Sun Guns Instead Of Lasers For Space Travel

[Spacescifi]

 

Is it not possible to make a laser beam out of concentrated sunlight via mirrors?

 

If so, why not do that?

Yeah I know... big mirrors, but at least you don't have to worry about power (it's the sun).

[StrandedonEarth]

Maybe if you have a Ringworld-sized superconducting grid to magnetically manipulate the Sun’s plasma, it might work…

[Shpaget]

No, because of conservation of étendue.

https://what-if.xkcd.com/145/

[KSK]

Not sure about conservation of hey-what’s-that, but solar pumped lasers are a thing.  Wikipedia article here and back when I was working for a nearby university, I knew a PI who was researching solar pumped semiconductor lasers.

I don’t believe they’re very efficient which might limit their practical real-life applications but an ‘advanced solar pumped laser’ (without looking too hard at what ‘advanced’ means) could work in a science fiction setting.

If you wanted to use laser ignition for fusion rockets though, they probably wouldn’t be much use for missions beyond Mars due to drop off in solar flux with distance.

[Gargamel]

27 minutes ago, KSK said:

knew a PI

Surely you don’t mean a private investigator? 

[KSK]

Heh.

Close - principal investigator.

Not sure if it’s a general title but in Britain, if you read about the latest breakthrough coming out of Professor X’s lab, then the good Prof X is most likely the principal investigator. They will usually have started the lab and will be responsible for its research direction although it’s day-to-day running may well be delegated to a senior postdoc.

[sevenperforce]

9 hours ago, Spacescifi said:

Is it not possible to make a laser beam out of concentrated sunlight via mirrors?

No, because laser beams are not created by focusing light with mirrors.

In the business world it is common to hear people discuss “laser focus” which always makes me chuckle. I suppose people assume that a handheld laser pointer consists of a tiny flashlight and a complex array of mirrors that turns the diffuse flashlight rays into a parallel stream of light. But that is not the case at all. The light coming out of a laser pointer did not originate from a tiny flashlight or other conventional light source.

What do mercury lamps, lightning bugs, an old-fashioned TVs have in common? One thing: fluorescence. Numerous elements and chemicals have the ability to absorb energy (in the form of heat, electricity, chemical reactions, or high-frequency light) and then radiate it out as low-frequency light. Even your skin has this capability.

Lasers do use mirrors, but they use mirrors to trap light, not focus it. Lasers consist of a fluorescent material which is placed in a mirrored cavity such that only a specific wavelength of fluorescent light can escape. The material is then “pumped” full of energy from an outside source, producing a perfectly parallel beam of escaping fluorescent light.

Now, you could certainly use mirrors to focus sunlight to provide the “pumping” mechanism for a laser. But the laser itself needs to be made of some fluorescent gain medium material. 

[HvP]

@sevenperforce It sounds like you're describing light amplification by the stimulated emission of radiation.

[wumpus]

In  other words, what you want is a solar sail (directly absorbing the momentum of the solar energy).  Using the solar energy to pump a laser makes no sense.  Even the laser bit makes no sense, you'd be better off with high efficiency blue(ish) LEDs.  You want the momentum, and coherence doesn't buy you any.

And as far as "light powered rockets" the two possible uses are: lifting off a planet with abundant energy (so the rocket doesn't carry its engine and fuel) and interstellar travel (because the Isp of light is near infinity).  The energy efficiency is so bad you'd never use it to travel between planets.

[kerbiloid]

The solar-powered beam energy transfer was possible in KSPI-E long ago for propulsion.

Why shouldn't it be irl?

[KSK]

2 hours ago, wumpus said:

In  other words, what you want is a solar sail (directly absorbing the momentum of the solar energy).  Using the solar energy to pump a laser makes no sense.  Even the laser bit makes no sense, you'd be better off with high efficiency blue(ish) LEDs.  You want the momentum, and coherence doesn't buy you any.

And as far as "light powered rockets" the two possible uses are: lifting off a planet with abundant energy (so the rocket doesn't carry its engine and fuel) and interstellar travel (because the Isp of light is near infinity).  The energy efficiency is so bad you'd never use it to travel between planets.

That seems a bit harsh. Laser boosted solar sails have a pretty impeccable pedigree in sci-fi - okay the coherence doesn't help you but the low dispersion over distance does.  Similarly, beamed power for interplanetary flight - at least for the outward journey - sounds fine to me.  Solar pumped lasers aren't terribly efficient, as you say,  but on the other hand, there's plenty of room in space to build lots of inefficient lasers and make up with quantity what each individual laser lacks in quality. Or heck, given that we have some leeway because this is fiction, I can imagine us developing vastly more efficient lasers well before we develop other space  propulsion systems.

For clarity, when I'm thinking about beamed power, I'm thinking about using a laser to heat propellant at a distance. Much like a nuclear-thermal rocket except that the propellant is passed through a plain block of metal - which is heated by the beam - rather than a nuclear reactor.

If you haven't got fusion and fission is a no-no, then beamed power based on orbital solar pumped lasers seems like a reasonable, if infrastructure heavy, sci-fi propulsion system to supplement chemical propulsion. Usual caveats apply about building giant orbiting laser cannons as transport infrastructure and pinky-swearing not to use them as weapons.

Edited June 22, 2022 by KSK