So the idea is some hyper advanced civilization has figured out a way to send multi generational ships through space to other planets. The ships are self contained living ecosystems with near perfect efficiency in regards to resources - but they need sunlight from the star to keep the whole thing alive. Rather than store huge amounts of energy on board, they create (through means that aren't explored), a huge lens that floats in space out somewhere between Earth and Mars (or wherever).

This lens focuses the sunlight into a straight sided beam, like from a spotlight, so light therefore maintains nearly all of its intensity through the interstellar void, providing light and heat to power the ecosystem inside the ship.

Please tell me all the science about how this would/wouldn't work.

Also what are similar ideas that currently exist in science-fiction (SF)?

EDIT/UPDATE: So... what I'm hearing is it would be fantastically difficult, and therefore probably not ever actually possible, but what I am not hearing is that is logically inconsistent or that it breaks any laws of physics?

  • $\begingroup$ shouludn't this be in sf se? $\endgroup$
    – NL628
    Commented Dec 30, 2017 at 8:09
  • $\begingroup$ Not posting as an answer because it doesn't address the main question, but, for the "similar ideas" part, you may want to google "laser propulsion" if you haven't already done so. $\endgroup$ Commented Dec 30, 2017 at 10:22
  • $\begingroup$ Take look at Isaac Arthur youtube channel. he had a video about interstellar travel there was laser interstellar road discussion, do not recall how the concept is called. quite good content, so maybe you can find more info for yourself there. $\endgroup$
    – MolbOrg
    Commented Dec 30, 2017 at 16:38

2 Answers 2


You are probably underestimating light propagation over huge distances.

If you give a look at the best collimated light beam we can produce, a.k.a. lasers, they have a very low divergence, about a thousandth of radiant. Over large distances this very small divergence results in the light beam spreading over a large surface.

For your reference, in the Lunar Laser Ranging Experiment, where scientists fire a laser beam to the Moon surface, just 400 thousands km away:

At the Moon's surface, the beam is about 6.5 kilometers (4.0 mi) wide

If you are instead thinking of wrapping the Sun (or another star) into a reflective sphere and let the light out only from a narrow exit, you can find some more details here:

When the beam of light hit the atmosphere, it would heat a pocket of air to millions of degrees in a fraction of a second. That air would turn to plasma and start dumping its heat as a flood of x-rays in all directions. Those x-rays would heat up the air around them, which would turn to plasma itself and start emitting infrared light. It would be like a hydrogen bomb going off, only much more violent. This radiation would vaporize everything in sight, turn the surrounding atmosphere to plasma, and start stripping away the Earth's surface.

  • $\begingroup$ Well, with lasers it depends on your wavelength and aperture size. Make it big enough and in principle there's no reason it couldn't focus over interstellar distances. Easier said than done, but we're talking about a hyperadvanced civilization here and this is one of the more grounded concepts for interstellar travel. $\endgroup$
    – Elukka
    Commented Dec 31, 2017 at 20:35

I'm going to reference https://what-if.xkcd.com/145/ for my answer:

You can't use lenses and mirrors to make something hotter than the surface of the light source itself

In other words, in order to double the amount of light that a distant observer receives from a star, you need an optical component that's as big - from the observer's point of view - as the star itself. That's quite a daunting task - and doubling the Sun's output will achieve spectacularly little. You need a bigger lens. Also, if you also don't want to put the lens directly in front of the Sun, as that would block direct sunlight, you'll have to figure out the lens shape so that its optical axis is outside of the lens.

Maybe you could use gas to form the lens shape? But how do you keep it in place? You don't need much optical power, but even then I feel like 1 atom per cm^3 of hydrogen won't cut it. You need a physical thing. The size of a star. That isn't a star. Yay. Take #3?

With a bit of mass fiddling, you could put a black hole in the middle of the optical path and use gravitational lensing instead of refraction. However, the last time I checked, perfect-size black holes aren't exactly easy to haul into place.

Perhaps your best bet would be to use a swarm of optical devices? And, because planetary-scale lenses are hard to make, you should probably use mirrors instead. Flat mirrors will do.

So.. how many? Let's be generous and say that you can craft a light sail the size of Earth - and keep it in orbit around the Sun and keep it visible and correctly oriented most of the time. I've been taught the radius of Earth is 6378 km. For Sun, that's 695 500 km. Let's round that to 637 800 km. So... in order to double Sun's output, you'd need a touch over 10000 ginormous solar sails. And that's just to give you a nice tan on Mars. It won't make your plants grow half-way to Alpha Centauri.

You could use active components to bypass the law of conservation of étendue. But even then, optics are against you. A point-like source will emit in all directions equally. My EWAG says that a phased array of microwave emitters the size of Sun will do the job (but better is easier in this case). Powering them won't be hard, but establishing microsecond synchronization will be a fun challenge when the Sun is much brighter than any radio antenna you could put on Earth - and a heliocentric polar 1-AU orbit isn't too cheap either. Still better than the equivalent amount of Earth-sized mirrors, though.

Also, as L. Dutch says, don't even think about putting your contraption on Earth.

I'd rather pack a couple tons of plutonium and use a low-power brand of cryogenic chambers for the trip.


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