wiki/Airy_disk in theory by putting a bunch of laser emitters in solar orbit and operating them as a phased array you can make an aperture the size of the solar system. 1.22λ/d at, say, 400nm and 2 astronomical units of diameter is about 1.6 attoradians of divergence; at a radius of 4.3 light years (the distance to proxima centauri) this works out to an airy disk focal point of 66 microns. optical phased array lasers have been demonstrated https://en.wikipedia.org/wiki/Phased-array_optics https://en.wikipedia.org/wiki/Phased-array_optics but nobody knows how to make one out of satellites yet i've only glanced at the proposal https://arxiv.org/pdf/2309.07061.pdf https://arxiv.org/pdf/2309.07061.pdf and they're proposing 4.1-meter-diameter 3.6-gram probes, which would allow you to use a phased array about a million times smaller, about 5 million km, 13 times the distance to the moon. i don't see where they're proposing how they propose to beam 100 gigawatts onto the probes so i don't know if this is their proposal dispersion, by contrast, which means wavelength-dependent indices of refraction, doesn't affect lasers and doesn't happen in a vacuum; probably the person who mentioned it was using the wrong term by accident and meant divergence
  - jchanimal 3y ago
    Less than 100 microns! You can lithograph on Proxima Centauri with that setup. Maybe PKD…
    - adrianN 3y ago
      One simple trick von Neumann probes love? Just build more probes remotely using a fricking laser?
  - mlyle 3y ago
    > and they're proposing 4.1-meter-diameter 3.6-gram probes, If you put 100 gigawatts on a 4.1 meter diameter very thin reflector, you're going to melt it. That's 10 gigawatts/square meter. Most of the light misses the targets. > i don't see where they're proposing how they propose to beam 100 gigawatts onto the probes so i don't know if this is their proposal A phased array on Earth, several kilometers in scale. > but nobody knows how to make one out of satellites yet Probably no one ever will make a huge (solar system scale) coherent power transmission system out of satellites, because of the thinned array curse-- https://en.wikipedia.org/wiki/Thinned-array_curse https://en.wikipedia.org/wiki/Thinned-array_curse It would be cool if we made a very large scale telescope, where a sparse array is less of a problem.
    - kragen 3y ago
      thank you, i didn't know about the thinned-array curse, and now that i understand it i agree that it would not be useful for this

Not an optics expert but I consult for a company that makes lasers. Afaik not currently possible. The beam divergence is only one of many limiting factors

by 6d6b73 3y ago

Not an optics expert but I consult for a company that makes lasers. Afaik not currently possible. The beam divergence is only one of many limiting factors

wombatpm 3y ago

I am not a physicist. Does beam divergence decrease in a vacuum? Could we put the lasers in space at L5 to power the swarm? Or L5 for Venus for the increased solar flux?

selecsosi 3y ago
Beam divergence, while happening at greater rate not in a vacuum, is a property of how photons propagate, so it's not something that can be eliminated
jiggawatts 3y ago
It's almost entirely a function of wavelength and aperture diameter. It's basic wave mechanics, and is almost impossible to improve upon. I just recently watched a great video explaining how lithography machines push the limits of what's possible: https://www.youtube.com/watch?v=rdlZ8KYVtPU&t=1151s https://www.youtube.com/watch?v=rdlZ8KYVtPU&t=1151s Note that the best ASML machines can eke out an improvement on the order of a factor of two at best, not two thousand, which is what would be likely needed to power space probes at light-year distances.
kragen 3y ago
most laser systems get close to diffraction-limited divergence, which is governed by the size of your optics. vacuum vs. air affects it, but the effect is so small that you need sensitive instruments to detect the effect https://en.wikipedia.org/wiki/Gaussian_beam#Beam_waist https://en.wikipedia.org/wiki/Gaussian_beam#Beam_waist the divergence angle is inversely proportional to your aperture diameter https://en.wikipedia.org/wiki/Airy_disk https://en.wikipedia.org/wiki/Airy_disk in theory by putting a bunch of laser emitters in solar orbit and operating them as a phased array you can make an aperture the size of the solar system. 1.22λ/d at, say, 400nm and 2 astronomical units of diameter is about 1.6 attoradians of divergence; at a radius of 4.3 light years (the distance to proxima centauri) this works out to an airy disk focal point of 66 microns. optical phased array lasers have been demonstrated https://en.wikipedia.org/wiki/Phased-array_optics https://en.wikipedia.org/wiki/Phased-array_optics but nobody knows how to make one out of satellites yet i've only glanced at the proposal https://arxiv.org/pdf/2309.07061.pdf https://arxiv.org/pdf/2309.07061.pdf and they're proposing 4.1-meter-diameter 3.6-gram probes, which would allow you to use a phased array about a million times smaller, about 5 million km, 13 times the distance to the moon. i don't see where they're proposing how they propose to beam 100 gigawatts onto the probes so i don't know if this is their proposal dispersion, by contrast, which means wavelength-dependent indices of refraction, doesn't affect lasers and doesn't happen in a vacuum; probably the person who mentioned it was using the wrong term by accident and meant divergence
- jchanimal 3y ago
  Less than 100 microns! You can lithograph on Proxima Centauri with that setup. Maybe PKD…
  - adrianN 3y ago
    One simple trick von Neumann probes love? Just build more probes remotely using a fricking laser?
- mlyle 3y ago
  > and they're proposing 4.1-meter-diameter 3.6-gram probes, If you put 100 gigawatts on a 4.1 meter diameter very thin reflector, you're going to melt it. That's 10 gigawatts/square meter. Most of the light misses the targets. > i don't see where they're proposing how they propose to beam 100 gigawatts onto the probes so i don't know if this is their proposal A phased array on Earth, several kilometers in scale. > but nobody knows how to make one out of satellites yet Probably no one ever will make a huge (solar system scale) coherent power transmission system out of satellites, because of the thinned array curse-- https://en.wikipedia.org/wiki/Thinned-array_curse https://en.wikipedia.org/wiki/Thinned-array_curse It would be cool if we made a very large scale telescope, where a sparse array is less of a problem.
  - kragen 3y ago
    thank you, i didn't know about the thinned-array curse, and now that i understand it i agree that it would not be useful for this