The Fourth Dimension

GA 324a — 31 May 1905, Berlin

Fifth Lecture

Last time, we tried to get an idea of a four-dimensional space. To visualize it, we reduced it to a three-dimensional one. First, we started by transforming a three-dimensional space into a two-dimensional one. We used colors instead of dimensions. We formed the idea in such a way that a cube appeared in three colors along the three dimensions. Then we laid the boundaries of a cube on the plane, which resulted in six squares in different colors. Through the diversity of colors on the individual sides, we obtained the three different dimensions in two-dimensional space. We had three colors, and with that we had represented the three dimensions.

We then imagined that we were passing a square cube into the third dimension, as if we were passing it through a colored fog and it reappeared on the other side. We imagined that we had pass squares, so that the square cubes move through these squares and are thereby tinged [with the color of the pass square]. This is how we tried to imagine the [three-dimensional] cube [by means of a two-dimensional color representation]. [For the one-dimensional representation of the] surfaces, we thus have two boundary colors and [for the two-dimensional representation of the] cube, three colors. [To represent a four-dimensional spatial structure in three-dimensional space, we must] then add a fourth boundary color.

Now we have to imagine in the same way that a cube, which, analogous to our square, has two different colors as boundary sides, has three different colors in its boundary surfaces. And finally, each cube moves through another cube that has the corresponding fourth color. In doing so, we let it disappear into the fourth color dimension. So, according to Hinton's analogy, we let the respective boundary cubes pass through the new [fourth] color, which then reappears on the other side, emerging in their [original] own color.

Now I will give you another analogy and first reduce the three dimensions back to two, so that we will then be able to reduce four dimensions to three. To do this, we have to imagine the following. The cube can be put together at its boundary surfaces from its six boundary squares; but instead of doing it in succession, as we did recently, it will now be done in a different way. I will also draw this figure (Figure 31). You see, we have now spread out the cube in two systems, each of which lies in the plane and consists of three squares. Now we have to be clear about how these different areas will lie when we actually put the cube together. I ask you to consider the following. If I now want to reassemble the cube from these six squares, I have to place the two sections on top of each other so that square 6 comes to rest on square 5. When square 5 is placed at the bottom, I have to fold up squares 1 and 2, while folding down squares 3 and 4 (Figure 32). In doing so, we get certain corresponding lines that overlap. The lines marked in the figure with the same color [here in the same line quality and in the same number of lines] will coincide. What lies here in the plane, in two-dimensional space, coincides to a certain extent when I move into three-dimensional space.

The square consists of four sides, the cube of six squares, and the four-dimensional area would then have to consist of eight cubes.? We call this four-dimensional area a tessaract [after Hinton]. Now, the point is that these eight cubes cannot simply be reassembled into a cube, but that one of them should always pass through the fourth dimension in the appropriate way.

If I now want to do the same with the tessaract as I just did with the cube, I have to follow the same law. The point is to find analogies of the three-dimensional to the two-dimensional and then of the four-dimensional to the three-dimensional. Just as I obtained two systems of [three squares each] here, the same thing happens with the tessaract with [two systems of four cubes each] when I fold a four-dimensional tessaract into three-dimensional space. The system of eight cubes is very ingeniously devised. This structure will then look like this (Figure 33).

Each time, these four cubes in three-dimensional space are to be taken exactly as these squares in two-dimensional space.

You just have to look carefully at what I have done here. When the cube was folded into two-dimensional space, a system of six squares resulted; when the corresponding procedure is carried out on the tessaract, we obtain a system of eight cubes (Figure 34). We have transferred the observation from three-dimensional space to four-dimensional space. [Folding up and joining the squares in three-dimensional space corresponds to folding up and joining the cubes in four-dimensional space.] In the case of the folded-down cube, [in the two-dimensional plane] different corresponding lines were obtained, which coincided when it was folded up again later. The same occurs with the surfaces of our individual cubes of the tessaract. [When the tessaract is folded down in three-dimensional space, corresponding surfaces appear on the corresponding cubes.] So, for example, in the case of the tessaract, the upper horizontal surface of

cube 1—by observing [mediation] the fourth dimension—with the front face of cube 5.

In the same way, the right face of cube 1 coincides with the front square of cube 4, and likewise the left square of cube 1 with the front square of cube 3 [as well as the lower square of cube 1 with the front square of cube 6]. The same applies to the other cube surfaces. The remaining cube, 7, is enclosed by the other six.

You see that here again we are concerned with finding analogies between the third and fourth dimensions. Just as a fifth square enclosed by four squares remains invisible to the being that can only see in two dimensions, as we saw in the corresponding figure of the previous lecture (Figure 29), so it is the case here with the seventh cube: it remains hidden from the three-dimensional eye. Corresponding to this seventh cube in the tessaract is an eighth cube, which, since we have a four-dimensional body here, lies as a counterpart to the seventh in the fourth dimension.

All analogies lead us to prepare for the fourth dimension. Nothing forces us to add the other dimensions to the usual dimensions [within the mere spatial view]. Following Hinton, we could also think of colors here and think of cubes put together in such a way that the corresponding colors come together. It is hardly possible in any other way [than by such analogies] to give a description of how to think of a four-dimensional entity.

Now I would like to mention another way [of representing four-dimensional bodies in three-dimensional space], which may also give you a better understanding of what we are actually dealing with here. This is an octahedron bounded by eight triangles, with the sides meeting at obtuse angles (Figure 35).

If you visualize this structure here, I ask you to follow the following procedure with me in your mind. You see, here one surface is always intersected by another. Here, for example, in AB, two side surfaces meet, and here in EB, two meet. The entire difference between an octahedron and a cube lies in the angle of intersection of the side surfaces. If surfaces intersect as they do in a cube [at right angles], a cube is formed. But if they intersect as they do here [obtuse], then an octahedron is formed. The point is that we can have surfaces intersect at the most diverse angles, and then we get the most diverse spatial structures."

Now imagine that we could also make the same faces of the octahedron intersect in a different way. Imagine this face here, for example AEB, continued on all sides, and this lower one here, BCF, also (Figure 36). Then likewise the ADF and EDC lying backwards. Then these faces must also intersect, and in fact they intersect here in a doubly symmetrical way. If you extend these surfaces in this way, [four of the original boundary surfaces] are no longer needed: ABF, EBC and, towards the back, EAD and DCF. So of the eight surfaces, four remain. And the four that remain give this tetrahedron, which is also called half of an octahedron. It is therefore half of an octahedron because it intersects half of the faces of the octahedron. It is not the case that you cut the octahedron in half. If you bring the other four faces of the octahedron to the cut, the result is also a tetrahedron, which together with the first tetrahedron has the octahedron as a common intersection. In stereometry [geometric crystallography], it is not the part that is halved that is called the half, but the one that is created by halving the [number of] faces. With the octahedron, this is quite easy to imagine.

If you imagine halving the cube in the same way, that is, if you allow one face to intersect with the corresponding other face, you will always get a cube. Half of a cube is a cube again. I would like to draw an important conclusion from this, but first I would like to use something else to help me.

Here I have a rhombic dodecahedron (Figure 37). You can see that the surfaces adjoin each other at certain angles. At the same time, we can see a system of four wires, which I would like to call axial wires, and which run in opposite directions to each other [i.e. connect certain opposite corners of the rhombic dodecahedron, and are therefore diagonals]. These wires now represent a system of axes in a similar way to the way in which you imagined a system of axes on the cube.

You get the cube when you create sections in a system of three perpendicular axes by introducing blockages in each of these axes.

If the axes are made to intersect at other angles, a different spatial figure is obtained. The rhombic dodecahedron has axes which intersect at angles other than right angles.

The cube reflects itself in half.

But this applies only to the cube. The rhombic dodecahedron, cut in half, also gives a different spatial structure.

Now let us take the relation of the octahedron to the tetrahedron. And I will tell you what is meant by this. This becomes clear when we gradually let the octahedron merge into the tetrahedron. For this purpose, let us take a tetrahedron, which we cut off at one vertex (Figure 38). We continue this process until the cut surfaces meet at the edges of the tetrahedron; then what remains is the indicated octahedron. In this way we obtain an eight-sided figure from a three-dimensional figure bounded by four surfaces, provided we cut off the corners at corresponding angles.

What I have done here with the tetrahedron, you cannot do with the cube.

The cube has very special properties, namely that it is the counterpart of three-dimensional space. Imagine the entire universe structured in such a way that it has three perpendicular axes. If you then imagine surfaces perpendicular to these three axes, you will, under all circumstances, get a cube (Figure 39). That is why, when we speak of the cube, we mean the theoretical cube, which is the counterpart of three-dimensional space. Just as the tetrahedron is the counterpart of the octahedron when I make the sides of the octahedron into certain sections, so the single cube is the counterpart of the whole of space.” If you think of the whole of space as positive, the cube is negative. The cube is the polar opposite of the whole of space. Space has in the physical cube its actually corresponding structure.

Now suppose I would not limit the [three-dimensional] space by two-dimensional planes, but I would limit it in such a way that I would have it limited by six spheres [thus by three-dimensional figures].

I first define two-dimensional space by having four circles that go inside each other [i.e., two-dimensional shapes]. You can now imagine that these four circles are getting bigger and bigger [as the radius gets longer and longer and the center point moves further and further away]; then, over time, they will all merge into a straight line (Figure 40). You then get four intersecting lines, and instead of the four circles, a square.

Now imagine that the circles are spheres, and that there are six of them, forming a kind of mulberry (Figure 41). If you imagine the spheres in the same way as the circles, that they get larger and larger in diameter, then these six spheres will ultimately become the boundary surfaces of a cube, just as the four circles became the boundary lines of a square.

The cube has now been created from the fact that we had six spheres that have become flat. So the cube is nothing more than a special case of six interlocking spheres – just as the square is nothing more than a special case of four interlocking circles.

If you are clear in your mind about how to imagine these six spheres, that they correspond to our earlier squares when brought into the plane, and if you imagine an absolutely round shape passing into a straight one, you will get the simplest spatial form. The cube can be imagined as the flattening of six spheres pushed into each other.

You can say of a point on a circle that it must pass through the second dimension if it is to come to another point on the circle. But if you have made the circle so large that it forms a straight line, then every point on the circle can come to every other point on the circle through the first dimension.

We are considering a square bounded by figures, each of which has two dimensions. As long as each of the four boundary figures is a circle, it is therefore two-dimensional. Each boundary figure, when it has become a straight line, is one-dimensional.

Each boundary surface of a cube is formed from a three-dimensional structure in such a way that each of the six boundary spheres has one dimension removed. Such a boundary surface has therefore been created by the third dimension being reduced to two, so to speak bent back. It has therefore lost a dimension. The second dimension was created by losing the dimension of depth. One could therefore imagine that each spatial dimension was created by losing a corresponding higher dimension.

Just as we obtain a three-dimensional figure with two-dimensional boundaries when we reduce three-dimensional boundary figures to two-dimensional ones, so you must conclude that when we look at three-dimensional space, we have to think of each direction as being flattened out, and indeed flattened out from an infinite circle; so that if you could progress in one direction, you would come back from the other. Thus, each [ordinary] spatial dimension has come about through the loss of the corresponding other [dimension]. In our three-dimensional space, there is a three-axis system. These are three perpendicular axes that have lost the corresponding other dimensions and have thus become flat.

So you get three-dimensional space when you straighten each of the [three] axis directions. If you proceed in reverse, each spatial part could become curved again. Then the following series of thoughts would arise: If you curve the one-dimensional structure, you get a two-dimensional one; by curving the two-dimensional structure, you get a three-dimensional one. If you finally curve a three-dimensional structure, you get a four-dimensional structure, so that the four-dimensional can also be imagined as a three-dimensional structure curved on itself.*

And with that, I come from the dead to the living. Through this bending, you can find the transition from the dead to the living. Four-dimensional space is so specialized [at the transition into three dimensions] that it has become flat. Death is [for human consciousness] nothing more than the bending of the three-dimensional into the four-dimensional. [For the physical body taken by itself, it is the other way around: death is a flattening of the four-dimensional into the three-dimensional.]