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I am setting up a very futuristic world for a sci-fi story, where having FTL tech would be very practical. For a bit of context, here is what I am imagining:

It is a type 3.2 - 3.3 civilisation, where there is a swarm of satellites with mirrors built around almost every star in the galaxy (aka a dyson sphere, but in the story it is called a dyson swarm). The light of the sun is reflected to one point where extreme heat builds up and mass is released in form of stellar winds, where it is collected. The helium and hydrogen is then used for many purposes including powering a caplan thruster and as material for building things.

The civilisation does not use asteroid mining since they consider that primitive and inefficient. Instead, they fuse the hydrogen enough times until they get the material they want. In the story, stellar mining is a big business and the civilisation now controls the stars, since one by one, they went to each star, mined the mass, built a new caplan thruster and went to the next one. The stars are controlled by the IMC which stands for the intergalactic monitoring center. The IMC is also incharge of controlling blackhole powerplants which have a mirror surrounding the ergosphere, terraforming planets, stellar mining activites, blackhole bomb monitors action and etc.. but I think I am now getting away from the point of the question.

Almost all the FTL technologies I have seen or either completely pure fiction or require negative mass which possibly doesn't exist. However, recently a new theory of the Alcubierre drive from Erik Lentz was released where the negative mass requirement was removed. I was looking for the required positive mass needed, but couldn't find anything. I did however, find this paper which mentions that it would require serval times the amount of mass of the Earth, but did not mention how much it needed exactly. A few times the mass of Earth may seem like a lot, but Earth is nothing compared to the size of a star.

My question: Is it possible for an Alcubierre drive which can travel at FTL speeds to be made by starlifting, since it could possibly cover the mass requirements or are there other issues?

FYI, many ideas of this story I found from watching Isaac Arthur and some Kurzgesagt videos

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  • $\begingroup$ That was mentioned here before a year or more ago. I'll see if I can track it down. There's an answer here though I'm a bit uncertain without following the links it's the same thing. That being said, you could always invent your own new paradigmatic theory that fits the story - no need to go into the maths. There's another here, that and the comments below confound my understanding of how it's supposed to work. $\endgroup$ Feb 29 at 22:00
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    $\begingroup$ A Kardashev 3.5 civilization, you say? The answer may simply be "automatically yes". That's energy comparable to 10,000 galaxies, presuming the definition of galaxy is 10^37 watts like on the original Kardashev scale, where controlling 1 galaxy's energy denotes a Type 3 civilization. At this point the distinction between a member of this civilization and a capital-G god breaks down — assuming a population of 1 quadrillion entities and perfectly equal distribution of control over energy, each individual member is a Type 2 as defined by the scale. $\endgroup$
    – KEY_ABRADE
    Mar 1 at 0:49
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    $\begingroup$ The Alcubierre drive has four problems you still haven't solved: 1) you can't start moving, 2) you can't stop moving, 3) you can't steer, and 4) if you somehow manage to stop, your destination gets destroyed by radiation. $\endgroup$
    – Mark
    Mar 1 at 3:02
  • $\begingroup$ @KEY_ABRADE in the story, they have colonised the galaxies in the local group. So that isn’t quite 10000 meaning that this might be a 3.2 or 3.3 civilisation, not a 3.5 civilisation. I will change that $\endgroup$ Mar 1 at 5:29
  • $\begingroup$ The mass of the Local Group's dwarves is negligible compared to that of Andromeda and the Milky Way (and Triangulum, which is ~1% each's mass), so one may think for Kardashev scale purposes this is only 2 galaxies. However, Andromeda has roughly 10x the Milky Way's stars, so this civilization is likely along the lines of a 3.1 rather than a 3.0. Under my assumptions, each member controls roughly 50 million times modern humanity's power, and they still count as very low-end Type 2s if brown dwarves count as stars for the purpose of the definition. Still gods, likely still "automatically yes". $\endgroup$
    – KEY_ABRADE
    Mar 1 at 9:22

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Well yes, but actually hm ?

Energy

Curving spacetime isnt hard, a pencil can do it, warping it at small scales with strong curvature only requires a lot of energy. Whenever these papers talk about Earth masses, they are referring to mass energy, all $E = mc^2$. The paper you mentioned predicts a mass energy equivalent of $E_{tot} = Cv^2_{s}\frac{R^2}{w}$. With apparently works out to a couple of Earth masses for a Radius of 100 meters.

The issue is not getting this much mass, its getting an energy source capable of outputting this much energy on such a small scale. The Energy density here is absurd. We can do the math for this rather easily, a 100 meter sphere has a volume of $4188790 m^3$, one Earth mass is $5.9722\times 10^{24} kg$,

$E = mc^2$

$E = 5.9722\times 10^{24} kg \times 299792458 \frac{m}{s}^2$

$E = 1.7904\times 10^{41} J$

$E_{\rho} = \frac{E}{V}$

$E_{\rho} = \frac{ 1.7904\times 10^{41} J}{4188790 m^3}$

$E_{\rho} = 4.274\times 10^{34} \frac{J}{m^3}$

or in other terms, $4.274 \times 10^{25}$ tons of TNT per cubic meter. This is an absurd amount of Energy per cubic meter. The Energy contained in one cubic meter of this Energy source is an order of magnitude more than the Asteroid that killed the Dinosaurs released in total. That was a 20 kilometer wide rock.

Certainly not impossible, but absurdly dangerous. No known reaction can release this much energy. But there are many things which release more energy as a sum of many small reactions. The Sun obviously releases more than this, but the sun is not 100 meters across !

So, can your guys do it ? If you accept there is a way to make and contain whatever Energy source they are using than yeah. Should be doable in the context of a Sci Fi setting.

Personally, i would argue the less you say about the Energy source the better. You wont be able to create something that actually works, because the only thing i can think of is a Black Hole generator and that is a whole other can of worms. The only known method of extracting Energy from a Black hole is the Penrose process (leaving aside Hawking radiation), and with such an enormous energy expenditure you will burn through the angular momentum of any black hole really quickly. Keep in mind, small black holes cannot contain that much angular momentum. Earth has more Angular momentum than a black hole of its mass can even theoretically have. So if you dont plan to haul a Multi stellar mass black hole with you, even a black hole generator wont do the trick. Not to mention that you would need to somehow feed the black hole faster than your drive burns through energy, without emitting such intense Gamma Radiation half the galaxy is sterilized.

Short Tangent - Alcubierre is not FTL

I want to include this section so that you avoid this older than time mistake. Alcubierre, Wormholes and similar exact solutions to Einstein´s field equations are categorically not Faster-Than-Light. The definition of FTL is, according to the Wiki;

Faster-than-light (also FTL, superluminal or supercausal) travel and communication are the conjectural propagation of matter or information faster than the speed of light (c).

Otherwise known as, FTL is the process by which the velocity of a massive or massless particle exceeds c.

In General Relativity there are two definitions for Velocity. The local and coordinate velocity. The Local Velocity describes the velocity of massive and massless particles through spacetime, while the coordinate velocity is the "speed" of spacetime itself. Both relative to some observer obviously.

The local velocity is constraint by c. While the coordinate velocity is not. Furthermore, these two velocities are independent. Your local velocity can be $0 \frac{m}{s}$ while the coordinate velocity is $10c$. This is possible because the Coordinate Velocity is not subject to any relativistic effects. There is no maximum velocity the underlying spacetime can have.

Wormholes, Alcubierre and so on exploit this fact. They construct spacetimes in which the coordinate velocity is really high, while the local velocity is 0. Take Alcubierre, the spacetime inside the Warp Bubble is flat with no local velocity at all. Meanwhile the overall bubble can move at an arbitary multiple of c, without violating any principles of GR. Because GR only stipulates that the local velocity cannot be bigger than c. Hence why there is no time dilation inside the bubble as well.

Moreover, these metrics (Solutions) do not violate Causality. Which is a corner stone of actual FTL.

Imagine Bob shooting at Alice. The bullet travels at true FTL speeds, its local velocity is > c. From Alices frame of reference, the bullet arrives before the information that the gun was fired. So from her POV, the effect (Bullet hitting her) literally happens before the cause (Gun fired). This breaks Causality because Alice can mathematically prove that from her frame of reference Cause and Effect were reversed.

Imagine the same scenario, but this time Bob fires a Alcubierre bullet. The moment the gun is fired the bullet is enveloped by a Warp drive that travels at the same Coordinate Velocity as the local velocity in the previews example. So v > c. Why doesn't this violate Causality ? While the overall Warp bubble may have v > c, the bullet itself inside the bubble is not moving. Its local velocity is 0, meanwhile light still travels at c. So as the Warp bubble propagates, light containing the information about the event (Gun being fired) constantly leaves the bubble. The bubble travels faster than light so the light ends up damming up in front of the bubble. When the Bubble arrives at Alice, the first thing she sees is the extremely blue shifted light at the front of the Warp bubble, containing the causal information before the bullet hits. This happens no matter the frame of reference. The light may only arrive a fraction of a nanosecond before the bullet, but it always arrives first.

Alcubierre violate Causality in other ways, and only some of the time. This dosnt matter for this discussion, just be aware that these systems are Categorically not FTL.

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  • $\begingroup$ I already knew the difference between the local velocity and coordinate velocity, but I think you did a good job explaining it, especially for readers who don’t the difference. $\endgroup$ Mar 1 at 8:04
  • $\begingroup$ I see how the energy is an issue, but would the rotational energy of a supermassive blackhole in the Center of a galaxy or a quasar have enough energy to support the Alcubierre drive? $\endgroup$ Mar 1 at 8:05
  • $\begingroup$ @TheRocketfan a lot of things technically have enough energy. The problem is density. SMBH´s are not very "dense", their Event Horizons are huge. So i am not sure how you would contain that. Id say just leave the Power source as a black box. $\endgroup$
    – ErikHall
    Mar 1 at 8:10
  • $\begingroup$ A quick google search says that antimatter is the largest specific energy density material known to humankind. The specific energy of antimatter is 180 MJ/μg pubs.aip.org/aip/acp/article-abstract/552/1/939/572933/… $\endgroup$ Mar 1 at 8:39
  • $\begingroup$ @TheRocketfan That depends on what Antimatter you have. The most energy you can get out of Antimatter is dictated by E = mc². Where m is related to the density. Anti Osmium is 22590 kg/m³. Which works out to 2.03x10^21 Joule/m³. Which is a couple of orders of magnitude less than needed. $\endgroup$
    – ErikHall
    Mar 1 at 8:47

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