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To make it easier to explain, I'm going to use 2-D and 3-D.

Imagine you have a stationary sphere in a 3-dimensional space crossing through a 2-dimensional space (meaning the 2-D perspective has a circle that is a cross-section of that sphere).

If the inhabitants of the 2-D universe start applying force to the circle, what happens? Does the entire sphere move in the 3rd dimension? Does it roll?

Note: the 2-D/3-D is purely to simplify the explanation. I'm guessing this could be expanded to any other size dimensional space. For example, a 1-D force on the sphere or a 3-D force on a 4-D object.

  • Consider for example an electric charge. This would exercise a force on all charged objects in the 3D world. – NDewolf Aug 29 '18 at 14:40
  • @NDewolf I was thinking of a 3D pit where the 2D force moves the ball enough that it falls out of their dimension entirely. – David Starkey Aug 29 '18 at 15:09

3 Answers3

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Physics does not have such an easy concept of "universes within universes".

In classical physics, (if we even allow the notion of varying the number of spatial dimensions) the number of dimensions of the universe dictates a lot of fundamental behaviour. The prime example is the behaviour of the electrostatic or gravitational forces, which scale as $\propto 1/r^2$ with the distance from their sources, essentially because the surface of a sphere goes with $\propto r^2$ in 3 spatial dimensions. Consequently, one expects that universes with $d$ spatial dimensions have force laws that scale as $\propto 1/r^{d-1}$.

But this means that you can't just "embed" one universe in another! A charge (or mass) cannot simultaneously exist in a universe where the force it exerts scales with $1/r$ and one where it scales with $1/r^2$, since then the 3d force does not properly restrict to the 2d force. So the concept of embedding universes, if such a thing exists, must be more subtle than one would naively assume.

In modern field theory there is however, both classically and quantumly, the notion of dimensional reduction or compactification. The simplest instance is Kaluza-Klein theory, where a four-dimensional universe with gravity and electromagnetism emerges from a five-dimensional universe with only gravity whose fifth dimension is compact, which in this case means it has the form of a (very small) circle instead of being infinitely extended like the dimensions in flat space $\mathbb{R}^3$.

In this scenario (which has many other variations on it such as brane cosmology with the Randall-Sundrum model as its archetypal example), the physics in the higher-dimensional universe (the bulk) and the lower-dimensional universe (the brane) are markedly different. The physics of one are not the naive restriction or extension of the other - they observe different fields, different forces, essentially entirely different laws of physics. It is crucial to note that the physics of the brane are entirely derived from the physics in the bulk - there is nothing that is "solely on the brane".

In this context, it is not clear how one would interpret your question, since there are no genuine "lower-dimensional objects" here - everything that lives in the lower-dimensional universe is induced by objects in the higher-dimensional universe, and still follows the laws of physics in the bulk, however different they may appear if one just restricts to the brane. There is no interaction like "pushing on the circle only in 2d", since both the circle and the objects pushing on it are all essentially slices from the higher-dimensional manifold, and what manifests as "pushing in 2d" has a corresponding - but not necessarily recognizable - action that happens in the bulk.

ACuriousMind
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  • Re. "But this means that you can't just "embed" one universe in another!" Does this mean there is a problem with Hadamard's method of descent where he gets the solution to the 2D wave equation by taking a slice thro the 3D solution? – user45664 Aug 30 '18 at 17:04
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    @user45664 Of course Hadamard's method is perfectly valid - it doesn't involve any physical argument, it is pure math. Note that the method doesn't claim that any slice through any solution to the wave equation is a valid solution in the lower dimension - it only claims this for higher-dimensional solutions that do not depend on the extra dimension at all. So sure, if your higher-dimensional universe doesn't do anything with its extra dimension, then you can embed into it naively, but what's the point? – ACuriousMind Aug 30 '18 at 17:25
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If the inhabitants of the 2-D universe apply a force to the circle then they apply a force to the ball, since the circle is the ball. So the circle/ball will accelerate in both universes. If the ball is on the edge of a 3-D pit and slides or rolls down into the pit, it will leave the 2-D universe and they will see the circle's diameter shrink quickly to zero and disappear. (Since their universe is a plane slicing through the sphere, they will always see a circle no matter the shape of the pit.)

Really, there is only one 'universe' in your question: a 3-D space with inhabitants occupying an embedded 2-D plane.

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@David Starkey has given an adequate answer to the question.

None can affect anything in higher dimensions from a lower one. However, a higher dimensional entity can affect lower dimensional entities.

  • A point has no dimension but has a conceptual location, imperceptible to us.
  • Length added to a point is one dimensional (line), and
    perceptible to us.
  • Height added to a Length is two dimensional(plane), and perceptible to us.
  • Depth added to a plane is three dimensional (space), also
    perceptible to us.
  • Another dimension added to space is four dimensional,imperceptible to us.

It is clear from the above statements that a lower dimensional entity won't have complete reach in its immediate higher dimension. However, a higher dimensional entity contains all of its lower dimension/s and can affect or influence anything in lower dimension/s.

As for example, we (being three dimensional) can sense (view and hear) a video from a two dimensional screen but anything from us is not sensible to the entities within the two dimensional screen.