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I am writing a science fiction novel therefore I am using the idea of the Alcubierre drive for warp travel which involves some handwavium but has roots in theoretical physics (which is my goal). While looking at the NASA design for their hypothetical warp ship:

enter image description here

I noticed the two rings (which I assumed it would be for compression of space in front and expansion of space in back). This got me wondering about the negative mass requirement. I understand that using negative mass would allow a repulsion, therefore allowing expansion in the back. However why would the ring in front also need negative mass if it compressing space? Would it make theoretical sense to use dark matter instead (I know dark matter is called dark due to us not knowing exactly what it is but we do know that it is a strong attractive force). If not then why do both rings need the negative mass? Thank you for the help and thoughts.

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    $\begingroup$ FWIW: I think dark matter is called "dark" because it does not interact with EM radiation, and thus is "nonluminous," not because we are ignorant of its true nature. (We are, but that's not the reason behind the name.) $\endgroup$
    – Tom
    Mar 6, 2022 at 7:27
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    $\begingroup$ Why do you think both rings need negative mass? $\endgroup$
    – Daron
    Mar 6, 2022 at 12:53
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    $\begingroup$ Just curious, I see no links.. I wonder where you looked at this "NASA design", I would prefer an illustration here, explaining the rings. When I go search I find this critical oneliner nasa.gov/centers/glenn/technology/warp/warp.html which refers to some dreams of the interstellar future coffeeordie.com/alcubierre-white-warp-drive but there are no concrete (science based) designs. The Wiki reference in the Alcubierre topic (2012) refers to a non-existing page. Maybe I missed something ? $\endgroup$
    – Goodies
    Mar 6, 2022 at 13:03
  • $\begingroup$ .. NASA has been silent about the subject since 2015. It is easier to find references to NASA from science fiction websites.. maybe you refer to futurism.com ? page is 2014, NASA is presented as a source, various designs are presented futurism.com/meet-nasas-newest-design-for-a-warp-drive-ship $\endgroup$
    – Goodies
    Mar 6, 2022 at 13:09
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    $\begingroup$ @H.M It sounds like no one here can think of a reason both rings need negative mass either. It makes more sense for the back one to be negative and the front one to be positive. $\endgroup$
    – Daron
    Mar 7, 2022 at 11:48

2 Answers 2

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I know dark matter is called dark due to us not knowing exactly what it is but we do know that it is a strong attractive force

The problem with dark matter is that it is very much not a strong attractive force. It only seems to interact with the rest of the universe via the weak nuclear force (which is very weak, and quite short ranged) and via gravity (which is weak, but long ranged).

Because of this, there's no convenient way in which to corral a useful amount of dark matter in a reasonable volume... you can't keep it in a container, because it will pass straight through the walls basically unimpeded, and you can't trap it in an electromagnetic field which only really leaves gravitational attraction. And if you can generate nice strong gravitational fields in a compact volume, then you don't actually need the gravitational mass provided by dark matter in the first place!

looking at the NASA design for their hypothetical warp ship, I noticed the two rings

These sorts of things are always artist's impressions, because no-one has any idea what the nature of a warp field generator would be. Treat them as inspiration, not engineering guidelines.

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    $\begingroup$ It's not even clear that dark matter interacts via the weak nuclear force. Perhaps only through gravity. $\endgroup$
    – doneal24
    Mar 6, 2022 at 18:35
  • $\begingroup$ @doneal24 there's some possibility that we'll detect weakly interacting dark matter particles, but it needs more sensitive detectors than we have now. I wouldn't put money on any new particles being discovered though. $\endgroup$
    – Starfish Prime
    Mar 6, 2022 at 19:06
  • $\begingroup$ @StarfishPrime I liked the design for inspiration since it makes sense to me for there is be two rings for the compression and expansion of a warp drive. I am just stuck on the compression aspect since I handwave a material that can produce the negative mass for the repulsion in the back and I was wondering if I would need to handwave a material that has similar affects to dark matter for the compression since all the papers said they just need a ton of negative mass or energy. I hope that made sense xD $\endgroup$
    – H. M
    Mar 6, 2022 at 20:16
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    $\begingroup$ @H.M its difficult to really describe or envisage what is happening in a warp field in 3d space, but the implication of at least some of the things I've looked at is that the negative-energy-density-violating-unobtanium is the only thing you actually need for an Alcubierre-style warp bubble. You arrange in in a suitable shape and it effectively distorts space by both expanding and compressing it. An additional source of gravity is not required. $\endgroup$
    – Starfish Prime
    Mar 6, 2022 at 21:41
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Saddling over space time

The ring on the back will push, the ring on the front will pull

The Alcubierre drive is based on deformation of space time itself, creating a means of forward propulsion by folding (or warping) space time into a saddle surface around the ship, which looks like this (projected in 3d)

enter image description here

Note the ship is not sticked to the surface, as would be the case in classic models. The ship actually travels straight over space time, and the bent surface in front of the ship provides the shortcut over long distances. The amount of FTL depends on the amount of spacetime "bridged" by the ship's forward movement. The steeper the gradients are, the more space time will be shortcut and the faster your ship will travel.

Rings are different polarity

To create a space time valley in front of the ship, there needs to be a "positive mass", but that is not a real, physical mass, or any object mass. The ship's rings contain field generators that generate an artificial gravitational field, which causes the saddle surface in spacetime to exist. The ship "falls" into forward direction.

The saddle surface cannot have a single lobe. On the back of the ship, you have a similar generator that has opposite polarity: it will project a negative mass behind the ship, so the gradient (or perceived gravity) is negative.

The saddle surface is huge, it can span light years. But that mode of operation is only safe in interstellar space. In the first weeks of travel, the saddle surface will gradually expand, keeping the middle part tilted, in effect accelerating the ship. At some point it will travel FTL over the valley.. when arrived on the other side a new saddle is generated and the procedure is repeated, until the target is reached.

How you hit the breaks or change direction when using an Alcubierre drive

The ship is actually pushed moving over spacetime. That push ends when a platform is formed. The platform will look as follows:

enter image description here

While the ship is "on" the platform, it will experience 0G and travel with a single speed. It can decelerate, weaken the gradients, or change direction. During travel, the saddle's shape, allows some velocity control, but the ship can't change direction. The two wheels make the ship only travel straight forward, during FTL.

Mind the exhaust: gravitational wave interference

One little note I can leave about this ancient FTL warp technology: there will be side effects from using it. Your ship will affect the space it travels through. While it expands, the saddle surface will invoke gravitational waves, which are not healthy for life, when subjected to them at close range. For that reason, you can't switch an Alcubierre drive on, anywhere near inhabited planets. A distance of about 80 AU is advised, which is about 2x Pluto's orbit.

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