Good gas to trace lasers in a vacuum

[Melbourne Guy]

I'm excitedly close to the end of proofing my latest novel, and as happens, tweaking of passages occurs. In this instance, a fleet of warships travel together in a collective warp space bubble that has inconsistent gravity, especially around the edges. To facilitate shuttle transfers between ships, they map the gravity, and I am looking to use lasers and some kind of gas that the ships release to make the path of the lasers visible. From the path, they identify safe zones of ship-to-ship travel, and those zones of crushing gravity they need to avoid.

I've done some research but can't readily find an indication of the density required for Rayleigh scattering to occur. Being a story, I can pick any wavelength laser and assume high-precision optics spread across a fleet of 16 ships to detect the laser light. They are traveling in a roughly spherical bubble three-hundred klicks in diameter, which seems a large volume if the gas needs to be reasonably dense.

Are there any combinations of gas and wavelength that would operate particularly well in this scenario?

 

[berkeman]

To facilitate shuttle transfers between ships, they map the gravity, and I am looking to use lasers and some kind of gas that the ships release to make the path of the lasers visible.

Using laser light to map gravitation changes seems clumsy to me (and not very accurate, given how little gravity displaces light paths).

Instead, I would fire little 3-D cube cornter reflectors between the ships, and track them with radar/lidar. Those cube corner masses will be obviously influenced by changes in the local gravitational field, and their range and position can be tracked very accurately.

 

[dlgoff]

... how little gravity displaces light paths ...

This is the coolest stuff IMO. Wikipedia has a nice page gravitational lensing.

 

[256bits]

which seems a large volume if the gas needs to be reasonably dense.

Not sure of the aspects of your gravity field, but would not the gas have dense and rarified regions, which, with your laser, that density should be able to be detected somehow. Oxygen/ ozone cycle might be something to look at, but you will have to wrangle some sort of odd system or physics to detect using ultraviolet light the rarifeid and dense regions. https://en.wikipedia.org/wiki/Ozone_layer

You could also sprinkle dust in the bubble. Think dust in the atmosphere at sunrise or sunset.

 

[Rive]

You could also sprinkle dust in the bubble.

I think that would be the best. Maybe not 'dust' but some frozen liquid vapour so collision would be less of a problem. Could be traced on radars to have data about their movement.

The issue with the light bending thing is, that any measurable warp already means really huge gravitational well (far bigger than your average planet). To have any reasonably sized shuttle to travel safe you need something far more sensitive than that.

 

[Melbourne Guy]

Using laser light to map gravitation changes seems clumsy to me (and not very accurate, given how little gravity displaces light paths).

Thanks @berkeman. I'd not included that gravity at the edges can be hundreds of gees, but your retroreflectors do seem more straightforward.

You could also sprinkle dust in the bubble.

I had wondered about that, @256bits, just wasn't sure about the structural integrity of dust in a vacuum, but as I write this reply, I realize how naïve that is given there are literally dust clouds in space

some frozen liquid vapour so collision would be less of a problem. Could be traced on radars to have data about their movement

Radar tracking, that's very practical, @Rive, and it aligns with ship operations, you'd not need a different system to keep an eye on the gravitational effects.

 

[berkeman]

Maybe combine the dust and retroreflector ideas -- spray out small dust particles that are actually tiny retroflectors and use scanning lasers to track the cloud to map out the 3-D graviational field. Now we're getting somewhere...

 

[sophiecentaur]

What about using a laser frequency that matches an absorption line of the gas? The re-radiated light would be visible and identifiable. The only problem with that sort of method is that it involves a net loss of material from the ships. You could run out of 'marker' gas (unless you have transmutation equipment on board).

 

[Melbourne Guy]

The only problem with that sort of method is that it involves a net loss of material from the ships

Yeah, I was wondering about that, too, @sophiecentaur. I'm assuming high-tech, which includes replicators that I can put to work transmuting matter, but they still need raw materials and these are warships, they tend to formal supply chains. I guess a lumpy asteroid might do in a pinch, but that's dragging the story off base.

Likely, I'll go with a fleet of robust, self-propelled bots that fan out and survey the gravity field then return to the ship so they're not consumed. Then I can have a bit of fun with units that get too close to the very high gees at the edges and are smeared by their encounter.

 

[Drakkith]

I'd say the retroreflector drones is probably the best idea. The idea of releasing a gas is interesting, but I question how much gas would actually be needed considering you're releasing it into a vacuum that is, presumably, at least billions or trillions of times larger than the volume of space occupied by the gas containers on board your ships. For comparison, the volume of Earth is about 1x10^21 m^3, whereas an olympic swimming pool has a volume of about 2.5 x 10^3 m^3. That's 17 or 18 orders of magnitude difference. Even assuming a compression of 1,000x ( a scuba tank is usually about 200x) you're still looking at 14+ orders of magnitude difference. Scale things up or down as necessary for the volume of space you're mapping vs the volume of the gas tanks onboard your ships.

 

[Rive]

units that get too close to the very high gees at the edges and are smeared by their encounter.

Be careful with the 'high G kills' misconception, then. Those drones will feel nothing about any high G forces: unless their engines working, they will just speeding away, but still in weightlessness.
Tidal forces can be different, but to get deadly tidal forces in the size of a (reasonably built) drone, you need far more G around than what you are asking for here.

 

[sophiecentaur]

billions or trillions of times larger than the volume of space occupied by the gas containers on board your ships.

Yes; it's a very low vacuum but so is a visible Nebula. There again, the total Mass of 'a nebula' can be enough to spawn a whole family of stars. But you'd have to do he sums, which is something that SF writers seldom have time for.

 

[Melbourne Guy]

I question how much gas would actually be needed

Me too, @Drakkith. The volume inside the bubble is around 113M km³, and while there are sixteen ships in the fleet, any gas they can disperse is going to be very diffuse. I could just wave it away, but that seems like an unnecessary cheat.

Be careful with the 'high G kills' misconception, then.

Hmm, I need to think about that, @Rive. Bubble boundaries are high gee zones, hundreds of gees typically, but gravity drops off faster than in real space, so I can make them assumed tidal forces. The topology within a bubble is only vaguely defined for narrative purposes, and does not conform to current physics. I'm happy to take a bit of license with the degree of realism, but it does need to be consistent within the story, so I might have to define my terms more definitively lest I trip myself up along the way.

 

[Drakkith]

I could just wave it away, but that seems like an unnecessary cheat.

Depends on your style of writing. If you're writing something that you want to be very 'technical' then you can feel free to describe how everything works. Otherwise a simple sentence of, "We've mapped the bubble's safety corridors and can now begin the crew transfer" works just fine.

People get too stuck on 'cheats' or 'handwaves' when it comes to futuristic or fantasy elements in stories. But they don't think about all the cheats and handwaves used when writing about contemporary elements. If I'm writing a story where a character is taken to an aircraft carrier, I have soooo many choices on how to write that scene. I could describe the actions of half the deck crew, in detail, if it suits my novel. Or I could say absolutely nothing except that the character arrived by helicopter, stayed for lunch, then flew back to land.

 

[Melbourne Guy]

Depends on your style of writing.

I was going to add a bit of technology, but working through all the observations in this post, I'm now going for the offhand, vague approach, @Drakkith. Specifically:

Even something as seemingly simple as firing a missile could backfire, given the chaotic interior space. The missile might bend back the way it came, or crumple a klick from the ship, there was no way to tell without extensive survey work. Some of that was done to identify safe corridors for ship-to-ship transfers, but those corridors were traversed by slow-moving shuttles, and any missile flying in the same way would be easily shot down.

 

[sophiecentaur]

I'm now going for the offhand, vague approach,

It's all you can ever do, with safety. In SciFi, you always need a Wizard's Staff at some stage. If the story itself and the characterisations are worth reading then the Staff is always valid. Read the Foundation Trilogy; full of nonsense but it keeps you at it right up to the end.

 

[Melbourne Guy]

It's all you can ever do, with safety. In SciFi, you always need a Wizard's Staff at some stage. If the story itself and the characterisations are worth reading then the Staff is always valid. Read the Foundation Trilogy; full of nonsense but it keeps you at it right up to the end.

Yep, that pretty much describes my approach to writing, @sophiecentaur. Make the characters relatable, the story engaging, and don't insult the readers with deus ex machina, and you should have a passable novel, at least.