Spacetime manipulation vs a fall.

[dendros]

Let's imagine a scifi scenario in which a space vessel is falling on a planet due to a degraded state but remains functional enough to create an emergency warp bubble as a shield, basically the bubble makes the geometry of the vessel unchangeable in order to survive the impact. In other words, the local spacetime region around the vessel becomes "rigid".
Let's also say the vessel is made from a metal with tungsten-like density and it's a sphere with a diameter of 4 km, hollow inside with the shell 160 m thick. The speed of the fall is about 20 km/s.
What would happen at impact?

 

[.Scott]

Anything you want. The Physics is all yours. It isn't real Physics.
But you may want to make yourself (and any other occupants) similarly "unchangeable" or else the high G's on impact will leave you unpleasantly "changed".

 

[dendros]

Of course this is fiction but I want to give it at least a semblance of realism.
The high G's are not a problem, inside the sphere are dampening fields.
The vessel is quite advanced, it has a warp core, onboard fabricators (they're not like star trek replicators, rather they convert matter into any other matter), advanced sensors that can perceive spacetime directly in addition to standard sensing, life-preserving modules, advanced AI.
What I'm trying to imagine is how would the impact look like for an outside observer, so to speak. The planet in this scenario is uninhabited.

 

[Matterwave]

I might just point out that for me as a reader and general enjoyer of physics, the two parts of your two posts that seem most implausible to me are that "spacetime becomes rigid" and "perceive spacetime directly".

Spacetime is not a "thing" which I would generally describe as capable of the adjectives "rigid" and "detectable".

That being said, you are indeed free to write the physics however you like! :)

 

[dendros]

@Matterwave, notice the quotes around "rigid". Basically I'm saying the warp bubble preserves the geometry enclosed by it, in that sense it becomes "rigid". This means deformations are effectively forbidden.
The vessel is basically an advanced alcubierre drive.

 

[Halc]

basically the bubble makes the geometry of the vessel unchangeable in order to survive the impact. In other words, the local spacetime region around the vessel becomes "rigid".

Sounds like Niven's stasis field. It stops time inside the enclosed space, not turning off until it's safe. The occupants notice only a discontinuity, possibly billions of years later if conditions remain hostile long enough.

Let's also say the vessel is made from a metal with tungsten-like density and it's a sphere with a diameter of 4 km,

Sounds like Niven's universe General Products #3 hull, immune to almost anything except antimatter. But without the stasis field, the occupants are still subject to potentially fatal proper acceleration. All the GP hulls (different sizes/shapes) are like that, and are transparent, so occupants also need to be protected from harsh light.

If you don't know what I'm talking about, read the original Ringworld book, which references both things, as do several other of his books set in the same universe.

 

[dendros]

Not exactly this. The hull is not "immune to almost anything" and not exactly in a stasis field.
Rather, it is in a region of spacetime whose geometry was made "rigid" by the warp bubble. It's not about time but about geometry.
The hull does not experience an impact, the bubble does - and survives it by keeping its geometry intact, overriding the local spacetime geometry. Other than that, the hull is strong but not indestructible. Without the warp bubble, the hull would definitely disintegrate.

 

[Matterwave]

@Matterwave, notice the quotes around "rigid". Basically I'm saying the warp bubble preserves the geometry enclosed by it, in that sense it becomes "rigid". This means deformations are effectively forbidden.
The vessel is basically an advanced alcubierre drive.

Yeah, again, you can come up with whatever rules you would like, I'm not trying to constrain you. I'm only giving my opinion on what might seem incredible to me from a physics point of view.

Spacetime is the background in which matter moves. It's hard for me to envision how to make "rigid" a valid adjective that describes it since it is not a physical object.

 

[dendros]

Well, in GR spacetime is something that is deformed by matter and energy. And theoretically such deformations can allow ftl travel (the alcubierre metric).
The idea is simply to expand this concept to other applications, in that case it's used to enforce an unchanged geometry, that is, it's kept stable even in an extreme event such as an impact at hyper-velocity.

 

[Matterwave]

I mean, sure. "My spacetime is rigid so it enforces unchanged geometry" is a sentence that can be written. You can include this in your story. And I think probably some (many?) people would find nothing wrong with that sentence.

For me, reading a sentence like that is like reading "the color purple tastes like lemons".

 

[dendros]

You missed the quotes. It was a figure of speech.
If I say "the spacetime metric around the vessel is forcefully preserved so that deformations are impossible", is that better?

 

[Matterwave]

It feels like you are considering spacetime to be the thing that dictates geometries of physical objects. It is not.

The metric of spacetime, far away from any massive object, is basically always just flat (Minkowski). There is no change to that spacetime however your ship moves in it. That doesn't stop your ship from deforming to exploding or doing whatever.

I want to emphasize again though that I'm not here to tell you how to write your story. And I am not saying I wouldn't enjoy reading the story if it was well written.

 

[dendros]

The spacetime "bubble" generated by the warp core of the vessel is what dictates geometry. You can say the energy of the warp core imposes that the spacetime metric around the hull is preserved so that an impact cannot deform the hull.
Not because the hull is magically made indestructible but simply because the spacetime around the hull does not allow deformations, geometrically speaking.
In the story, the vessel survives the impact thanks to this spacetime manipulation, sinks deeply into the crust until it stops. Then, it initiates a slow repair with the goal of restoring the warp core so that it can generate a stronger, better controlled bubble. Once this is achieved, in let's say a century in the local time, it begins a slow ascent (in about 1 year) to the surface.