Imagine a world where you can only move forwards and backwards along a line. You’d see nothing but single points in front of you, and nothing but single points behind you—a one-dimensional world. Now expand this to a second dimension—you can move forwards and backwards, and also left and right, experiencing the world as flat lines moving around one another. But you couldn’t drill a hole into something in this universe—you’d split it in half. Add a third dimension, and you’ve got depth—things take on visible forms and can move around one another, becoming the rich world we live in today. Then there’s a fourth dimension, time, through which we can only experience forward motion.

What if there was a fifth dimension through which things could move? What would it be like? Does it exist somewhere else, obscured by our four-dimensional brains? Or what if the universe secretly has eleven dimensions, some of which are curled up and only experienced by the smallest units of mass?

Like anyone else who’s taken a physics class (or smoked weed) we wondered whether a fifth dimension could exist somewhere in the universe, and if so, what it would be like. We’ve reached out to physicists and mathematicians to help us figure it out and it turns out there’s a lot of disagreement surrounding the question.

### Djuna Croon

*Postdoctoral researcher at in TRIUMF Theory Group, Vancouver BC, Canada*

The existence of a fifth dimension is an interesting possibility that may or may not be realized in nature. In particular, an extra dimension which behaves like space (instead of like time) but is compact rather than infinite features in many theories of particle physics. The extra dimension may for example explain properties of particles that we observe in our experiments. It may also predict the existence of other particles, or explain how gravity behaves.

Whether the fifth dimension is “real” or not, it can provide a useful mathematical device. The 5D picture allows us to calculate things that are much harder to calculate in 4D.

We will probably never directly see the fifth dimension, but that does not mean we cannot find convincing evidence. For example, no one has ever detected an isolated quark, but the quark model does an incredible job at explaining the properties of larger, composite particles (hadrons). Because of this, scientists today agree that quarks are fundamental particles. Likewise, if we would collect enough evidence that 5D theories “work”—explain the existing data and make successful predictions—then the existence of a fifth dimension would be very well motivated.

### Timm Wrase

*Physicist at the Institute for Theoretical Physics of the Vienna University of Technology*

I am a string theorist and as such am trying to combine Einstein’s theory of general relativity with quantum mechanics. String theory is actually only consistent, if you have not only a fifth dimension but also a sixth, seventh, eighth, ninth and tenth dimension. If you do not have these extra dimensions then you cannot write down a theoretically consistent theory that describes our universe.

How do extra dimensions look like? Since we can’t see them, they have to very small. The classical example is a straw. From far away it looks like a 1-dimensional object but if you look at it closely you see that it has a second dimension that is a circle. So the fifth dimension that would be everywhere in space should be a circle that is very small. Since it is so small we cannot see it. There are more elaborate ways of experimentally searching for extra dimension that don’t use light and a microscope but they all have in common that they can only give an upper bound on the length of a circular extra dimension. So depending on how small or large such an extra dimension is, we could either discover it in the near future or potentially never.

If you have more than one extra dimension then they can be all circular but in principle they can also have more complicated shapes. So for example two extra dimensions could take the shape of a donut (this is essentially two circles) or they can be for example a sphere.

The big question is, if there is one or more extra dimensions then why are they so different from the three spatial dimensions that we are used to. These are not small circles but very large. They could be gigantic circles and it is possible that if you fly with your spaceship for a very long time in one direction that you will come back to Earth. We have no evidence for such circularity of any of the three dimensions but we cannot exclude it either. Definitely our three dimensions are very different from tiny extra dimensions and there are speculations to why that is but nothing of this is really understood. A problem with for example having only two large dimension (left-right and up-down but no back and forth) is that life as we know it could not exist. Similar problems might or might not exist for more than three large space dimensions. So it could be that life as we know it favors one time and three large space dimensions (but again this is far from really understood).

### Sabine Hossenfelder

*Research fellow at the Frankfurt Institute for Advanced Studies*

We have zero evidence for the existence of a fifth dimension of space. Physicists certainly pursue ideas according to which there might be a fifth dimension (or more than five dimensions) but since we have no evidence, they always follow up those ideas with excuses for why haven’t seen the new dimension(s). Typically it’s because we would only see new dimensions at distances shorter or longer than what we have measured so far.

There are now a lot of theoretical physicists who make a living calculating details of models with extra dimensions like, say, if a fifth dimension existed with these specific properties what would it do to this bump in the cosmic microwave background or such. But absent any evidence, or even plausible reason, for the existence of those dimensions you may as well debate whether unicorns shit rainbows.

### Peter Woit

*Senior Lecturer in the Mathematics department at Columbia University*

There are lots of ways of adding a fifth dimension, the problem is just that if it’s anything like the other four, there’s no experimental evidence for it at all, and no viable theory that uses a hidden fifth dimension to explain anything convincingly. The Standard Model of elementary particles is a geometrical theory, and as such, involves geometrical dimensions beyond the usual four space-time ones. But these “internal” dimensions behave quite differently, don’t follow at all the physical laws that govern space and time.

**Sean Carroll**

*Research professor in the Department of Physics at the California Institute of Technology*

We have no idea whether there’s a fifth (or sixth, seventh...) dimension or not. The possibility is predicted by some very promising theories, such as string theory, but there’s no experimental evidence in favor of it as yet. If there are extra dimensions, they have to somehow be hidden from our notice. That could be either because they are curled into a little ball, or because the particles and forces of which we are made are confined to a three-dimensional “brane” inside a larger space. Physicists are actively conducting experimental searches for all the various possibilities.

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