It seems like a pretty fundamental question, the modern-day physicists’ equivalent of knowing whether the world is round or flat. But the question, “How many dimensions are there in the universe?” turns out to be a tricky one — enough so that physicists strongly disagree about the answer.
Proponents of string theory — a highly speculative but popular explanation of the universe’s detailed structure — argue for 10 dimensions. Some theorists have even argued for more, up to an indefinite number of possible dimensions.
Other physicists suggest that experimental results have thrown cold water on the case for higher dimensions, leaving us only with the familiar three dimensions of length, width and height, plus the dimension of time.
Yet, others have suggested that additional dimensions exist but are crumpled up like a piece of paper — so crumpled, in fact, that we can’t detect them.
Dimensions in Perspective
As if these complexities weren’t enough, there’s another level of complication to asking how many dimensions there are in the universe: When physicists talk about dimensions, the word doesn’t quite mean what most of us think it means.
To a physicist, dimension simply means direction — or more precisely, a pair of directions opposite to each other and at right angles to other dimensions, as Phys.org reports. The everyday three dimensions can be described in three different ways:
- Length, width and height
- X, Y and Z
- Forward/backward, right/left and up/down
The fourth dimension, time, is likewise future/past — with the important constraint that in the time dimension you only get to go one way and can’t double back for another try.
In popular culture, we mostly leave mere directions to road signs and use dimensions in a somewhat grander way. As Smithsonian magazine notes, we picture interdimensional portals as leading to not merely some extra directions, but to weird and colorful worlds — worlds where the dinosaurs still rule, or Napoleon won the battle of Waterloo, or where Captain Kirk and the crew of the original Enterprise are the bad guys.
These different meanings of dimension matter a lot when you start asking how many dimensions are there in the universe. But there is a connection, even if — appropriately enough for the whole dimension thing — it’s somewhat roundabout.
A Quick Dimensional Tour
In string theory, all 10 dimensions posited have a specific role, as Phys.org points out. The first four we already know about. But with the fifth dimension, things do get interesting. As Phys.org explains, if we could see it, “we would see a world slightly different from our own that would give us a means of measuring the similarity and differences between our world and other possible ones.” For all we know, the differences might be living dinosaurs, or Evil Captain Kirk.
In the string model, the sixth dimension would let us see all the variant worlds that began with the same initial conditions and only came to evolve differently later on. A peek into the seventh dimension would let us see worlds that differed from ours from the very beginning, and the eighth would let us see all of them, spread out for us to examine.
The ninth dimension would let us “compare all the possible universe histories,” while in the final 10th dimension, “we arrive at the point in which everything possible and imaginable is covered.” At that point, the technical and popular-culture meanings of dimension become pretty much interchangeable.
A Cold Shower for Higher Dimensions?
A universe with 10 dimensions worth of “possible and imaginable” worlds sounds pretty cool, but as Livescience reports, a collision of neutron stars observed in 2017 may have splashed some cold water on all those dimensions.
String theory argues that gravity waves should weaken over very large (intergalactic!) distances, causing the expansion of the universe to gradually accelerate. In string theory, this weakening is due to gravitational energy “leaking” into all those other dimensions. But the neutron star collision observations show no hint of such leakage on scales ranging from about one mile up to 80 million light years.
These findings suggest that the higher dimensions are either pretty small or so vast that a mere 80 million light years isn’t enough to be noticed.
A Multitude of Surprises
The higher dimensions may exist, but they’re likely so crumpled up that they’re too small to easily detect. Phys.org reports a topological form, called a Calabi-Yau manifold, that could confine the higher dimensions to a subatomic scale. Yet, in spite of being so confined, the extra dimensions may still be indirectly detectable by measuring the subtle effects they’ve had over the history of the universe.
CERN, a European nuclear-energy organization, reports that other tools may be available for investigating these dimensions. Gravitons, or hypothesized (but not yet detected) particles produced by gravitation, are one possibility. Examination of atomic-sized black holes — also hypothesized but not yet detected — could provide yet another tool.
How many dimensions are there in the universe? The short answer is that we don’t know yet. And finding the answer is likely to generate a multitude of new questions waiting for us to explore.