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I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. There is a summary of arguments on Wikipedia.

For example, a common argument for why >3 dimensions is too many is that the gravitational law cannot result in stable orbital motion. A common argument for <3 dimensions being too few is that one cannot have a gastrointestinal tract, or more generally, a hole that doesn't split an organism into two.

Am I being overly skeptical in thinking that while the force of gravity may not be able to hold objects in stable orbits, there most certainly exist sets of physical laws in higher dimensions which result in formation of stable structures at all scales? It may be utterly different to our universe, but who said a 4D universe must be the same as ours with one extra dimension?

Similarly, isn't it very easy to conceive of a 2D universe in which organisms can feed despite not having any holes, or not falling apart despite having them? For example, being held together by attractive forces, or allowing certain fundamental objects of a universe to interpenetrate, and thus enter a region of the body in which they become utilized. Or, conceive of a universe so incomprehensibly different to ours that feeding is unnecessary, and self-aware structures form through completely different processes.

While I realise that this is sort of a metaphysical question, is 3+1 dimensions really widely acknowledged to be particularly privileged by respected physicists?

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Well, it seems like the main point of your question is whether it's possible for stable bound structures to form in higher-dimensional spaces, which is a perfectly fine (and not at all metaphysical) question. – David Z Jun 1 '11 at 1:15
If one wants to ask whether 3+1 is privileged, one should also consider the possibility of more than 1 temporal dimensions. :-) – Willie Wong Sep 23 '11 at 17:58
Nature likes to minimize things, its a universal feature. Imagine living in a 3343432411111111110122-dimensional universe, that would require an explanation, not 3+1. – Hobo Dec 2 '12 at 14:54
Related: My answer to: – centralcharge Jun 17 '13 at 10:23
Related MO.SE question: – Qmechanic Feb 27 at 16:50

No. While there are some arguments for why 3 spatial dimensions are a good place to live in, the answer to the question why our universe has 3 large spatial dimensions is presently not known.

Karch & Randall wrote a paper on the issue some years back: They consider some higher dimensional space filled with objects of different dimensions that have some interactions among each other and argue that 3 dimensional ones are among those most likely to dominate. It's an argument though that is not widely accepted due to the assumptions they have to make for this to work.

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Science fiction writer (but also published physicist) Greg Egan has put quite a bit of work into investigated a universe with 4+0 dimensions: Some of it is quite ingenious, eg. assuming a compact universe guarantees that the (modified) wave equation doesn't have exponentially growing solutions and time appears, without the -1 in the spacetime metric, as the local gradient of entropy.

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but how does he prevent a moving object from rotating so that it is going backwards in time? There is no way to prevent this without disconnecting the rotation group. – Ron Maimon Sep 9 '11 at 17:29
He's a bit quiet on this topic:… I'm expecting this topic to be touched in the sequel :-) – Dan Piponi Sep 9 '11 at 18:49

I would like to share my view on this issue.

I think some answers with the word "anthropic" need not to be dismissed, but could be interpreted them in a deeper sense.

Anthropic should not be something derogatory, "just humans", as if we were not part of universe, instead perhaps it could be treated as concepts like "inertial frame of reference" are treated. A measure, a way to measure, a point of view, a frame of reference.

An imagination exercise:

Suppose one day a networking software is self aware.

Then is make some self replicas, and they ask themselves :

"Why we are on layer 7 of the OSI model?"

"Does it have something special?"

One of them would say "Because we can't live in lower layers then if the universe would be lower layered we wouldn't be asking things like this"

Another might say : "To live in layer 7, previous layer must exist to allow us, but, think on layer 0, our conversation are ultimately travelling through a cable for example, then we are at the same time, layer 0, layer 1, ... layer 7, the universe is not layer 7!!, its one or all layer at same time, depending "who" is measuring it, we can see it till layer 7, but the top we see doesn't mean it's the whole that exist, perhaps there are higher layers than 7, and lower than 0, that are forbidden to us, and can't be known at all"

I think 3D+1 is the top that our natural senses are aware of, with technology we could know or suspect other dimensions, as far we know, "conscious beings" can't rise in lower dimensions, but that perhaps is a prejudice, because whatever we call 3D+1 perhaps can be parsed in just 1D! (similar as in the above story), so we should review our statements, of course beings could exist in higher dimensions too (if they do not exist already, they would).

A single matrix in a paper although is within a 3D+1 it could contain higher dimensions, of course a matrix in a paper is not conscious, but nobody knows if a computer program will be aware of itself someday, that day, it will "live" and even "measure" a higher dimension, and again as the matrix in the paper, we would know that it coexist in a lower dimension too.

It's a very interesting topic, I've asked about this before, you could read the answer to that question too

what are dimensions?


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Not sure if an adequate answer, but at least a very innovative one! – arivero Sep 15 '11 at 14:01
I find the analogy between spatial dimensions and layers of abstraction in protocols to be... questionable, if not wholly inapplicable. – Timwi Jul 31 '12 at 12:56

It is the minimum dimension required for the Weyl Tensor $C_{abcd}$to exist in the decomposition of the (completely covariant) Riemann Curvature Tensor $R_{abcd}$. That is kind of privileged. Or else, there would be no gravity in a vacuum (and thus, no long distance gravity, and no orbits, no free-fall)! And if it were any more, the gravity would weaken too quick (the inverse square law would become the inverse cube law, etc.)

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You seem to be implying that General Relativity is particularly necessary for life to exist? Long distance gravity and stable orbits can trivially arise without GR; furthermore, long distance gravity and stable orbits don't sound like something that is absolutely required in the first place. – romkyns Jun 17 '13 at 9:18
@Romkyns: Nope, I didn't say that. General Relativity explains gravity, Newtonian clearly does not EXPLAIN it, but it only describes it. It is just that Newtonian Gravity cannot explain why there is no long-distance gravity in lower than 4 spacetime dimensions. – centralcharge Jun 17 '13 at 10:20
You should clarify "too quick"--- it is difficult to know how quick is too quick. The orbits wouldn't work. But perhaps you could have 4+1d life, with another thing, not planets. Who knows. – Ron Maimon Aug 22 '13 at 22:27
@RonMaimon: By too quick, I meant quicker than 3+1 or 2+1 dimensions, . – centralcharge Aug 23 '13 at 2:04
This answer assumes that physics works the same in a 2 dimensional universe - something that the asker asked us to abandon, which maybe makes the question useless in this context, but still. – Travis May 2 '14 at 15:17

If you look at you'll see a line of reasoning that doesn't seem to depend on the number of spacelike dimensions, yet still arrives at an inverse square law. I realize this isn't exactly a rigorous QED calculation (for which I feel far too stupid) but it makes me reconsider my former belief in non-privilege. If d = 3 is the only case that allows both radiation and conservation of energy, then that's just... wow.

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I think that the uncertainty principle is a pretty strong assumption to start with if you're trying to justify three spatial dimensions. – Eric Aug 29 '11 at 18:58
The argument on Wikipedia is totally wrong. It is assuming that the particle absorption is independent of the geometry, and it is assuming that to get from point A to point B a virtual particle needs to travel at the speed of light. Unfortunately, these two idiotic mistakes cancel out each other in three dimensions. – Ron Maimon Sep 10 '11 at 20:14

I am not sure that being in 3+1-D is a privilege. Actually, all the troubles with Feynmann integrals come from 4D. Secondly, the QFT is integrable only in 2+1-D. From the mathematical point of view, the 4D differentiable manifolds are most problematic.

On contrary, I also heard that if the space is not 3D then the signal cannot be transmitted, but at the moment I don't know the proof. This is significant, since without signal transmission, our world has a bigger problem than to be able to tie a knot from a string.

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You mean that if the space is not 3D then a signal cannot be transmitted using the exact same electromagnetic radiation, right? I mean, you're not saying that signal transmission is fundamentally impossible in 5D, right? – romkyns Mar 30 '14 at 12:40

I think that many different dimensions and metric signatures have their specific “privileges”. More general, different geometries in a broader sense, and, even more general, different underlying mathematical structures (such as fields other than ℝ) also could be models for space-time of some alternative physics. But is was just (necessary for me) philosophical preface.

One time and PDEs

What is special for Lorentzian manifolds with their (locally) one temporal and three spatial dimensions? First of all, a metric signature with one temporal dimension but some (one or more) spatial dimensions is something very special (I deliberately ignore the question of mathematical sign, whether $t^2$ is positive and $x^2$ negative or versa — it doesn’t make difference where the time is distinguished). There are two cases where a Cauchy problem can be solved for degree-2 partial differential equation, for a reasonably broad class of initial and boundary conditions. In $t^2 - x^2$-like metrics hyperbolic differential equations live. Notoriously, the other case are parabolic differential equations that are degree-2 by space but degree-1 by time and correspond to Galilean time; so it is also one-time-many-space-dimensions universe. In parabolic case, of course, there is no non-degenerate quadratic metric.

What is special with Cauchy problem? It is a natural formulation of evolution problem. We specified an initial state of the field, we specified boundary conditions, and we can predict evolution. And even without boundary conditions hyperbolic equations (but not parabolic) admit a solution in a cone-like domain, of space-time points where initial conditions traverse all the past cone. Hyperbolic PDE is the only case that allows exact prediction (is certain spacetime domain) in spite of spatially-bounded knowledge of initial conditions.

For more that one time solutions will not be unique. For a “time”, geometrically, no different from space, a solution will not always exist.

Specificity of 3 + 1

Let’s think we proved that exactly one time dimension is a requisite. Why is special to have exactly 3 spatial dimensions, D = 3? In the case of quadratic metric (corresponding to abovementioned hyperbolic PDEs) the answer is simple: orthogonal group is the Lorentz group. Its unity component is isomorphic to Möbius group. The universal cover of said unity component is SL(2, ℂ) – it is very convenient for quantum field theory and other applications.

The case of D = 1 is inconvenient for numerous reasons (not only symmetry-related). In the case of D = 2, apart of not having the full geometric SU(2), we’d have more types of quantum statistics than two types that we have in our universe (fermions and bosons). We could have particles with arbitrary angular momentum; it’s IMHO not for good. But photons couldn’t have helicity. What all this quantum stuff is for, indeed? Although D = 2 can be, in principle, habitable, it unlikely will be a quantum world.

What about D > 3, indeed? Geometrical gains are insignificant. There are some theories that requires extra dimensions but… in 4+ dimensional spaces we should have more than 2-component spinors. It is an unnecessary complication, isn’t it?

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