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Say there is a fourth spatial dimension ( let's call it the q axis as in x,y,z axis ) and that humans are still 3D beings. Could we use an Alcubierre drive to travel through the fourth dimension? As in, our Q position is 1, but now it's 2 and suddenly we can't see Earth anymore because we have different Q positions.

For sake of simplicity, let's just assume the drive has infinite power, and can never overheat.

EDIT : And by fourth dimension I don't mean time.

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  • $\begingroup$ Welcome worldbuilding, to get yourself familiar with this community, please take the tour and visit the help center. Be sure you also give a look at this, to have clear what is on topic here. Other than that, have fun! $\endgroup$ – L.Dutch Aug 11 '18 at 9:44
  • $\begingroup$ x and y are already dimensions, don't use them to indicate other positions. Also, we are already in a 4 dimensional space (x,y,z and t). $\endgroup$ – L.Dutch Aug 11 '18 at 9:59
  • $\begingroup$ I already knew about time being a dimension, is just I'm not that good at phrasing questions it seems. $\endgroup$ – Bogdan 705 Aug 11 '18 at 10:04
  • $\begingroup$ And by doing a quick google search I found out that together, x,y,z and t form space time, t being a temporal dimension and x,y,z being spatial dimensions. In my question I did state "a fourth spatial dimension". $\endgroup$ – Bogdan 705 Aug 11 '18 at 10:11
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The following is a try to find an explanation to how this works "scientifically", skip to the last paragraph for the practical implementation\mechanics of this.

An Alcubierre drive is a theoretically plausible device that bends spacetime geometry to move "faster than light" to an outside observer. With some logical jumps you can say that it can move you through other dimensions I guess, and by some theories there are up to 10 or even 20 dimensions.

but, to our knowledge, all dimensions are continuous. As such, to actually be able to return to earth assuming it's in only one position you'll have to say the Q dimension is finite, so you can be in position 1, 2, 3, 4 or even 0, -1, -2 and so on, but never in position 1.5 or any non - whole dimension.
Also another problem is that gravity and other forces propagate through dimensions: moving in the x axis doesn't make a magnet stop responding to another magnet. You can say that the gravity field is not continuous through this "discrete dimension", or say it's like dark matter and has negligible effect on other dimensions.

So, in conclusion: the Q dimension is discrete, like the popular representation of "parallel universes", and you cannot interact through its different dimensions meaningfully through any classical ways (gravity, electricity..). The Alcubierre drive can bend the spacetime geometry enough to move between two different Q "slices", using something similar to a wormhole.

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    $\begingroup$ The Alcubierre drive doesn't change the topology of spacetime. It changes the geometry of spacetime $\endgroup$ – John Rennie Aug 11 '18 at 14:38
  • $\begingroup$ I don't think this is right. The Alcubierre drive bends space-time in order to travel through it. To travel through a 4th dimension it would need to also manipulate that dimension, which means that gravity does indeed interact through it. $\endgroup$ – Julian L Aug 14 '18 at 19:20
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No. The Alcubierre drive would warp the same spacetime as we're in. No extra dimensions involved.

However ...

The normal description of our spacetime is 3+1 (that's 3 spatial and 1 temporal dimension). If we discovered that our spacetime was actually 4+1 (an extra spatial dimension) then of course any "warp" like drive would be traveling through those dimensions as well. But everything outside the warp would also be in 4+1 space.

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Let's ignore the name "Alcubierre" for a moment

You see, the Alcubierre drive doesn't exist. For all intents and purposes, it's simply a name... a piece of technobabble. We have an idea of how it might operate in really general terms, but without a working model, that's all just more science fiction.

Which is good for you!

Let's address what I believe is your real question. You want to know if it's plausible (aka, "believable") to describe a fourth geometric dimension that our fictional drive can travel through?

It's perfectly satisfactory to throw out time as the forth dimension. After all, the "first three" all describe geometric relationships, why not the fourth? The idea isn't new, a tesseract is to a 3D cube what a 3D cube is to a 2D square. So, we have a geometric 4D universe where there exists a tesseract with 4 equal length legs extending from all 8 verticies.

The question becomes, if you can dtravel through such a unverse, along that 4th leg, would it buy you anything in terms of travel?

The answer is complex, but cool1

Answer #1: No Remember that tesseract? the only way to get from the 3D "inner cube"2 to 3D "outer cube" is by traversing through 4D space. To make it simple, we'll only travel vertex-to-vertex along legs. The shortest distance from any vertex of the inner cube to the closest vertex of the outer cube where the length of all the legs is "x" is 2x. In fact, as we'll see later, this is actually costing you distance and time to traverse the 4th D.

Answer #2: Yes Except that 4D space will allow manipulation that can't be achieved in 3D space. Let's assume a piece of paper represents an infinitely thin 2D plane. Our 2D people want to get from one point to another very distant point. We godlike beings in the 3D universe can easily see that the paper can be bent such that the distance travelled through the 3D universe is very short... even ZERO.... We can bend the paper such that it touches.

Now, this assumes that the 2D plane actually can be bent. We have (and this is REALLY IMPORTANT) not one single example of an actual 2D universe. Not one, nada, zilch. We hypothesize that we can do it because we can easily bend a sheet of paper, but paper isn't actually a 2D object or a 2D universe. In fact, the reality that we do not (or, at least, have not yet) perceived a true 2D universe in any way strongly suggests (but doesn't prove) either (a) there are no other traversible dimensions than the 3rd dimension or (b) No one inhabiting a dimension (2D, 3D, 4D...) can perceive any other dimension.3

And this is OK, because the ability to perceive the 2D universe would mean the ability to manipulate the 2D universe and that would be very godlike and I'm not feeling that today.

Answer #3: Well, no, kinda... yes... but... The next realization from our 3D perspective is that between the top of my head and the soles of my feet is an infinite number of 2D planes. a 2D creature could conceivably travel a distance through 3D space... but the little tyke will end up on another 2D plane. They can pass a distance through 3D space and take an enormous amount of time along a parabolic course to arrive at some point on their own plane (it would always be a longer time due to the shortest-path-between-two-points issue).

In this case we can't bend our universe (the 2D universe) but can travel through the infinite "planar" universes that comprise the 3D universe. In like manner, we travel through the infinite planar 3D universes that make up the 4th D. But it would always take a longer period of time and a longer physical distance.

But the Alcubierre drive is all about bending that piece of paper...

And that's where the magic of fiction comes in. We have this mathematical theory that suggests we can do it. The hint of the possibility makes eyes grow and mouths drool. If we can bend that piece of paper, the only way it can be bent is through a 4th material dimension, just as bending 2D space would need to happen through a 3rd material dimension. (Ignoring Footnote #2 and the ramifications that it's not actually distance that's being manipulated....)

So, ultimately, if you use the Alcubierre drive as your reference, the answer is Yes.

As for whether or not things like magnetism and gravity flow through the 4th dimension, that's a good question. Mathematically they do, but we don't have a shred of evidence. What does it mean to a densizen of 2D land to have a gravitational force in the 3rd direction? It means nothing. What would "gravity" caused by an infinitely thin 2D "mass" mean to a 3D universe? We have no example of anything doing such. And what would be the impact of "gravity" caused by a 4D mass mean to us? So far it means absoltuely nothing at all. All the gravity we can detect is accounted for by 3D objects.

In short, while some tend to treat mathematics as written-in-stone proof, I tend to treat things that only exist mathematically as cool suggestions and hints until actual proof arrives. Violating a mathematical theory before physical proof cannot make your idea suddenly "unrealistic."


1If HDE 226868 disagrees with anything I've said here, he's right. His left big toe knows more about this than I ever will.

2Really, this is from the 3D perspective of the Teserract where it's often drawn with an inner and an outer cube and the reader is expacted to deal with the mind-bending problem of the two cubes having the same physical size and volume. It's a bit like thinking about Dr. Who's Tardis and it's often to take it on faith. I'm using this reference simply to make a point and hope I'm not being too confusing in the process.

3By the way, this is REALLY STRONG EVIDENCE supporting time as the true 4th dimension. We do have evidence that "space-time" is bent by gravity. This isn't a shortening of the geometric distance between two points, but the relativistic perception of the distance travelled due to the bending of space-time. In short, we know space-time can be bent, we can prove it. That kinda disproves the 4th D being geometric, at least in my mind. But that shouldn't stop you from writing your story at all.

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An Alcubierre drive, by definition, can only travel in our dimensions, so the short answer is no. Anything that you can think of to travel through a hypothetical fourth dimension would certainly not be an Alcubierre drive. Inventing an "Alcubierre-like" drive that works in four spatial dimensions would require solving General Relativity equations in four spatial dimensions, which I won't do, but for the sake of argument, I will assume your question is "Can we travel through a fourth spatial dimension by bending space-time?"

First, to travel through dimensions, you need to interact with them. The Alcubierre drive uses space-time curvature (gravitational interaction) to do so, so if you want an Alcubierre-like drive, gravity would need to interact through that fourth dimension.

As in, our Q position is 1, but now it's 2 and suddenly we can't see Earth anymore because we have different Q positions.

For you to literally not see Earth it is required that the electromagnetic interaction (photons) cannot change their Q coordinate. This means that in this 4-dimensional universe, a geodesic (trajectory of shortest length between two points, which is the same as the trajectory of light) never goes through multiple Q values. This means that it is impossible to connect two points with different Q through space-time, they are effectively in separate parallel universes.

It is impossible, through curvature, to connect two points that exist in completely unconnected universes, so we can't travel through different Q coordinates by bending space-time, giving the long answer: not with our current understanding of gravity.

Some alternatives:

  • Your drive can create a wormhole which transports the ship trough the fourth dimension.
  • In your universe, humanity has discovered a new particle which travels through a fourth spatial dimension. They learn how this new particle and its fundamental interaction works and somehow learn to build a drive which uses this interaction to travel through the fourth dimension, similar to how an Alcubierre travels through our three dimensions.
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